The importance of Fcγ and C-type lectin receptors in host immune responses during Pneumocystis pneumonia.

Pneumocystis jirovecii pneumonia (PJP) remains a significant cause of morbidity and mortality during AIDS. In AIDS, the absence of CD4 immunity results in exuberant and often fatal PJP. In addition, organism clearance requires a balanced macrophage response since excessive inflammation promotes lung injury and respiratory failure. Corticosteroids given in addition to antibiotics significantly improve outcomes during PJP. However, concerns exist that corticosteroids further suppress immunity and increase co-infections. New strategies to promote killing and clearance of Pneumocystis while balancing lung inflammation are required. Prior studies have shown that innate immunity to Pneumocystis is mediated by C-type lectin receptors (CLRs) on macrophages and involves downstream CARD9 activation. CARD9 can be targeted by a novel specific small molecule inhibitor (BRD5529) that significantly reduces inflammatory signaling by macrophages. CARD9 serves as the central intracellular molecule through which Dectin-1, Dectin-2, Mincle, and other CLRs signal. Dectin-1 CLR is activated through its own intracytoplasmic domain, whereas other innate CLRs (e.g., Dectin-2 and Mincle) require interactions with a common Fc-gamma receptor (FcγR) accessory chain to mediate responses. We now observe that mice double deficient in both Dectin-1 and Fcer1g (which lack the FcγR gamma chain) exhibit markedly reduced organism clearance compared with Card9^-/- infected animals. These mice also possess deficiencies in immunoglobulin (Ig) Fc receptors directly mediating antibody responses, further implicating altered humoral responses in Pneumocystis killing. We further demonstrate in the Pneumocystis pneumonia (PCP) mouse model that BRD5529 administration successfully suppresses inflammatory cytokines. Our data support that innate immune responses through the CLR-CARD9 axis and humoral response act together to mediate effective responses resulting in optimal organism killing and generation of host inflammatory responses. Furthermore, host lung inflammation during PCP may be successfully reduced with a novel CARD9 small molecule inhibitor.IMPORTANCEPneumocystis pneumonia (PCP) causes severe respiratory impairment in hosts with suppressed immunity, particularly those with CD4 deficiencies, such as HIV. In addition to lymphocytic immunity, both innate and humoral immunities also participate in host defense against Pneumocystis. In the current studies, we defined the relative roles of CLR receptor-mediated inflammation, as well as FcgR-related inflammation and clearance of Pneumocystis organisms. Our studies reveal important roles for CLR activities for inducing lung inflammation, which can be ameliorated with a novel small molecule inhibitor of the CARD9 adaptor protein that is necessary for CLR signaling. In contrast, FcgR has a dominant role in organism clearance, underscoring an integral role of humoral responses for the elimination of this infection.