Fas ligand-induced apoptosis of lymphocytes in a privileged site induces immune tolerance

Injection of antigen into the anterior chamber (AC) of the eye results in the establishment of immune deviation where DTH responses are tolerized and antibody responses are normal. Recently we showed that Fas ligand (FasL) expression on non-lymphoid cells in the eye induced apoptosis in invading inflammatory cells following viral infection of the eye. Mice defective in FasL expression had uncontrolled cell proliferation and invasion of important ocular structures without apoptosis. Here we show that lymphoid cell death is important for induction of systemic immune tolerance. In B6 animals, where inflammatory cells undergo apoptosis in the eye following infection with HSV-1, immune deviation was induced. B6-lpr (Fas')and B6-gld (FasL") mice, where inflammatory cell death was absent, did not display immune tolerance following AC injection. Using a system to induce immune deviation by AC injection of TNP-coupled spleen cells (TNP-spl) we found that apoptosis induced in Fas⁺ TNP-spl by FasL⁺ ocular tissue was essential for tolerance. Further studies showed that it was not specifically Fas/FasL induced cell death, but cellular apoptosis that was critical. Cells killed by non-apoptotic means, or cells unable to undergo apoptosis, did not induce immune tolerance. Insight into the mechanism was gained when we demonstrated that apoptotic cells were engulfed by macrophages in the eye, suggesting that presentation of antigens from apoptotic cells may lead to tolerance induction. Our results demonstrate that cellular apoptosis in an immune privileged site can lead to the induction of immune tolerance.