HIV-specific CD8⁺ T cells and HIV eradication

After the success of combination antiretroviral therapy (cART) to treat HIV infection, the next great frontier is to cure infected persons, a formidable challenge. HIV persists in a quiescent state in resting CD4⁺ T cells, where the replicative enzymes targeted by cART are not active. Although low levels of HIV transcripts are detectable in these resting cells, little to no viral protein is produced, rendering this reservoir difficult to detect by the host CD8⁺ T cell response. However, recent advances suggest that this state of latency might be pharmacologically reversed, resulting in viral protein expression without the adverse effects of massive cellular activation. Emerging data suggest that with this approach, infected cells will not die of viral cytopathic effects, but might be eliminated if HIV-specific CD8⁺ T cells can be effectively harnessed. Here, we address the antiviral properties of HIV-specific CD8⁺ T cells and how these cells might be harnessed to greater effect toward achieving viral eradication or a functional cure.

Viruses are typically eliminated by virus-specific CD8⁺ T cells, which recognize processed viral proteins that are presented as a complex with an HLA class I molecule at the surface of an infected cell. Recognition through the T cell receptor (TCR) initiates a cascade of activation events, ultimately leading to the release of granzymes and perforin and killing of the infected cell, which can occur before infectious progeny virions are produced (10). Additionally, TCR activation leads to the release of a variety of cytokines including IFN-γ, TNF-α, macrophage inflammatory proteins 1α and 1β (MIP-1α and MIP-1β), and RANTES (CCL5), which have antiviral effects.

Numerous lines of evidence suggest that HIV-specific CD8⁺ T cells exert potent antiviral effects. The magnitude and rapidity of HIV-specific CD8⁺ T cell activation in hyperacute infection correlate inversely with the viral load set point (4), indicating that these cells mediate antiviral pressure during peak viremia (2, 3). Antiviral pressure is further indicated by rapid evolution of escape variants within targeted viral CD8⁺ T cell epitopes following acute infection (11, 12). In vitro models provide additional evidence for an antiviral effect, showing that these cells potently inhibit viral replication (10, 13). This is consistent with animal model data showing that depletion of CD8⁺ T cells following acute infection leads to high-level viremia that decreases as CD8⁺ T cells reappear (14). Genetic studies indicate that HLA class I alleles are associated with differences in set-point viremia (15, 16), modulated by the nature of viral peptide binding to the class I groove (16). Studies of viral fitness indicate that CD8⁺ T cell–induced mutations can diminish viral fitness, particularly those in epitopes restricted by protective HLA alleles such as B*27 and B*57, suggesting a persisting antiviral effect, even in cases of immune escape (17–19). Together, these studies indicate that CD8⁺ T cells are capable of potent antiviral function and provide a strong rationale for enlisting these responses in eradication strategies.

In the mid-1990s, several groups recognized that HIV could establish infection in resting CD4⁺ T cells in patients (20–24). As detectable virion production does not occur in these resting cells but can be induced by mitogens and other agents, these cells were defined as being “latently” infected. Longitudinal studies indicated that this reservoir was extremely stable under cART, with a half-life of 44 months (22). The ability of these cells to persist for years to decades without being killed by either viral cytopathic effects or immune effectors, such as CD8⁺ T cells, and then to re-seed systemic viremia is accepted as a primary mechanism by which infection persists despite effective cART. The expression of HIV antigens, which is either absent or low in these quiescent cells, is a prerequisite to their targeting and elimination by CD8⁺ T cells. To be effective at eradicating this reservoir, CD8⁺ T cells will likely need to be combined with LRAs, which induce antigen expression (see below).

While the role of latency in protecting cells from viral cytopathicity is clear, its status as an absolute barrier to cytotoxic T lymphocyte–mediated (CTL-mediated) killing is perhaps less self-evident. As T cells can detect even a single MHC-peptide complex on a cell surface (25), a remarkably strict state of latency would need to be maintained over years for CD8⁺ T cell killing of latently infected target cells to be absent in antiretroviral-treated (ARV-treated) subjects. While there are major defects in transcriptional initiation and elongation in resting CD4⁺ T cells (26–29), both unspliced and multiply spliced HIV transcripts can be detected in resting CD4⁺ T cells from HIV-infected individuals (29–34). The degree to which these transcripts result in the translation of HIV gene products may be limited by several factors, including retention in the nucleus (33), transcriptional interference, and “read-through” transcription (35). The studies described above support the possibility that low-level HIV antigen expression may occur in at least a subset of resting CD4⁺ T cells.

Currently, it is not possible to directly detect the minute amount of HIV antigen needed to trigger CD8⁺ T cells that might be expressed on the approximately 1 of 10⁶ resting CD4⁺ T cells that typically harbor an inducible intact provirus in ex vivo samples. However, studies in primary cell latency models have revealed that some expression of HIV Gag does occur in this system (36, 37), suggesting that some cells are transcriptionally active. Given the multitude of factors limiting the potency of CD8⁺ T cells in untreated infection that restricts their ability to control viremia, latency may not fully explain the inability of CD8⁺ T cells to eradicate persistent reservoirs in ARV-treated subjects. Further study of the interaction between HIV-specific CD8⁺ T cells and latently infected cells is warranted, as is further consideration of the possibility that these cells may play an ongoing role in limiting the viral reservoir in ARV-treated subjects.

Suppression of HIV replication by cART leads to the reduction or elimination of antigen expression, thereby affecting the magnitude and breadth of effector CD8⁺ T cell responses that would be poised to kill virus-infected cells. Although cART often results in a transient rebound of detectable CD8⁺ T cell responses in individuals with advanced immunosuppression (38), in subjects with less advanced disease, CD8⁺ T cell responses decline rapidly in the first weeks of ARV initiation (39). Once viremia is fully suppressed, HIV-specific CD8⁺ T cell responses in the peripheral blood continue to decay with a t_1/2 of 38.8 weeks for at least 2 years (40–43). The overall implications of cART-associated decay kinetics for HIV eradication strategies are not entirely clear without knowing the density of HIV-specific CD8⁺ T cells that would be required to clear viral reservoirs at tissue sites of viral reactivation and how this relates to frequencies of HIV-specific CD8⁺ T cells in the peripheral blood. In one sense, it is encouraging that CD8⁺ HIV-specific T cell responses are readily detectable in the majority of individuals, even after several years of ARV therapy, and that cART reduces the frequency of proapoptotic HIV-specific CD8⁺ T cells (44). However, CD8⁺ T cell responses may need to be activated to an effector phenotype and/or expanded in frequency in order to effectively contribute to eradication.

In addition to allowing for the contraction of HIV-specific CD8⁺ T cell responses, the reduction or elimination of HIV antigen in ARV-treated subjects leads to alterations in the phenotypic and functional profiles of the remaining cell populations. In untreated HIV infection, as in other chronic viral infections, persistent exposure to antigens leads to the progressive dysfunction of virus-specific CD8⁺ and CD4⁺ T cells, a phenomenon termed “T cell exhaustion” (reviewed in refs. 45, 46). In the early stages of exhaustion, T cells exhibit impaired proliferation in response to antigen and reduced polyfunctionality (the ability to produce multiple cytokines) (47). This is followed at late stages by apoptosis of HIV-specific CD8⁺ T cells (48). This exhausted state is associated with the upregulation of multiple activation and coinhibitory molecules (see below) (49–55). Several studies have demonstrated that prolonged ARV therapy results in some restoration of polyfunctionality and at least partial downregulation of activation and exhaustion markers (49–52, 56–60). It is tempting to speculate that, despite being reduced in numbers, the HIV-specific CD8⁺ T cells that remain in ARV-treated subjects may be more functional, less restrained by coinhibition, and less proapoptotic. Thus, on a per-cell basis, the remaining cells may be more able to eliminate reactivated HIV-infected cells than pre-ARV CD8⁺ T cells.

