Post-translational attainment of allelic exclusion of the T cell receptor α chain in a T cell clone

: Two in-frame rearranged mouse TCR alpha chain genes from a single CD4+ T cell clone, MS202, specific for self-class II MHC antigen were transfected into a TCR-negative T cell hybridoma together with a beta chain gene derived from the same T cell clone. Both alpha chain genes were efficiently transcribed and translated in the cytoplasm of host cells, but only one alpha chain (V alpha 5) was expressed on the cell surface in association with the partner beta chain (V beta 4). The other alpha chain gene (V alpha 4) was translated into a mature form of the alpha chain but was unable to make a pair with the beta chain, being prohibited from the surface expression. The supertransfection of the CD4 gene into the alpha beta transfectants did not alter the transcription and expression of both combinations of TCR alpha and beta genes. The original self-class II reactivity was, however, reconstituted only in the cells expressing V alpha 5 and V beta 4 genes supertransfected with CD4. These results indicate that the allelic exclusion of MS202 was achieved by a post-translational mechanism where the product of an in-frame rearranged alpha chain was unable to be expressed on the cell surface, allowing further rearrangement and expression of the other alpha chain gene. The self-class II reactivity of TCR was dependent on the co-expression of CD4 molecules.