Disseminated C. albicans infection in patients who have a weakened immune system is life-threatening. In hospitals 40% of bloodstream infections (candidemia) are caused by Candida spp. Despite the availability of several anti-fungal drugs, invasive candidiasis still has a high mortality rate ranging from 45 to 75%. The high morbidity and mortality associated with disseminated candidiasis are mainly due to the lack of early and accurate diagnostic tools, the limited anti-fungal drugs, and the emergence of drug resistance, thus highlighting the need to further understand host-pathogen interactions and the mechanisms of immune resistance to fungal spread, and to develop alternative immune-based strategies to combat candidemia. In normal hosts, C. albicans is controlled after activation of innate immune cells via cell surface pattern recognition receptors (PRRs) such as TLR2 and C-type lectin receptors (CLRs) that detect the infecting fungi. The CLRs Dectin-1 and Dectin-2/3 recognize C. albicans yeast cells and hyphae by binding to the surface ?-glucans and ?- mannans of the two fungal forms, respectively. Recognition of these molecules results in release of inflammatory cytokines from innate immune cells, which is critical for anti-fungal immunity. The mechanisms that control this CLR-mediated innate immune response to fungal infection are completely unknown. Our preliminary results showed that bone marrow-derived macrophages (BMDMs) from Cblb-/- mice infected with C. albicans yeast and hyphae produce more inflammatory cytokines than do BMDMs from wild type (WT) mice. This response involves signaling via Dectin-1, -2, and/or -3 but not via the TLRs. This finding suggests that Cbl-b acts through the Dectin CLRs but not the TLRs in macrophages. Consistent with this notion, BMDMs from Cblb-/- mice have impaired Dectin-1, -2, and -3 degradation upon interaction with C. albicans yeast and hyphae. In support of this finding, Dectin-1 undergoes poly-ubiquitination in macrophages upon infection with C. albicans yeasts, whereas this ubiquitination is abrogated in BMDMs lacking Cbl-b. Furthermore, Cblb-/- mice are protected from infection with a lethal dose of C. albicans. Based on these results, we hypothesize that during C. albicans infection, Cbl-b is recruited to Dectin CLRs, and this targets Dectins for ubiquitination which dampens CLR-mediated innate immune responses against fungi. To test this hypothesis, we propose to determine (1) whether and how Cbl-b regulates Dectin down-modulation via protein ubiquitination in vitro; and (2) whether Cbl-b dampens host effective responses against C. albicans mediated by the Dectin family of CLRs. Successful completion of this project will reveal a previously unknown host defense mechanism and provide molecular insight into the host defense machinery and ultimately facilitate the identification of candidate targets for appropriate modulation of anti-fungal responses and therapeutic options.

Public Health Relevance

Candida albicans, one of the clinically most relevant fungal pathogens can disseminate systematically and thereby accounts for significant morbidity and mortality. This project will investigate whether an enzyme called Cbl-b which directs the proteins at the cell surface of macrophages and other immune cells that recognizes the cell wall component(s) of Candida albicans for degradation controls host defense against Candida albicans infection.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Special Emphasis Panel (ZRG1-IMM-J (90)S)
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Jiang, Chao
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Ohio State University
Schools of Medicine
United States
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Tang, Rong; Langdon, Wallace Y; Zhang, Jian (2018) Regulation of immune responses by E3 ubiquitin ligase Cbl-b. Cell Immunol :
Tang, Juan; Lin, Guoxin; Langdon, Wallace Y et al. (2018) Regulation of C-Type Lectin Receptor-Mediated Antifungal Immunity. Front Immunol 9:123
Xiao, Yun; Tang, Juan; Guo, Hui et al. (2016) Targeting CBLB as a potential therapeutic approach for disseminated candidiasis. Nat Med 22:906-14