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

Abstract

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.