Alveolar macrophages in tissue homeostasis, inflammation, and infection: evolving concepts of therapeutic targeting

Alveolar macrophages (AMs) are the sentinel cells of the alveolar space, maintaining homeostasis, fending off pathogens, and controlling lung inflammation. During acute lung injury, AMs orchestrate the initiation and resolution of inflammation in order to ultimately restore homeostasis. This central role in acute lung inflammation makes AMs attractive targets for therapeutic interventions. Single-cell RNA-Seq and spatial omics approaches, together with methodological advances such as the generation of human macrophages from pluripotent stem cells, have increased understanding of the ontogeny, function, and plasticity of AMs during infectious and sterile lung inflammation, which could move the field closer to clinical application. However, proresolution phenotypes might conflict with proinflammatory and antibacterial responses. Therefore, therapeutic targeting of AMs at vulnerable time points over the course of infectious lung injury might harbor the risk of serious side effects, such as loss of antibacterial host defense capacity. Thus, the identification of key signaling hubs that determine functional fate decisions in AMs is of the utmost importance to harness their therapeutic potential.

AMs as central regulators of the innate immune defense in respiratory diseases. The tolerogenic programming of AMs is overrun upon infection, when inertly antiinflammatory AMs overcome local immunosuppressive signals and switch to a more proinflammatory phenotype (20, 43). Given the plasticity of AMs and their spatial distribution in the lung, AMs play indispensable and quite versatile roles responding to the inhaled pathogens they are faced with.

AMs can initiate leukocyte recruitment (44, 45) and directly eliminate the pathogen using multiple pathogen-specific mechanisms (15). For example, upon encounter of Legionella pneumophila, AMs activate the interleukin-1β (IL-1β) pathway to recruit neighboring monocytes (46). Upon challenge with Aspergillus fumigatus, murine AMs control infection by activating the NADPH oxidase and ROS pathway (47, 48). When the overall bacterial load exceeds their capacity for pathogen degradation in the phagolysosomes, as observed with Streptococcus pneumoniae, AMs can switch on the apoptosis pathway in order to prevent bacterial dissemination, through the classical process of phagocytosis, as observed in a murine infection model (49). Well-adapted pathogens like Mycobacterium tuberculosis are capable of subverting macrophage containment strategies to ultimately promote their dissemination (50).

AMs are critical coordinators of the immune response against viral pathogens, via secretion of proinflammatory cytokines/chemokines such as IL-6, IL-8, or CXCL10, initiation of type I interferon (IFN) signaling, and enhanced expression of pattern recognition receptors, as well as inhibition of nuclear export of viral genome as shown in a series of elegant studies on influenza virus infection (51–55). Further murine infection models also revealed that AMs can utilize other molecular mechanisms, such as arginase-1 expression, phagocytosis of fungal conidia (48, 56), directed migration toward and phagocytosis of bacterial pathogens, and release of IFN-γ (15, 57), to support pathogen clearance from the alveoli, which is eventually performed by recruited neutrophils and bone marrow–derived macrophages (BMDMs). Depletion of the AM pool, either experimentally (e.g., by clodronate) or during the natural course of disease (often observed in respiratory viral infection) (58–60), is linked to increased morbidity and mortality (61–67). Increased AM death and reduced self-renewal capacity (68) might account for this, yet the molecular mechanisms of AM depletion are not fully elucidated.

Depending on the type of injury and the extent of AM loss, the alveolar niche can be replenished either through local proliferation or through the recruitment of BMDMs from the periphery, as a response to chemoattractants such as CCL2 (69). These newly recruited macrophages exhibit several transcriptional differences from their resident alveolar counterparts regarding metabolism, proliferation, and inflammation signaling (70, 71). AMs are capable of self-renewal and mainly use the tricarboxylic acid cycle and amino acid metabolism in order to maintain their functions, whereas recruited BMDMs are governed by glycolysis and arginine metabolism, while adopting a more proinflammatory profile (66, 70, 72). This profile is characterized by the abundant release of molecules such as TNF-α, IL-6, and inducible NOS, which are instrumental for pathogen clearance during the acute infection phase (53, 73). As opposed to that, BMDMs can also drive inflammatory tissue damage characterized by epithelial injury, degradation of extracellular matrix, and cytokine-dependent prolonged inflammation, which aggravate barrier loss and gas exchange function and may worsen disease outcome (60, 64, 66, 73–79). Addressing specific targets related to the proinflammatory response of these cells could, therefore, mitigate their injurious potential without compromising host defense and pathogen clearance. In this regard, blocking of macrophage-derived TRAIL attenuated epithelial cell apoptosis and improved edema reabsorption and survival following severe influenza A virus infection in mice (73, 76), while membrane-tethered matrix metalloprotease (MT1-MMP) inhibition attenuated tissue damage without altering the immune response (79).

Generally, various types of injury over an individual’s lifespan result in the gradual replacement of the original, yolk sac–derived AMs by BMDMs (80), creating a mosaic of resident macrophages of different ontogeny, with distinct transcriptome signature, metabolic profile, and responses to infection (81). This heterogeneous constitution must be taken into account in considering macrophage-based therapeutic interventions in pneumonia or lung injury. Given that BMDM-replenished resident AMs may retain defined lineage- or inflammation-imprinted BMDM characteristics (35, 66), it is of utmost importance to pinpoint the correct timing and definition of the functional macrophage phenotype required to be targeted or promoted in a specific type of injury.

AMs as drivers of inflammation resolution. AMs actively contribute to inflammation resolution and to the repair of the denuded airways through secretion of immunomodulatory cytokines such as IL-10, CCL22, and a plethora of growth factors, including TGF-β, VEGF, trefoil factor 2, and PDGF (35, 82–84). However, alveolar cues that initiate the resolving properties of AMs are largely unknown. Interestingly, TNF-α released by AMs was shown to promote the release of GM-CSF by alveolar epithelial cells and thus support alveolar epithelial repair (85). In a model of helminth infection, IL-4 and IL-13, together with apoptotic cells, upregulated antiinflammatory and tissue-repair genes within AMs (86). Genetic deletion of AXL and MERTK, which are crucial receptors for phagocytic uptake, impaired resolution of inflammation in this model, underlining the importance of efferocytosis.

Repair processes following injury require a tight balance between inflammation resolution, epithelial barrier restoration, and the prevention of aberrant remodeling. Newly replenished, pro-regenerative AMs exhibit a profibrotic potential in animal models of bleomycin-induced fibrosis, in patients with pulmonary fibrosis, and during infection (87–91). In the context of COVID-19, human monocyte-derived AM subsets contribute to immunopathology by perpetuating inflammation and promoting post-resolution profibrotic pathways, leading to aberrant lung remodeling and an idiopathic pulmonary fibrosis–like phenotype (92–94). The specific mechanisms involved remain a matter of intensive research, aimed at dissecting the intertwined roles of innate and adaptive immunity in these processes (60, 78, 94). Reprogramming of AMs and targeting of fibrosis-related intracellular pathways in favor of a coordinated and accelerated epithelial repair, therefore, emerge as potential therapeutic strategies for a clinical phenotype with remarkably limited treatment options (90, 95, 96).

Aging adds an additional level of complexity to the aforementioned differentiation trajectories and AM phenotype. The transcription profile of AMs heavily depends on age, with cell cycle–related genes being downregulated in aging animals and humans (61, 81). Upregulation of genes related to injury resolution and lung remodeling is a characteristic of resident AMs found in the lungs of aged mice (97). Local microenvironment is an important factor shaping AM functions with age (66, 97). Hyaluronan increase in the extracellular matrix of the lungs of aged animals diminishes the pro-proliferative effect of GM-CSF on macrophages, which could explain the gradual decrease in AM numbers over time (81). Aging additionally impairs the phagocytosis and efferocytosis capacity of AMs, leading to retention of activated neutrophils within the alveoli with prolonged lung damage in mice (61). Adoptive transfer of rejuvenated AMs into an aged lung could, therefore, constitute a potential therapeutic intervention.

Pneumonia results in pathogen- and injury-dependent reprogramming of AM functions. Inflammation-derived lung injury is characterized by enhanced susceptibility to secondary bacterial hits in the aftermath of infection (98–101) or acid pneumonitis (102). This susceptibility can primarily be attributed to diminished numbers, impaired bactericidal properties, and the adoption of a pro-resolution phenotype by AMs (45, 58, 102–107). Human and murine infection studies showed that as inflammation subsides, T lymphocytes orchestrate an IFN-γ–driven downregulation of macrophage receptor with collagenous structure (MARCO) expression on AMs (82, 108–110), diminishing antibacterial properties of AMs (111). Depletion of IFN-γ was, therefore, shown to restore bacterial control in this particular context (110). AMs also actively terminate neutrophil influx in murine models of bacterial and LPS-induced inflammation (112–114). NO production is reduced, while the 15-lipoxygenase pathway, which is initiated by AMs during efferocytosis, further promotes inflammation resolution (115, 116). However, this shift toward an antiinflammatory phenotype comes at the cost of bacterial clearance (117, 118). Medeiros et al. delineated in an animal study that prostaglandin E2 (PGE2) dampens bacterial clearance (118). In vitro, pharmacological interference with this pathway restored antibacterial control. Notably, PGE₂ inhibition in influenza virus–infected mice enhanced immunity against infection (119), illustrating the impact of immunomodulation during acute lung injury.