While it may be true that on cART, HIV-specific CD8⁺ T cells exhibit improved function, our knowledge of the functional and phenotypic features of HIV-specific CD8⁺ T cells with the greatest potential for eradicating reservoirs is inadequate to draw conclusions. Eradication may require only relatively rare and diffuse encounters between CD8⁺ T cells and residual infected cells with limited capacity to re-seed themselves, suggesting that the most desirable effectors will be highly specialized for cytotoxicity, even at the expense of proliferative capacity or the ability to produce cytokines and chemokines to recruit or support other cells. Such considerations may have implications for the timing of therapies aimed at disrupting latency, whereby CD8⁺ T cells may be either gradually improving or regressing in their potential to eliminate exposed reservoirs.

Despite evidence of potent antiviral function of HIV-specific CD8⁺ T cells, these cells are unable to fully clear infection, with plasma viremia persisting in most untreated infected persons. Under the best of circumstances, a détente is reached, in which plasma viral load is maintained at undetectable levels in plasma, although evidence indicates that tissue replication continues to occur in the vast majority of so-called “elite controllers” (reviewed in ref. 61). Additional barriers to CTL-mediated eradication are discussed below and summarized in Figure 1.

Figure 1

Barriers to CTL-mediated elimination of persistent viral reservoirs. Shown is a representation of the major barriers preventing CTL from effectively recognizing and eliminating HIV-infected cells. These issues will likely have to be solved in parallel in order to achieve reductions in the viral reservoir. For example, the enhancement of T cell function by blockade of immune checkpoints can only be effective if the rescued T cells are targeted against nonescaped viral epitopes.

Sequence variability and immune escape. Ongoing replication is partly due to immune escape, which is made probable by the error-prone HIV reverse transcriptase. The ability to rapidly acquire mutations that confer escape to otherwise effective immune responses is a hallmark of HIV (62–67) and has plagued efforts to develop a vaccine (68). In the setting of effective cART, viral replication is arrested, abrogating any appreciable viral evolution (69). Nonetheless, escape mutations of autologous T cell responses that were acquired prior to the initiation of therapy are preserved in proviral reservoirs. The degree to which viral reservoirs are recognizable to autologous HIV-specific CD8⁺ T cells diminishes as a function of the time between infection and ARV initiation. A recent study reported that, while escape mutations of common CD8⁺ T cell epitopes are relatively rare in individuals treated during acute infection, more than 98% of proviruses in patients treated during chronic infection harbored escape mutations in dominant epitopes that rendered the proviruses unrecognizable to CD8⁺ T cells (70). Nonetheless, subdominant CD8⁺ T cell responses targeted against nonescaped epitopes were identified in each of the subjects tested, and in vitro studies confirmed the elimination of cells infected with autologous HIV by corresponding expanded CD8⁺ T cell lines (70). Thus, in individuals treated during chronic infection, strategies to specifically augment CD8⁺ T cell responses to nonescaped epitopes may be key to eradication efforts.

CD8⁺ T cell exhaustion. HIV-specific CD8⁺ T cell efficacy is partly limited by the effects of chronic immune stimulation on CD8⁺ T cell function. A variety of coinhibitory molecules, including programmed death 1 (PD-1), T cell Ig and mucin domain 3 (TIM-3), CD160, the NK cell receptor 2B4, lymphocyte activation gene 3 (LAG-3), and cytotoxic T lymphocyte antigen 4 (CTLA-4), which impair the antiviral function of HIV-specific CD8⁺ T cells that negatively regulate immune function, are expressed under conditions of chronic antigenic stimulation (49–55). Simultaneous expression of multiple coinhibitory molecules may result in even more profound functional immune impairment (71). Since cART only partially reverses the upregulation of these molecules and the epigenetic program at the PD-1 locus becomes fixed after long-term TCR stimulation by HIV (72), CD8⁺ T cells can be expected to remain functionally impaired, even after prolonged cART.

Suboptimal epitope targeting. The observation that natural control of HIV replication is associated with certain HLA class I alleles suggested that some aspect of CD8⁺ T cell targeting may distinguish the most effective CD8⁺ T cell responses (15, 73–76). It was recently confirmed that HIV control is partly mediated by CD8⁺ T cell targeting of specific epitopes (76). More generally, apart from the phenomenon of elite control, strong T cell responses against the gene product Gag have been associated with control of viremia, while those targeting Env are associated with rapid progression (77). Although these observations are related to targeting of viral regions that cannot tolerate mutations (78, 79), high-avidity IFN-γ–expressing CD8⁺ T cells targeted against nonescaped epitopes also persist in cases of poor virologic control. These and other lines of evidence have led to an appreciation for the considerable heterogeneity in the antiviral functionalities of HIV-specific CD8⁺ T cell responses, giving rise to the paradigm of “driver” CD8⁺ T cell responses, which lead to control over viral replication and/or selection of escape mutations, versus “passenger” responses, which exert only weak pressure (80, 81). While epitope presentation kinetics may play a role in this phenomenon, efforts to further define what distinguishes the most effective CD8⁺ T cell responses in this setting remain a highly active area of research. In the meantime, the evidence discussed above provides guidance for the design of immunotherapeutic strategies, such as preferentially targeting the Gag protein, that are aimed at controlling viremia in untreated infection.

The features that define an effective CD8⁺ T cell response in the context of reservoir elimination are likely distinct from those needed to control active viremia and should also be considered in the context of therapeutic immunization. In the setting of active viremia, it is critical that CD8⁺ T cells target a conserved epitope, such that either escape cannot occur, or CTL-induced mutations in vulnerable regions reduce viral fitness (78, 79). In ARV-suppressed patients, the lack of viral replication negates the issue of ongoing viral escape, though sequence diversity in the existing reservoir is an important consideration. In unsuppressed patients, it is critical for CD8⁺ T cells to be able to recognize infected cells quickly, before virus can be produced (82–85). In ARV-treated subjects, rapid killing may not be important, provided that transmission of infection to other cells is suppressed by ARVs. High-avidity CD8⁺ T cells may also be important for eradication, whereby LRAs may induce only low-level antigen expression. The above-mentioned points are speculation provided to illustrate a gap in knowledge that must be addressed to most effectively harness CD8⁺ T cells for HIV eradication.

CD8⁺ T cell compartmentalization and the viral reservoir. Compartmentalization likely limits the ability of HIV-specific CD8⁺ T cells to eliminate infected cells. Pioneering studies in HIV-infected subjects demonstrated that HIV-specific CD8⁺ T cells were largely excluded from lymph node follicles, and CD4⁺ T cells in the lymphoid follicle were, on average, 31-fold more likely to be productively infected as compared with those in the paracortex (86). Studies performed in the SIV-infected rhesus macaque model have confirmed and extended these observations. The SIV model of elite control is driven by highly effective virus-specific CD8⁺ T cells that are able to recognize and eliminate at least a subset of infected cells systemically and in the lymph node paracortex without the aid of LRAs, with compartmentalization constituting a primary barrier to eradication (87, 88).

While the lymphoid follicles provide a clear-cut example of HIV persistence facilitated by the lack of HIV-specific CD8⁺ T cell access, similar scenarios may also exist in immune-privileged sites such as the testicles and CNS (89–92). Additionally, immune compartmentalization may contribute to HIV persistence in more subtle ways. CD4⁺ and CD8⁺ tissue–resident memory T cells (Trm) that are clonally expanded in tissue sites and do not readily circulate have recently been identified (93–100). The contributions of infected CD4⁺ Trm to HIV persistence remain unknown, as does the potential for CD8⁺ Trm to contribute to eradication at these tissue sites. One potential consequence is that infected CD4⁺ Trm that are localized in sites with restricted CD8⁺ T cell access will not expose themselves to killing by periodic egress. A second consequence is that infected CD4⁺ Trm in a given site may harbor escape mutations in CD8⁺ T cell epitopes that are not well represented in the peripheral blood. On an optimistic note, strategies to mobilize CD8⁺ Trm in tissues may have the potential to contribute to eradication, particularly if these cells have desirable specificities or functional profiles that are not represented in the circulation.

There are multiple challenges to harnessing CD8⁺ T cells to eradicate a reservoir, as outlined above and as evidenced by the inability of these cells to eradicate the reservoir in treated or untreated natural infection. Nevertheless, there are a number of compelling strategies that are worthy of pursuit.

