ANTIBODIES TO SEVERAL CONFORMATION-DEPENDENT EPITOPES OF GP120 GP41 INHIBIT CCR-5-DEPENDENT CELL-TO-CELL FUSION MEDIATED BY THE NATIVE ENVELOPE GLYCOPROTEIN OF A PRIMARY MACROPHAGE-TROPIC HIV-1 ISOLATE/

The beta-chemokine receptor CCR-5 is essential for the efficient entry of primary macrophage-tropic HIV-1 isolates into CD4(+) target cells. To study CCR-5-dependent cell-to-cell fusion, we have developed an as say system based on the infection of CD4(+) CCR-5(+) HeLa cells with a Semliki Forest virus recombinant expressing the gp120/gp41 envelope ( Env) from a primary clade B HIV-1 isolate (BX08), or from a laboratory T cell line-adapted strain (LAI), In this system, gp120/gp41 of the ' 'nonsyncytium-inducing,'' primary, macrophage-tropic HIV-1(BX08) isola te, was at least as fusogenic as that of the ''syncytium-inducing'' HI V-1(LAI) strain, BX08 Env-mediated fusion was inhibited by the beta-ch emokines RANTES (regulated upon activation, normal T cell expressed an d secreted) and macrophage inflammatory proteins 1 beta (MIP-1 beta) a nd by antibodies to CD4, whereas LAI Env-mediated fusion was insensiti ve to these beta-chemokines. In contrast soluble CD4 significantly red uced LAI, but not BX08 Env-mediated fusion, suggesting that the primar y isolate Env glycoprotein has a reduced affinity for CD4. The domains in gp120/gp41 involved in the interaction with the CD4 and CCR-5 mole cules were probed using monoclonal antibodies, For the antibodies test ed here, the greatest inhibition of fusion was observed with those dir ected to conformation-dependent, rather than linear epitopes. Efficien t inhibition of fusion was nut restricted to epitopes in any one domai n of gp120/gp41. The assay was sufficiently sensitive to distinguish b etween antibody-and beta-chemokine-mediated fusion inhibition using se rum samples from patient BX08, suggesting that the system may be usefu l for screening human sera for the presence of biologically significan t antibodies.