APC requirements of self-reactive, S-antigen-specific T cells: A model for peripheral self-tolerance to retinal autoantigens

The activation requirements of autoreactive T cells were examined using rat/mouse T-cell hybridomas specific for a pathogenic peptide of bovine retinal S-antigen, which induces experimental autoimmune uveoretinitis. These self-reactive hybridomas secreted IL-2 in response to an antigenic peptide presented by nonirradiated splenic antigen-presenting cells (APC), while the same peptide presented by irradiated splenic APC stimulated much less IL-2 secretion. IL-2 production by a non-self-reactive hybridoma, E3, was unaffected by irradiation of the APC. Antigen presentation by thymic APC did not stimulate Ag-specific IL-2 release by any hybridoma tested, while the non-self-reactive E3 hybridoma was highly responsive to peptide presentation by thymic APC. Pretreatment of splenic APC with phorbol ester or LPS and IL- 4 reversed the radiosensitivity of the activity required for IL-2 secretion, but was unable to confer this activity on thymic APC. The radiosensitive activity did not correlate with MHC class II expression; these molecules were neither down-regulated by γ-radiation nor upregulated by PMA. Failure of the T-cell hybridomas to secrete IL-2 was not due to lack of TCR occupancy, since thymic or irradiated splenic APC could induce Ag-specific growth inhibition of the hybridomas. Finally, the radiosensitive activity was not a costimulatory activity, as stimulation with anti-TCR Ab, in the absence of APC, elicited IL-2 secretion from the hybridomas. In summary, the data show that IL-2 secretion and growth inhibition of self-reactive T-cell hybridomas can be dissociated as parameters of activation. Growth inhibition required only Ag presentation, whereas IL-2 secretion required a tissue-specific, radiosensitive APC activity as well. Our data suggest that these two 'signals' are mediated through the T-cell receptor alone and probably reflect the differential outcomes of high and low affinity interactions.