Cryptococcus neoformans secretes small molecules that inhibit IL-1β inflammasome-dependent secretion

Abstract

Cryptococcus neoformans is an encapsulated yeast that causes disease mainly in immunosuppressed hosts. It is considered a facultative intracellular pathogen because of its capacity to survive and replicate inside phagocytes, especially macrophages. This capacity is heavily dependent on various virulence factors, particularly the glucuronoxylomannan (GXM) component of the polysaccharide capsule, that render the non- or poorly-activated macrophage ineffective against phagocytosed yeast. Strategies utilized by macrophages to prevent this scenario include pyroptosis (a rapid highly inflammatory cell death) and vomocytosis (the expulsion of the pathogen from the intracellular environment without lysis). Inflammasome activation in phagocytes is usually protective against fungal infections, including cryptococcosis. Nevertheless, recognition of C. neoformans by inflammasome receptors requires specific changes in morphology or the opsonization of the yeast, impairing a proper inflammasome function. In this context, we analyzed the impact of molecules secreted by C. neoformans B3501 strain and its acapsular mutant Δcap67 in an inflammasome activation in vitro model. Our results showed that conditioned media derived from B3501 was capable of inhibiting inflammasome dependent events (i. e. IL-1β secretion and LDH release via pyroptosis) more strongly than conditioned media from Δcap67, regardless of GXM presence. We also demonstrated that macrophages treated with conditioned media were less responsive against infection with the virulent strain H99, exhibiting lower rates of phagocytosis, increased fungal burdens and enhanced vomocytosis. Moreover, we showed that the aromatic metabolite DL-Indole-3-lactic acid (ILA) was present in B3501's conditioned media and that this fungal metabolite is involved in the regulation of inflammasome activation by C. neoformans. Overall, the results presented show that conditioned media from a wild-type strain can inhibit an important recognition pathway and subsequent fungicidal functions of macrophages, contributing to fungal survival in vitro and suggesting that this serves as an important role for secreted molecules during cryptococcal infections.