Influence of protein-quaternary structure on antigen processing.

T cell recognition of the quaternary structure of human chorionic gonadotropin (hCG), resulting from the association between its alpha (hCG-alpha) and beta (hCG-beta) subunits, was analyzed using hCG-alpha and hCG-beta T cell hybridomas produced in BALB/c mice. First, the fine specificity of these T cell hybridomas was determined, enabling us to divide hCG-alpha-specific T cell hybridomas into two groups. Group I recognized the hCG-alpha(61-81) region, and group II responded to the hCG-alpha(50-70) part of the molecule. Two groups of hCG-beta-specific T cell hybridomas, designated groups III and IV, were analyzed and found to respond to the C- and the N-terminal parts of the hCG-beta(1-22) peptide, respectively. Moreover, we observed that the nature of APC influenced Ag recognition by hCG-beta T cell hybridomas from group IV, but not by other selected T cell hybridomas. We then showed that recognition of the hCG alpha/beta dimer by alpha-specific T cell hybridomas was dramatically reduced compared to both free hCG-alpha and heat-dissociated hCG alpha/beta molecules. In contrast, hCG-beta hybridomas exhibited comparable responses to the free beta subunit and the hCG dimer. Experiments using a dimeric molecule assembled from the alpha-subunit of human follicle-stimulating hormone, which is identical to hCG-alpha, and the beta-subunit of human follicle-stimulating hormone, which is homologous to hCG-beta, confirmed that the three-dimensional structure of the complex rather than the primary structure of the beta-subunit plays a critical role in the processing pathway. Finally, kinetic experiments showed that the presentation of hCG-alpha T cell epitopes differed depending upon whether the alpha-subunit was in its free or combined form. In contrast, the kinetic expression of hCG-beta T cell epitopes appeared to be independent of the quaternary structure of hCG. Thus, conformational alterations resulting from the alpha/beta subunit association mainly influenced processing of the alpha-subunit in its complexed form, rather than processing of the combined beta-subunit. The effect of protein-quaternary structure on T cell recognition may represent a new element in our understanding of the processing and presentation of oligomeric molecules.