Inflammasome activation is critical for host defenses against various microbial infections. Activation of the NLRC4 inflammasome requires detection of flagellin or type III secretion system (T3SS) components by NLR family apoptosis inhibitory proteins (NAIPs); yet how this pathway is regulated is unknown. Here, we found that interferon regulatory factor 8 (IRF8) is required for optimal activation of the NLRC4 inflammasome in bone-marrow-derived macrophages infected with Salmonella Typhimurium, Burkholderia thailandensis, or Pseudomonas aeruginosa but is dispensable for activation of the canonical and non-canonical NLRP3, AIM2, and Pyrin inflammasomes. IRF8 governs the transcription of Naips to allow detection of flagellin or T3SS proteins to mediate NLRC4 inflammasome activation. Furthermore, we found that IRF8 confers protection against bacterial infection in vivo, owing to its role in inflammasome-dependent cytokine production and pyroptosis. Altogether, our findings suggest that IRF8 is a critical regulator of NAIPs and NLRC4 inflammasome activation for defense against bacterial infection.
Mice deficient in SHANK-associated RH domain-interacting protein (SHARPIN), a component of the linear ubiquitin chain assembly complex (LUBAC), develop a spontaneous inflammatory disorder with pathologic hallmarks similar to atopic dermatitis and psoriasis in humans. Previous studies identified the crucial role of components of the TNF and IL-1 signaling pathways in the progression of disease in SHARPIN-deficient mice. However, an innate immune adaptor or sensor that relates to the disease progression has remained unknown. In this study, we found that the genetic ablation of myeloid differentiation primary response 88 (MyD88) completely rescued skin inflammation in SHARPIN-deficient (Sharpin) mice. Systemic inflammation and immune cell dysregulation were partially rescued. Fibroblasts derived from SharpinMyd88 mice failed to provide protection against TNF-induced cell death. SharpinMyd88 mice had reduced TNF production in their skin. Furthermore, depletion of the microbiota through the oral administration of antibiotics (ABX) partially rescued both the skin inflammation and systemic inflammation, demonstrating a role for the gut microbiota in SHARPIN-deficient mice. Our findings suggest a detrimental role for the innate immune adaptor MyD88 in instigating skin inflammation in Sharpin mice.
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