Efferocytosis is a key event of inflammation resolution and the initiation of tissue repair (120–122). Macrophages are capable of responding to “find me” (123, 124) and “eat me” signals from apoptotic cells through a variety of scavenger or tyrosine kinase receptors, integrins, and complement receptors (125–127). Intriguingly, cell type–specific efferocytosis of neutrophils rewires mitochondrial metabolism to switch AMs to a pro-resolution phenotype. Neutrophil myeloperoxidase locked AMs in a state of pro-resolution, which precluded generation of mitochondrial ROS via uncoupling protein 2 upon bacterial encounter (128). Roquilly et al. described SIRPα-dependent impaired antibacterial properties in AMs months after bacterial pneumonia, conferred by the lung environment (129). AMs from SIRPα-knockout mice maintained phagocytic functions, and peripheral blood monocytes of patients exhibited improved phagocytosis after treatment with an anti-SIRPα antibody (129). In contrast, depending on the specific infection context, reprogramming of AMs may result in improved bactericidal properties, as a sign of trained immunity. Prior mycobacterial infection or adenovirus exposure improved AM defense against a secondary bacterial infection (130, 131), or resulted in heterologous protection against a rechallenge with M. tuberculosis (132). Guillon et al. revealed that after S. pneumoniae–induced pneumonia in mice, the newly acquired AM phenotype was long-lasting over a period of 6 months, regionally localized to the affected lung lobe, and resulted in enhanced AM-driven protection against another pneumococcal serotype (133).

Ex vivo culture models of murine AMs. Knowing the vital roles of AMs in regulating lung homeostasis and inflammation necessitates better understanding of the underlying mechanisms pertaining to AM functions. Although immortalized murine AM cell lines, such as MH-S cells, exist, these cells lack key hallmarks of AMs, such as expression of sialic acid–binding Ig-like lectin-F (Siglec-F), and the specific phenotype imprinted by the lung environment (134, 135). Studies using primary AMs have improved our knowledge of AM biology. However, lavage-isolated primary AMs do not reflect the whole AM pool, as AMs that are more adherent to the epithelium might be missed (136).

Recently, several groups investigated different cell culture models to replicate the AM phenotype and functions in ex vivo settings (Figure 1). Murine fetal liver cells can serve as a resource for the long-term culture of self-replicating AMs. Culturing fetal liver cells with GM-CSF gave rise to self-replicating AMs that could be sustained in culture for 8 weeks (137). Long-term culture of non-transformed AMs was also achieved from other cell sources, such as adult bone marrow cells (138). The synergistic effect of GM-CSF in combination with TGF-β and PPARγ agonist such as rosiglitazone was essential to reproduce the AM-like phenotype from cultured adult bone marrow cells. The generated AM-like cells from this culturing condition expressed typical AM surface markers (Siglec-F, CD11c), AM-specific genes such as carbonic anhydrase 4 (Car4), placenta-expressed transcript 1 (Plet1), and self-renewal genes (138). The triple combination of these growth factors resulted in prolonged expansion of mouse ex vivo–cultured AMs (mexAMs) over several months and similar functionality to primary AMs in terms of phagocytosis capacity and cytokine secretion upon challenge with S. pneumoniae or LPS (139). Intranasal transfer of mexAMs showed successful transfer, engraftment, and proliferation of the administered cells. Similarly, adoptive transfer of mexAMs into mice lacking AMs because of GM-CSF deficiency resulted in the restoration of lung homeostasis (139).

Figure 1

Depiction of diverse murine/human-based AM culturing models promoting translational AM research. The figure summarizes the various modern ex vivo culturing approaches that have used murine or human-derived AMs in combination with human induced pluripotent stem cells (hiPSCs) or murine pluripotent stem cells (mPSCs), mediating the transition to 3D culturing systems (organoids, precision-cut lung slices, etc.), all dedicated to studying the biology of AMs and potential therapies targeting these cells.

Such optimized murine AM models provide powerful tools to deepen our understanding of AM biology and behavior during homeostasis and disease. Nevertheless, single-cell RNA-Seq experiments strongly indicate that the lung AM populations are heterogeneous (6, 140). This heterogeneity might not be ideally reflected in models in which mice were maintained under tightly controlled, specific pathogen–free conditions, with limited exposure to lung insults, and breeding facility–specific microbiome that might substantially influence AM phenotype and function (141, 142). AMs in the lung niche of naive laboratory mice are mostly of embryonic origin. In contrast, the AM pool in humans is a mix of embryonic and BMDM-derived AMs (143, 144). Moreover, the lung macrophage compartment is much more diverse in humans, compared with mice, at least in the context of lung cancer (144). In fact, transcriptomic analysis revealed proliferating and non-proliferating AMs in healthy humans, in addition to the added diversity through the characterization of newly emerging AM subtypes within different disease entities, such as fibrosis and malignant lung disease (145).

Ex vivo culture models of human AMs. To overcome the shortcomings of murine-based systems and to provide a more clinically relevant model of AM culture, several approaches have been taken to either prolong the primary culture of AMs or identify a regenerative supply of macrophages from different stem cell sources. In this context, a technique was developed to culture human AMs ex vivo from resected lung tissues of patients with pulmonary tuberculosis, caused by M. tuberculosis (146). This method revealed important insights into the interaction between M. tuberculosis and human cells, identifying factors that can control mycobacterium infection in the lung microenvironment (147).

Recently, Pahari et al. demonstrated that human blood-derived monocytes cultured with lung lipids, GM-CSF, TGF-β, and IL-10 exhibited similar morphology, transcriptional profile, and functions to those of human AMs (148). While the ability to culture primary cells ex vivo is helpful, such approaches are limited by the lack of a three-dimensional environment that can mimic the cell-to-cell interaction and recapitulate the complex cellularity of the lung parenchyma and airways. Hence, other 3D ex vivo systems should be explored, such as spatially organized multicellular human lung organoids or human precision-cut lung slices (hPCLSs). PCLSs are slices of healthy or diseased lungs containing all cell types found in the desired tissue, while still mirroring the underlying changes in the context of different diseases affecting the extracellular matrix. Use of hPCLSs revealed important insights into the ability of Staphylococcus aureus to survive and grow in AMs. S. aureus cannot use AMs for intracellular growth during infection but is, rather, present in the epithelial and interstitial regions of hPCLSs, highlighting important antibacterial features of AMs in a preserved human lung environment (149).

While isolation of primary AMs from human samples is clinically relevant, it is also hampered by low yields and the inability to genetically engineer these cells for disease modeling. In addition, isolation techniques can have detrimental impacts on the quality and marker expression patterns of the isolated AMs, necessitating caution in interpreting the results. Along these lines, human induced pluripotent stem cells (hiPSCs) can meet the need for scalable, personalized, and standardized sources of human macrophages. Additionally, iPSCs have the advantage of being easily manipulated by genome editing methods (CRISPR/Cas9, TALENs), providing a powerful tool for disease modeling (150). iPSC-derived macrophages (iMacs) share the phenotypic, functional, and transcriptional hallmarks of professional phagocytes, while the transcriptional profile resembles that of primitive macrophages (151–153). In fact, iPSCs are thought to drive macrophage differentiation predominantly through the embryonic hematopoiesis pathway, in a MYB-independent but RUNX1- and SPI1 (PU.1)–dependent fashion. Correspondingly, TRMs, including AMs, are of embryonic origin, driven from their yolk sac progenitors in a MYB-independent fashion. This suggests that iMacs represent better models of TRMs compared with primary monocytes (153, 154).

Another advantage to the use of iMacs is the capacity to scale up iMac generation, allowing long-term production of industrial quantities of iMacs with consistent quality. iMacs can also be incorporated into 3D culture systems that can feature the human “lung in a dish.” Research efforts in human lung organoids resulted in major advancements in identifying the molecular cues needed to facilitate the establishment of various models with different spatial complexity and levels of epithelial cell commitment and differentiation. Currently, lung organoids stemming from adult human lung-committed progenitor cells such as basal cells or alveolar type II cells (giving rise to bronchiospheres and alveolospheres), from fetal lung progenitor cells, or from iPSCs can harbor multiple airway and alveolar cell types and are used in either 3D culture or in vivo transplantation models (155–158). iMacs can be successfully incorporated into lung organoids stemming from iPSCs, providing an indispensable tool to study the interaction of macrophages with the human lung environment in homeostasis and disease, including infection, while overcoming many of the limitations of in vivo animal models or ex vivo–derived murine and human primary cultures (Figure 1). In fact, incorporating iMacs into alveolar organoids resulted in higher levels of chemokines after LPS stimulation (159). Similarly, murine lung organoids complemented with AMs had higher chemokine and cytokine levels than organoids without AMs (160). iPSC-derived, iMac-complemented lung organoids harbor the additional advantage of generating patient-specific models for future personalized diagnostic or treatment approaches (161).

With the continuous advancement of currently existing translational AM models and the knowledge gained on AM biology, these cells are currently emerging as highly promising candidates for the treatment of pathogen-driven inflammatory lung diseases.

AMs as a cell-based therapeutic regimen. The regenerative feature of the pluripotent stem cell (PSC) system and the convenience of generating murine or human macrophages in upscaled quantities under good manufacturing practice conditions in a continuous fashion have promoted the application of AMs as a cell-based therapeutic option.

Several groups have examined the potential of macrophage transplant therapy in resolving different pulmonary diseases. In one study, AMs derived from murine PSCs were used to ameliorate acute and chronic lung injury. To mimic the acute lung injury (ALI) model, AMs were partially depleted, whereas the chronic lung injury was modeled using adenosine deaminase (ADA) knockout mice that typically suffer from a plethora of pulmonary abnormalities with pulmonary failure and poor survival. The intratracheal administration of PSC-AMs was a successful and safe strategy to resolve the acute and chronic injury by engulfing neutrophils and driving lung repair, thus prolonging survival, while AMs persisted in the airways for a minimum of 4 weeks (162).

In cases in which the hyperinflammatory environment is the main driver of ALI, the therapeutic intervention would vary, requiring the neutralization of the inflammatory environment instead of merely boosting of the AM niche. Hence, in an ALI model induced by LPS administration, skewing of the AM phenotype to an antiinflammatory profile was examined. This was achieved by pulmonary macrophage transfer of murine engineered macrophages (RAW264.7 macrophages or BMDMs) that can secrete IL-4. The sustained secretion of IL-4 from AMs polarized the macrophages to a pro-resolution phenotype, halted the harmful proinflammatory cascade, and activated tissue repair and remodeling programs (163).

