Characterizing the Mechanisms of Nonopsonic Uptake of Cryptococci by Macrophages

Abstract

The pathogenic fungus <i>Cryptococcus</i> enters the human host via inhalation into the lung and is able to reside in a niche environment that is serum- (opsonin) limiting. Little is known about the mechanism by which nonopsonic phagocytosis occurs via phagocytes in such situations. Using a combination of soluble inhibitors of phagocytic receptors and macrophages derived from knockout mice and human volunteers, we show that uptake of nonopsonized <i>Cryptococcus neoformans</i> and <i>C. gattii</i> via the mannose receptor is dependent on macrophage activation by cytokines. However, although uptake of <i>C. neoformans</i> is via both dectin-1 and dectin-2, <i>C. gattii</i> uptake occurs largely via dectin-1. Interestingly, dectin inhibitors also blocked phagocytosis of unopsonized Cryptococci in wax moth (<i>Galleria mellonella</i>) larvae and partially protected the larvae from infection by both fungi, supporting a key role for host phagocytes in augmenting early disease establishment. Finally, we demonstrated that internalization of nonopsonized Cryptococci is not accompanied by the nuclear translocation of NF-κB or its concomitant production of proinflammatory cytokines such as TNF-α. Thus, nonopsonized Cryptococci are recognized by mammalian phagocytes in a manner that minimizes proinflammatory cytokine production and potentially facilitates fungal pathogenesis.