Use of a human immortalized microglia cell line to study recognition, phagocytosis, and intracellular survival of Cryptococcus neoformans.

Cryptococcus neoformans, the etiological agent of cryptococcal meningitis (CM) is a globally distributed environmental yeast that mainly causes infections in immunocompromised individuals. Particularly in low-resource countries, the mortality rate of CM can reach 81% and accounts for ~19% of HIV/AIDS-related deaths each year. In immunocompromised individuals, once inhaled, C. neoformans escapes from the lungs and disseminates with special predilection for the central nervous system (CNS). Once in the brain, C. neoformans interacts with microglia, the tissue-resident macrophages of the CNS. Previous studies indirectly showed that microglia are ineffective at controlling this fungal infection. The mechanisms underlying this fungal survival and proliferation within the CNS, however, remain unclear. In this study, we use and validate the C20 immortalized human microglia cell line to study cryptococcal-microglia interactions. We show that microglia have limited phagocytic activity that is specific to C. neoformans and partly dependent on cryptococcal antiphagocytic proteins that alter cell size and cell wall structure. We also show that human microglia respond to cryptococcal strains differently than peripheral macrophages. Further, we show that human microglia are ineffective at killing phagocytosed C. neoformans, and that this could be due to the ability of this yeast to disrupt phagosome maturation and induce phagosome membrane damage in these cells. These findings provide us with fundamental knowledge regarding cryptococcal pathogenesis in the CNS, specifically the insight into how C. neoformans is recognized by microglia under different conditions and demonstrate the usefulness of C20 cells to further study how this yeast survives and replicates within the CNS environment. IMPORTANCE: While Cryptococcus neoformans is acquired through inhalation, the fatal pathology of cryptococcal infection occurs when the yeast disseminates to the central nervous system (CNS) and causes cryptococcal meningitis. Microglia are the first immune cells that C. neoformans will encounter once it reaches the CNS, and they are the largest population of macrophages in the brain. While microglia are professional phagocytes, they are unable to control C. neoformans infection. The mechanisms behind uncontrolled growth of C. neoformans within the CNS remain understudied, partly due to incomplete knowledge regarding microglia-cryptococcal interactions. This study provides fundamental knowledge into these interactions and establishes a powerful model to specifically study how C. neoformans is recognized by microglia and how cryptococcal phagosomes mature in these phagocytes. This work opens new avenues of research to further our understanding of cryptococcal-host interactions, which can be leveraged to develop more effective therapeutics for cryptococcal meningitis.