Beyond the use of AMs as a cellular therapy for lung tissue repair and homeostasis maintenance, adoptive transfer of AMs was also a successful strategy in combating pulmonary infections. In proof-of-concept studies, adoptive transfer of iMacs successfully rescued mice from Pseudomonas aeruginosa–mediated acute infections within 4–8 hours after intrapulmonary transplantation, with a reduction of the bacterial load (151). Similar findings were also provided by a murine pneumonia model of methicillin-resistant S. aureus (MRSA), where the transplanted iMacs were able to elicit a more than 10-fold reduction of the bacterial load within 20 hours of adoptive transfer (164).

Overall, these data highlight the antiinflammatory/pro-repair properties of AMs. However, proinflammatory functions have to be taken into account and deserve further studies, particularly in the setting of ALI, where inflammatory tissue damage determines outcome.

Correction of dysfunctional AM phenotypes. Cell-based therapy is of particular importance in individuals suffering from genetic diseases that cause malfunctioning AMs. Hematopoietic stem cell transplantation (HSCT) has been studied in that regard as a non-cell-specific strategy to correct deficiencies in the AM compartment. Cystic fibrosis (CF) is one prominent example of a genetic disease–causing mutation in the CF transmembrane conductance regulator (CFTR) protein, which ultimately leads to defective mucus clearance and heavy bacterial colonization of the lung by various species that develop antibiotic resistance. In addition, loss of CFTR in macrophages impacts proper phagosome acidification (165), and macrophages derived from CF patient–specific iPSC lines exhibit a dysregulated type I IFN response (166). Hence, in such a devastating disease course, healthy AMs may be key players in controlling the associated pulmonary infections.

HSCT was performed in CFTR-deficient mice suffering from acute airway infection with P. aeruginosa. The transfer of competent stem cells allowed the differentiation and successful homing of healthy AMs that cleared the bacterial infection (167). Another in vivo murine study demonstrated the promising potential of this intervention in the context of the Mendelian susceptibility to mycobacterial disease (MSMD) syndrome. Genetically impaired IFN-γ–mediated immunity characterizes this syndrome, manifesting within the lymphoid and myeloid compartment, and limits tissue macrophage capacity to clear mycobacteria. Gene-corrected hematopoietic stem cells that were transplanted into Ifnγr1^–/– mice infected with Mycobacterium bovis, Bacille Calmette-Guérin (BCG), gave rise to AMs with restored antibacterial activity against BCG, maintained lung integrity, and prolonged overall mouse survival (168).

AM-specific targeting approaches were used to investigate whether primary macrophages could be directly engineered to correct a pathological phenotype. To demonstrate the feasibility of this approach, a third-generation self-inactivating lentivirus was used to engineer primary human and murine macrophages inducing the expression of the human α₁-antitrypsin (hAAT). Such macrophages could be used to compensate for AAT deficiency in affected individuals suffering from pulmonary emphysema as a consequence of the deficiency. Overexpression of hAAT in murine AMs (an AM cell line and BMDM-derived AMs) resulted in secretion of hAAT in vivo, which was detected in the bronchoalveolar lavage fluid of AM-transplanted mice. Secretion of hAAT by the genetically corrected administered macrophages also correlated with increased overall survival after BCG infection, suggesting that genetic engineering of primary monocytes/macrophages could be a potentially valid approach to address pulmonary disease (169).

Hereditary pulmonary alveolar proteinosis (PAP) is another genetic disease that can benefit from AM-based cellular therapy. In this disease, recessive mutations in the GM-CSF receptor genes CSF2RA and CSF2RB lead to a subsequent defect in the GM-CSF receptor, which correlates with poor AM differentiation and function. Clinically, this is associated with a predisposition to respiratory infection and lung failure (170). Intratracheal application of iMacs in a humanized model of hereditary PAP showed promising therapeutic potential. Two months after engraftment, iMacs differentiated into functional AMs, reducing the surfactant levels and restoring lung homeostasis (170). Similarly, modeling hereditary PAP using Csf2ra-knockout mice revealed that the pulmonary transfer of genetically corrected macrophages expressing GM-CSF receptor α can improve the disease phenotype and mediate long-term AM engraftment lasting up to 6 months (171). While the aforementioned studies highlight the promising potential of adoptive macrophage transfer for cell-based therapy, many factors need to be determined to guarantee the successful clinical translation of such an approach. One of these factors is the durability of the administered treatment, as different studies report varying findings concerning how long transplanted AMs are sustained in the lung niche, with the longest reported period reaching up to 3 years after transplant (172).

Donor tolerance to the allograft is a further key determinant of cell transplantation success. Preconditioning of the lung environment prior to macrophage transplant using a liposomal macrophage-depleting agent like clodronate was shown to promote hematopoietic chimerism, resulting in better donor tolerance to the received cells (173). The possible occurrence of graft-versus-host disease upon the subsequent transfer of allogenic AMs must, however, be fully addressed before ex vivo findings can be transferred to the clinic. Current knowledge on this subject is limited, in particular regarding the extent of the immune reaction that such a transplantation therapy may elicit as well as the ideal immunosuppressive method. For instance, MHC I–mismatched AMs are able to elicit humoral and cellular immune responses to lung-associated self-antigens (172).

AMs as targets for drug delivery and for modification of their functional state. Several features facilitate therapeutic targeting of AMs in the lungs. AMs are positioned at an easily accessible site, allowing administration of drugs by nebulization techniques that generate small droplet sizes to reach the alveolar compartment, or targeting by drug-containing particles or other carriers.

Novel formulation techniques were used to promote AM-targeted antibiotic therapy. In a recent study, the authors used a polymeric ciprofloxacin prodrug with a linker for mannose ligands. The abundant expression of mannose receptors on the surface of AMs facilitated ciprofloxacin intake by AMs, and the formulation was more effective in protecting the mice from a Burkholderia pseudomallei clinical isolate as compared with the administration of the free drug (174). Furthermore, liposome-based formulations were very successful for AM-targeted antibiotic therapy. Liposomal amikacin was recently approved by the FDA for treatment of non-tuberculous mycobacterial biofilms with high efficacy of the antibiotic in the infected AMs (175). Lipid nanocarriers encapsulating levofloxacin were conjugated with fucosyl residues, which enabled the targeting of the mannose receptor–positive AMs. As in other studies, the encapsulated antibiotic showed superior activity compared with the “free” antibiotic in the mycobacterial clearance of infected AMs. Likewise, ciprofloxacin in liposomal formulation showed protective effects against pneumonic plague in vivo (176).

Other AM-targeting therapeutic strategies have aimed at activating different cellular programs (pro- or antiinflammatory) of AMs based on the needed role in the given disease state. For example, the activation of PPARγ controlled host response and lung pathology during influenza virus pneumonia in mice by skewing AM polarization status toward the anti–inflammatory macrophage subtype (177). Similar findings were seen in a bleomycin-induced lung injury model, where PPARγ activation was linked to an enhanced AM efferocytic potential and a tight regulation of IL-10 and TGF-β secretion.

The functional (re)polarization of AMs is of particular importance in infectious and inflammatory lung injury, due to the critical role of macrophages in all stages of the disease. Recent efforts have focused on repurposing drugs for inflammatory lung injury and infections to switch the proinflammatory state of AMs to an antiinflammatory, pro-resolution phenotype, or to increase macrophage host defense, as reviewed in ref. 178. Zanubrutinib targets Bruton’s tyrosine kinase (BTK) activation, inhibiting JAK2/STAT1 and TLR4/MyD88/NF-κB signaling pathways and promoting activation of STAT6 and PI3K/Akt signaling pathways (179). Canagliflozin is an SGLT2 inhibitor with pronounced antiinflammatory actions that inhibits the IL-1β–stimulated secretion of IL-6 and impacts glucose metabolism in macrophages (180). As these strategies were applied in sterile lung inflammation models, the impact of such strategies with regard to AM host defense capacities during lung infection remains unknown (179, 180).

Alternative efficient approaches target AM capacity to clear pathogens via the activation of different cellular death programs. In the context of chronic obstructive pulmonary disease, an ongoing study is screening for compounds that can activate macrophage efferocytosis (ClinicalTrials.gov NCT04775394). Additionally, several studies highlighted the potential of triggering autophagy in infected AMs (181, 182). Using autophagy inducers like all-trans-retinoic acid (ATRA) helped reduce the bacterial burden in M. tuberculosis–infected AMs (183). Autophagy can also be activated by the counteracting of autophagy-inhibitory signals, such as the inhibition of mTORC1 to prevent autophagosome formation. Several mTOR inhibitors (rapamycin, everolimus) are promising in controlling the bacterial burden of M. tuberculosis infection (181).

GM-CSF, an indispensable factor for AM differentiation, survival, and homeostasis functions, improved AM host defense in bacterial and viral pneumonia models when locally deposited into the lungs of mice either by therapeutic administration or by genetic overexpression in the alveolar epithelium (63, 184–189). Intrapulmonary deposition of GM-CSF also prevented bacterial superinfection after influenza in mice (58, 190) and improved anti-influenza adaptive immunity by targeting lung-resident dendritic cells (191). Notably, GM-CSF exerts a direct effect on the injured alveolar epithelium in sterile and infectious lung injury, which improves outcome independently of the myeloid cell compartment by driving proliferation of epithelial progenitor cells and resealing the alveolar barrier (85, 192). Inhaled recombinant human GM-CSF (sargramostim, molgramostim) has recently emerged as a successful strategy for driving a beneficial AM phenotype and improving barrier function in pneumonia-associated acute respiratory distress syndrome, including COVID-19 (193–196), with the first phase II trials revealing encouraging results and a beneficial safety profile (ClinicalTrials.gov NCT02595060, NCT04569877). On the other hand, blocking the GM-CSF receptor with mavrilimumab in cases of severe lung damage due to late-stage COVID-19 was suggested to reduce inflammation and control lung injury (197, 198), highlighting the dual role of the GM-CSF signaling pathway in tailoring macrophage polarization within disease progression. In summary, several approaches demonstrated the potential of AMs for cell-based therapy. The aforementioned strategies highlight encouraging proof-of-concept studies, laying the foundation for further investigation of the promising potential of AMs in clinical applications.

