SIGIRR promotes resistance against Pseudomonas aeruginosa keratitis by down-regulating type-1 immunity and IL-1R1 and TLR4 signaling

Pseudomonas aeruginosa keratitis destroys the cornea in susceptible Th1 responder C57BL/6 (B6), but not resistant Th2 responder (BALB/c) mice. To determine whether single Ig IL-IR-related molecule (SIGIRR) played a role in resistance, mRNA and protein expression levels were tested. Both were constitutively expressed in the cornea of the two mouse groups. X disparate mRNA and protein expression pattern was detected in the cornea of BALB/c vs B6 mice after infection. SIGIRR protein decreased significantly in BALB/c over B6 mice at 1 day postinfection. Thus, BALB/c mice were injected with an anti-SIGIRR Ab or IgG control. Anti-SIGIRR Ab over control-treated mice showed increased corneal opacity, stromal damage, and bacterial load. Corneal mRNA levels for IL-1 beta, MIP-2, IL-1R1, TLR4, IL-18, and IFN-gamma and protein levels for IL-1 beta and MIP-2 also were significantly upregulated in anti-SIGIRR Ab over control mice, while no changes in polymorphonuclear cell number, IL-4, or IL-10 mRNA expression were detected. To further define the role of SIGIRR, RAW264.7 macrophage-like cells were transiently transfected with SIGIRR and stimulated with heat-killed P. aeruginosa or LPS. SIGIRR transfection significantly decreased mRNA levels for IL-1R1, TLR4, and type 1 immune response-associated cytokines (IL-12, IL-18, and IFN-gamma) as well as proinflammatory cytokines IL-10 and MIP-2 protein expression. SIGIRR also negatively regulated IL-1 and LPS, but not poly(I:C)-mediated signaling and NF-kappa B activation. These data provide evidence that SIGIRR is critical in resistance to P. aeruginosa corneal infection by downregulating type 1 immunity, and that it negatively regulates IL-1 and TLR4 signaling.