Abstract
Abstract: :
Purpose: Ocular immune privilege is illustrated by the observation that the immune response is capable of eliminating tumors injected into the skin but fails to eliminate the same tumors injected into the anterior chamber (a.c.) of the eye. Herein, we developed an ocular tumor model with a single defined tumor antigen to determine the mechanisms of immune evasion by tumors developing within the eye Methods:E.G7–OVA are an EL–4 thymoma (H–2b,Thy1.2+) transduced to express chicken ovalbumin (OVA). E.G7–OVA (0.4 – 1.0 X 105) were injected into the a.c. or skin of B6.PL (H–2b,Thy1.1+) mice and tumor growth was monitored by caliper measurements in the skin or by enumeration of tumor cells in collagenase digested eyes by flow cytometry utilizing anti–Thy1.2 antibodies. OVA–specific CD8+ T cells were also enumerated in the submandibular lymph nodes of B6.PL recipients of OVA–specific CD8+ T cells (OT–I) by flow cytometry utilizing OVA257–264, H–2Kb tetramers. Results: B6.Pl mice injected with E.G7–OVA a.c. uniformly developed ocular tumors (6/6; 100%) whereas only a minority of mice (1/6; 17%) developed skin tumors when injected with the same number of E.G7–OVA in the skin. By contrast, immunodeficient Rag–/– mice developed skin tumors (3/4; 75%) when injected with E.G7–OVA in the skin indicating that the adaptive immune response was responsible for tumor rejection. Nine days after E.G7–OVA cells were injected into the a.c. of B6.PL recipients of OT–I T cells, a significant 12 fold expansion of OT–I T cells was observed in the ipsilateral submandibular lymph nodes suggesting that antigens escaped the eye and primed OVA–specific CD8+ CTL precursors. In support of that interpretation, B6 mice previously injected with E.G7–OVA completely rejected a subsequent injection with E.G7–OVA in the opposite eye and these eyes contained 6.5 fold higher numbers of CD8+ leukocytes compared to eyes of mice previously given PBS a.c. prior to E.G7–OVA challenge. E.G7–OVA isolated after seven days of growth in the a.c. and E.G7–OVA grown in standard growth medium were rejected when injected into the skin of B6 mice (0/4; 0%, and 1/6; 17% respectively) suggesting that the antigenicity of E.G7–OVA did not change within the eye. Conclusions: These data suggest that CD8+ CTL migrate to the eye and are capable of exerting their effector function within the immune privileged a.c. However, unlike the skin the a.c. is lined by cells expressing Fas ligand which induces apoptosis of activated Fas expressing CD8+ CTL. The expression of Fas ligand in the eye may decrease the frequency of CD8+ CTL infiltrating ocular tumors allowing for tumor growth to exceed immune elimination.
Keywords: immune tolerance/privilege • tumors • immunomodulation/immunoregulation