Thymic expression of myelin basic protein (MBP). Activation of MBP-specific T cells by thymic cells in the absence of exogenous MBP.

Previous studies have shown golli-myelin basic protein (MBP) mRNA to be expressed in the thymus of normal SJL mice, but translation of the mRNA was not assessed. To test for the presence of immunoreactive protein, single cell suspensions were prepared from adult SJL thymus and cultured with syngeneic MBP-specific T cells. After 48 h [3H]thymidine was added to the microcultures to assess T cell proliferation. MBP-specific T cell lines proliferated strongly (stimulation index range, 13-31). T cell lines specific for MBP exon 2, MBP peptide 89-101, proteolipid protein peptide 139-151, and OVA gave stimulation indices of 10-13, 5-6, 2-3, and 2-3, respectively. Stimulatory activity could be abrogated by irradiation of either the thymic cells or the MBP-specific T cells. Stimulatory activity was a property of a minor population of plastic-adherent thymic cells. Monoclonal anti-I-As Ab added to the microcultures inhibited the reaction by 77%. MBP-specific T cells cultured with syngeneic nonirradiated thymus cells in the absence of added MBP transferred experimental autoimmune encephalomyelitis adoptively to syngeneic recipients. These findings indicate that golli-MBP mRNA is translated in normal SJL thymus, and that peptides reactive with MBP-specific T cells in the context of class II MHC molecules are expressed.