C3B COVALENTLY ASSOCIATED TO TETANUS TOXIN MODULATES TT PROCESSING AND PRESENTATION BY U937 CELLS

Complement protein C3, like C4 and alpha(2)-macroglobulin (alpha(2) M) , is a potentially bivalent ligand: (1) its proteolytic fragment, C3b, is able to interact covalently with antigens, and (2) this bound frag ment is able to interact non-covalently with specific complement recep tors of antigen presenting cells (APC). The formation of antigen-C3b c omplexes frequently occurs in vivo at inflammatory sites during the ea rly stages of an immune response. Tetanus toxin (TT)-C3b covalent comp lexes, prepared from purified proteins, were used to study how C3b ass ociation influences the handling of Tr by U937 cells used as APC. TT-s pecific T cell proliferation following TT-C3b processing was observed at a concentration when TT alone was inefficient. Whereas TT pinocytic uptake was low, TT-C3b uptake, through the help of complement recepto r CRI, was three times higher. Free TT was rapidly transported to the lysosomes where it was proteolysed, whereas TT-C3b complexes were firs t retained in the endosomes and underwent only limited proteolysis. Wh ile the ester link of the complexes was fairly stable in the endosomes , it was gradually hydrolysed in the lysosomes with ensuing efficient proteolysis of the two proteins. This reflects the fact that associate d C3b escorts TT during intracellular trafficking in the APC, and infl uences antigen processing. A triple role of C3b escorting antigen resi des at the level of antigen uptake, routing, and proteolysis inside U9 37 cells, thus modulating antigen-dependent T cell proliferation.