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.