Effect of CARD9 Deficiency on Neutrophil-Mediated Host Defense against Pulmonary Infection with Streptococcus pneumoniae.

Streptococcus pneumoniae is a major causative bacterium of community-acquired pneumonia. Dendritic cell-associated C-type lectin-2 (dectin-2), one of the C-type lectin receptors (CLRs), was previously reported to play a pivotal role in host defense against pneumococcal infection through regulating phagocytosis by neutrophils while not being involved in neutrophil accumulation. In the present study, to elucidate the possible contribution of other CLRs to neutrophil accumulation, we examined the role of caspase recruitment domain-containing protein 9 (CARD9), a common adaptor molecule for signal transduction triggered by CLRs, in neutrophilic inflammatory response against pneumococcal infection. Wild-type (WT), CARD9 knockout (KO), and dectin-2 KO mice were infected intratracheally with pneumococcus, and the infected lungs were histopathologically analyzed to assess neutrophil accumulation at 24 h postinfection. Bronchoalveolar lavage fluids (BALFs) were collected at the same time point to count the neutrophils and assess the production of inflammatory cytokines and chemokines. Neutrophil accumulation was significantly decreased in CARD9 KO mice, but not in dectin-2 KO mice. Tumor necrosis factor alpha (TNF-α), keratinocyte-derived chemokine (KC), and macrophage inflammatory protein-2 (MIP-2) production in BALFs were also attenuated in CARD9 KO mice, but not in dectin-2 KO mice. Production of TNF-α and KC by alveolar macrophages stimulated with pneumococcal culture supernatants was significantly attenuated in CARD9 KO mice, but not in dectin-2 KO mice, compared to that in each group's respective control mice. In addition, pneumococcus-infected CARD9 KO mice showed larger bacterial burdens in the lungs than did WT mice. These data indicate that CARD9 is required for neutrophil migration after pneumococcal infection, as well as inflammatory cytokine and chemokine production by alveolar macrophages, and suggest that a CLR distinct from dectin-2 may be involved in this response.

Limited Role of Mincle in the Host Defense against Infection with Cryptococcus deneoformans.

Cryptococcus deneoformans is an opportunistic fungal pathogen that frequently causes fatal meningoencephalitis in patients with impaired cell-mediated immune responses such as AIDS. Caspase-associated recruitment domain 9 (CARD9) plays a critical role in the host defense against cryptococcal infection, suggesting the involvement of one or more C-type lectin receptors (CLRs). In the present study, we analyzed the role of macrophage-inducible C-type lectin (Mincle), one of the CLRs, in the host defense against C. deneoformans infection. Mincle expression in the lungs of wild-type (WT) mice was increased in the early stage of cryptococcal infection in a CARD9-dependent manner. In Mincle gene-disrupted (Mincle KO) mice, the clearance of this fungus, pathological findings, Th1/Th2 response, and antimicrobial peptide production in the infected lungs were nearly comparable to those in WT mice. However, the production of interleukin-22 (IL-22), tumor necrosis factor alpha (TNF-α), and IL-6 and the expression of AhR were significantly decreased in the lungs of Mincle KO mice compared to those of WT mice. In in vitro experiments, TNF-α production by bone marrow-derived dendritic cells was significantly decreased in Mincle KO mice. In addition, the disrupted lysates of C. deneoformans, but not those of whole yeast cells, activated Mincle-triggered signaling in an assay with a nuclear factor of activated T cells (NFAT)-green fluorescent protein (GFP) reporter cells expressing this receptor. These results suggest that Mincle may be involved in the production of Th22-related cytokines at the early stage of cryptococcal infection, although its role may be limited in the host defense against infection with C. deneoformans.