THE N-TERMINAL END OF THE C(H)2 DOMAIN OF CHIMERIC HUMAN IGG1 ANTI-HLA-DR IS NECESSARY FOR C1Q, FC-GAMMA-RI AND FC-GAMMA-RIII BINDING

We have found that amino acid residues necessary for Clq and Fc gamma R binding of human IgG1 are located in the N-terminal region of the C( H)2 domain, residues 231-238, using a matched set of engineered antibo dies based on the anti-HLA-DR antibody L243. Changing the leucine 235 in the C(H)2 region of IgG3 and IgG4 to glutamic acid was already know n to abolish Fc gamma RI binding. We have confirmed this for IgG1 and also found a concomitant abolition of human complement lysis with rete ntion of Fc gamma RIII-mediated function. Changing the glycine at 237 to alanine of IgG1 also abolished Fc gamma RI binding and reduced huma n complement lysis and Fc gamma RIII-mediated function. Exchanging the whole region 233-236 with the sequence found in human IgG2, abolished Fc gamma RI binding and human complement lysis and reduced Fc gamma R III-mediated function of IgG1. In contrast, a change in the previously described Clq-binding motif, from lysine at 320 to alanine, had no ef fect on IgG1-mediated complement lysis.