Specific AM-targeting or AM-based cell therapies are still in their infancy, but several promising and clinically effective strategies are emerging (Figure 2). Such diverse targeting strategies, however, pose a challenge in identifying whether the current disease stage necessitates a harnessed, suppressed, or disease-specifically modified AM. Therefore, a deep understanding of the given disease pathology, together with proper identification of inflammation onset and the disease-specific course of AM reprogramming, is needed. An increasing knowledge of the mechanistic regulations of AM functions, together with the possibility of generating stem cell–derived human macrophages, might provide therapeutic options to promote lung repair without impairing pathogen control.

Figure 2

AM-targeting strategies for the treatment of pulmonary diseases. Different approaches to support and/or repair the pool of endogenous AMs are pursued. AMs can either be targeted by viral and nonviral approaches to restore and/or enhance their function in vivo (left), or be modified by different molecules to influence and/or promote a certain stage of activation to promote lung tissue repair or antibacterial function (right). An alternative and innovative approach would be the direct transfer of macrophages into the lungs of patients (middle). Suitable macrophages can be derived from human iPSCs (iMacs) or classical monocytes (monocyte-derived macrophages [MDMs]) by new immune cell farming strategies.

CM, NL, UM, and SH conceptualized the project. CM and SA performed the investigation. NL, UM, and SH acquired resources for the project. CM and SMA wrote the original draft of the manuscript. CM, SMA, NL, UM, and SH reviewed and edited the manuscript. SMA and NL visualized the project. NL, UM, and SH acquired funding for the project. All authors contributed to the article and approved the submitted version. The order of the co-first authors was decided based on responsibility for covering different manuscript aspects (introduction, main text, concluding remarks, figures, references) as well as different stages of manuscript preparation (manuscript writing and editing, coordination between the 2 sites).

The work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy - EXC 2155 - project number 390874280 and DFG support LA 3680/9-1 as well as REBIRTH Research Center for Translational Regenerative Medicine “Förderung aus Mitteln des Niedersächsischen Vorab” (grant: ZN3340). The work received further funding by the Mukoviszidose Institute under project ‘Pulmonary transplantation of macrophages as a cell-based therapy to treat chronic infections in the cystic fibrosis lung’, project number 1905. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 852178) and is funded also by the European Union (grant agreement no. 101100859). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council (ERC). Neither the European Union nor the granting authority can be held responsible for them. This work was also supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under following projects: KFO309 project number 284237345; SFB-TR84 project number 114933180; SFB1021 project number 197785619; the Hessen State Ministry of Higher Education, Research and the Arts (Landes-Offensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz, LOEWE), grant “diffusible signals”, LOEWE CoroPan, P2; LOEWE-Förderlinie 4a, project ID: III L7 – 519/05.00.002, the German Center for Lung Research (Deutsches Zentrum für Lungenforschung, DZL), and the Cardiopulmonary Institute (CPI; DFG EXC 2026).

Address correspondence to: Susanne Herold, Department of Internal Medicine V, University Clinic of Giessen, Klinikstrasse 33, 35392 Giessen, Germany. Phone: +49 (0)641 985 57061. Email: Susanne.Herold@innere.med.uni-giessen.de.

Conflict of interest: NL has filed a patent application on the generation of human iPSC-derived macrophages (IP PCT/EP2018/061574). NL and SMA have filed intellectual property for the use of iPSC-derived macrophages in the framework of a monocyte activation test (PCT/EP2021/083371).

Copyright: © 2023, Malainou et al. This is an open access article published under the terms of the Creative Commons Attribution 4.0 International License.

Reference information: J Clin Invest. 2023;133(19):e170501. https://doi.org/10.1172/JCI170501.

