COVALENT BINDING OF C3B TO MONOCLONAL-ANTIBODIES SELECTIVELY UP-REGULATES HEAVY-CHAIN EPITOPE RECOGNITION BY T-CELLS

Protein C3 of the complement system is known for its role in the nonsp ecific immune response. Covalent binding of C3b to antigen upon comple ment activation also plays a significant role in specific T cell immun e response. C3b-antigen complexes can bind to complement receptors on the antigen-presenting cell, and the C3b antigen link (most often an e ster link) remains fairly stable inside the cells. In this study, IgG1 , kappa and IgG2a, kappa murine monoclonal antibodies (mAb) were used as antigens; covalent complexes between mAb and C3b were produced and purified in vitro from purified proteins; human B cell lines and T cel l clones were raised from tumor patients who received mAb injections f or cancer therapy or diagnosis. Recognition of epitopes of these mAb b y T cell clones when the mAb were processed alone or bound to C3b was compared. IgG or IgG-C3b complexes presented by B cell lines were able to stimulate proliferation of kappa light chain-specific T cell clone s at similar concentrations. In contrast, IgG-C3b complex recognition by heavy chain-specific T cell clones required 100-fold less IgG-C3b t han uncomplexed IgG. As C3b was shown to be covalently bound only to t he IgG heavy chains in the complexes, C3b chaperoning is restricted to only the IgG heavy chain and selectively influences intracellular ste ps of IgG heavy chain processing. This differential modulation of C3b suggests an early dissociation of IgG heavy and light chains in antige n-presenting cells.