COVALENT BINDING OF C3B TO TETANUS TOXIN - INFLUENCE ON UPTAKE INTERNALIZATION OF ANTIGEN BY ANTIGEN-SPECIFIC AND NONSPECIFIC B-CELLS/

Antigen opsonization by the C3b fragment of complement is a significan t event in the modulation of cell-mediated immune response, but its me chanism is still largely unknown. The structural characteristics of C3 b allow it to act as a bifunctional ligand between antigen and cells v ia their membrane C3b receptors, it was thus of interest to study the influence of the covalent link between C3b and antigen on the fixation and internalization of this antigen by antigen-presenting cells. Teta nus toxin (TT) was used as antigen, either free or covalently linked t o C3b (TT-C3b), The antigen-presenting cells were TT specific (4.2) or non-specific (BL15) Epstein-Barr virus (EBV)-transformed B cells, C3b was found to play an important role in antigen fixation and internali zation by both antigen-specific and antigen non-specific cells. Covale nt binding of C3b on TT (1) permitted fixation and internalization of this antigen by non-specific cells via their complement receptors; (2) enhanced antigen fixation and resulted in cross-linking between membr ane immunoglobulins and complement receptors on antigen-specific cells , The consequences of covalent C3b binding to TT were analysed using a ntigen-specific and antigen-nonspecific cells, In both cases, a net in crease in antigen fixation was observed. At the intracellular level, c ovalent C3b binding to TT resulted in a large TT incorporation in endo somes of nonspecific cells, similar to that observed in antigen-specif ic cells. Thus, C3b covalently linked to antigen enlarges the array of B-cell types capable of presenting antigen, including non-specific ce lls.