Resident tissue macrophages within the normal rat iris lack immunosuppressive activity and are effective antigen-presenting cells

Despite extensive study of the numerous immunoregulatory mechanisms that co ntribute to the immune-privileged nature of the anterior chamber (AC) of th e eye, little is known of the functional nature of antigen-presenting cells (APC) present in the tissues adjoining the AC. In the present study, we ha ve compared the antigen-presenting capacity of dendritic cells (DC) and mac rophages isolated from the normal rat iris. Whereas iris DC exhibited a pot ent ability to stimulate resting allogeneic T cells in MLR cultures (an in- vitro correlate of the ability to induce primary T cell responses), residen t iris macrophages displayed negligible MLR-stimulatory capacity. Significa ntly, iris macrophages could efficiently elicit proliferation of primed ant igen-specific T cells (an in-vitro correlate of the ability to act as local APC in secondary responses). This antigen-presenting activity was approxim ately half that of fully mature iris DC and considerably greater than that of freshly isolated iris DC. A key contributor to the effectiveness of resi dent iris macrophage antigen presentation was considered to be the absence of lymphocytostatic control of T cell proliferation exerted by these cells. The results indicate dichotomous but complementary roles for DC (immune su rveillance) and macrophages (local antigen presentation in secondary respon ses) in this tissue.