Investigation of Genetic Susceptibility to Blastomycosis Reveals Interleukin-6 as a Potential Susceptibility Locus

Genetic differences are hypothesized to underlie ethnic disparities in incidence rates of the endemic systemic mycoses, including blastomycosis. Individuals of Hmong ancestry display elevated risk for this serious fungal infection. Here, we interrogated the genomes of Wisconsin (WI) Hmong blastomycosis patients using homozygosity mapping to uncover regions of the genome that are likely shared among the greater Hmong population and filtered for variants with high potential to affect disease susceptibility. This approach uncovered 113 candidate susceptibility variants, and among the most promising are those in genes involved in the interleukin-17 (IL-17) response. In particular, we identified 25 linked variants near the gene encoding IL-6 (IL6). We validated differences in cytokine production between Hmong and European volunteers and formally demonstrated a critical role for IL-6 in the development of adaptive immunity to Blastomyces dermatitidis Our findings suggest that the dysregulation of IL-17 responses underlies a recently reported and poorly understood ethnic health disparity.IMPORTANCE Blastomycosis is a potentially life-threatening infection caused by the fungus Blastomyces dermatitidis As with related fungal diseases, blastomycosis is noted to affect some populations more than others. These patterns of illness are often not related to predisposing conditions or exposure risks; thus, genetic differences are thought to underlie these health disparities. People of Hmong ancestry in Wisconsin are at elevated risk of blastomycosis compared to the general population. We studied the genetic codes of Hmong blastomycosis patients and identified candidate sites in their genomes that may explain their susceptibility to this infection. We further studied one particular region of the genome that is involved with the immune processes that fight B. dermatitidis Our work revealed population differences in the response to fungi. A better understanding of the genetic underpinnings of susceptibility to infectious diseases has broader implications for community health, especially in the paradigm of personalized medicine.

Fungal Mimicry of a Mammalian Aminopeptidase Disables Innate Immunity and Promotes Pathogenicity

Systemic fungal infections trigger marked immune-regulatory disturbances, but the mechanisms are poorly understood. We report that the pathogenic yeast of Blastomyces dermatitidis elaborates dipeptidyl-peptidase IVA (DppIVA), a close mimic of the mammalian ectopeptidase CD26, which modulates critical aspects of hematopoiesis. We show that, like the mammalian enzyme, fungal DppIVA cleaved C-C chemokines and GM-CSF. Yeast producing DppIVA crippled the recruitment and differentiation of monocytes and prevented phagocyte activation and ROS production. Silencing fungal DppIVA gene expression curtailed virulence and restored recruitment of CCR2(+) monocytes, generation of TipDC, and phagocyte killing of yeast. Pharmacological blockade of DppIVA restored leukocyte effector functions and stemmed infection, while addition of recombinant DppIVA to gene-silenced yeast enabled them to evade leukocyte defense. Thus, fungal DppIVA mediates immune-regulatory disturbances that underlie invasive fungal disease. These findings reveal a form of molecular piracy by a broadly conserved aminopeptidase during disease pathogenesis.

The adaptor CARD9 is required for adaptive but not innate immunity to oral mucosal Candida albicans infections (MPF3P.805)

Oropharyngeal candidiasis (OPC, thrush) is an opportunistic infection caused by the commensal fungus Candida albicans. OPC is common in individuals with HIV/AIDS, infants, and individuals with congenital immune defects. Immunity to OPC is dependent on the IL-23/IL-17R axis, as mice and humans with defects in IL-17R signaling (IL17F, ACT1, IL-17RA) or in genes that direct Th17 differentiation (STAT3, STAT1, CARD9) are prone to mucocutaneous candidiasis. Conventional Th17 cells are induced in response to C. albicans via signals from C-type lectin receptors, which signal through the adaptor CARD9 leading to production of Th17-inducing cytokines such as IL-6, IL-1 and IL-23. Recent data indicates that IL-17 can also be made by numerous innate cell subsets. These innate ‘Type 17’ cells resemble conventional Th17 cells, but they can be activated without need for prior antigen exposure. Because C. albicans is not a commensal organism in rodents and thus mice are naïve to this fungus, we had the opportunity to assess the role of CARD9 in innate versus adaptive responses using an OPC infection model. As expected, CARD9-/- mice failed to mount an adaptive Th17 response following oral Candida infection. Surprisingly, however, CARD9-/- mice had preserved innate IL-17-dependent responses and were almost fully resistant to OPC. Thus, CARD9 is important primarily for adaptive immunity to C. albicans, whereas alternate recognition systems appear to be needed for effective innate responses.

“Natural” Th17 cells control Candida albicans infections in the oral mucosa (MUC7P.749)

Oropharyngeal candidiasis (OPC) is an opportunistic mucosal infection caused by the commensal fungus Candida albicans. To date, there is a paucity of research on oral mucosal immunity in general and to fungi in particular. Humans with rare mutations have revealed a vital protective role of IL-17 signaling to OPC. It has long been assumed that conventional “iTh17” cells produce IL-17. However, many innate populations also produce IL-17, but their role in oral immunity is unknown. Since rodents are naïve to C. albicans, we evaluated OPC in a murine model. T cell receptor rearrangement is required for immunity, since Rag1-/-, SCID and IL-7Rα-/- mice are susceptible to OPC. However, iTh17, NK and NKT cells were dispensable. We identified a CD3+CD4+CD44hiTCRβ+ tissue-resident population in tongue that produces IL-17 rapidly upon Candida exposure. These were phenotypically identical to ‘natural Th17’ cells, a recently described, thymically derived innate lymphocyte population whose function is highly enigmatic. Development of nTh17 and iTh17 cells involves differential AKT/mTOR pathways, with nTh17 cells requiring mTORC2, and iTH17 cells requiring mTORC1. Consistently, mice deficient in the mTORC2 subunit Rictor were highly susceptible to OPC, whereas mice treated with rapamycin, which selectively targets mTORC1, were similar to WT mice. Therefore, innate immunity to Candida requires nTh17 cells, whose presence in the oral mucosa ideally positions them to control oral pathogens.

Th17 cells confer long-term adaptive immunity to oropharyngeal candidiasis (P3308)

Oropharyngeal candidiasis (OPC) is an opportunistic infection of the oral cavity caused primarily by Candida albicans. OPC particularly afflicts HIV+ patients with low CD4 counts, suggesting a role for CD4+ T cells in host defense. Experimental models of OPC have strongly suggested that immunity to OPC is IL-17-dependent, and humans with rare genetic defects that result in Th17 deficiency are associated with the development of OPC. However, other sources of IL-17 have been described, particularly in the innate compartment, but the relative contribution of adaptive and innate sources of IL-17 during OPC has not been well articulated. Here, we showed that development of adaptive immune responses in OPC results in a ~1 log decrease in fungal burden, tightly associated with an increased frequency of antigen-specific CD4+IL-17+ (Th17) cells. Importantly, adoptive transfer of Candida-primed Th17 cells into Rag1-/- mice resulted in protection from disease. Surprisingly, however, CD4-/- mice were resistant to OPC, which was linked, at least in part, with compensatory IL-17 production by both CD8+ and double-negative (DN) T cells. The adoptive transfer of CD8+IL-17+ or DN cells into Rag1-/- mice resulted in protection from OPC. Therefore, Th17 cells contribute to C. albicans clearance but their absence can be compensated by alternative sources of IL-17. These findings implicate CD8+IL-17+ and DN cells as vaccine targets in immunocompromised populations.