TRAIL–induced apoptosis as a mechanism of ocular immune privilege

Abstract

Abstract: : Purpose: To determine whether tumor necrosis factor–related apoptosis–inducing ligand (TRAIL) contributes to maintaining the immune privileged environment of the eye by inducing apoptosis of inflammatory cells of the innate immune apparatus. Methods: The in vitro Annexin V assay was used to determine macrophage and neutrophil sensitivity to TRAIL–induced apoptosis. A three–color assay was performed to determine if human corneal endothelial cells, a known expressor of TRAIL, induce apoptosis of human macrophages. Western blot analysis was performed to evaluate TRAIL expression in human aqueous humor. Results: A human macrophage cell line demonstrated 35% apoptosis following exposure to TRAIL (p=0.001), and this effect was further enhanced to 71% apoptosis by the oligimerization of TRAIL protein with a cross–linking antibody (p<0.001). When human macrophages were activated with gamma interferon and lipopolysaccharide, TRAIL mediated 42% more apoptosis than resting macrophages (p=0.0055). Human corneal endothelial cells induced 59% apoptosis in human macrophages (p<0.001). Primary murine macrophages showed 45% apoptosis following TRAIL administration (p<0.001). A murine macrophage cell line demonstrated 42% (p=0.021) and 20% (p=0.0271) apoptosis following exposure to murine and human TRAIL, respectively, and both effects were enhanced following oligimerization. Primary murine neutrophils underwent 25% apoptosis following exposure to murine TRAIL protein (p=0.0345). Resting and activated murine T cells were not sensitive to TRAIL induced apoptosis, supporting the hypothesis that TRAIL may contribute to ocular immune privilege only against inflammatory cells of the innate immune response. TRAIL protein was present in 2 out of 3 human aqueous humor samples tested. Conclusions: The results support the feasibility of TRAIL functioning to maintain immune privilege in the eye by inducing apoptosis of inflammatory cells of the innate immune response. TRAIL that is present in the eye is not only isolated to surfaces of ocular tissue, but is also found to be expressed in the aqueous humor, which may also contribute to maintaining immune privilege.

Keywords: immune tolerance/privilege • inflammation • apoptosis/cell death