Activation of antigen-specific T lymphocyte lines requires presentation of the relevant antigen by syngeneic accessory cells (AC) that express Class II MHC gene products. To determine if the T cell activation signal was AC associated or was shed into the medium, supernatants from rat thymic AC populations pulsed with guinea pig basic protein (GP-BP) or PPD were used to stimulate resting BP- or PPD-reactive T cell lines derived from Lewis or BN rats. Cellfree supernatants were found to stimulate the T cell lines in an antigen-specific, strain-restricted manner, reflecting the pattern of stimulation observed with intact AC used. The activity of the cellfree supernatant could be enhanced five to 20 times in the presence of activated T lymphocytes, the optimal production occurring over a 6-hr period. The cellfree T lymphocyte activation signal produced in the presence of activated T cell products contained both an antigen-specific, MHC-restricted component (85%) and an antigen-independent, mitogenic component (15%). Supernatants containing GP-BP but not bovine BP or PPD induced highly significant proliferation of the Lewis rat-derived BP-1 T lymphocyte line, and transfer of these supernatant-activated cells produced clinical signs of experimental autoimmune encephalomyelitis (EAE) and DTH reactions to GP-BP. The GP-BP component was not inhibited by either of two monoclonal antibodies directed at determinants on either side of the epitope(s) recognized by BP-1 cells. However, stimulation was inhibited by an anti-I-A but not an anti-I-E monoclonal antibody, suggesting the involvement of a Class II MHC gene product on the T cell activation signal. The supernatant activity could be separated by ultracentrifugation at 100,000 X G for 4 hr into a microsomal pellet and an ultrasupernatant, and both fractions had greatly reduced activity on resting T cells until they were reconstituted by vigorous mixing. These results suggest that T effector cells can be activated by AC-derived microsomal fragments bearing Class II antigens that associate noncovalently with processed antigen. This cellfree signal is sufficient to activate encephalitogenic T lymphocytes to transfer clinical EAE and DTH reactions without need for a direct T cell-AC interaction.

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