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

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.

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 RT-PCR, and ELISA.

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-1beta, MIP-2, IL-6, IL-1R1, and Th1-type cytokine such as IFN-gamma. Protein levels for IL-1beta, 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.

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.