Activation of Autoreactive T Cells by Endogenous Commensal Microflora Provokes Spontaneous Autoimmunity in the Immunologically Privileged Eye

Abstract

Purpose: Autoimmune uveitis is believed to be driven by autoreactive T cells. A conceptual paradox is that although the retina-specific T cells that cause uveitis must be activated to penetrate the blood-retinal barrier, the antigens that can activate them are sequestered inside the immunologically privileged eye. Microbial triggers have been suspected, but never proven.

Methods: To address this question, we developed a robust and highly penetrant mouse model of spontaneous uveitis. R161H mice express a transgenic retina-specific T cell receptor (TCR) and develop 100% spontaneous uveitis by 2-3 months of age. Th1 and Th17 cells are present in their eyes and intestinal lamina propria (LP). Mice treated with a broad-spectrum antibiotic, or reared under germ free conditions, as well as control specific-pathogen-free (SPF) mice, were examined for uveitis and immunological responses. IRBP-specific T cells (~25% of peripheral CD4+ cells) were detected with an antigen-MHC.II-Ig dimer.

Results: Antibiotic treated and germ free R161H mice developed a markedly attenuated uveitis and had fewer Th17 cells in their LP. Interestingly, the retina-specific R161H T cells appeared to be activated more strongly in the gut than polyclonal T cells, by several criteria: (a) under SPF conditions, R161H mice consistently displayed 2-5x more Th17 cells in their LP than wild type littermates, even when crossed onto an IRBP-/- background; (b) in the LP of R161H mice, Th17 cells were more frequent among R161H TCR+ T cells than among non-specific T cells, and (c) R161H TCR+ T cells in culture were more readily activated by proteins extracted from intestinal contents than polyclonal T cells, suggesting involvement of the IRBP-specific TCR in their activation. Experiments designed to test TCR signaling (Nur77 reporter mice and staining for pZap70) revealed clear evidence that the R161H TCR is being engaged in the gut environment.

Conclusions: We propose that circulating retina-specific T cells which cause spontaneous autoimmune uveitis are activated in the intestine by commensal microflora, through a process that may involve antigenic mimicry. These data not only have implications for etiology of human uveitis, but also raise the possibility that activation of autoreactive TCRs by endogenous microflora may be a more common trigger of autoimmune diseases than is currently appreciated.

Keywords: 432 autoimmune disease • 555 immunomodulation/immunoregulation • 557 inflammation