An in vitro model to study the naive human peripheral blood mononuclear cell response to Cryptococcus neoformans infection

Cryptococcus neoformans is a facultative intracellular pathogenic yeast which causes cryptococcal meningitis. Infection most commonly occurs via the lungs in humans and is cleared without clinical signs in the immunocompetent, but may cause life-threatening infection in immunocompromised. C. neoformans can be phagocytosed by host phagocytes, but may manipulate the intracellular niche of phagocytes for replication and dissemination. The interaction of macrophages with cryptococcal cells has been studied in detail but little is known about the interaction between human peripheral blood mononuclear cells (PBMC) and C. neoformans. PBMCs are rapidly recruited to the site of initial infection in the lung, peripheral tissues, and also respond to cryptococci that disseminate via the bloodstream. Therefore, deciphering the interactions between PBMCs and cryptococci is an important but neglected aspect of our understanding of the immune response during cryptococcal infection. Here, using time lapse imaging of primary human PBMCs in vitro, we are able to measure the PBMC response to cryptococci. Using this approach we find that naive, undifferentiated human monocytes phagocytose cryptococcal cells, and that aggregates (swarms) of monocytes and T cells often form in response to engulfment of cryptococci. Interestingly, we find a correlation between the size of the PBMC aggregates and proliferative ability of the cryptococci within. While these aggregates slow intracellular cryptococcal growth, cryptococci are able to replicate within this niche and escape from PBMCs to replicate extracellularly. These results provide evidence for PBMC control of cryptococcal infection and provide a model for the in vitro study of cryptococcal granuloma biology.