Shock and kill. A promising eradication strategy involves combining LRAs, such as histone deacetylase inhibitors (HDACIs), cytokines, TLR agonists, or others, with CD8⁺ T cells (or other immune effectors) in order to induce antigen expression from quiescent cells and then eliminate these exposed targets (101). When combined with expanded HIV-specific CD8⁺ T cell lines, this approach has been shown to drive the elimination of infected cells from a primary cell model of latency and from patient samples in vitro (8, 102). Despite evidence that the administration of certain HDACIs disrupted HIV latency in patients, none of these studies revealed detectable reservoir depletion. One potential explanation for this finding could be that HDACIs impaired CD8⁺ T cell function in vivo, thereby interfering with the ability of these cells to eliminate exposed target cells. Both panobinostat and romidepsin have been shown to interfere with multiple CD8⁺ T cell functions, including elimination of HIV-infected cells, when tested in vitro at pharmacologically relevant concentrations (103). Additionally, HDACIs exhibit immunosuppressive activities in animal models of GVHD, experimental autoimmune encephalomyelitis, and other diseases for which they may be of therapeutic benefit (104–106). Encouragingly, ex vivo assessment of CD8⁺ T cell responses in clinical trials involving HIV-infected participants has thus far shown a lack of detectable impairment following the administration of panobinostat or vorinostat, though increases in CD4⁺ Treg frequencies were observed (32, 107). The question remains as to whether the degrees of latency reversal observed in these trials were sufficient to expose latently infected cells to immune recognition. If not, then the potential to negatively impact CD8⁺ T cell function with higher dosing regimens may define an upper limit on the therapeutic windows of these agents. Moreover, as activation through TCR stimulation sensitizes T cells to romidepsin and panobinostat toxicity in vitro (103), it is possible that CD8⁺ T cells that have been recently boosted by therapeutic vaccination may be preferentially killed by subsequent HDACI treatment. Moving forward, it will be important to continue to assess the potential impact of LRAs on the immune effectors with which they will need to work in concert in order to either mitigate potential interference or capitalize on potential enhancements of immune function. It will also likely be important to combine LRAs with strategies to address the other limitations of CD8⁺ T cells described above, including epitope escape and the diminished magnitude of responses observed in cART-treated subjects.

Therapeutic immunization. Peripheral blood mononuclear cells (PBMCs) from cART-treated subjects exhibit fairly weak ex vivo CTL-mediated killing of HIV-infected cells. Moreover, cART-associated reductions in viremia skew cells toward a memory phenotype. CD8⁺ T cell effector activity can be substantially enhanced by short-term expansion with HIV antigens (8, 102, 108). Successful in vitro demonstrations of the shock-and-kill concept have utilized such expanded HIV-specific CD8⁺ T cell lines (8, 102) and can be replicated in vivo by administering therapeutic vaccines aimed at boosting cellular immunity prior to administering LRAs (reviewed in ref. 109). The issue of immune escape presents an unfortunate complexity, in which — with the exception of subjects whose cART was initiated during their primary infection — immunodominant CD8⁺ T cell responses that might be preferentially boosted by therapeutic immunization are largely targeted against escaped epitopes and are therefore of no utility to HIV eradication (9). Therapeutic immunization to enhance cure efforts will likely require expanding the breadth of responses to include subdominant epitopes that have not already escaped. A related approach would involve the de novo priming of novel HIV-specific T cell responses that had not been elicited during the untreated infection period. New vaccine technologies, such as peptide-amphiphile vaccines that elicit robust T cell responses to peptides in animal models (110), have the potential to make important contributions to these efforts. Dendritic cells (DCs) are also likely to play critical roles in refocusing the immune response. In vitro studies have demonstrated the ability of DCs to expose and boost CD8⁺ T cell responses against subdominant autologous variants of HIV epitopes in cART-treated subjects (111). DC vaccines involving the ex vivo manipulation and reinfusion of HIV antigen–loaded DCs have also been shown to boost HIV-specific T cell responses in cART-treated patients, resulting in significantly reduced viral load set points following cART interruption (112). Effective enhancement of CD8⁺ T cell responses will also almost certainly require augmentation of HIV-specific CD4⁺ Th cell responses that are both critical for maintaining effective CD8⁺ T cell function and able to reverse some of the functional defects acquired during prolonged viral exposure (113).

Cell therapy. Ex vivo expansion and reinfusion of antigen-specific T cells has shown tremendous promise as a safe and effective means of augmenting antiviral immunity to CMV and EBV and as a therapeutic modality for cancer (114–116). A limited number of attempts have been made to translate this approach to HIV (117–121). The sole study that infused oligoclonal-expanded natural T cells into HIV-infected patients was performed in the early days of ARV therapy, when suppression was poor and showed a trend toward increased CD4 counts and decreased viremia in the absence of toxicity (120). The strategy of ex vivo expansion and reinfusion of virus-specific CTLs offers a superior measure of control over epitope specificity and functional characteristics that is particularly well suited to focusing responses against nonescaped epitopes (reviewed in refs. 122, 123). Cell therapy additionally offers the intriguing possibility of addressing issues related to compartmentalization, as particular homing profiles can be imprinted on CD8⁺ T cells by ex vivo culture conditions. For example, expanding T cells in the presence of retinoic acid results in subsequent homing of these T cells to the gut (124). T cell therapy involving the expansion of natural virus-specific responses has an excellent safety record (125), can be performed for approximately $6,000 per patient (126), and can establish populations of long-lived memory cells.

As an alternative to expanding natural HIV-specific T cell responses, cell therapy products can consist of T cells that have been redirected to recognize HIV-infected cells by genetic modification. This can be achieved by transducing cells with either transgenic HIV-specific TCRs or chimeric antigen receptors (CARs). These approaches offer several potential advantages, including the possibility of engineering high-avidity TCRs that may have enhanced abilities to detect viral reservoirs (127) and freedom from MHC-I restriction in the case of CAR T cells (128). However, unlike the expansion of naturally occurring HIV-specific T cell populations, these approaches must also address safety considerations regarding the possibility of unintentional targeting of self-antigens.

Coinhibitory blockade. Coinhibitory receptors, including PD-1, TIM-3, CD160, 2B4, LAG-3, and CTLA-4, play a critical role in the maintenance of exhaustion (49–55). Blockade of these receptors — either alone or in combination — has enhanced T cell function in vitro and viral control in multiple animal models (49–55, 129–132), providing a rationale for testing coinhibitory pathway blockade as an immunotherapeutic strategy in HIV infection. Additional enthusiasm for this approach can be drawn from advances in cancer immunotherapy, in which Abs that block the PD-1 and CTLA-4 pathways have been highly successful and are considered breakthrough drugs in the treatment of solid tumors (133). While treatment with cART results in some level of downregulation of coinhibitory receptor levels in the majority of HIV-infected subjects, these levels do not fully normalize in peripheral blood T cells, and persistent upregulation may be more pronounced in lymphoid tissues (49–54, 56, 57, 134). Thus, therapeutic blockade of coinhibitory pathways represents a promising approach to enhancing the abilities of CD8⁺ T cells to clear persistent viral reservoirs.

Additional immunotherapeutics. As an alternative or adjunct to blocking inhibitory pathways, CD8⁺ T cell function can be enhanced by the provision of cytokines or other immunostimulatory agents. IL-15 agonists are of particular interest in this regard, having been shown to enhance CD8⁺ T cell and NK cell activity in a number of preclinical models (135–139), and the IL-15 superagonist ALT-803 is moving into a clinical trial in cART-treated HIV-infected subjects (ClinicalTrials.gov identifier: NCT02191098). Other immunostimulatory agents include TLR-2 agonists, which reverse CD8⁺ T cell exhaustion and enhance both tumor- and pathogen-specific T cell responses in vivo (140–142), and agonistic Abs against 41BB or CD40 (143–145), among others.

