The Functional Role of the Tetraspanin CD82/Kai1 in Fungal Innate Immunity

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Project Summary: Invasive fungal infections (IFIs) represent a major threat to critically ill and immunocompromised patients and are associated with significant morbidity and mortality. Aspergillus and Candida are opportunistic fungi that cause the majority of these life-threatening infections. In order to exert their deleterious effects, pathogens including viruses, bacteria, parasites, and fungi can hijack tetraspanins for cell invasion or intracellular trafficking. Tetraspanins comprise a family of proteins that are expressed on the plasma membrane, intracellular membranes, and exosomes from nearly all cell types. As their name implies, tetraspanins span the membrane four times and have short intracellular amino and carboxyl-termini and two extracellular loops. Tetraspanins regulate diverse biological processes including cell migration, adhesion, and signaling events by serving as organizers of multimolecular complexes; they form promiscuous associations with one another and other membrane proteins and lipids to generate tetraspanin-enriched microdomains. Elucidating the precise function of tetraspanins has been difficult due to molecular redundancy, a lack of catalytic activity, and insufficient specific antibodies. Despite these challenges, several tetraspanins have been implicated as key regulators of cancer progression and immunity. In particular, CD82 acts as a potent suppressor of tumor metastasis. Tetraspanins including CD82 are also widely expressed in immune cells, but their exact role in undefined. We have made several key observations that begin to define the role of CD82 in fungal immunity. Following phagocytosis by macrophages, CD82 is specifically recruited to phagosomes containing fungal pathogens prior to lysosomal fusion. Remarkably, LysM-Cre CD82 knockout mice infected with Candida albicans have significantly reduced survival compared to wild-type controls, indicating that CD82 plays an important functional role in myeloid cells in response to systemic fungal infection. We developed fungal-like particles to probe directly the immune response to carbohydrates expressed on the fungal cell wall, such as β- 1,3 glucan which is recognized by the pathogen recognition receptor, Dectin-1. We demonstrated that Dectin-1 is critical for phagolysosomal maturation and controls recruitment of the autophagy-related protein, LC3, to fungal containing phagosomes through Syk activation. We also found that CD82 associates with Dectin-1 in macrophages and is important for downstream signaling events in response to fungal infection including Syk activation and ROS generation. To determine the molecular mechanism of CD82 in fungal innate immunity in macrophages, we propose the following two specific aims: (1) Determine the role of CD82 in Dectin-1 signaling by macrophages stimulated with β-1,3 glucan, and (2) Define the role of CD82 in LC3 associated phagocytosis in macrophages challenged by C. albicans.