B7+ Iris Pigment Epithelium Induce CD8+ T Regulatory Cells; Both Suppress CTLA-4+ T Cells

Abstract

Ocular pigment epithelia contribute to immune privilege by suppressing T cell activation and converting T cells into regulatory T regulatory cells (Tregs) that inhibit bystander T cell activation. Iris pigment epithelium (IPE) does so through direct cell-cell contact with naive T cells, and this suppressive contact is via interactions between B7 expressed constitutively on IPE cells and CTLA-4 expressed on a subpopulation of CD8+ T cells. We have now examined whether TGFβ is required in this process. We report that IPE produces both soluble and membrane-bound active TGFβ, but that only the latter is actually delivered to CD8+ T cells. In turn, these T cells become IPE Tregs by up-regulating their own expression of B7-1/B7-2 and soluble and membrane-bound TGFβ. IPE Tregs through their expression of B7 are able to engage CTLA-4+ bystander T cells, and thus precisely, target delivery of membrane-bound TGFβ. We propose that this mechanism of suppression via TGFβ ensures that soluble active TGFβ is not released into the ocular microenvironment where it can have unregulated and deleterious effects, including elevation of intraocular pressure and development of glaucoma.