The cellular immune response has evolved to specifically target and eliminate intracellular pathogens, primarily viruses. In this Review, we have taken the pragmatic approach of mainly focusing on the barriers to the effective targeting of CD8⁺ T cells against the viral reservoir that persists in the setting of cART (Figure 2). Faced with the challenge of achieving HIV eradication, we do, however, draw considerable inspiration from the other precedents that illustrate the power of a cellular immune response that has been properly unleashed. Several recent examples of this come from the oncology setting, in which both checkpoint blockade inhibition (146) and T cell–based therapies have resulted in substantial clinical benefits (128, 147). These successes include some dramatic cases, in which, for example, a single dose of anti–CTLA-4 Ab resulted in the eradication of a large tumor mass (148). An additional example emanates from the SIV rhesus macaque model of HIV infection, in which a subset of animals that received rhesus CMV–vectored SIV vaccine went on to have seemingly eradicated nascent infections (149). While clearance in this model has only been demonstrated when the vaccine was given prophylactically, these studies provide a critical proof of principle for immune-mediated eradication of a lentiviral infection. With recent advances in cell therapy and vaccine platforms and growing clinical experience with immunotherapeutics, we have multiple tools available to attempt to overcome the factors limiting CD8⁺ T cell efficacy in preclinical models and in the clinic. A particular challenge may be the need to address multiple barriers in parallel in order to achieve a measurable benefit. For example, boosting the total magnitude of the CD8⁺ T cell response and enhancing function on a per-cell basis may be ineffective without a component that targets these responses against nonescaped epitopes.

Figure 2

Factors governing the abilities of CTLs to recognize and eliminate infected cells. Shown is a summary of the major factors known to govern the outcome of an interaction between an HIV-specific CD8⁺ T cell and a persistently HIV-infected target cell.

This Review has focused on the prospects for harnessing CD8⁺ T cell responses to contribute to the eradication of infection, i.e., to achieve a “sterilizing cure.” An equally important strategy proposes to enlist cellular immune responses to exert long-term control of viremia without eliminating all reservoirs, i.e., to achieve a “functional cure.” Unlike sterilizing cures, functional cures have precedents in natural infection of patients who are elite controllers and in viremic controllers who maintain low levels of viremia in the absence of cART therapy. An additional precedent comes from the phenomenon of post-treatment control, as observed in the VISCONTI (viro-immunologic sustained control after treatment interruption) study cohort, in which subjects who were treated with cART during primary HIV infection showed a disproportionately high likelihood of maintaining low levels of viremia upon stopping cART (150). Many of the challenges in harnessing CD8⁺ T cells to achieve functional cures overlap with those involved in sterilizing cures, such as mitigating T cell exhaustion. There are, however, distinctions. For example, while achieving sterilizing cures will only have to address a fixed level of viral sequence diversity (established before initiation of cART), functional cures will have to address some level of ongoing viral evolution. Pursuing both of these related objectives in parallel will allow for cross-fertilization of lessons learned and maximize the potential for novel therapeutics that will improve the lives of people living with HIV/AIDS.

Finally, while the optimization of various arms of the immune system needs to be considered separately, we stress the importance of developing combination therapies that bring together both the cellular and humoral arms of adaptive immunity with innate immune mechanisms in order to overcome the tenacity of a virus that manages to persist for decades, even in an environment rendered inhospitable to its replication by cART.

B.D. Walker is an Investigator of the Howard Hughes Medical Institute. R.B. Jones gratefully acknowledges the support of the Ontario HIV Treatment Network and of amfAR’s generationCURE initiative. This work was also supported by a grant from the NIH (R37 AI067073, to B.D. Walker).

Address correspondence to: Bruce D. Walker, Ragon Institute of MGH, MIT and Harvard, 400 Technology Square, Room 870, Cambridge, Massachusetts 02139, USA. Phone: 857.268.7072; E-mail: bwalker@partners.org.

Conflict of interest: R.B. Jones is an inventor on a patent claiming the blockade of the TIM-3 pathway as a potential therapy for chronic viral infection.

Reference information: J Clin Invest. 2016;126(2):455–463. doi:10.1172/JCI80566.

