Major histocompatibility complex (MHC) class II molecules are cell surface heterodimeric glycoproteins consisting of one alpha and one beta polypeptide chain of similar size. These molecules play a critical role in immune recognition by displaying processed antigens to CD4-positive T helper cells. Several attempts to express the MHC class II molecules by recombinant methods in various systems resulted in either failure or poor recovery of the intact heterodimer. The present study describes our successful effort to refold and reconstitute HLA DR2 heterodimer from individually expressed alpha and beta polypeptide chains lacking the transmembrane hydrophobic regions in Escherichia coli, in the presence of an immunodominant epitope analog from human myelin basic protein (b-MBP(83-102)Y83). The reconstituted DR2 heterodimer complex was selectively purified from unfolded alpha and beta chains using heterodimer-specific monoclonal antibody (L243) coupled to a solid support. The detection of two polypeptide chains in the purified refolded DR2-peptide complex preparations was accomplished by Western blot analysis and enzyme-linked immunosorbent assay using heterodimer- and chain-specific polyclonal antibodies, and the presence of equimolar amounts of both alpha chain and beta chain in the reconstituted complex preparation was confirmed by a double label experiment. The quantitation of the bound peptide in complex preparation was measured by incubating two chains in the presence of 125I-labeled peptide. An increase in the yield of refolded and reconstituted DR2-peptide complexes was observed with increasing peptide concentration in the reaction mixture. Finally, the functional activity of the reconstituted DR2 complexes was measured by their ability to stimulate gamma-interferon production by SS8T cloned T cells in an antigen-specific and dose-dependent manner. These results demonstrate that biologically active complexes of human DR2.b-MBP (83-102)Y83 can be prepared by proper folding of human leukocyte antigen DR2 alpha and beta chains in the presence of antigenic peptide. The yield of such DR2 heterodimers with bound peptide is several thousand-fold higher over native DR2 purified from transformed B cells. Since purified MHC class II-peptide complexes have been shown to prevent autoimmune diseases in various animal models, reconstituted heterodimer complexes may have significant clinical relevance in antigen-specific treatment of various autoimmune diseases. In addition, such complexes with increased yield will provide better understanding of the trimolecular interactions between MHC-peptide and T cell receptor.