IFN-γ blocks CD4+CD25+ Tregs and abolishes immune privilege of minor histocompatibility mismatched corneal allografts

Th1 CD4+ cells are believed to be the primary mediators of corneal allograft rejection. However, rejection of fully allogeneic C57BL/6 corneal allografts soared from 50% to 90% in both interferon-gamma (IFN-γ)(-/-) and anti-IFN-γ-treated BALB/c mice. In contrast, similar deficits in IFN-γ in BALB/c hosts enhanced immune privilege of BALB.B (minor histocompatibility [minor H] antigen-matched, major histocompatibility complex [MHC]-mismatched) and NZB (MHC-matched, minor H antigen-mismatched) corneal allografts-decreasing rejection from 80% to ~20%. This effect of IFN-γ was independent of CD4+ T cell lineage commitment as both anti-IFN-γ-treated acceptor and rejector mice displayed a Th2 cytokine profile. The presence of IFN-γ prevented the generation of alloantigen-specific CD4+CD25+ T regulatory cells (Tregs) in hosts receiving either MHC only mismatched BALB.B or minor only histocompatibility (minor H)-mismatched NZB corneal allografts. Tregs in these hosts promoted corneal allograft survival by suppressing Th2 effector cells. By contrast, IFN-γ was necessary for the generation of CD4+CD25+ Tregs that prevented rejection of fully allogeneic C57BL/6 corneal allografts in BALB/c hosts. These findings suggest that MHC-matching in combination with blockade of IFN-γ holds promise as a means of enhancing corneal allograft survival.