Hostile takeover: fungal protein promotes host cell invasion

The ability to suppress the immune system has lead to great advances in transplant technology and treatment of autoimmune diseases. Unfortunately, the immunosuppression of these patients has led to the rise of opportunistic infections by organisms that are recalcitrant to current prophylactic strategies. One such example is the increase of mucormycosis, an invasive infection caused by filamentous fungi of the order Mucorales. In this issue of the JCI, Gebremariam and colleagues determined that spore coat homolog (CotH) proteins are required for angioinvasion and that these proteins are unique to Mucorales. Their findings provide a potential therapeutic target for prevention and treatment of mucormycosis.

Recent medical advances have made remarkable progress in treating previously refractory conditions. More aggressive and targeted cancer chemotherapies have vastly improved outcomes for many malignancies. Inhibition of TNF-α activity now affords better control of various autoimmune disorders. Moreover, advances in solid organ transplantation have dramatically improved the lives of many patients with organ failure syndromes. However, these revolutionary therapies considerably impair patient immunity.

Because of the increased infection risk in patients with highly immunocompromised states, clinicians have adopted concrete strategies for infectious disease prevention in many of these patient populations. Unfortunately, as our ability to suppress infections by the most common microbial pathogens has improved, other, less well-characterized infectious agents have begun to fill this clinical void. One very important example of this phenomenon is the increasing incidence of mucormycosis, an invasive infection caused by the Mucorales order of filamentous fungi (1, 2). Human pathogens in this fungal group include Rhizopus, Mucor, and Cunninghamella species.

The increased incidence of mucormycosis has been attributed to many factors, including the fact that Mucorales are much less susceptible to current antifungal agents than other fungal pathogens. Therefore, the use of standard antifungal drugs in prophylactic strategies is unlikely to successfully prevent this type of infection (3, 4). Mucormycosis occurs in association with a wide range of disorders. In addition to classical immunocompromised states, such as prolonged neutropenia and organ transplantation, conditions such as diabetic ketoacidosis (DKA), undernutrition, and iron chelation therapy also predispose patients to mucormycosis (2). Once established, invasive infections due to Mucorales frequently take an aggressive clinical course characterized by rapid tissue destruction. These infections are difficult to treat, requiring high-dose antifungal therapy and surgical debridement. Mortality in mucormycosis remains high, despite combined medical/surgical therapy (5, 6). Therefore, new insights into the pathogenesis of this fungal infection are needed in order to envision innovative strategies for prevention and treatment.

The study in this issue by Gebremariam et al. (7) began with a classical clinical observation about mucormycosis that was used to infer unique aspects regarding the establishment and propagation of this disease. This infection tends to display angioinvasion, or progression along the micro- and macrovasculature (8). Clinically, angioinvasion results in tissue infarction, which complicates the infection and explains many of the principal manifestations of this disease. The authors decided to pursue the angioinvasive behavior of Mucorales to determine whether this fungal feature could offer insight into approaches to altering disease procession. Thus, they sought to determine whether specific molecular interactions exist between the fungal cell surface and the mammalian endothelium. Historically, there has been some debate among clinicians and pathologists as to whether fungal angioinvasion is the result of a specific interaction between host and fungus, or whether this process simply involves microbial growth along the path of least resistance within the infected tissue. The data now presented by Gebremariam et al. (7) strongly argue that the interaction of Rhizopus species and endothelial cells involves targeted molecular recognition between fungal and host cells, promoting microbial invasion and disease progression.

The authors previously identified glucose-regulated protein 78 (GRP78) as a component of mammalian cells that bound Rhizopus oryzae hyphae (9). GRP78 colocalized with the fungal hyphae during invasion of endothelial cells. Additionally, anti-GRP78 antibodies effectively prevented the adherence and endocytosis of R. oryzae, but not other fungal pathogens. Expression of GRP78 was induced in mice during DKA and in cultured epithelia exposed to excess glucose and iron, conditions that favor mucormycosis. Blocking this putative endothelial cell receptor with antibodies protected DKA mice from experimental mucormycosis (9).

In the present study, the authors identified fungal ligands that bind GRP78 (7). The data presented here compellingly argue that bacterial spore coat protein homolog (CotH) proteins present on the R. oryzae cell surface mediate a specific interaction with GRP78, promoting fungal adherence and invasion (Figure 1). Heterologous expression of the R. oryzae CotH proteins resulted in mammalian cell invasion by the typically nonpathogenic yeast Saccharomyces cerevisiae. Additionally, interruption of CotH function in R. oryzae disrupted its invasive potential. Interference of the CotH/GRP78 interaction with anti-CotH antibodies resulted in a protective effect in a murine DKA model of mucormycosis. Interestingly, these CotH proteins were present on the surface of Mucorales, but do not appear to be produced by other fungal pathogens (7). This may explain the more extensive degree of angioinvasion observed in mucormycosis compared with other invasive fungal infections.

Figure 1

CotH proteins on the fungal surface promote angioinvasion. CotH proteins on the surface of Mucorales specifically bind to GRP78 on the surface of host endothelial cells. This interaction facilitates fungal invasion of the cell. Damage to the endothelial cells promotes angioinvasion and dissemination.

Many questions naturally arise when considering the results of these investigations. Mucorales are not adapted human pathogens, but rather environmental molds that infect patients with specific defects in immunity; therefore, the native role of CotH proteins remains to be determined. Do these fungal proteins promote interaction with potential food sources in the environment? Alternatively, do these fungal surface proteins provide some sort of protection against natural predators?

In addition to the important questions in fungal biology raised by these findings, there are equally important clinical implications. The specificity of the CotH proteins to Mucorales suggests that they could be effective therapeutic targets for preventing and treating mucormycosis. Would therapies directed against fungal angioinvasion be effective once these infections are well established in tissues? It would also be interesting to study whether disruption of fungal adherence and invasion could be used in various prevention strategies, in addition to targeted therapies of active Mucorales infections.

These studies also highlight the larger context of recent investigations into the interaction of the fungal cell surface with the infected host. Fungal surface molecules that act as adhesins, invasins, and immunogens are being increasingly characterized in diverse human pathogens. Vaccine strategies against various fungal surface epitopes have been explored, and clinical trials of resulting fungal vaccines are now being pursued (10). Additionally, the fungal cell surface often determines the nature and degree of immune activation during invasive fungal infections. Exposure of highly immunogenic fungal surface components may be beneficial in some settings, allowing infected hosts to recognize and rapidly clear invading microorganisms. Conversely, excessive immune activation could potentially accelerate host damage, which would result in allergic responses for superficial fungal conditions and worsen morbidity in invasive mycoses (11–13). Therefore, continued investigations are warranted into this important intersection of microbiology and host immune response.

Conflict of interest: The author has declared that no conflict of interest exists.

Reference information: J Clin Invest. 2014;124(1):74–76. doi:10.1172/JCI73585.

See the related article at CotH3 mediates fungal invasion of host cells during mucormycosis.

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