Complement C3b fragment covalently linked to tetanus toxin increases lysosomal sodium dodecyl sulfate-stable HLA-DR dimer production.

Processing and presentation of covalently linked C3b-tetanus toxin (TT) complexes, as compared to unlinked C3b + TT, lead to increased T cell proliferation. The aim of this study was to analyze the effect of coupling C3b to TT on the efficiency of TT peptide loading on HLA-DR1 molecules. In the Epstein-Barr virus-transformed B cell line HOM 2, we detected a significant increase of sodium dodecyl sulfate (SDS)-stable major histocompatibility complex (MHC) class II molecules after exposure to C3b-TT as compared to unlinked C3b and TT. The ratio of compact form/unbound form (C/U ratio) obtained with C3b-TT as antigen (Ag) is about twice that obtained with uncomplexed TT + C3b as Ag. Similar results were obtained using HLA-DR1-transfected fibroblasts that do not express C3b complement receptors, indicating that the SDS-stable HLA-DR1 increase did not result simply from C3b opsonization but rather from a direct effect of C3b-TT linkage on peptide generation. Exposure of HOM 2 cells to C3b-TT resulted in an increase in concentration of SDS-stable HLA-DR molecules in lysosomes but not in endosomes. Thus, C3b attachment to Ag induces a redistribution of peptide/MHC complex which results in a higher efficiency of Ag presentation by MHC class II molecules.