1. Whitney JB, et al. Rapid seeding of the viral reservoir prior to SIV viraemia in rhesus monkeys. Nature. 2014;512(7512):74–77.
   View this article via: PubMed Google Scholar
2. Borrow P, Lewicki H, Hahn BH, Shaw GM, Oldstone MB. Virus-specific CD8⁺ cytotoxic T-lymphocyte activity associated with control of viremia in primary human immunodeficiency virus type 1 infection. J Virol. 1994;68(9):6103–6110.
   View this article via: PubMed Google Scholar
3. Koup RA, et al. Temporal association of cellular immune responses with the initial control of viremia in primary human immunodeficiency virus type 1 syndrome. J Virol. 1994;68(7):4650–4655.
   View this article via: PubMed Google Scholar
4. Ndhlovu ZM, et al. Magnitude and kinetics of CD8⁺ T cell activation in hyperacute HIV infection impact viral set point. Immunity. 2015;43(3):591–604.
   View this article via: PubMed CrossRef Google Scholar
5. Hutter G, et al. Long-term control of HIV by CCR5 Δ32/Δ32 stem-cell transplantation. N Engl J Med. 2009;360(7):692–698.
   View this article via: PubMed CrossRef Google Scholar
6. Allers K, et al. Evidence for the cure of HIV infection by CCR5Δ32/Δ32 stem cell transplantation. Blood. 2011;117(10):2791–2799.
   View this article via: PubMed CrossRef Google Scholar
7. Yukl SA, et al. Challenges in detecting HIV persistence during potentially curative interventions: a study of the Berlin patient. PLoS Pathog. 2013;9(5):e1003347.
   View this article via: PubMed CrossRef Google Scholar
8. Shan L, et al. Stimulation of HIV-1-specific cytolytic T lymphocytes facilitates elimination of latent viral reservoir after virus reactivation. Immunity. 2012;36(3):491–501.
   View this article via: PubMed CrossRef Google Scholar
9. Deng K, et al. Broad CTL response is required to clear latent HIV-1 due to dominance of escape mutations. Nature. 2015;517(7534):381–385.
   View this article via: PubMed CrossRef Google Scholar
10. Yang OO, et al. Suppression of human immunodeficiency virus type 1 replication by CD8⁺ cells: evidence for HLA class I-restricted triggering of cytolytic and noncytolytic mechanisms. J Virol. 1997;71(4):3120–3128.
    View this article via: PubMed Google Scholar
11. Goonetilleke N, et al. The first T cell response to transmitted/founder virus contributes to the control of acute viremia in HIV-1 infection. J Exp Med. 2009;206(6):1253–1272.
    View this article via: PubMed CrossRef Google Scholar
12. Liu MK, et al. Vertical T cell immunodominance and epitope entropy determine HIV-1 escape. J Clin Invest. 2013;123(1):380–393.
    View this article via: JCI PubMed CrossRef Google Scholar
13. Chen H, et al. Differential neutralization of human immunodeficiency virus (HIV) replication in autologous CD4 T cells by HIV-specific cytotoxic T lymphocytes. J Virol. 2009;83(7):3138–3149.
    View this article via: PubMed CrossRef Google Scholar
14. Schmitz JE, et al. Control of viremia in simian immunodeficiency virus infection by CD8⁺ lymphocytes. Science. 1999;283(5403):857–860.
    View this article via: PubMed CrossRef Google Scholar
15. Fellay J, et al. A whole-genome association study of major determinants for host control of HIV-1. Science. 2007;317(5840):944–947.
    View this article via: PubMed CrossRef Google Scholar
16. Pereyra F, et al. The major genetic determinants of HIV-1 control affect HLA class I peptide presentation. Science. 2010;330(6010):1551–1557.
    View this article via: PubMed CrossRef Google Scholar
17. Miura T, et al. HLA-associated alterations in replication capacity of chimeric NL4-3 viruses carrying gag-protease from elite controllers of human immunodeficiency virus type 1. J Virol. 2009;83(1):140–149.
    View this article via: PubMed CrossRef Google Scholar
18. Martinez-Picado J, et al. Fitness cost of escape mutations in p24 Gag in association with control of human immunodeficiency virus type 1. J Virol. 2006;80(7):3617–3623.
    View this article via: PubMed CrossRef Google Scholar
19. Carlson JM, et al. HIV transmission. Selection bias at the heterosexual HIV-1 transmission bottleneck. Science. 2014;345(6193):1254031.
    View this article via: PubMed CrossRef Google Scholar
20. Chun TW, Finzi D, Margolick J, Chadwick K, Schwartz D, Siliciano RF. In vivo fate of HIV-1-infected T cells: quantitative analysis of the transition to stable latency. Nat Med. 1995;1(12):1284–1290.
    View this article via: PubMed CrossRef Google Scholar
21. Chun TW, et al. Presence of an inducible HIV-1 latent reservoir during highly active antiretroviral therapy. Proc Natl Acad Sci U S A. 1997;94(24):13193–13197.
    View this article via: PubMed CrossRef Google Scholar
22. Finzi D, et al. Identification of a reservoir for HIV-1 in patients on highly active antiretroviral therapy. Science. 1997;278(5341):1295–1300.
    View this article via: PubMed CrossRef Google Scholar
23. Wong JK, et al. Recovery of replication-competent HIV despite prolonged suppression of plasma viremia. Science. 1997;278(5341):1291–1295.
    View this article via: PubMed CrossRef Google Scholar
24. Finzi D, et al. Latent infection of CD4⁺ T cells provides a mechanism for lifelong persistence of HIV-1, even in patients on effective combination therapy. Nat Med. 1999;5(5):512–517.
    View this article via: PubMed CrossRef Google Scholar
25. Irvine DJ, Purbhoo MA, Krogsgaard M, Davis MM. Direct observation of ligand recognition by T cells. Nature. 2002;419(6909):845–849.
    View this article via: PubMed CrossRef Google Scholar
26. Lin X, et al. Transcriptional profiles of latent human immunodeficiency virus in infected individuals: effects of Tat on the host and reservoir. J Virol. 2003;77(15):8227–8236.
    View this article via: PubMed CrossRef Google Scholar
27. Adams M, et al. Cellular latency in human immunodeficiency virus-infected individuals with high CD4 levels can be detected by the presence of promoter-proximal transcripts. Proc Natl Acad Sci U S A. 1994;91(9):3862–3866.
    View this article via: PubMed CrossRef Google Scholar
28. Adams M, Wong C, Wang D, Romeo J. Limitation of Tat-associated transcriptional processivity in HIV-infected PBMC. Virology. 1999;257(2):397–405.
    View this article via: PubMed CrossRef Google Scholar
29. Lassen KG, Bailey JR, Siliciano RF. Analysis of human immunodeficiency virus type 1 transcriptional elongation in resting CD4+ T cells in vivo. J Virol. 2004;78(17):9105–9114.
    View this article via: PubMed CrossRef Google Scholar
30. Chun TW, et al. Gene expression and viral prodution in latently infected, resting CD4⁺ T cells in viremic versus aviremic HIV-infected individuals. Proc Natl Acad Sci U S A. 2003;100(4):1908–1913.
    View this article via: PubMed CrossRef Google Scholar
31. Archin NM, et al. Administration of vorinostat disrupts HIV-1 latency in patients on antiretroviral therapy. Nature. 2012;487(7408):482–485.
    View this article via: PubMed CrossRef Google Scholar
32. Elliott JH, et al. Activation of HIV transcription with short-course vorinostat in HIV-infected patients on suppressive antiretroviral therapy. PLoS Pathog. 2014;10(10):e1004473.
    View this article via: PubMed Google Scholar
33. Lassen KG, Ramyar KX, Bailey JR, Zhou Y, Siliciano RF. Nuclear retention of multiply spliced HIV-1 RNA in resting CD4⁺ T cells. PLoS Pathog. 2006;2(7):e68.
    View this article via: PubMed CrossRef Google Scholar
34. Hermankova M, et al. Analysis of human immunodeficiency virus type 1 gene expression in latently infected resting CD4⁺ T lymphocytes in vivo. J Virol. 2003;77(13):7383–7392.
    View this article via: PubMed CrossRef Google Scholar
35. Bullen CK, Laird GM, Durand CM, Siliciano JD, Siliciano RF. New ex vivo approaches distinguish effective and ineffective single agents for reversing HIV-1 latency in vivo. Nat Med. 2014;20(4):425–429.
    View this article via: PubMed CrossRef Google Scholar
36. Graf EH, et al. Gag-positive reservoir cells are susceptible to HIV-specific cytotoxic T lymphocyte mediated clearance in vitro and can be detected in vivo [corrected]. PLoS One. 2013;8(8):e71879.
    View this article via: PubMed CrossRef Google Scholar
37. Pace MJ, et al. Directly infected resting CD4⁺ T cells can produce HIV Gag without spreading infection in a model of HIV latency. PLoS Pathog. 2012;8(7):e1002818.
    View this article via: PubMed CrossRef Google Scholar
38. Rinaldo CR Jr, et al. Anti-human immunodeficiency virus type 1 (HIV-1) CD8(+) T-lymphocyte reactivity during combination antiretroviral therapy in HIV-1-infected patients with advanced immunodeficiency. J Virol. 2000;74(9):4127–4138.
