Exploiting Type 3 Complement Receptor for TNF-α Suppression, Immune Evasion, and Progressive Pulmonary Fungal Infection

TNF-α is crucial in defense against intracellular microbes. Host immune cells use type 3 complement receptors (CR3) to regulate excess TNF-α production during physiological clearance of apoptotic cells. BAD1, a virulence factor of Blastomyces dermatitidis , is displayed on yeast and released during infection. BAD1 binds yeast to macrophages (Mφ) via CR3 and CD14 and suppresses TNF-α, which is required for resistance. We investigated whether blastomyces adhesin 1 (BAD1) exploits host receptors for immune deviation and pathogen survival. Soluble BAD1 rapidly entered Mφ, accumulated intracellularly by 10 min after introduction to cells, and trafficked to early and late endosomes. Inhibition of receptor recycling by monodansyl cadaverine blocked association of BAD1 with Mφ and reversed TNF-α suppression in vitro. Inhibition of BAD1 uptake with cytochalasin D and FcR-redirected delivery of soluble BAD1 as Ag-Ab complexes or of wild-type yeast opsonized with IgG similarly reversed TNF-α suppression. Hence, receptor-mediated entry of BAD1 is requisite in TNF-α suppression, and the route of entry is critical. Binding of soluble BAD1 to Mφ of CR3−/− and CD14−/− mice was reduced to 50 and 33%, respectively, of that in wild-type mice. Mφ of CR3−/− and CD14−/− mice resisted soluble BAD1 TNF-α suppression in vitro, but, in contrast to CR3−/− cells, CD14−/− cells were still subject to suppression mediated by surface BAD1 on wild-type yeast. CR3−/− mice resisted both infection and TNF-α suppression in vivo, in contrast to wild-type and CD14−/− mice. BAD1 of B. dermatitidis thus co-opts normal host cell physiology by exploiting CR3 to subdue TNF-α production and foster pathogen survival.