Neutralization synergy of human immunodeficiency virus type 1 primary isolates by cocktails of broadly neutralizing antibodies

Several reports have described the existence of synergy between neutralizin g monoclonal antibodies (MAbs) against human immunodeficiency virus type 1 (HIV-1). Synergy between human MAbs b12, 2G12, 2F5, and 4E10 in neutralizat ion of primary isolates is of particular interest. Neutralization synergy o f these MAbs, however, has not been studied extensively, and the mechanism of synergy remains unclear. We investigated neutralization synergy among th is human antibody set by using the classical approach of titrating antibodi es mixed at a fixed ratio as well as by an alternative, variable ratio appr oach in which the neutralization curve of one MAb is assessed in the presen ce and absence of a fixed, weakly neutralizing concentration of a second an tibody. The advantage of this second approach is that it does not require m athematical analysis to establish synergy. No neutralization enhancement of any of the MAb combinations tested was detected for the T-cell-line-adapte d molecular HIV-1 clone HxB2 using both assay formats. Studies of primary i solates (89.6, SF162, and JR-CSF) showed neutralization synergy which was r elatively weak, with a maximum of two- to fourfold enhancement between anti body pairs, thereby increasing neutralization titers about 10-fold in tripl e and quadruple antibody combinations. Analysis of b12 and 2G12 binding to oligomeric envelope glycoprotein by using flow cytometry failed to demonstr ate cooperativity in binding between these two antibodies. The mechanism by which these antibodies synergize is, therefore, not yet understood. The re sults lend some support to the notion that an HIV-1 vaccine that elicits mo derate neutralizing antibodies to multiple epitopes may be more effective t han hereto supposed, although considerable caution in extrapolating to a va ccine situation is required.