    View this article via: PubMed CrossRef Google Scholar
39. Ogg GS, et al. Decay kinetics of human immunodeficiency virus-specific effector cytotoxic T lymphocytes after combination antiretroviral therapy. J Virol. 1999;73(1):797–800.
    View this article via: PubMed Google Scholar
40. Gray CM, et al. Frequency of class I HLA-restricted anti-HIV CD8⁺ T cells in individuals receiving highly active antiretroviral therapy (HAART). J Immunol. 1999;162(3):1780–1788.
    View this article via: PubMed Google Scholar
41. Casazza JP, Betts MR, Picker LJ, Koup RA. Decay kinetics of human immunodeficiency virus-specific CD8⁺ T cells in peripheral blood after initiation of highly active antiretroviral therapy. J Virol. 2001;75(14):6508–6516.
    View this article via: PubMed CrossRef Google Scholar
42. Kalams SA, et al. Levels of human immunodeficiency virus type 1-specific cytotoxic T-lymphocyte effector and memory responses decline after suppression of viremia with highly active antiretroviral therapy. J Virol. 1999;73(8):6721–6728.
    View this article via: PubMed Google Scholar
43. Addo MM, et al. Comprehensive epitope analysis of human immunodeficiency virus type 1 (HIV-1)-specific T-cell responses directed against the entire expressed HIV-1 genome demonstrate broadly directed responses, but no correlation to viral load. J Virol. 2003;77(3):2081–2092.
    View this article via: PubMed CrossRef Google Scholar
44. Yan J, et al. HIV-specific CD8⁺ T cells from elite controllers are primed for survival. J Virol. 2013;87(9):5170–5181.
    View this article via: PubMed CrossRef Google Scholar
45. Kuchroo VK, Anderson AC, Petrovas C. Coinhibitory receptors and CD8 T cell exhaustion in chronic infections. Curr Opin HIV AIDS. 2014;9(5):439–445.
    View this article via: PubMed CrossRef Google Scholar
46. Kahan SM, Wherry EJ, Zajac AJ. T cell exhaustion during persistent viral infections. Virology. 2015;479–480:180–193.
    View this article via: PubMed Google Scholar
47. Betts MR, et al. HIV nonprogressors preferentially maintain highly functional HIV-specific CD8⁺ T cells. Blood. 2006;107(12):4781–4789.
    View this article via: PubMed CrossRef Google Scholar
48. Mueller YM, et al. Increased CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells. Immunity. 2001;15(6):871–882.
    View this article via: PubMed CrossRef Google Scholar
49. Day CL, et al. PD-1 expression on HIV-specific T cells is associated with T-cell exhaustion and disease progression. Nature. 2006;443(7109):350–354.
    View this article via: PubMed CrossRef Google Scholar
50. Jones RB, et al. Tim-3 expression defines a novel population of dysfunctional T cells with highly elevated frequencies in progressive HIV-1 infection. J Exp Med. 2008;205(12):2763–2779.
    View this article via: PubMed CrossRef Google Scholar
51. Kaufmann DE, et al. Upregulation of CTLA-4 by HIV-specific CD4+ T cells correlates with disease progression and defines a reversible immune dysfunction. Nat Immunol. 2007;8(11):1246–1254.
    View this article via: PubMed CrossRef Google Scholar
52. Nikolova MH, et al. The CD160+ CD8high cytotoxic T cell subset correlates with response to HAART in HIV-1+ patients. Cell Immunol. 2005;237(2):96–105.
    View this article via: PubMed CrossRef Google Scholar
53. Tian X, et al. The upregulation of LAG-3 on T cells defines a subpopulation with functional exhaustion and correlates with disease progression in HIV-infected subjects. J Immunol. 2015;194(8):3873–3882.
    View this article via: PubMed CrossRef Google Scholar
54. Trautmann L, et al. Upregulation of PD-1 expression on HIV-specific CD8⁺ T cells leads to reversible immune dysfunction. Nat Med. 2006;12(10):1198–1202.
    View this article via: PubMed CrossRef Google Scholar
55. Petrovas C, et al. PD-1 is a regulator of virus-specific CD8⁺ T cell survival in HIV infection. J Exp Med. 2006;203(10):2281–2292.
    View this article via: PubMed CrossRef Google Scholar
56. Grabmeier-Pfistershammer K, Steinberger P, Rieger A, Leitner J, Kohrgruber N. Identification of PD-1 as a unique marker for failing immune reconstitution in HIV-1-infected patients on treatment. J Acquir Immune Defic Syndr. 2011;56(2):118–124.
    View this article via: PubMed CrossRef Google Scholar
57. Kassu A, Marcus RA, D’Souza MB, Kelly-McKnight EA, Palmer BE. Suppression of HIV replication by antiretroviral therapy reduces TIM-3 expression on HIV-specific CD8(+) T cells. AIDS Res Hum Retroviruses. 2011;27(1):1–3.
    View this article via: PubMed CrossRef Google Scholar
58. Streeck H, et al. Antigen load and viral sequence diversification determine the functional profile of HIV-1-specific CD8⁺ T cells. PLoS Med. 2008;5(5):e100.
    View this article via: PubMed CrossRef Google Scholar
59. Conrad JA, et al. Antiretroviral therapy reduces the magnitude and T cell receptor repertoire diversity of HIV-specific T cell responses without changing T cell clonotype dominance. J Virol. 2012;86(8):4213–4221.
    View this article via: PubMed CrossRef Google Scholar
60. Rehr M, et al. Emergence of polyfunctional CD8⁺ T cells after prolonged suppression of human immunodeficiency virus replication by antiretroviral therapy. J Virol. 2008;82(7):3391–3404.
    View this article via: PubMed CrossRef Google Scholar
61. Cockerham LR, Hatano H. Elite control of HIV: is this the right model for a functional cure? Trends Microbiol. 2015;23(2):71–75.
    View this article via: PubMed CrossRef Google Scholar
62. Borrow P, et al. Antiviral pressure exerted by HIV-1-specific cytotoxic T lymphocytes (CTLs) during primary infection demonstrated by rapid selection of CTL escape virus. Nat Med. 1997;3(2):205–211.
    View this article via: PubMed CrossRef Google Scholar
63. Goulder PJ, et al. Late escape from an immunodominant cytotoxic T-lymphocyte response associated with progression to AIDS. Nat Med. 1997;3(2):212–217.
    View this article via: PubMed CrossRef Google Scholar
64. Koup RA. Virus escape from CTL recognition. J Exp Med. 1994;180(3):779–782.
    View this article via: PubMed CrossRef Google Scholar
65. Leslie AJ, et al. HIV evolution: CTL escape mutation and reversion after transmission. Nat Med. 2004;10(3):282–289.
    View this article via: PubMed CrossRef Google Scholar
66. McMichael AJ, Borrow P, Tomaras GD, Goonetilleke N, Haynes BF. The immune response during acute HIV-1 infection: clues for vaccine development. Nat Rev Immunol. 2010;10(1):11–23.
    View this article via: PubMed CrossRef Google Scholar
67. Phillips RE, et al. Human immunodeficiency virus genetic variation that can escape cytotoxic T cell recognition. Nature. 1991;354(6353):453–459.
    View this article via: PubMed CrossRef Google Scholar
68. Goulder PJ, Watkins DI. HIV and SIV CTL escape: implications for vaccine design. Nat Rev Immunol. 2004;4(8):630–640.
    View this article via: PubMed CrossRef Google Scholar
69. von Stockenstrom S, et al. Longitudinal genetic characterization reveals that cell proliferation maintains a persistent HIV Type 1 DNA pool during effective HIV therapy. J Infect Dis. 2015;212(4):596–607.
    View this article via: PubMed CrossRef Google Scholar
70. Deng K, et al. Broad CTL response is required to clear latent HIV-1 due to dominance of escape mutations. Nature. 2015;517(7534):381–385.
    View this article via: PubMed CrossRef Google Scholar
71. Blackburn SD, et al. Coregulation of CD8⁺ T cell exhaustion by multiple inhibitory receptors during chronic viral infection. Nat Immunol. 2009;10(1):29–37.
    View this article via: PubMed CrossRef Google Scholar
72. Youngblood B, et al. Cutting edge: Prolonged exposure to HIV reinforces a poised epigenetic program for PD-1 expression in virus-specific CD8 T cells. J Immunol. 2013;191(2):540–544.
    View this article via: PubMed CrossRef Google Scholar
73. Carrington M, et al. HLA and HIV-1: heterozygote advantage and B*35-Cw*04 disadvantage. Science. 1999;283(5408):1748–1752.
    View this article via: PubMed CrossRef Google Scholar
74. International HIVCS, et al. The major genetic determinants of HIV-1 control affect HLA class I peptide presentation. Science. 2010;330(6010):1551–1557.
    View this article via: PubMed CrossRef Google Scholar
75. Kaslow RA, et al. Influence of combinations of human major histocompatibility complex genes on the course of HIV-1 infection. Nat Med. 1996;2(4):405–411.
    View this article via: PubMed CrossRef Google Scholar
