Functions of Peptidoglycan Recognition Proteins (Pglyrps) at the Ocular Surface: Bacterial Keratitis in Gene-Targeted Mice Deficient in Pglyrp-2, -3 and -4

Purpose Functions of antimicrobial peptidoglycan recognition proteins (Pglyrp1-4) at the ocular surface are poorly understood. Earlier, we reported an antibacterial role for Pglyrp -1 in Pseudomonas aeruginosa keratitis. Here we investigated functions of three other related genes Pglyrp -2, -3 and -4 in a mouse model of P . aeruginosa keratitis. Methods Wild type (WT) and each of the Pglyrp -null genotypes were challenged with P . aeruginosa keratitis. The eyes were scored in a blinded manner 24 and 48h post infection. Viable bacterial counts and inflammatory factors (IL-12, TNF-α, IFN-γ, CCL2, IL-6 and IL-10) were measured in whole eye homogenates using cytometric bead arrays. Expressions of Pglyrp-1-4 , mouse beta defensins ( mBD)-2 , -3 , cathelicidin-related antimicrobial peptide (CRAMP) were determined by qRTPCR in total RNA extracts of uninfected and infected eyes of WT and each of the Pglyrp -null mouse types. Results The Pglyrp-2 -/- mice showed reduced disease and lower induction of pro-inflammatory TNF-α ( p = 0.02) than WT or the other Pglyrp null mice. Viable bacterial yield was significantly lower in the Pglyrp -2 -/- ( p = 0.0007) and the Pglyrp -4 -/- ( p = 0.098) mice. With regards to expression of these antimicrobial genes, Pglyrp -2 expression was induced after infection in WT mice. Pglyrp -3 expression was low before and after infection in WT mice, while Pglyrp -4 expression was slightly elevated after infection in WT, Pglyrp -2 and -3 null mice. Pglyrp -1 expression was slightly elevated after infection in all genotypes without statistical significance. Transcripts for antimicrobial peptides mBD2, mBD3 and CRAMP were elevated in infected Pglyrp-2 -/- males without statistical significance. Conclusions Efficient resolution of keratitis in the Pglyrp-2 -/- mice may be due to a reduced pro-inflammatory microenvironment and synergistic antibacterial activities of defensins, CRAMP and Pglyrp-1. Therefore, in ocular infections the pro-inflammatory functions of Pglyrp -2 must be regulated to benefit the host.