1. Hume PS, et al. Localization of macrophages in the human lung via design-based stereology. Am J Respir Crit Care Med. 2020;201(10):1209–1217.
   View this article via: CrossRef PubMed Google Scholar
2. Aegerter H, et al. Biology of lung macrophages in health and disease. Immunity. 2022;55(9):1564–1580.
   View this article via: CrossRef PubMed Google Scholar
3. Bain CC, MacDonald AS. The impact of the lung environment on macrophage development, activation and function: diversity in the face of adversity. Mucosal Immunol. 2022;15(2):223–234.
   View this article via: CrossRef PubMed Google Scholar
4. Hou F, et al. Diversity of macrophages in lung homeostasis and diseases. Front Immunol. 2021;12:753940.
   View this article via: CrossRef PubMed Google Scholar
5. Guilliams M, et al. Alveolar macrophages develop from fetal monocytes that differentiate into long-lived cells in the first week of life via GM-CSF. J Exp Med. 2013;210(10):1977–1992.
   View this article via: CrossRef PubMed Google Scholar
6. Hashimoto D, et al. Tissue-resident macrophages self-maintain locally throughout adult life with minimal contribution from circulating monocytes. Immunity. 2013;38(4):792–804.
   View this article via: CrossRef PubMed Google Scholar
7. van de Laar L, et al. Yolk sac macrophages, fetal liver, and adult monocytes can colonize an empty niche and develop into functional tissue-resident macrophages. Immunity. 2016;44(4):755–768.
   View this article via: CrossRef PubMed Google Scholar
8. Scott CL, Guilliams M. Tissue unit-ed: lung cells team up to drive alveolar macrophage development. Cell. 2018;175(4):898–900.
   View this article via: CrossRef PubMed Google Scholar
9. T’Jonck W, et al. Niche signals and transcription factors involved in tissue-resident macrophage development. Cell Immunol. 2018;330:43–53.
   View this article via: CrossRef PubMed Google Scholar
10. Scott CL, et al. The transcription factor ZEB2 is required to maintain the tissue-specific identities of macrophages. Immunity. 2018;49(2):312–325.
    View this article via: CrossRef PubMed Google Scholar
11. Gautier EL, et al. Gene-expression profiles and transcriptional regulatory pathways that underlie the identity and diversity of mouse tissue macrophages. Nat Immunol. 2012;13(11):1118–1128.
    View this article via: CrossRef PubMed Google Scholar
12. Lavin Y, et al. Tissue-resident macrophage enhancer landscapes are shaped by the local microenvironment. Cell. 2014;159(6):1312–1326.
    View this article via: CrossRef PubMed Google Scholar
13. Gordon S, et al. Macrophage heterogeneity in tissues: phenotypic diversity and functions. Immunol Rev. 2014;262(1):36–55.
    View this article via: CrossRef PubMed Google Scholar
14. Wynn TA, et al. Macrophage biology in development, homeostasis and disease. Nature. 2013;496(7446):445–455.
    View this article via: CrossRef PubMed Google Scholar
15. Neupane AS, et al. Patrolling alveolar macrophages conceal bacteria from the immune system to maintain homeostasis. Cell. 2020;183(1):110–125.
    View this article via: CrossRef PubMed Google Scholar
16. Meliopoulos VA, et al. An epithelial integrin regulates the amplitude of protective lung interferon responses against multiple respiratory pathogens. PLoS Pathog. 2016;12(8):e1005804.
    View this article via: CrossRef PubMed Google Scholar
17. Westphalen K, et al. Sessile alveolar macrophages communicate with alveolar epithelium to modulate immunity. Nature. 2014;506(7489):503–506.
    View this article via: CrossRef PubMed Google Scholar
18. Hussell T, Bell TJ. Alveolar macrophages: plasticity in a tissue-specific context. Nat Rev Immunol. 2014;14(2):81–93.
    View this article via: CrossRef PubMed Google Scholar
19. Peters DM, et al. TGF-β directs trafficking of the epithelial sodium channel ENaC which has implications for ion and fluid transport in acute lung injury. Proc Natl Acad Sci U S A. 2014;111(3):E374–E383.
    View this article via: CrossRef PubMed Google Scholar
20. Kopf M, et al. The development and function of lung-resident macrophages and dendritic cells. Nat Immunol. 2015;16(1):36–44.
    View this article via: CrossRef PubMed Google Scholar
21. Bissonnette EY, et al. Cross-talk between alveolar macrophages and lung epithelial cells is essential to maintain lung homeostasis. Front Immunol. 2020;11:583042.
    View this article via: CrossRef PubMed Google Scholar
22. Janssen WJ, et al. Surfactant proteins A and D suppress alveolar macrophage phagocytosis via interaction with SIRP alpha. Am J Respir Crit Care Med. 2008;178(2):158–167.
    View this article via: CrossRef PubMed Google Scholar
23. Coleman MM, et al. Alveolar macrophages contribute to respiratory tolerance by inducing FoxP3 expression in naive T cells. Am J Respir Cell Mol Biol. 2013;48(6):773–780.
    View this article via: CrossRef PubMed Google Scholar
24. Blumenthal RL, et al. Human alveolar macrophages induce functional inactivation in antigen-specific CD4 T cells. J Allergy Clin Immunol. 2001;107(2):258–264.
    View this article via: CrossRef PubMed Google Scholar
25. Chelen CJ, et al. Human alveolar macrophages present antigen ineffectively due to defective expression of B7 costimulatory cell surface molecules. J Clin Invest. 1995;95(3):1415–1421.
    View this article via: JCI CrossRef PubMed Google Scholar
26. Bernasconi E, et al. Airway microbiota determines innate cell inflammatory or tissue remodeling profiles in lung transplantation. Am J Respir Crit Care Med. 2016;194(10):1252–1263.
    View this article via: CrossRef PubMed Google Scholar
27. Brown RL, et al. The microbiota protects against respiratory infection via GM-CSF signaling. Nat Commun. 2017;8(1):1512.
    View this article via: CrossRef PubMed Google Scholar
28. Epelman S, et al. Origin and functions of tissue macrophages. Immunity. 2014;41(1):21–35.
    View this article via: CrossRef PubMed Google Scholar
29. Yu X, et al. The cytokine TGF-β promotes the development and homeostasis of alveolar macrophages. Immunity. 2017;47(5):903–912.
    View this article via: CrossRef PubMed Google Scholar
30. Hu G, Christman JW. Editorial: Alveolar macrophages in lung inflammation and resolution. Front Immunol. 2019;10:2275.
    View this article via: CrossRef PubMed Google Scholar
31. Bleriot C, et al. Determinants of resident tissue macrophage identity and function. Immunity. 2020;52(6):957–970.
    View this article via: CrossRef PubMed Google Scholar
32. Subramanian S, et al. Long-term culture-expanded alveolar macrophages restore their full epigenetic identity after transfer in vivo. Nat Immunol. 2022;23(3):458–468.
    View this article via: CrossRef PubMed Google Scholar
33. Svedberg FR, et al. The lung environment controls alveolar macrophage metabolism and responsiveness in type 2 inflammation. Nat Immunol. 2019;20(5):571–580.
    View this article via: CrossRef PubMed Google Scholar
34. Machiels B, et al. A gammaherpesvirus provides protection against allergic asthma by inducing the replacement of resident alveolar macrophages with regulatory monocytes. Nat Immunol. 2017;18(12):1310–1320.
    View this article via: CrossRef PubMed Google Scholar
35. Aegerter H, et al. Influenza-induced monocyte-derived alveolar macrophages confer prolonged antibacterial protection. Nat Immunol. 2020;21(2):145–157.
    View this article via: CrossRef PubMed Google Scholar
36. Ginhoux F, Guilliams M. Tissue-resident macrophage ontogeny and homeostasis. Immunity. 2016;44(3):439–449.
    View this article via: CrossRef PubMed Google Scholar
37. Mass E, et al. Tissue-specific macrophages: how they develop and choreograph tissue biology. [published online March 15, 2023]. Nat Rev Immunol. https://doi.org/10.1038/s41577-023-00848-y.
    View this article via: PubMed Google Scholar
38. Schultze JL, et al. Emerging principles in myelopoiesis at homeostasis and during infection and inflammation. Immunity. 2019;50(2):288–301.
    View this article via: CrossRef PubMed Google Scholar
39. Shapouri-Moghaddam A, et al. Macrophage plasticity, polarization, and function in health and disease. J Cell Physiol. 2018;233(9):6425–6440.
    View this article via: CrossRef PubMed Google Scholar
40. Viola A, et al. The metabolic signature of macrophage responses. Front Immunol. 2019;10:1462.
    View this article via: CrossRef PubMed Google Scholar
41. Anderson NR, et al. Macrophage-based approaches for cancer immunotherapy. Cancer Res. 2021;81(5):1201–1208.
    View this article via: CrossRef PubMed Google Scholar
42. Liu C, et al. Advances in the regulation of macrophage polarization by mesenchymal stem cells and implications for ALI/ARDS treatment. Front Immunol. 2022;13:928134.
    View this article via: CrossRef PubMed Google Scholar
43. Ghosh S, et al. MARCO regulates early inflammatory responses against influenza: a useful macrophage function with adverse outcome. Am J Respir Cell Mol Biol. 2011;45(5):1036–1044.
    View this article via: CrossRef PubMed Google Scholar
44. Schabbauer G, et al. Myeloid PTEN promotes inflammation but impairs bactericidal activities during murine pneumococcal pneumonia. J Immunol. 2010;185(1):468–476.
    View this article via: CrossRef PubMed Google Scholar
45. Didierlaurent A, et al. Sustained desensitization to bacterial Toll-like receptor ligands after resolution of respiratory influenza infection. J Exp Med. 2008;205(2):323–329.
    View this article via: CrossRef PubMed Google Scholar
46. Liu X, et al. Legionella-infected macrophages engage the alveolar epithelium to metabolically reprogram myeloid cells and promote antibacterial inflammation. Cell Host Microbe. 2020;28(5):683–698.
    View this article via: CrossRef PubMed Google Scholar
47. Shlezinger N, Hohl TM. Mitochondrial reactive oxygen species enhance alveolar macrophage activity against aspergillus fumigatus but are dispensable for host protection. mSphere. 2021;6(3):e0026021.
    View this article via: CrossRef PubMed Google Scholar
48. Philippe B, et al. Killing of Aspergillus fumigatus by alveolar macrophages is mediated by reactive oxidant intermediates. Infect Immun. 2003;71(6):3034–3042.
    View this article via: CrossRef PubMed Google Scholar
49. Aberdein JD, et al. Alveolar macrophages in pulmonary host defence the unrecognized role of apoptosis as a mechanism of intracellular bacterial killing. Clin Exp Immunol. 2013;174(2):193–202.
    View this article via: CrossRef PubMed Google Scholar
50. Cohen SB, et al. Alveolar macrophages provide an early mycobacterium tuberculosis niche and initiate dissemination. Cell Host Microbe. 2018;24(3):439–446.
    View this article via: CrossRef PubMed Google Scholar
51. Yu WC, et al. Viral replication and innate host responses in primary human alveolar epithelial cells and alveolar macrophages infected with influenza H5N1 and H1N1 viruses. J Virol. 2011;85(14):6844–6855.
    View this article via: CrossRef PubMed Google Scholar