76. Pereyra F, et al. HIV control is mediated in part by CD8⁺ T-cell targeting of specific epitopes. J Virol. 2014;88(22):12937–12948.
    View this article via: PubMed CrossRef Google Scholar
77. Kiepiela P, et al. CD8⁺ T-cell responses to different HIV proteins have discordant associations with viral load. Nat Med. 2007;13(1):46–53.
    View this article via: PubMed CrossRef Google Scholar
78. Dahirel V, et al. Coordinate linkage of HIV evolution reveals regions of immunological vulnerability. Proc Natl Acad Sci U S A. 2011;108(28):11530–11535.
    View this article via: PubMed CrossRef Google Scholar
79. Ferguson AL, Mann JK, Omarjee S, Ndung’u T, Walker BD, Chakraborty AK. Translating HIV sequences into quantitative fitness landscapes predicts viral vulnerabilities for rational immunogen design. Immunity. 2013;38(3):606–617.
    View this article via: PubMed CrossRef Google Scholar
80. Klenerman P, Wu Y, Phillips R. HIV: current opinion in escapology. Current Opin Microbiol. 2002;5(4):408–413.
    View this article via: PubMed CrossRef Google Scholar
81. Zafiropoulos A, Barnes E, Piggott C, Klenerman P. Analysis of ‘driver’ and ‘passenger’ CD8+ T-cell responses against variable viruses. Proc Biol Sci. 2004;271 Suppl 3:S53–S56.
    View this article via: PubMed Google Scholar
82. Payne RP, et al. Efficacious early antiviral activity of HIV Gag- and Pol-specific HLA-B 2705-restricted CD8⁺ T cells. J Virol. 2010;84(20):10543–10557.
    View this article via: PubMed CrossRef Google Scholar
83. Dinter J, et al. Different antigen-processing activities in dendritic cells, macrophages, and monocytes lead to uneven production of HIV epitopes and affect CTL recognition. J Immunol. 2014;193(9):4322–4334.
    View this article via: PubMed CrossRef Google Scholar
84. Sacha JB, et al. Differential antigen presentation kinetics of CD8+ T-cell epitopes derived from the same viral protein. J Virol. 2008;82(18):9293–9298.
    View this article via: PubMed CrossRef Google Scholar
85. Sacha JB, et al. Gag-specific CD8⁺ T lymphocytes recognize infected cells before AIDS-virus integration and viral protein expression. J Immunol. 2007;178(5):2746–2754.
    View this article via: PubMed CrossRef Google Scholar
86. Fukazawa Y, et al. B cell follicle sanctuary permits persistent productive simian immunodeficiency virus infection in elite controllers. Nat Med. 2015;21(2):132–139.
    View this article via: PubMed CrossRef Google Scholar
87. Folkvord JM, Armon C, Connick E. Lymphoid follicles are sites of heightened human immunodeficiency virus type 1 (HIV-1) replication and reduced antiretroviral effector mechanisms. AIDS Res Hum Retroviruses. 2005;21(5):363–370.
    View this article via: PubMed CrossRef Google Scholar
88. Connick E, et al. CTL fail to accumulate at sites of HIV-1 replication in lymphoid tissue. J Immunol. 2007;178(11):6975–6983.
    View this article via: PubMed CrossRef Google Scholar
89. Galvin SR, Cohen MS. Genital tract reservoirs. Curr Opin HIV AIDS. 2006;1(2):162–166.
    View this article via: PubMed Google Scholar
90. Gray LR, Roche M, Flynn JK, Wesselingh SL, Gorry PR, Churchill MJ. Is the central nervous system a reservoir of HIV-1? Curr Opin HIV AIDS. 2014;9(6):552–558.
    View this article via: PubMed CrossRef Google Scholar
91. Muldoon LL, et al. Immunologic privilege in the central nervous system and the blood-brain barrier. J Cereb Blood Flow Metab. 2013;33(1):13–21.
    View this article via: PubMed CrossRef Google Scholar
92. Schuppe HC, Meinhardt A. Immune privilege and inflammation of the testis. Chem Immunol Allergy. 2005;88:1–14.
    View this article via: PubMed Google Scholar
93. Gebhardt T, Wakim LM, Eidsmo L, Reading PC, Heath WR, Carbone FR. Memory T cells in nonlymphoid tissue that provide enhanced local immunity during infection with herpes simplex virus. Nat Immunol. 2009;10(5):524–530.
    View this article via: PubMed CrossRef Google Scholar
94. Jiang X, Clark RA, Liu L, Wagers AJ, Fuhlbrigge RC, Kupper TS. Skin infection generates non-migratory memory CD8⁺ T(RM) cells providing global skin immunity. Nature. 2012;483(7388):227–231.
    View this article via: PubMed CrossRef Google Scholar
95. Klonowski KD, Williams KJ, Marzo AL, Blair DA, Lingenheld EG, Lefrancois L. Dynamics of blood-borne CD8 memory T cell migration in vivo. Immunity. 2004;20(5):551–562.
    View this article via: PubMed CrossRef Google Scholar
96. Liu L, Zhong Q, Tian T, Dubin K, Athale SK, Kupper TS. Epidermal injury and infection during poxvirus immunization is crucial for the generation of highly protective T cell-mediated immunity. Nat Med. 2010;16(2):224–227.
    View this article via: PubMed CrossRef Google Scholar
97. Park CO, Kupper TS. The emerging role of resident memory T cells in protective immunity and inflammatory disease. Nat Med. 2015;21(7):688–697.
    View this article via: PubMed CrossRef Google Scholar
98. Sathaliyawala T, et al. Distribution and compartmentalization of human circulating and tissue-resident memory T cell subsets. Immunity. 2013;38(1):187–197.
    View this article via: PubMed CrossRef Google Scholar
99. Steinert EM, et al. Quantifying memory CD8 T cells reveals regionalization of immunosurveillance. Cell. 2015;161(4):737–749.
    View this article via: PubMed CrossRef Google Scholar
100. Teijaro JR, Turner D, Pham Q, Wherry EJ, Lefrancois L, Farber DL. Cutting edge: Tissue-retentive lung memory CD4 T cells mediate optimal protection to respiratory virus infection. J Immunol. 2011;187(11):5510–5514.
     View this article via: PubMed CrossRef Google Scholar
101. Deeks SG. HIV: Shock and kill. Nature. 2012;487(7408):439–440.
     View this article via: PubMed CrossRef Google Scholar
102. Sung JA, et al. Expanded cytotoxic T-cell lymphocytes target the latent HIV reservoir. J Infect Dis. 2015;212(2):258–263.
     View this article via: PubMed CrossRef Google Scholar
103. Jones RB, et al. Histone deacetylase inhibitors impair the elimination of HIV-infected cells by cytotoxic T-lymphocytes. PLoS Pathog. 2014;10(8):e1004287.
     View this article via: PubMed CrossRef Google Scholar
104. Camelo S, et al. Transcriptional therapy with the histone deacetylase inhibitor trichostatin A ameliorates experimental autoimmune encephalomyelitis. J Neuroimmunol. 2005;164(1–2):10–21.
     View this article via: PubMed Google Scholar
105. Dowdell KC, et al. Valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, diminishes lymphoproliferation in the Fas-deficient MRL/lpr(–/–) murine model of autoimmune lymphoproliferative syndrome (ALPS). Exp Hematol. 2009;37(4):487–494.
     View this article via: PubMed CrossRef Google Scholar
106. Garcia BA, Busby SA, Shabanowitz J, Hunt DF, Mishra N. Resetting the epigenetic histone code in the MRL-lpr/lpr mouse model of lupus by histone deacetylase inhibition. J Proteome Res. 2005;4(6):2032–2042.
     View this article via: PubMed CrossRef Google Scholar
107. Olesen RRT, Graversen M, Leth S, Ostergaard L, Sogaard O, Tolstrup M, Lichterfeld M. Conference on Retroviruses and Opportunistic Infections (CROI). Seattle; 2015.
108. Lam S, et al. Broadly-specific cytotoxic T cells targeting multiple HIV antigens are expanded from HIV+ patients: implications for immunotherapy. Mol Ther. 2015;23(2):387–395.
     View this article via: PubMed CrossRef Google Scholar
109. Mylvaganam GH, Silvestri G, Amara RR. HIV therapeutic vaccines: moving towards a functional cure. Curr Opin Immunol. 2015;35:1–8.
     View this article via: PubMed CrossRef Google Scholar
110. Liu H, et al. Structure-based programming of lymph-node targeting in molecular vaccines. Nature. 2014;507(7493):519–522.
     View this article via: PubMed CrossRef Google Scholar
111. Smith KN, et al. Dendritic cells restore CD8⁺ T cell reactivity to autologous HIV-1. J Virol. 2014;88(17):9976–9990.
     View this article via: PubMed CrossRef Google Scholar
112. Garcia F, et al. A dendritic cell-based vaccine elicits T cell responses associated with control of HIV-1 replication. Sci Transl Med. 2013;5(166):166ra2.
     View this article via: PubMed Google Scholar
113. Lichterfeld M, et al. Loss of HIV-1-specific CD8⁺ T cell proliferation after acute HIV-1 infection and restoration by vaccine-induced HIV-1-specific CD4⁺ T cells. J Exp Med. 2004;200(6):701–712.
     View this article via: PubMed CrossRef Google Scholar
114. Saglio F, Hanley PJ, Bollard CM. The time is now: moving toward virus-specific T cells after allogeneic hematopoietic stem cell transplantation as the standard of care. Cytotherapy. 2014;16(2):149–159.
