ST2 is essential for Th2 responsiveness and resistance to Pseudomonas aeruginosa keratitis

PURPOSE. To elucidate the role of ST2, a member of the TLR/IL-1R (TIR) superfamily, in protecting against Pseudomonas aeruginosa keratitis in BALB/c mice. METHODS. ST2 mRNA and protein expression levels were tested by real-time PCR and Western-blot in C57BL/6 (B6; susceptible) versus BALB/c (resistant) mice before and after P. aeruginosa ( strain 19660; American Type Culture Collection, Philadelphia, PA) challenge. Infected BALB/c mice also were tested after subconjunctival injection with recombinant murine (rm) ST2 or PBS. Disease was monitored by clinical score, slit lamp, bacterial plate count, a myeloperoxidase (MPO) assay to measure polymorphonuclear neutrophil (PMN) infiltrate, real-time RTPCR, and ELISA. RESULTS. ST2 mRNA and protein were constitutively expressed in the uninfected normal corneas of both mouse groups. ST2 levels in the cornea of BALB/c compared with B6 mice were elevated significantly at 1 to 3 days post infection ( PI), peaked at 3 and decreased at 5 days PI. BALB/c mice treated with rmST2 showed increased corneal opacity and perforation ( at 5 days PI) when compared with PBS controls. rmST2-versus PBS-injected mice exhibited increased bacterial load, PMN infiltrate, and higher corneal mRNA levels for IL-1 beta, MIP-2, IL-6, IL-1R1, and Th1-type cytokine such as IFN-gamma. Protein levels for IL-1 beta, MIP-2, and IL-6 also were significantly upregulated, whereas the Th2 cytokines IL-4 ( mRNA), IL-5 ( mRNA), and IL-10 ( mRNA and protein) were significantly reduced. CONCLUSIONS. ST2 is critical in resistance to P. aeruginosa keratitis, functioning to reduce corneal infection ( bacterial load) and inflammation by negatively regulating proinflammatory cytokines and inhibiting type-1 immunity, but upregulating type-2 cytokine production, particularly IL-10.