FUNCTIONALLY ACTIVE RECOMBINANT ALPHA-CHAIN AND BETA-CHAIN PEPTIDE COMPLEXES OF HUMAN MAJOR HISTOCOMPATIBILITY CLASS-II MOLECULES

Major histocompatibility (MHC) class II molecules are cell surface het erodimeric (alpha beta) glycoproteins that display processed antigens to T cell receptors (TCRs) of CD4-positive T cells. The present study describes that individual recombinant alpha and beta chains of human M HC class II molecules lacking the transmembrane region (alpha-Tm and b eta-Tm) are capable of binding antigenic peptide and that these comple xes of chain-peptide are recognized by TCRs to induce antigen-specific apoptosis in restricted T cells. The alpha-Tm and the beta-Tm of huma n HLA-DR2 (DRB50101) were cloned, expressed in Escherichia coli, and purified in large scale by conventional chromatographic methods. The i n vitro binding of an immunodominant epitope from the myelin basic pro tein (MBP-(83-102)Y-83) to purified DR2 alpha-Tm and DR2 beta-Tm was d emonstrated with biotinylated and fluoresceinated MBP-(83-102)Y-83 pep tide. The specificity of the MBP-(83-102)Y-83 peptide binding to both DR2 alpha-Tm and DR2 beta-Tm was demonstrated in a competitive peptide binding assay. When exposed to a transformed T cell clone (SS8T) rest ricted to DR2(DRB50101) and MBP-(84-102) peptide, complexes of DR2 al pha-Tm and DR2 beta-Tm with MBP-(83-102)Y-83 peptide were able to spec ifically recognize TCRs as measured by the increase in gamma-interfero n (gamma-IFN) cytokine. Such recognition of TCRs by soluble alpha-MBP- (83-102)Y-83 and beta-MBP-(83-102)Y-83 complexes led to the induction of antigen-specific apoptosis in SS8T cells as measured by double fluo rescence flow cytometry and electron microscop. These results provide the first evidence that soluble complexes of antigenic peptide and ind ividual chains of human MHC class II molecules lacking the transmembra ne region can recognize TCRs and induce antigen-specific apoptosis in T cells. Since activated CD4-positive T cells are involved in pathogen esis of various autoimmune diseases, the apoptosis triggered by indivi dual soluble chain-peptide complexes has significant potential for eli minating autoreactive T cells.