     View this article via: PubMed CrossRef Google Scholar
115. Leen AM, et al. Monoculture-derived T lymphocytes specific for multiple viruses expand and produce clinically relevant effects in immunocompromised individuals. Nat Med. 2006;12(10):1160–1166.
     View this article via: PubMed CrossRef Google Scholar
116. Hinrichs CS, Rosenberg SA. Exploiting the curative potential of adoptive T-cell therapy for cancer. Immunol Rev. 2014;257(1):56–71.
     View this article via: PubMed CrossRef Google Scholar
117. Chapuis AG, et al. HIV-specific CD8⁺ T cells from HIV+ individuals receiving HAART can be expanded ex vivo to augment systemic and mucosal immunity in vivo. Blood. 2011;117(20):5391–5402.
     View this article via: PubMed CrossRef Google Scholar
118. Brodie SJ, et al. In vivo migration and function of transferred HIV-1-specific cytotoxic T cells. Nat Med. 1999;5(1):34–41.
     View this article via: PubMed CrossRef Google Scholar
119. Tan R, et al. Rapid death of adoptively transferred T cells in acquired immunodeficiency syndrome. Blood. 1999;93(5):1506–1510.
     View this article via: PubMed Google Scholar
120. Lieberman J, et al. Safety of autologous, ex vivo-expanded human immunodeficiency virus (HIV)-specific cytotoxic T-lymphocyte infusion in HIV-infected patients. Blood. 1997;90(6):2196–2206.
     View this article via: PubMed Google Scholar
121. Koenig S, et al. Transfer of HIV-1-specific cytotoxic T lymphocytes to an AIDS patient leads to selection for mutant HIV variants and subsequent disease progression. Nat Med. 1995;1(4):330–336.
     View this article via: PubMed CrossRef Google Scholar
122. Hildebrandt M, Peggs K, Uharek L, Bollard CM, Heslop HE. Immunotherapy: opportunities, risks and future perspectives. Cytotherapy. 2014;16(4 suppl):S120–S129.
     View this article via: PubMed Google Scholar
123. Lam S, Bollard C. T-cell therapies for HIV. Immunotherapy. 2013;5(4):407–414.
     View this article via: PubMed CrossRef Google Scholar
124. Iwata M, Hirakiyama A, Eshima Y, Kagechika H, Kato C, Song SY. Retinoic acid imprints gut-homing specificity on T cells. Immunity. 2004;21(4):527–538.
     View this article via: PubMed CrossRef Google Scholar
125. Cruz CR, et al. Adverse events following infusion of T cells for adoptive immunotherapy: a 10-year experience. Cytotherapy. 2010;12(6):743–749.
     View this article via: PubMed CrossRef Google Scholar
126. Heslop HE, et al. Long-term outcome of EBV-specific T-cell infusions to prevent or treat EBV-related lymphoproliferative disease in transplant recipients. Blood. 2010;115(5):925–935.
     View this article via: PubMed CrossRef Google Scholar
127. Varela-Rohena A, et al. Control of HIV-1 immune escape by CD8 T cells expressing enhanced T-cell receptor. Nat Med. 2008;14(12):1390–1395.
     View this article via: PubMed CrossRef Google Scholar
128. Barrett DM, Singh N, Porter DL, Grupp SA, June CH. Chimeric antigen receptor therapy for cancer. Annu Rev Med. 2014;65:333–347.
     View this article via: PubMed CrossRef Google Scholar
129. Xiao L, et al. Enhancement of SIV-specific cell mediated immune responses by co-administration of soluble PD-1 and Tim-3 as molecular adjuvants in mice. Hum Vaccin Immunother. 2014;10(3):724–733.
     View this article via: PubMed CrossRef Google Scholar
130. Vargas-Inchaustegui DA, et al. Immune targeting of PD-1(hi) expressing cells during and after antiretroviral therapy in SIV-infected rhesus macaques. Virology. 2013;447(1–2):274–284.
     View this article via: PubMed Google Scholar
131. Dyavar Shetty R, et al. PD-1 blockade during chronic SIV infection reduces hyperimmune activation and microbial translocation in rhesus macaques. J Clin Invest. 2012;122(5):1712–1716.
     View this article via: JCI PubMed CrossRef Google Scholar
132. Velu V, et al. Enhancing SIV-specific immunity in vivo by PD-1 blockade. Nature. 2009;458(7235):206–210.
     View this article via: PubMed CrossRef Google Scholar
133. Callahan MK, Wolchok JD. At the bedside: CTLA-4- and PD-1-blocking antibodies in cancer immunotherapy. J Leuk Biol. 2013;94(1):41–53.
     View this article via: PubMed CrossRef Google Scholar
134. Rueda CM, Velilla PA, Chougnet CA, Montoya CJ, Rugeles MT. HIV-induced T-cell activation/exhaustion in rectal mucosa is controlled only partially by antiretroviral treatment. PLoS One. 2012;7(1):e30307.
     View this article via: PubMed CrossRef Google Scholar
135. Mathios D, et al. Therapeutic administration of IL-15 superagonist complex ALT-803 leads to long-term survival durable antitumor immune response in a murine glioblastoma model. Int J Cancer. 2016;138(1):187–194.
     View this article via: PubMed CrossRef Google Scholar
136. Gomes-Giacoia E, et al. Intravesical ALT-803 and BCG treatment reduces tumor burden in a carcinogen induced bladder cancer rat model; a role for cytokine production and NK cell expansion. PLoS One. 2014;9(6):e96705.
     View this article via: PubMed CrossRef Google Scholar
137. Wong HC, Jeng EK, Rhode PR. The IL-15-based superagonist ALT-803 promotes the antigen-independent conversion of memory CD8 T cells into innate-like effector cells with antitumor activity. Oncoimmunology. 2013;2(11):e26442.
     View this article via: PubMed CrossRef Google Scholar
138. Xu W, et al. Efficacy and mechanism-of-action of a novel superagonist interleukin-15: interleukin-15 receptor αSu/Fc fusion complex in syngeneic murine models of multiple myeloma. Cancer Res. 2013;73(10):3075–3086.
     View this article via: PubMed CrossRef Google Scholar
139. Seay K, et al. In vivo activation of human NK cells by treatment with an interleukin-15 superagonist potently inhibits acute in vivo HIV-1 infection in humanized mice. J Virol. 2015;89(12):6264–6274.
     View this article via: PubMed CrossRef Google Scholar
140. Jayakumar A, Castilho TM, Park E, Goldsmith-Pestana K, Blackwell JM, McMahon-Pratt D. TLR1/2 activation during heterologous prime-boost vaccination (DNA-MVA) enhances CD8⁺ T Cell responses providing protection against Leishmania (Viannia). PLoS Negl Trop Dis. 2011;5(6):e1204.
     View this article via: PubMed CrossRef Google Scholar
141. Asprodites N, Zheng L, Geng D, Velasco-Gonzalez C, Sanchez-Perez L, Davila E. Engagement of Toll-like receptor-2 on cytotoxic T-lymphocytes occurs in vivo and augments antitumor activity. FASEB J. 2008;22(10):3628–3637.
     View this article via: PubMed CrossRef Google Scholar
142. Hernandez-Ruiz J, et al. CD8 cells of patients with diffuse cutaneous leishmaniasis display functional exhaustion: the latter is reversed, in vitro, by TLR2 agonists. PLoS Negl Trop Dis. 2010;4(11):e871.
     View this article via: PubMed CrossRef Google Scholar
143. Yonezawa A, Dutt S, Chester C, Kim J, Kohrt HE. Boosting cancer immunotherapy with anti-CD137 antibody therapy. Clin Cancer Res. 2015;21(14):3113–3120.
     View this article via: PubMed CrossRef Google Scholar
144. Ellmark P, Mangsbo SM, Furebring C, Totterman TH, Norlen P. Kick-starting the cancer-immunity cycle by targeting CD40. Oncoimmunology. 2015;4(7):e1011484.
     View this article via: PubMed CrossRef Google Scholar
145. Mangsbo SM, et al. The human agonistic CD40 antibody ADC-1013 eradicates bladder tumors and generates T-cell-dependent tumor immunity. Clin Cancer Res. 2015;21(5):1115–1126.
     View this article via: PubMed CrossRef Google Scholar
146. Page DB, Postow MA, Callahan MK, Allison JP, Wolchok JD. Immune modulation in cancer with antibodies. Annu Rev Med. 2014;65:185–202.
     View this article via: PubMed CrossRef Google Scholar
147. Dudley ME, et al. Adoptive cell therapy for patients with metastatic melanoma: evaluation of intensive myeloablative chemoradiation preparative regimens. J Clin Oncol. 2008;26(32):5233–5239.
     View this article via: PubMed CrossRef Google Scholar
148. Chapman PB, D’Angelo SP, Wolchok JD. Rapid eradication of a bulky melanoma mass with one dose of immunotherapy. N Engl J Med. 2015;372(21):2073–2074.
     View this article via: PubMed CrossRef Google Scholar
149. Hansen SG, et al. Profound early control of highly pathogenic SIV by an effector memory T-cell vaccine. Nature. 2011;473(7348):523–527.
     View this article via: PubMed CrossRef Google Scholar
150. Saez-Cirion A, et al. Post-treatment HIV-1 controllers with a long-term virological remission after the interruption of early initiated antiretroviral therapy ANRS VISCONTI Study. PLoS Pathog. 2013;9(3):e1003211.
     View this article via: PubMed CrossRef Google Scholar