| Cryptococcus neoformans is an encapsulated fungal pathogen that is remarkable for its ability to cause central nervous system infections especially in AMS patients. We designed studies to explore the roles of microglial Fcγ receptors (FcγR) in the host defense against C. neoformans infection.; We found that microglia produce several chemokines stimulated by C. neoformans and this process involves FcγR since (1) C. neoformans-induced chemokine production required specific antibody; (2) immobilized immunoglobulin induced chemokines in the absence of antigen; and (3) C. neoformans-mAb mediated chemokine production was abolished in FcR null microglia.; We next explored microglial FcR activation with respect to the specific FcγR types involved. We found that FcγRI and III, but not FcγRII, are involved in the chemokine response by challenging microglia with C. neoformans complexed with IgG of various isotypes, with or without FcγRII and III blocking antibodies. Further, experiments with various FcγR-deficient murine microglia also support the result. Thus, FcγRI and III mediated the chemokine production in C. neoformans IC-challenged microglia.; While MIP-1α production was usually accompanied by C. neoformans phagocytosis, under certain circumstances we found the two could be dissociated. For instance, cytochalasin D, which abrogated phagocytosis, did not inhibit MIP-1α production, suggesting that the mechanisms involved in phagocytosis and chemokine production were not the same. Therefore, the microglial FcγR signal transduction pathways leading to these two processes were studied in depth. Using specific inhibitors, we observed that Src and Syk are involved early in FcγR signaling leading to both processes. By employing adenoviral mediated gene transfer and specific inhibitors of cell signaling pathways, we found that the PI3K and Ras/MEK/ERK pathways are involved in phagocytosis and MIP-1α induction, respectively. We found that NF-κB played no role in phagocytosis but affected microglial chemokine production.; Together, we studied the microglial Fc receptor activation in response to C. neoformans-mAb challenge and showed that FcγR activation is a powerful means of microglial gene induction. We also showed that distinct FcR subtypes and signaling pathways are involved in phagocytosis and chemokine expression. Our studies indicate a mechanism by which passive antibody can influence the course of cryptococcal disease. |
