Reciprocal regulation of TLR4, TLR3 and Macrophage Scavenger Receptor 1 regulates nonopsonic phagocytosis of the fungal pathogen Cryptococcus neoformans.
The opportunistic fungal pathogen Cryptococcus neoformans causes lethal infections in immunocompromised patients. Macrophages are central to the host response to cryptococci; however, it is unclear how C. neoformans is recognized and phagocytosed by macrophages. Here we investigate the role of TLR4 in the nonopsonic phagocytosis of C. neoformans. We find that loss of TLR4 function unexpectedly increases phagocytosis of nonopsonized cryptococci. The increased phagocytosis observed in Tlr4-/- cells was dampened by pre-treatment of macrophages with either a TLR3 inhibitor or oxidised-LDL, a known ligand of scavenger receptors. The scavenger receptor, macrophage scavenger receptor 1 (MSR1) (also known as SR-A1 or CD204) was upregulated in Tlr4-/- macrophages and there was a 75% decrease in phagocytosis of nonopsonized cryptococci by Msr1-/- macrophages. Furthermore, immunofluorescence imaging revealed colocalization of MSR1 and internalised cryptococci. Together, these results identify MSR1 as a key receptor for the phagocytosis of nonopsonized C. neoformans and demonstrate TLR4/MSR1 crosstalk in the phagocytosis of C. neoformans.