Characterization of a Novel Model of NOD2-Dependent Ocular Inflammation

Abstract

Purpose:: CARD15 (NOD2) senses a component of bacterial peptidoglycan, muramyl dipeptide (MDP), and is expressed in various cell types and tissues including the uveal vascular endothelium. Specific mutations in the human NOD2 gene are associated with an autosomal dominant form of uveitis. Since the in vivo effectsof MDP are incompletely studied and the susceptibility of the eye to mutations in NOD2 is unexplained, we sought to investigate the role of NOD2 in uveitis using a mouse model system.

Methods:: Saline or 100ug MDP was injected into the vitreous of BALB/c mice or mice deficient in L-selectin or NOD2 and their wild-type control strains. The intravascular inflammatory response within the iris was assessed by intravital microscopy at different times following MDP treatment and the numbers of rolling and adherent cells were ascertained. Infiltration of leukocytes in the aqueous was assessed by histological examination of eye tissue sections.

Results:: MDP treatment elicited a transient inflammatory response within the eye, as the number of rolling (859+170 cells/mm²/min vs 142+95 cells/mm²/min in saline controls) and adherent leukocytes (385+93 cells/min vs 24+19 cells/min in saline controls) along the iris vascular endothelium was significantly increased by 6 hours and returned to baseline levels within 24 hours following MDP treatment. This intravascular inflammatory response translated to a minor increase in leukocytes within the aqueous of mice at 12 hours following MDP treatment. The increase in rolling leukocytes in MDP treated mice was dependent on the adhesion molecule L-selectin, as L-selectin knock-out mice showed a significant decrease in the number of rolling cells within the iris vasculature in response to MDP. Importantly, both the intravascular inflammatory response and the cellular infiltration induced by MDP treatment were markedly reduced in NOD2 knockout mice which had been backcrossed for at least 10 generations to BALB/c.

Conclusions:: Our findings indicate that the bacterial product, MDP, induces uveitis in mice. In analogy to Toll-like receptors, innate immune responses mediated by NOD2 play an essential role in the ocular inflammatory response elicited by MDP. Thus, NOD2 may participate in the development of uveitis in response to bacterial products through its ability to enhance intravascular interactions between leukocytes and the endothelium in the eye in an L-selectin dependent fashion. This model will clarify the role of NOD2 in regulating inflammatory mediators in ocular inflammation.

Keywords: immunomodulation/immunoregulation • inflammation