52. Kumagai Y, et al. Alveolar macrophages are the primary interferon-alpha producer in pulmonary infection with RNA viruses. Immunity. 2007;27(2):240–252.
    View this article via: CrossRef PubMed Google Scholar
53. Tumpey TM, et al. Pathogenicity of influenza viruses with genes from the 1918 pandemic virus: functional roles of alveolar macrophages and neutrophils in limiting virus replication and mortality in mice. J Virol. 2005;79(23):14933–14944.
    View this article via: CrossRef PubMed Google Scholar
54. Meischel T, et al. Influenza A virus interactions with macrophages: lessons from epithelial cells. Cell Microbiol. 2020;22(5):e13170.
    View this article via: CrossRef PubMed Google Scholar
55. Tavares LP, et al. CXCR1/2 antagonism is protective during influenza and post-influenza pneumococcal infection. Front Immunol. 2017;8:1799.
    View this article via: CrossRef PubMed Google Scholar
56. Bhatia S, et al. Rapid host defense against Aspergillus fumigatus involves alveolar macrophages with a predominance of alternatively activated phenotype. PLoS One. 2011;6(1):e15943.
    View this article via: CrossRef PubMed Google Scholar
57. Steiner DJ, et al. Protective role for macrophages in respiratory Francisella tularensis infection. Infect Immun. 2017;85(6):e00064-17.
    View this article via: CrossRef PubMed Google Scholar
58. Ghoneim HE, et al. Depletion of alveolar macrophages during influenza infection facilitates bacterial superinfections. J Immunol. 2013;191(3):1250–1259.
    View this article via: CrossRef PubMed Google Scholar
59. Grant RA, et al. Circuits between infected macrophages and T cells in SARS-CoV-2 pneumonia. Nature. 2021;590(7847):635–641.
    View this article via: CrossRef PubMed Google Scholar
60. Liao M, et al. Single-cell landscape of bronchoalveolar immune cells in patients with COVID-19. Nat Med. 2020;26(6):842–844.
    View this article via: CrossRef PubMed Google Scholar
61. Wong CK, et al. Aging impairs alveolar macrophage phagocytosis and increases influenza-induced mortality in mice. J Immunol. 2017;199(3):1060–1068.
    View this article via: CrossRef PubMed Google Scholar
62. Cardani A, et al. Alveolar macrophages prevent lethal influenza pneumonia by inhibiting infection of type-1 alveolar epithelial cells. PLoS Pathog. 2017;13(1):e1006140.
    View this article via: CrossRef PubMed Google Scholar
63. Schneider C, et al. Alveolar macrophages are essential for protection from respiratory failure and associated morbidity following influenza virus infection. PLoS Pathog. 2014;10(4):e1004053.
    View this article via: CrossRef PubMed Google Scholar
64. Tate MD, et al. Critical role of airway macrophages in modulating disease severity during influenza virus infection of mice. J Virol. 2010;84(15):7569–7580.
    View this article via: CrossRef PubMed Google Scholar
65. Purnama C, et al. Transient ablation of alveolar macrophages leads to massive pathology of influenza infection without affecting cellular adaptive immunity. Eur J Immunol. 2014;44(7):2003–2012.
    View this article via: CrossRef PubMed Google Scholar
66. Li F, et al. Monocyte-derived alveolar macrophages autonomously determine severe outcome of respiratory viral infection. Sci Immunol. 2022;7(73):eabj5761.
    View this article via: CrossRef PubMed Google Scholar
67. Santos LD, et al. TNF-mediated alveolar macrophage necroptosis drives disease pathogenesis during respiratory syncytial virus infection. Eur Respir J. 2021;57(6):2003764.
    View this article via: CrossRef PubMed Google Scholar
68. Zhu B, et al. Uncoupling of macrophage inflammation from self-renewal modulates host recovery from respiratory viral infection. Immunity. 2021;54(6):1200–1218.
    View this article via: CrossRef PubMed Google Scholar
69. Maus UA, et al. Resident alveolar macrophages are replaced by recruited monocytes in response to endotoxin-induced lung inflammation. Am J Respir Cell Mol Biol. 2006;35(2):227–235.
    View this article via: CrossRef PubMed Google Scholar
70. Mould KJ, et al. Cell origin dictates programming of resident versus recruited macrophages during acute lung injury. Am J Respir Cell Mol Biol. 2017;57(3):294–306.
    View this article via: CrossRef PubMed Google Scholar
71. O’Neill LA, et al. A guide to immunometabolism for immunologists. Nat Rev Immunol. 2016;16(9):553–565.
    View this article via: CrossRef PubMed Google Scholar
72. Woods PS, et al. Tissue-resident alveolar macrophages do not rely on glycolysis for LPS-induced inflammation. Am J Respir Cell Mol Biol. 2020;62(2):243–255.
    View this article via: CrossRef PubMed Google Scholar
73. Herold S, et al. Lung epithelial apoptosis in influenza virus pneumonia: the role of macrophage-expressed TNF-related apoptosis-inducing ligand. J Exp Med. 2008;205(13):3065–3077.
    View this article via: CrossRef PubMed Google Scholar
74. Peteranderl C, et al. Human influenza virus infections. Semin Respir Crit Care Med. 2016;37(4):487–500.
    View this article via: CrossRef PubMed Google Scholar
75. Perrone LA, et al. H5N1 and 1918 pandemic influenza virus infection results in early and excessive infiltration of macrophages and neutrophils in the lungs of mice. PLoS Pathog. 2008;4(8):e1000115.
    View this article via: CrossRef PubMed Google Scholar
76. Peteranderl C, et al. Macrophage-epithelial paracrine crosstalk inhibits lung edema clearance during influenza infection. J Clin Invest. 2016;126(4):1566–1580.
    View this article via: JCI CrossRef PubMed Google Scholar
77. Szabo PA, et al. Longitudinal profiling of respiratory and systemic immune responses reveals myeloid cell-driven lung inflammation in severe COVID-19. Immunity. 2021;54(4):797–814.
    View this article via: CrossRef PubMed Google Scholar
78. Schulte-Schrepping J, et al. Severe COVID-19 is marked by a dysregulated myeloid cell compartment. Cell. 2020;182(6):1419–1440.
    View this article via: CrossRef PubMed Google Scholar
79. Talmi-Frank D, et al. Extracellular matrix proteolysis by MT1-MMP contributes to influenza-related tissue damage and mortality. Cell Host Microbe. 2016;20(4):458–470.
    View this article via: CrossRef PubMed Google Scholar
80. Schneider JL, et al. The aging lung: physiology, disease, and immunity. Cell. 2021;184(8):1990–2019.
    View this article via: CrossRef PubMed Google Scholar
81. McQuattie-Pimentel AC, et al. The lung microenvironment shapes a dysfunctional response of alveolar macrophages in aging. J Clin Invest. 2021;131(4):e140299.
    View this article via: JCI CrossRef PubMed Google Scholar
82. Herold S, et al. Influenza virus-induced lung injury: pathogenesis and implications for treatment. Eur Respir J. 2015;45(5):1463–1478.
    View this article via: CrossRef PubMed Google Scholar
83. Cheng P, et al. Macrophages in lung injury, repair, and fibrosis. Cells. 2021;10(2):436.
    View this article via: CrossRef PubMed Google Scholar
84. Hung LY, et al. Macrophages promote epithelial proliferation following infectious and non-infectious lung injury through a Trefoil factor 2-dependent mechanism. Mucosal Immunol. 2019;12(1):64–76.
    View this article via: CrossRef PubMed Google Scholar
85. Cakarova L, et al. Macrophage tumor necrosis factor-alpha induces epithelial expression of granulocyte-macrophage colony-stimulating factor: impact on alveolar epithelial repair. Am J Respir Crit Care Med. 2009;180(6):521–532.
    View this article via: CrossRef PubMed Google Scholar
86. Bosurgi L, et al. Macrophage function in tissue repair and remodeling requires IL-4 or IL-13 with apoptotic cells. Science. 2017;356(6342):1072–1076.
    View this article via: CrossRef PubMed Google Scholar
87. Misharin AV, et al. Monocyte-derived alveolar macrophages drive lung fibrosis and persist in the lung over the life span. J Exp Med. 2017;214(8):2387–2404.
    View this article via: CrossRef PubMed Google Scholar
88. Zhou Y, et al. Chitinase 3-like 1 suppresses injury and promotes fibroproliferative responses in Mammalian lung fibrosis. Sci Transl Med. 2014;6(240):240ra76.
    View this article via: CrossRef PubMed Google Scholar
89. Cao Z, et al. Targeting of the pulmonary capillary vascular niche promotes lung alveolar repair and ameliorates fibrosis. Nat Med. 2016;22(2):154–162.
    View this article via: CrossRef PubMed Google Scholar
90. Joshi N, et al. A spatially restricted fibrotic niche in pulmonary fibrosis is sustained by M-CSF/M-CSFR signalling in monocyte-derived alveolar macrophages. Eur Respir J. 2020;55(1):1900646.
    View this article via: CrossRef PubMed Google Scholar
91. Reyfman PA, et al. Single-cell transcriptomic analysis of human lung provides insights into the pathobiology of pulmonary fibrosis. Am J Respir Crit Care Med. 2019;199(12):1517–1536.
    View this article via: CrossRef PubMed Google Scholar
92. Wendisch D, et al. SARS-CoV-2 infection triggers profibrotic macrophage responses and lung fibrosis. Cell. 2021;184(26):6243–6261.e27.
    View this article via: CrossRef PubMed Google Scholar
93. Melms JC, et al. A molecular single-cell lung atlas of lethal COVID-19. Nature. 2021;595(7865):114–119.
    View this article via: CrossRef PubMed Google Scholar
94. Wauters E, et al. Discriminating mild from critical COVID-19 by innate and adaptive immune single-cell profiling of bronchoalveolar lavages. Cell Res. 2021;31(3):272–290.
    View this article via: CrossRef PubMed Google Scholar
95. Xu D, et al. PLG nanoparticles target fibroblasts and MARCO+ monocytes to reverse multiorgan fibrosis. JCI Insight. 2022;7(5):e151037.
    View this article via: JCI Insight CrossRef PubMed Google Scholar
96. Singh A, et al. Nanoparticle targeting of de novo profibrotic macrophages mitigates lung fibrosis. Proc Natl Acad Sci U S A. 2022;119(15):e2121098119.
    View this article via: CrossRef PubMed Google Scholar
97. Angelidis I, et al. An atlas of the aging lung mapped by single cell transcriptomics and deep tissue proteomics. Nat Commun. 2019;10(1):963.
    View this article via: CrossRef PubMed Google Scholar
98. McCullers JA. The co-pathogenesis of influenza viruses with bacteria in the lung. Nat Rev Microbiol. 2014;12(4):252–262.
    View this article via: CrossRef PubMed Google Scholar
99. Dreyfuss D, Ricard JD. Acute lung injury and bacterial infection. Clin Chest Med. 2005;26(1):105–112.
    View this article via: CrossRef PubMed Google Scholar
100. Peruzzi WT, et al. Concurrent bacterial lung infection in patients with AIDS, PCP, and respiratory failure. Chest. 1992;101(5):1399–1403.
     View this article via: CrossRef PubMed Google Scholar
101. Kitsios GD, Morris A. Seek and ye shall find: COVID-19 and bacterial superinfection. Am J Respir Crit Care Med. 2021;204(8):875–877.
     View this article via: CrossRef PubMed Google Scholar
102. Matt U, et al. Bbeta(15-42) protects against acid-induced acute lung injury and secondary pseudomonas pneumonia in vivo. Am J Respir Crit Care Med. 2009;180(12):1208–1217.
