Purpose: Nucleotide oligomerization domain-2 (NOD2) plays an important role in innate immunity to sense muramyl dipeptide (MDP), a component of bacterial cell walls. Notably, NOD2 is linked to eye inflammation because mutations in NOD2 cause a granulomatous type of uveitis called Blau syndrome. A mouse model of NOD2-dependent ocular inflammation was employed to test the role of a cytokine strongly implicated in granuloma formation, IFN-gamma, in order to gain insight into downstream functional consequences of NOD2 activation within the eye triggering uveitis.
Methods: Mice deficient in IFN-gamma, NOD2, or CD11b and their wild-type controls were treated with intravitreal injection of MDP in the presence or absence of IFN-gamma. IFN-gamma production in the eye was measured by ELISA. The intravascular inflammatory response within the iris was quantified by intravital microscopy.
Results: NOD2 activation resulted in the production of IFN-gamma within the eye. Deficiency in IFN-gamma diminished the development of MDP-induced uveitis, indicating its crucial role in downstream inflammatory events triggered by NOD2. Moreover, exogenous IFN-gamma markedly exacerbated MDP-induced ocular inflammation in a NOD2-dependent mechanism. The potential of IFN-gamma to enhance inflammation required the adhesion molecule CD11b because CD11b-deficient mice failed to show the synergistic effects of IFN-gamma and MDP cotreatment on adhering and infiltrating cells.
Conclusions: IFN-gamma was identified as a downstream mediator of NOD2-driven inflammation and the capacity of IFN-gamma in vivo to enhance the inflammatory potential of NOD2 was demonstrated. Extrapolation of these findings in mice suggests that the dysregulation of IFN-gamma may occur in patients with Blau syndrome, thereby contributing to the granulomatous nature of the disease.