Vasoactive Intestinal Peptide Balances Pro- and Anti-Inflammatory Cytokines in the Pseudomonas aeruginosa-Infected Cornea and Protects against Corneal Perforation

Abstract

Corneal infection with Pseudomonas aeruginosa perforates the cornea in susceptible C57BL/6 (B6), but not resistant BALB/c, mice. To determine whether vasoactive intestinal peptide (VIP) played a role in development of the resistant response, protein expression levels were tested by immunocytochemistry and enzyme immunoassay in BALB/c and B6 corneas. Both mouse strains showed constitutive expression of corneal VIP protein and nerve fiber distribution. However, disparate expression patterns were detected in the cornea after infection. VIP protein was elevated significantly in BALB/c over B6 mice at 5 and 7 days postinfection. Therefore, B6 mice were injected with rVIP and subsequently demonstrated decreased corneal opacity and resistance to corneal perforation compared with PBS controls. rVIP- vs PBS-treated B6 mice also demonstrated down-regulation of corneal mRNA and/or protein levels for proinflammatory cytokines/chemokines: IFN-gamma, IL-1beta, MIP-2, and TNF-alpha, whereas anti-inflammatory mediators, IL-10 and TGF-beta1, were up-regulated. Treatment with rVIP decreased NO levels and polymorphonuclear neutrophil (PMN) number. To further define the role of VIP, peritoneal macrophages (Mphi) and PMN from BALB/c and B6 mice were stimulated with LPS and treated with rVIP. Treatment of LPS-stimulated Mphi from both mouse strains resulted in decreased IL-1beta and MIP-2 protein levels; PMN responded similarly. Both cell types also displayed a strain-dependent differential response to rVIP, whereby B6 Mphi/PMN responded only to a higher concentration of VIP compared with cells from BALB/c mice. These data provide evidence that neuroimmune regulation of the cytokine network and host inflammatory cells functions to promote resistance against P. aeruginosa corneal infection.