A cell-to-cell HIV transfer assay identifies humoral responses with broad neutralization activity.

Background: Cell-to-cell HIV spread through virological synapses proceeds in two steps, first HIV particles are rapidly transferred to target cells in a CD4-dependent manner and then coreceptor-dependent events allow for infection or death of single target cells and cell-to-cell fusion.

Methods: 293T or MOLT cells producing HIV particles were cocultured with primary CD4 T-cells or reporter cell lines. The extent of HIV transfer, cell fusion and target cell death was assessed. Inhibition by sera from 19 HIV-infected patients was evaluated and compared with cell-free HIV neutralization using different envelopes from clades A, B, C and E.

Results: Sera showed different abilities to protect CD4 T-cells from cell-to-cell transfer, fusion or death when cocultured with HIV producing 293T cells. Some sera were able to block all parameters (a property of IgGb12), while other showed lower activity against HIV transfer despite being able to block fusion and death (a property of antibodies blocking post-CD4 binding steps). Neutralization of cell-to-cell HIV transfer strongly correlated with IgG binding to native Env. Interestingly, sera that efficiently blocked HIV transfer showed broader neutralizing response, as they neutralized a higher percentage of the viruses tested compared with sera showing low CD4 binding site responses (P=0.01). Similar results were observed in a model of T cell-T cell HIV transmission, although this experimental model showed lower capacity to discriminate broadly neutralizing responses.

Conclusion: Sera showed different abilities to protect CD4 T-cells from cell-to-cell transfer, fusion or death when cocultured with HIV producing 293T cells. Some sera were able to block all parameters (a property of IgGb12), while other showed lower activity against HIV transfer despite being able to block fusion and death (a property of antibodies blocking post-CD4 binding steps). Neutralization of cell-to-cell HIV transfer strongly correlated with IgG binding to native Env. Interestingly, sera that efficiently blocked HIV transfer showed broader neutralizing response, as they neutralized a higher percentage of the viruses tested compared with sera showing low CD4 binding site responses (P=0.01). Similar results were observed in a model of T cell-T cell HIV transmission, although this experimental model showed lower capacity to discriminate broadly neutralizing responses.