AECs secrete soluble factors that inhibit cells in both the innate and adaptive immune systems.

Human uveal melanoma arises in an immune privileged ocular environment in which both adaptive and innate immune effector mechanisms are suppressed. Uveal melanoma is the most common intraocular tumor in adults and is derived from tissues in the eye that produce macrophage migration-inhibitory factor (MIF), a cytokine that has recently been demonstrated to produce immediate inhibition of NK cell-mediated lytic activity. Although NK cell-mediated lysis of uveal melanomas is inhibited in the eye, melanoma cells that disseminate from the eye are at risk for surveillance by NK cells. Moreover, uveal melanoma cells demonstrate a propensity to metastasize to the liver, an organ with one of the highest levels of NK activity in the body. Therefore, we speculated that uveal melanomas produced MIF as a means of escaping NK cell-mediated lysis. Accordingly, seven primary uveal melanoma cell lines and two cell lines derived from uveal melanoma metastases were examined for their production of MIF. MIF was detected in melanoma culture supernatants by both ELISA and the classical bioassay of macrophage migration inhibition. Melanoma-derived MIF inhibited NK cell-mediated lysis of YAC-1 and uveal melanoma cells. Cell lines derived from uveal melanoma metastases produced approximately twice as much biologically active MIF as cultures from primary uveal melanomas. Inhibition of NK cell-mediated killing by uveal melanoma-derived MIF was specifically inhibited in a dose-dependent manner by anti-MIF Ab. The results suggest that human uveal melanoma cells maintain a microenvironment of immune privilege by secreting active MIF that protects against NK cell-mediated killing.

The results suggest that CD4(+) T cells function directly as effector cells and not as helper cells in the rejection of corneal allografts. Although the corneal endothelium is highly susceptible to Fas-induced apoptosis, this is apparently not the primary mechanism of CD4(+) T-cell-dependent rejection.

Ocular immune privilege is the result of several unique features of the eye, including the systemic down-regulation of Th1 immune responses to Ags encountered in the anterior chamber of the eye—a phenomenon termed anterior chamber-associated immune deviation (ACAID). The induction of ACAID requires the participation of three cell populations: the ocular ACAID APC, the splenic B cell, and the splenic T cell. Because B cells have been implicated in tolerogenic Ag presentation in other systems, we hypothesized that B cells were responsible for the induction of regulatory T cells in ACAID. The central hypothesis for this study is that APC from the eye migrate to the spleen where they release antigenic peptides (OVA) that are captured and presented to T cells by splenic B cells. A combination of in vitro and in vivo studies demonstrated that splenic B cells, incubated with ACAID APC in vitro, were capable of inducing ACAID when transferred to naive mice. The induction of ACAID required the normal expression of β2-microglobulin on both the B cell and ACAID APC, but not on the T suppressor cells. Moreover, the induction of ACAID regulatory cells required histocompatibility between the B cells and regulatory T cells at the TL/Qa region. The results indicate that: 1) B cells are necessary for the induction of ACAID; 2) ACAID B cells do not directly suppress the expression of delayed-type hypersensitivity; and 3) the induction of Ag-specific regulatory T cells by ACAID B cells requires histocompatibility at the TL/Qa region.

A large body of evidence suggests that corneal allograft rejection is mediated by a type 1 Th cell response and that deviation toward type 2 immunity favors graft survival. However, clinical observations indicate that patients with severe ocular allergies have increased risk of corneal allograft rejection. We used a mouse model of atopic conjunctivitis to evaluate the effects of Th2 immune deviation on corneal allograft survival and possible mechanisms of graft rejection. Our results reveal the following novel findings: 1) atopic conjunctivitis promotes systemic Th2 immune responses to corneal graft donor alloantigens; 2) corneal allografts in atopic host eyes have an increased incidence and swifter tempo of rejection; 3) increased rejection is associated with alterations in systemic T cell-mediated responses to donor alloantigens; and 4) corneal allograft rejection in atopic hosts does not require the direct involvement of infiltrating eosinophils.