ISOTYPIC RESIDUES IN THE MEMBRANE PROXIMAL DOMAIN OF MHC CLASS-II BETA-CHAINS CONTROL ACTIVATION OF CD4-CELLS( T)

To investigate the role that isotypic residues play during interaction s between CD4+ T cells and MHC class II molecules, interisotypic MHC c lass II beta-chains have been generated in which the beta1 domain is d erived from I-A and the beta2, transmembrane, and intracytoplasmic reg ions are derived from I-E. Interisotypic or wild-type Abeta genes have been transfected into L cells with the genes encoding the wild-type A alpha-chain. Transfectants bearing the recombinant beta-chain thus exp ress an MHC class II dimer in which the amino-terminal domains that co ntrol TCR and peptide-binding interactions are wild-type Aalpha and Ab eta, whereas the membrane proximal domains are derived from Aalpha and Ebeta. L cells expressing this recombinant class II molecule or wild- type AalphaAbeta have been compared functionally for their ability to stimulate Ag-specific T cell hybridomas and normal T cell clones and t o activate primary alloreactive and superantigen-specific T cells. Ag- specific T cell hybridomas vary in their ability to be activated by th e recombinant class II molecule, and sensitivity to the isotypic form of the membrane proximal domain correlates with expression of the CD4 molecule. CD4+ T cells distinguish between the wild-type and recombina nt dimers, whereas CD4- T cells react equivalently with both. The reco mbinant class II molecules are defective in activation of normal T cel l clones and are totally deficient in activation of primary alloreacti ve and superantigen-reactive T cells, but stimulate CD4- alloreactive T cell hybridomas equivalently. Together, the results from these exper iments suggest that the interisotypic dimers possess normal TCR and pe ptide interactions, but altered CD4-dependent accessory interactions n ecessary for activation of normal T cells. These findings indicate tha t isotypic residues in the membrane proximal domains of MHC class II c ontrol CD4-linked accessory function and that this accessory function is most critical for freshly isolated CD4 lymphocytes.