Stat3 is a transcription factor mediating anti-inflammatory properties of IL-10. In the present study, we demonstrate a pivotal role of Stat3 expressed in innate immune cells during septic peritonitis induced by cecal ligation and puncture (CLP). Mice with targeted disruption of Stat3 in macrophages and neutrophils were succumbed to septic peritonitis induced by CLP. The mice displayed an excessive local and systemic inflammation relative to the control mice, an event that was accompanied by substantial increases in the level of multiple cytokines. Hepatic and renal injury was significantly exacerbated in mice with Stat3 deficiency. Despite enhanced inflammatory responses, the mice failed to facilitate bacterial clearance as compared with the control mice. In addition, the mice exhibited an increased lethality after i.p. inoculation of live bacteria recovered from CLP-mice. In vitro, resident peritoneal macrophages from mice with Stat3 deficiency impaired bactericidal activity relative to the control whereas productions of inflammatory cytokines were significantly augmented when cells were stimulated with a synthetic lipopeptide, macrophage-activating lipopeptide-2 and LPS. Elicited macrophages and neutrophils with Stat3 deficiency also impaired bactericidal activity as compared with those with Stat3. Lysosomal enzyme release, an effector molecule for bacterial clearance, was significantly decreased in elicited leukocytes with Stat3 deficiency while increasing the production of inflammatory cytokines. Altogether, these results suggest that macrophage/neutrophil-specific STAT3 is crucial in not only modulating multiple organ failure associated with systemic inflammation but also intensifying the bactericidal activity, which highlight the significance of cell-specific Stat3 in the protective immunity during sepsis.

Inflammation is counterbalanced by anti-inflammatory cytokines such as IL-10, in which Stat3 mediates the signaling pathway. In this study, we demonstrate that resident macrophages, but not other cell types, are important targets of IL-10 in a murine model of acute peritonitis. Injection of thioglycollate i.p. induced a considerable number of neutrophils and macrophages in the peritoneum, which was significantly augmented in mice with a cell-type specific disruption of the Stat3 gene in macrophages and neutrophils (LysMcre/Stat3flox/− mice). The augmented leukocyte infiltration was accompanied by increased peritoneal levels of TNF-α, MIP-2, KC chemokine (KC), and MCP-1/CCL2. Stat3 was tyrosine phosphorylated in peritoneal resident macrophages as well as infiltrating leukocytes in the littermate controls, suggesting that Stat3 in either or both of these cells might play a regulatory role in inflammation. The peritoneal levels of TNF-α, MIP-2, KC, and MCP-1 were similarly elevated in LysMcre/Stat3flox/− mice rendered leukopenic by cyclophosphamide treatment as compared with the controls. Adoptive transfer of resident macrophages from LysMcre/Stat3flox/− mice into the control littermates resulted in increases in the peritoneal level of TNF-α, MIP-2, KC, and MCP-1 after i.p. injection of thioglycollate. Under these conditions, control littermates harboring LysMcre/Stat3flox/− macrophages exhibited an augmented leukocyte infiltration relative to those received control macrophages. Taken together, these data provide evidence that resident macrophages, but not other cell types, play a regulatory role in inflammation through a Stat3 signaling pathway. Stat3 in resident macrophages appears to function as a repressor protein in this model of acute inflammation.