Influence of heavy chain constant regions on antigen binding and HIV-1 neutralization by a human monoclonal antibody.

F105, a neutralizing IgG1 kappa human mAb, is reactive with a discontinuous epitope within the gp120 CD4 binding site. Because isotype usage may affect Ab function, we examined the effect of isotype on Ag/Ab interactions and HIV-1 neutralization. An IgG3 kappa Ab was prepared by linking the variable regions of F105 to cloned human kappa and gamma 3 constant regions. Immunoreactivity of F105 IgG1 and IgG3 with IIIB-, MN-, and RF-infected cells was equivalent. Inhibition of binding and fusion of IIIB to uninfected cells and neutralization of IIIB virus was comparable for F105 IgG1 and IgG3, with 14 to 23 micrograms/ml required for 90% neutralization. In contrast, F105 IgG3 was marginally more effective at inhibition of MN binding/fusion and significantly more effective at neutralization of MN virus (62 micrograms/ml for IgG3 and > 100 micrograms/ml for IgG1 to achieve 90% neutralization). Despite high affinity binding to RF-infected cells, F105 IgG1 minimally neutralizes free RF virus. F105 IgG3 is dramatically more effective against the RF isolate, with 2 to 20 micrograms/ml of Ab required for 50% neutralization. Both isotypes were relatively ineffective at inhibition of RF binding/fusion. Thus, whereas affinity with native Ags on the surface of HIV-1-infected cells was unaffected by heavy chain constant regions, Ab isotype can strongly influence virion neutralization. Structural changes in gp120, as a result of increased flexibility conferred by the elongated IgG3 hinge region, are suggested as a possible mechanism to increase neutralization of selected HIV-1 isolates. These results may have significant implications in the design of immunotherapeutic and vaccine agents.