     View this article via: CrossRef PubMed Google Scholar
103. Verma AK, et al. Influenza infection induces alveolar macrophage dysfunction and thereby enables noninvasive Streptococcus pneumoniae to cause deadly pneumonia. J Immunol. 2020;205(6):1601–1607.
     View this article via: CrossRef PubMed Google Scholar
104. Collini PJ, et al. HIV gp120 in the lungs of antiretroviral therapy-treated individuals impairs alveolar macrophage responses to pneumococci. Am J Respir Crit Care Med. 2018;197(12):1604–1615.
     View this article via: CrossRef PubMed Google Scholar
105. Sun K, Metzger DW. Influenza infection suppresses NADPH oxidase-dependent phagocytic bacterial clearance and enhances susceptibility to secondary methicillin-resistant Staphylococcus aureus infection. J Immunol. 2014;192(7):3301–3307.
     View this article via: CrossRef PubMed Google Scholar
106. Roquilly A, et al. Local modulation of antigen-presenting cell development after resolution of pneumonia induces long-term susceptibility to secondary infections. Immunity. 2017;47(1):135–147.
     View this article via: CrossRef PubMed Google Scholar
107. Shibata T, et al. Respiratory syncytial virus infection exacerbates pneumococcal pneumonia via Gas6/Axl-mediated macrophage polarization. J Clin Invest. 2020;130(6):3021–3037.
     View this article via: JCI CrossRef PubMed Google Scholar
108. Wu M, et al. Immunomodulators targeting MARCO expression improve resistance to postinfluenza bacterial pneumonia. Am J Physiol Lung Cell Mol Physiol. 2017;313(1):L138–L153.
     View this article via: CrossRef PubMed Google Scholar
109. Braciale TJ, et al. Regulating the adaptive immune response to respiratory virus infection. Nat Rev Immunol. 2012;12(4):295–305.
     View this article via: CrossRef PubMed Google Scholar
110. Sun K, Metzger DW. Inhibition of pulmonary antibacterial defense by interferon-gamma during recovery from influenza infection. Nat Med. 2008;14(5):558–564.
     View this article via: CrossRef PubMed Google Scholar
111. Arredouani MS, et al. MARCO is the major binding receptor for unopsonized particles and bacteria on human alveolar macrophages. J Immunol. 2005;175(9):6058–6064.
     View this article via: CrossRef PubMed Google Scholar
112. Herold S, et al. Exudate macrophages attenuate lung injury by the release of IL-1 receptor antagonist in gram-negative pneumonia. Am J Respir Crit Care Med. 2011;183(10):1380–1390.
     View this article via: CrossRef PubMed Google Scholar
113. Dean RA, et al. Macrophage-specific metalloelastase (MMP-12) truncates and inactivates ELR+ CXC chemokines and generates CCL2, -7, -8, and -13 antagonists: potential role of the macrophage in terminating polymorphonuclear leukocyte influx. Blood. 2008;112(8):3455–3464.
     View this article via: CrossRef PubMed Google Scholar
114. Knapp S, et al. Alveolar macrophages have a protective antiinflammatory role during murine pneumococcal pneumonia. Am J Respir Crit Care Med. 2003;167(2):171–179.
     View this article via: CrossRef PubMed Google Scholar
115. Mukundan L, et al. PPAR-delta senses and orchestrates clearance of apoptotic cells to promote tolerance. Nat Med. 2009;15(11):1266–1272.
     View this article via: CrossRef PubMed Google Scholar
116. Ariel A, Serhan CN. New lives given by cell death: macrophage differentiation following their encounter with apoptotic leukocytes during the resolution of inflammation. Front Immunol. 2012;3:4.
     View this article via: CrossRef PubMed Google Scholar
117. Morioka S, et al. Living on the edge: efferocytosis at the interface of homeostasis and pathology. Immunity. 2019;50(5):1149–1162.
     View this article via: CrossRef PubMed Google Scholar
118. Medeiros AI, et al. Efferocytosis impairs pulmonary macrophage and lung antibacterial function via PGE2/EP2 signaling. J Exp Med. 2009;206(1):61–68.
     View this article via: CrossRef PubMed Google Scholar
119. Coulombe F, et al. Targeted prostaglandin E2 inhibition enhances antiviral immunity through induction of type I interferon and apoptosis in macrophages. Immunity. 2014;40(4):554–568.
     View this article via: CrossRef PubMed Google Scholar
120. Horckmans M, et al. Neutrophils orchestrate post-myocardial infarction healing by polarizing macrophages towards a reparative phenotype. Eur Heart J. 2017;38(3):187–197.
     View this article via: PubMed CrossRef Google Scholar
121. Chang CF, et al. Erythrocyte efferocytosis modulates macrophages towards recovery after intracerebral hemorrhage. J Clin Invest. 2018;128(2):607–624.
     View this article via: JCI CrossRef PubMed Google Scholar
122. Yurdagul A Jr, et al. Macrophage metabolism of apoptotic cell-derived arginine promotes continual efferocytosis and resolution of injury. Cell Metab. 2020;31(3):518–533.
     View this article via: CrossRef PubMed Google Scholar
123. Elliott MR, et al. Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance. Nature. 2009;461(7261):282–286.
     View this article via: CrossRef PubMed Google Scholar
124. Truman LA, et al. CX3CL1/fractalkine is released from apoptotic lymphocytes to stimulate macrophage chemotaxis. Blood. 2008;112(13):5026–5036.
     View this article via: CrossRef PubMed Google Scholar
125. Grimsley C, Ravichandran KS. Cues for apoptotic cell engulfment: eat-me, don’t eat-me and come-get-me signals. Trends Cell Biol. 2003;13(12):648–656.
     View this article via: CrossRef PubMed Google Scholar
126. Perretti M, D’Acquisto F. Annexin A1 and glucocorticoids as effectors of the resolution of inflammation. Nat Rev Immunol. 2009;9(1):62–70.
     View this article via: CrossRef PubMed Google Scholar
127. Mevorach D, et al. Complement-dependent clearance of apoptotic cells by human macrophages. J Exp Med. 1998;188(12):2313–2320.
     View this article via: CrossRef PubMed Google Scholar
128. Better J, et al. Neutrophil efferocytosis reprograms mitochondrial metabolism to switch alveolar macrophages to a pro-resolution phenotype at the cost of bacterial control [preprint]. https://doi.org/10.1101/2023.03.07.528464 Posted on bioRxiv March 27, 2023.
129. Roquilly A, et al. Alveolar macrophages are epigenetically altered after inflammation, leading to long-term lung immunoparalysis. Nat Immunol. 2020;21(6):636–648.
     View this article via: CrossRef PubMed Google Scholar
130. Yao Y, et al. Induction of autonomous memory alveolar macrophages requires T cell help and is critical to trained immunity. Cell. 2018;175(6):1634–1650.
     View this article via: CrossRef PubMed Google Scholar
131. Mata E, et al. Pulmonary BCG induces lung-resident macrophage activation and confers long-term protection against tuberculosis. Sci Immunol. 2021;6(63):eabc2934.
     View this article via: CrossRef PubMed Google Scholar
132. Nemeth J, et al. Contained Mycobacterium tuberculosis infection induces concomitant and heterologous protection. PLoS Pathog. 2020;16(7):e1008655.
     View this article via: CrossRef PubMed Google Scholar
133. Guillon A, et al. Pneumonia recovery reprograms the alveolar macrophage pool. JCI Insight. 2020;5(4):e133042.
     View this article via: JCI Insight CrossRef PubMed Google Scholar
134. Mbawuike IN, Herscowitz HB. MH-S, a murine alveolar macrophage cell line: morphological, cytochemical, and functional characteristics. J Leukoc Biol. 1989;46(2):119–127.
     View this article via: CrossRef PubMed Google Scholar
135. Brenner TA, et al. Immortalized MH-S cells lack defining features of primary alveolar macrophages and do not support mouse pneumovirus replication. Immunol Lett. 2016;172:106–112.
     View this article via: CrossRef PubMed Google Scholar
136. Bhattacharya J, Westphalen K. Macrophage-epithelial interactions in pulmonary alveoli. Semin Immunopathol. 2016;38(4):461–469.
     View this article via: CrossRef PubMed Google Scholar
137. Fejer G, et al. Nontransformed, GM-CSF-dependent macrophage lines are a unique model to study tissue macrophage functions. Proc Natl Acad Sci U S A. 2013;110(24):E2191–E2198.
     View this article via: CrossRef PubMed Google Scholar
138. Luo M, et al. Development of an optimized culture system for generating mouse alveolar macrophage-like cells. J Immunol. 2021;207(6):1683–1693.
     View this article via: CrossRef PubMed Google Scholar
139. Gorki AD, et al. Murine ex vivo cultured alveolar macrophages provide a novel tool to study tissue-resident macrophage behavior and function. Am J Respir Cell Mol Biol. 2022;66(1):64–75.
     View this article via: CrossRef PubMed Google Scholar
140. Evren E, et al. Origin and ontogeny of lung macrophages: from mice to humans. Immunology. 2020;160(2):126–138.
     View this article via: CrossRef PubMed Google Scholar
141. Kim YG, et al. Gut dysbiosis promotes M2 macrophage polarization and allergic airway inflammation via fungi-induced PGE(2). Cell Host Microbe. 2014;15(1):95–102.
     View this article via: CrossRef PubMed Google Scholar
142. Thaiss CA, et al. The microbiome and innate immunity. Nature. 2016;535(7610):65–74.
     View this article via: CrossRef PubMed Google Scholar
143. Yona S, et al. Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis. Immunity. 2013;38(1):79–91.
     View this article via: CrossRef PubMed Google Scholar
144. Zilionis R, et al. Single-cell transcriptomics of human and mouse lung cancers reveals conserved myeloid populations across individuals and species. Immunity. 2019;50(5):1317–1334.
     View this article via: CrossRef PubMed Google Scholar
145. Lambrechts D, et al. Phenotype molding of stromal cells in the lung tumor microenvironment. Nat Med. 2018;24(8):1277–1289.
     View this article via: CrossRef PubMed Google Scholar
146. Ufimtseva E, et al. Ex vivo expansion of alveolar macrophages with Mycobacterium tuberculosis from the resected lungs of patients with pulmonary tuberculosis. PLoS One. 2018;13(2):e0191918.
     View this article via: CrossRef PubMed Google Scholar
147. Ufimtseva EG, et al. Mycobacterium tuberculosis load in host cells and the antibacterial activity of alveolar macrophages are linked and differentially regulated in various lung lesions of patients with pulmonary tuberculosis. Int J Mol Sci. 2021;22(7):3452.
     View this article via: CrossRef PubMed Google Scholar
148. Pahari S, et al. A new tractable method for generating human alveolar macrophage-like cells in vitro to study lung inflammatory processes and diseases [preprint]. https://doi.org/10.1101/2023.04.05.535806 Posted on bioRxiv April 6, 2023.
149. Brann KR, et al. Infection of primary human alveolar macrophages alters staphylococcus aureus toxin production and activity. Infect Immun. 2019;87(7):e00167-19.
     View this article via: CrossRef PubMed Google Scholar
150. Xue H, et al. Genetic modification in human pluripotent stem cells by homologous recombination and CRISPR/Cas9 system. Methods Mol Biol. 2016;1307:173–190.
     View this article via: CrossRef PubMed Google Scholar
