Enhancement of retroviral infectionin vitroby anti-LeyIgG: reversal by humanization of monoclonal mouse antibody

Monoclonal mouse IgG3 antibody (ABL 364) against the carbohydrate Le(y) antigen enhanced infection in vitro with HTLV-1 and with HIV-1 when propagated in both transformed and normal lymphocytes. Enhancement was independent of complement, occurred with both lymphocytes and monocytes as target cells, and did not use either L(ey) epitopes on target cells for cross-linkage of virus to the cell or the Fc part of the antibody as a ligand for any cellular receptor. For enhancement to occur, binding of anti-Le(y) antibody to virus was required to take place before virus binding to its specific receptor with no indication of any alternative pathway of infection, as evidenced by abrogation of enhancement by anti-CD4 MAb or soluble recombinant CD4, and also the inability of anti-Le(y) MAb to mediate HIV infection of HSB-2 cells in which HTLV-1/HIV pseudovirus infection was enhanced. While F(ab)2 fragments of ABL 364 also enhanced infection, a human/mouse chimeric antibody and a fully humanized antibody had no enhancing effect on free virus infection. We suggest that binding of anti-Le(y) ABL 364 or its F(ab)2 fragment induced a conformational change in the gp120 oligomers facilitating the process of infection, and that this function was abrogated by the IgG1 Fc of the chimeric and the humanized antibodies. The observations indicate that the non-paratope domains of antiviral antibodies can influence their function as neutralizing or enhancing for infection.