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 disconti nuous epitope within the gp120 CD4 binding site, Because isotype usage may affect Ab function, we examined the effect of isotype on Ag/Ab in teractions 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 f usion of IIIB to uninfected cells and neutralization of IIIB virus was comparable for F105 IgG1 and IgG3, with 14 to 23 mu g/ml required for 90% neutralization. In contrast, F105 IgG3 was marginally more effect ive at inhibition of MN binding/fusion and significantly more effectiv e at neutralization of MN virus (62 mu g/ml for IgG3 and >100 mu g/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 mu g/ml of Ab required for 50% neutralization. Both isotypes w ere relatively ineffective at inhibition of RF binding/fusion. Thus, w hereas affinity with native Ags on the surface of HIV-1-infected cells was unaffected by heavy chain constant regions, Ab isotype can strong ly 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 neutralizati on of selected HIV-1 isolates. These results may have significant impl ications in the design of immunotherapeutic and vaccine agents.