Disulfide linkage between C3b and tetanus toxin on tetanus toxin-specific EBV-transformed B cells

EBV-transformed B cells specific for tetanus toxin were found to bind C3b in excess over the expected figures based on the number of complement receptors CR1. This was confirmed by analysis of cell extracts by SDS-PAGE giving evidence for C3b-membrane protein complexes that were disrupted upon reduction. Alkylation of C3b-free cysteine abolished formation of these complexes and only a noncovalent binding of C3b to CR1 was observed, which could be inhibited by mAb to CR1. When C3b was incubated with the same cells coated with tetanus toxin bound to their specific membrane Ig, preferential formation of disulfide-bonded complexes between tetanus toxin and C3b was observed. These observations correspond to a novel capacity of C3b to interact covalently through its cysteine 1010 with free SH groups of protein acceptors. One hypothesis is that the disulfide bond formation is catalyzed by a thioredoxin-like protein secreted and expressed on the membrane of EBV-transformed B cells. In the context of Ag processing and presentation by B cells, disulfide binding of chaperone C3b to Ag is likely to persist during transcytosis and to play a significant role in the modulation of the processing.