IRAK-M Knockout Exacerbates Inflammation in Mice Infected with Streptococcus equi subsp. zooepidemicus: Observation in the Jejunum.

Streptococcus equi subsp. zooepidemicus (SEZ) causes severe infections, but the role of the negative regulator interleukin-1 receptor-associated kinase M (IRAK-M) in SEZ-induced inflammation is unclear. In this study, male C57BL/6 mice received a single intraperitoneal injection of SEZ at 2 × 107 CFU and were examined 24 h post-injection. SEZ infection triggered overt jejunal hemorrhage and upregulation of IL-1β, IL-6, and TNF-α mRNA, confirming local inflammation, with a concurrent marked reduction in both mRNA and protein levels of IRAK M in the jejunum, hinting at its possible role. We next used IRAK-M knockout (IRAK-M-/-) mice and found that, compared with the WT+SEZ group, the knockout group displayed more severe jejunal hemorrhage, histopathological scores, and neutrophil infiltration, along with a higher bacterial burden and further elevated pro-inflammatory cytokines and reduced anti-inflammatory cytokine expression. Mechanistically, IRAK-M deficiency in the jejunum further increased the mRNA expression of MyD88, TRIF, TRAF6, IKKα+β, NF-κBp50, and NF-κBp65 and decreased IκBα mRNA compared with the WT+SEZ group. These findings were paralleled at the protein level, as demonstrated by Western blotting for MyD88, TRAF6, and IKKα+β, and by immunofluorescence in peritoneal macrophages. Importantly, IRAK-M deficiency drove a shift toward the M1 macrophage phenotype in both jejunal tissues and peritoneal macrophages, characterized by upregulation of CD16, CD32, CD80, CD86, iNOS, and MHC II, and downregulation of the M2-associated markers Arg1 and CD206. These findings reveal that SEZ infection causes jejunal inflammation and that IRAK-M knockout worsens this condition, pointing to a role for IRAK-M in regulating inflammatory readouts in this model.