151. Ackermann M, et al. Bioreactor-based mass production of human iPSC-derived macrophages enables immunotherapies against bacterial airway infections. Nat Commun. 2018;9(1):5088.
     View this article via: CrossRef PubMed Google Scholar
152. Ackermann M, et al. Continuous human iPSC-macrophage mass production by suspension culture in stirred tank bioreactors. Nat Protoc. 2022;17(2):513–539.
     View this article via: CrossRef PubMed Google Scholar
153. Buchrieser J, et al. Human induced pluripotent stem cell-derived macrophages share ontogeny with MYB-independent tissue-resident macrophages. Stem Cell Reports. 2017;8(2):334–345.
     View this article via: CrossRef PubMed Google Scholar
154. Takata K, et al. Induced-pluripotent-stem-cell-derived primitive macrophages provide a platform for modeling tissue-resident macrophage differentiation and function. Immunity. 2017;47(1):183–198.
     View this article via: CrossRef PubMed Google Scholar
155. Sachs N, et al. Long-term expanding human airway organoids for disease modeling. EMBO J. 2019;38(4):e100300.
     View this article via: CrossRef PubMed Google Scholar
156. McCauley KB, et al. Efficient derivation of functional human airway epithelium from pluripotent stem cells via temporal regulation of Wnt signaling. Cell Stem Cell. 2017;20(6):844–857.
     View this article via: CrossRef PubMed Google Scholar
157. Miller AJ, et al. Generation of lung organoids from human pluripotent stem cells in vitro. Nat Protoc. 2019;14(2):518–540.
     View this article via: CrossRef PubMed Google Scholar
158. Lim K, et al. Organoid modeling of human fetal lung alveolar development reveals mechanisms of cell fate patterning and neonatal respiratory disease. Cell Stem Cell. 2023;30(1):20–37.
     View this article via: CrossRef PubMed Google Scholar
159. Seo HR, et al. Human pluripotent stem cell-derived alveolar organoid with macrophages. Int J Mol Sci. 2022;23(16):9211.
     View this article via: CrossRef PubMed Google Scholar
160. Vazquez-Armendariz AI, et al. Multilineage murine stem cells generate complex organoids to model distal lung development and disease. EMBO J. 2020;39(21):e103476.
     View this article via: CrossRef PubMed Google Scholar
161. Bose S, et al. Promises and challenges of organoid-guided precision medicine. Med. 2021;2(9):1011–1026.
     View this article via: CrossRef PubMed Google Scholar
162. Litvack ML, et al. Alveolar-like stem cell-derived Myb(-) macrophages promote recovery and survival in airway disease. Am J Respir Crit Care Med. 2016;193(11):1219–1229.
     View this article via: CrossRef PubMed Google Scholar
163. Liu H, et al. Efficacy of pulmonary transplantation of engineered macrophages secreting IL-4 on acute lung injury in C57BL/6J mice. Cell Death Dis. 2019;10(9):664.
     View this article via: CrossRef PubMed Google Scholar
164. Rafiei Hashtchin A, et al. Human iPSC-derived macrophages for efficient Staphylococcus aureus clearance in a murine pulmonary infection model. Blood Adv. 2021;5(23):5190–5201.
     View this article via: CrossRef PubMed Google Scholar
165. Meoli A, et al. Impact of CFTR modulators on the impaired function of phagocytes in cystic fibrosis lung disease. Int J Mol Sci. 2022;23(20):12421.
     View this article via: CrossRef PubMed Google Scholar
166. Gillan JL, et al. CAGE sequencing reveals CFTR-dependent dysregulation of type I IFN signaling in activated cystic fibrosis macrophages. Sci Adv. 2023;9(21):eadg5128.
     View this article via: CrossRef PubMed Google Scholar
167. Brinkert K, et al. Rescue from pseudomonas aeruginosa airway infection via stem cell transplantation. Mol Ther. 2021;29(3):1324–1334.
     View this article via: CrossRef PubMed Google Scholar
168. Hetzel M, et al. Hematopoietic stem cell gene therapy for IFNγR1 deficiency protects mice from mycobacterial infections. Blood. 2018;131(5):533–545.
     View this article via: CrossRef PubMed Google Scholar
169. Janosz E, et al. Pulmonary transplantation of alpha-1 antitrypsin (AAT)-transgenic macrophages provides a source of functional human AAT in vivo. Gene Ther. 2021;28(9):477–493.
     View this article via: CrossRef PubMed Google Scholar
170. Happle C, et al. Pulmonary transplantation of human induced pluripotent stem cell-derived macrophages ameliorates pulmonary alveolar proteinosis. Am J Respir Crit Care Med. 2018;198(3):350–360.
     View this article via: CrossRef PubMed Google Scholar
171. Arumugam P, et al. Long-term safety and efficacy of gene-pulmonary macrophage transplantation therapy of PAP in Csf2ra(-/-) mice. Mol Ther. 2019;27(9):1597–1611.
     View this article via: CrossRef PubMed Google Scholar
172. Nayak DK, et al. Long-term persistence of donor alveolar macrophages in human lung transplant recipients that influences donor-specific immune responses. Am J Transplant. 2016;16(8):2300–2311.
     View this article via: CrossRef PubMed Google Scholar
173. Li Z, et al. The macrophage-depleting agent clodronate promotes durable hematopoietic chimerism and donor-specific skin allograft tolerance in mice. Sci Rep. 2016;6:22143.
     View this article via: CrossRef PubMed Google Scholar
174. Chavas TEJ, et al. A macrophage-targeted platform for extending drug dosing with polymer prodrugs for pulmonary infection prophylaxis. J Control Release. 2021;330:284–292.
     View this article via: CrossRef PubMed Google Scholar
175. Zhang J, et al. Amikacin liposome inhalation suspension (ALIS) penetrates non-tuberculous mycobacterial biofilms and enhances amikacin uptake into macrophages. Front Microbiol. 2018;9:915.
     View this article via: CrossRef PubMed Google Scholar
176. Hamblin KA, et al. Inhaled liposomal ciprofloxacin protects against a lethal infection in a murine model of pneumonic plague. Front Microbiol. 2017;8:91.
     View this article via: CrossRef PubMed Google Scholar
177. Huang S, et al. PPAR-γ in macrophages limits pulmonary inflammation and promotes host recovery following respiratory viral infection. J Virol. 2019;93(9):e00030-19.
     View this article via: CrossRef PubMed Google Scholar
178. Deng L, et al. Macrophage polarization: an important candidate regulator for lung diseases. Molecules. 2023;28(5):2379.
     View this article via: CrossRef PubMed Google Scholar
179. Li X, et al. Zanubrutinib ameliorates lipopolysaccharide-induced acute lung injury via regulating macrophage polarization. Int Immunopharmacol. 2022;111:109138.
     View this article via: CrossRef PubMed Google Scholar
180. Lin F, et al. Canagliflozin alleviates LPS-induced acute lung injury by modulating alveolar macrophage polarization. Int Immunopharmacol. 2020;88:106969.
     View this article via: CrossRef PubMed Google Scholar
181. Gupta A, et al. Inhalable particles containing rapamycin for induction of autophagy in macrophages infected with Mycobacterium tuberculosis. Mol Pharm. 2014;11(4):1201–1207.
     View this article via: CrossRef PubMed Google Scholar
182. [No author listed]. Correction to Lancet Respir Med 2021; published online Feb 26. https://doi.org/10.1016/S2213-2600(20)30448-3. Lancet Respir Med. 2021;9(6):E55.
     View this article via: PubMed CrossRef Google Scholar
183. Coleman MM, et al. All-trans retinoic acid augments autophagy during intracellular bacterial infection. Am J Respir Cell Mol Biol. 2018;59(5):548–556.
     View this article via: CrossRef PubMed Google Scholar
184. Umstead TM, et al. Lower respiratory tract delivery, airway clearance, and preclinical efficacy of inhaled GM-CSF in a postinfluenza pneumococcal pneumonia model. Am J Physiol Lung Cell Mol Physiol. 2020;318(4):L571–L579.
     View this article via: CrossRef PubMed Google Scholar
185. Standiford LR, et al. TLR4-dependent GM-CSF protects against lung injury in Gram-negative bacterial pneumonia. Am J Physiol Lung Cell Mol Physiol. 2012;302(5):L447–L454.
     View this article via: CrossRef PubMed Google Scholar
186. Steinwede K, et al. Local delivery of GM-CSF protects mice from lethal pneumococcal pneumonia. J Immunol. 2011;187(10):5346–5356.
     View this article via: CrossRef PubMed Google Scholar
187. Huang FF, et al. GM-CSF in the lung protects against lethal influenza infection. Am J Respir Crit Care Med. 2011;184(2):259–268.
     View this article via: CrossRef PubMed Google Scholar
188. Sever-Chroneos Z, et al. GM-CSF modulates pulmonary resistance to influenza A infection. Antiviral Res. 2011;92(2):319–328.
     View this article via: CrossRef PubMed Google Scholar
189. Subramaniam R, et al. Delivery of GM-CSF to protect against influenza pneumonia. PLoS One. 2015;10(4):e0124593.
     View this article via: CrossRef PubMed Google Scholar
190. Subramaniam R, et al. Protecting against post-influenza bacterial pneumonia by increasing phagocyte recruitment and ROS production. J Infect Dis. 2014;209(11):1827–1836.
     View this article via: CrossRef PubMed Google Scholar
191. Unkel B, et al. Alveolar epithelial cells orchestrate DC function in murine viral pneumonia. J Clin Invest. 2012;122(10):3652–3664.
     View this article via: JCI CrossRef PubMed Google Scholar
192. Baleeiro CE, et al. GM-CSF and the impaired pulmonary innate immune response following hyperoxic stress. Am J Physiol Lung Cell Mol Physiol. 2006;291(6):L1246–L1255.
     View this article via: CrossRef PubMed Google Scholar
193. Herold S, et al. Inhaled granulocyte/macrophage colony-stimulating factor as treatment of pneumonia-associated acute respiratory distress syndrome. Am J Respir Crit Care Med. 2014;189(5):609–611.
     View this article via: CrossRef PubMed Google Scholar
194. Rosler B, Herold S. Lung epithelial GM-CSF improves host defense function and epithelial repair in influenza virus pneumonia—a new therapeutic strategy? Mol Cell Pediatr. 2016;3(1):29.
     View this article via: CrossRef PubMed Google Scholar
195. Bosteels C, et al. Loss of GM-CSF-dependent instruction of alveolar macrophages in COVID-19 provides a rationale for inhaled GM-CSF treatment. Cell Rep Med. 2022;3(12):100833.
     View this article via: CrossRef PubMed Google Scholar
196. Paine R, et al. Inhaled sargramostim (recombinant human granulocyte-macrophage colony-stimulating factor) for COVID-19-associated acute hypoxemia: results of the phase 2, randomized, open-label trial (iLeukPulm). Mil Med. 2022;188(7-8):e2629–e2638.
     View this article via: CrossRef PubMed Google Scholar
197. De Luca G, et al. GM-CSF blockade with mavrilimumab in severe COVID-19 pneumonia and systemic hyperinflammation: a single-centre, prospective cohort study. Lancet Rheumatol. 2020;2(8):e465–e473.
     View this article via: CrossRef PubMed Google Scholar
198. Cremer PC, et al. Mavrilimumab in patients with severe COVID-19 pneumonia and systemic hyperinflammation (MASH-COVID): an investigator initiated, multicentre, double-blind, randomised, placebo-controlled trial. Lancet Rheumatol. 2021;3(6):e410–e418.
     View this article via: CrossRef PubMed Google Scholar