DEFICIENT ANTIGEN-PROCESSING OF A PROTEIN QUATERNARY STRUCTURE CAN BEOVERCOME BY RECEPTOR-MEDIATED UPTAKE

Human chorionic gonadotropin (hCG) is a dimer of non-covalently associ ated alpha (hCG-alpha) and beta (hCG-beta) subunits. This molecule was used to study whether receptor-mediated uptake influences the present ation of a protein quaternary structure. Unprimed splenocytes and a B cell lymphoma were capable of presenting only the free (hCG-alpha) but not the combined (hCG) alpha subunit to hCG-alpha. T cell hybridomas, while hCG-alpha-primed lymph node cells (LNC) responded to both hCG-a lpha and hCG. As antigen (Ag)-specific antigen-presenting cells (APC) present in the hCG-alpha-primed LNC population may be potentially effe ctive for presenting hCG, we investigated the role of specific Ag capt ure, through mIg and FcgammaR, in the processing and presentation of h CG and hCG-alpha to HAG 5, a T cell hybridoma directed against the imm unodominant region (amino acids 61-81) of hCG-alpha. Results showed th at only B cells bearing membrane immunoglobulin capable of recognizing hCG-alpha and hCG, and present in hCG-alpha-primed mice, were extreme ly effective in presenting the free as well as the combined alpha subu nit. The effect of FcR-mediated uptake was analyzed using a B cell lin e transfected with the FcgammaRII-B2 gene to present immune complexes of either hCG-alpha or hCG. We found that hCG-alpha and hCG were prese nted equally well,whatever the Ag-binding site of each antibody to hCG or its alpha subunit. Using HBG 6, an hCG-beta T cell hybridoma, we p erformed similar experiments with the FcgammaRII-B2 cell line and dete rmined that the potentiation of hCG presentation to HBG 6 was similar to that observed with HAG 5. Then kinetic experiments were performed t o examine the effect of Ag uptake through FcR on processing. Results d emonstrated that the uptake pathway drastically influenced the express ion of alpha T cell determinants in the alpha/beta dimer. In addition, treatment with cycloheximide, a protein synthesis inhibitor, only imp aired the ability of APC to present specifically captured Ag. Thus, th e processing pathway for specifically captured Ag might be different f rom the pathway used to process nonspecifically captured Ag. This obse rvation might explain why receptor-enhanced uptake bypasses the ineffi cient processing of the hCG quaternary structure and enables similar e fficiency in the presentation of alpha and beta T cell specificities. These findings provide new insight into the antigenicity of oligomeric molecules, which is modified whether antigen capture is specific or n ot.