Diversity and function of maternal HIV-1-specific antibodies at the time of vertical transmission

Infants of HIV positive mothers can acquire HIV infection by various routes, but even in the absence of antiviral treatment, the majority of these infants do not become infected. There is evidence that maternal antibodies may provide some protection from infection, but gestational maternal antibodies have not yet been characterized in detail. One of the most studied vertically-infected infants is BG505, as the virus from this infant yielded an Envelope protein that was successfully developed as a stable trimer. Here, we isolated and characterized 39 HIV-specific neutralizing monoclonal antibodies (nAbs) from MG505, the mother of BG505, at a time point just prior to vertical transmission. These nAbs belonged to 21 clonal families, employed a variety of VH genes, many were specific for the HIV-1 Env V3 loop, and this V3 specificity correlated with measurable antibody-dependent cellular cytotoxicity (ADCC) activity. The isolated nAbs did not recapitulate the full breadth of heterologous nor autologous virus neutralization by contemporaneous plasma. Notably, we found that the V3-targeting nAb families neutralized one particular maternal Env variant even though all tested variants had low V3 sequence diversity and were measurably bound by these nAbs. None of the nAbs neutralized the BG505 transmitted virus. Furthermore, the MG505 nAb families were found at relatively low frequencies within the maternal B cell repertoire: all less than 0.25% of total IgG sequences. Our findings demonstrate the diversity of HIV-1 nAbs that exist within a single mother, resulting in a collection of antibody specificities that can shape the transmission bottleneck.