PROJECT SUMMARY Invasive fungal infections represent a major threat to immunocompromised patients and despite the availability of anti- fungal antibiotics, mortality rates remain as high as 50%. Candida spp. is fifth among hospital-acquired pathogens and fourth among bloodstream infections. Human SNPs in the type I interferon (IFN) pathway have been associated with increased susceptibility to candidemia. While these data suggest a significant role of type I IFNs in C. albicans host defense, the signaling pathway that licenses type I IFN production and regulation during C. albicans infection remains elusive. Although small in numbers, plasmacytoid dendritic cells (pDCs) are the primary producers of type I IFNs (IFNα and IFNβ) in response to viral and bacterial pathogens though other cells (i.e. macrophages and monocytes) contribute to type I IFN production as well. Upon activation, toll-like receptors (TLRs) 7 and 9 can upregulate type I IFN production or chemokine/cytokine production via IRF-7 and NF-κB pathways, respectively. Although induction of type I IFNs are well described in response to viral and bacterial pathogens, a crucial knowledge gap remains with respect to the mechanisms by which this pathway affects fungal pathogenesis. We have made several key observations to define the role of type I IFNs in response to C. albicans. We show that pDCs from mice infected with C. albicans intravenously significantly upregulate the activation markers, CD40 and CD86, as compared to uninfected mice. TLR9 and Dectin-1 are required for IFNα/β production. TLR9 trafficking to fungal endosomes require Dectin-1 and Syk signaling. Furthermore, a microarray analysis of wild-type and TLR9-knockout macrophages revealed IFN inducible family genes (IFI203, Mnda, and Ifi1) as dependent on TLR9, implicating its role in the regulation of IFN signaling. Type I IFNs improve killing capacity of neutrophils in response to C. albicans. We additionally demonstrated that in the absence of critical IFN signaling components (i.e. STING, cGAS, IRF-3, and IFN receptor) mice demonstrate striking resistance to candidemia, but at the cost of a higher fungal burden. These exciting data suggest that dysregulation of IFN signaling significantly affects the outcomes of invasive Candida infections. Lastly, our preliminary studies implicate a role for STING in the production of the negative feedback regulator SOCS1 (suppressor of cytokine signaling). Thus, our long-term goal is to understand the regulation and role of type I IFNs in host defense against invasive candidiasis. To address our long-term goal, we propose the following three aims: (1) elucidate the signaling pathway for TLR9-dependent type I IFN production in response to C. albicans, (2) determine the impact of cGAS, STING, and IRF-3 in the host defense against candidemia, and (3) identify the mechanism of C. albicans-induced SOCS1 to block TLR9-dependent type I IFN production. This work will provide greater understanding of...