Defining the role of maternal neutralizing antibodies in mother-to-child HIV transmission

From the Overbaugh laboratory, Human Biology Division

The development of a vaccine against HIV has been a global health priority since the mid-1980s, but, to this day, a successful vaccine remains elusive. An effective HIV vaccine would have to stimulate the immune system to produce antibodies that can bind to the virus and prevent infection of target cells –an event known as neutralization. HIV-specific neutralizing antibodies (nAbs) bind to the HIV envelope (Env), a protein on the surface of the viral particle that binds CD4 receptors on T-cells and mediates entry into the cell. Such nAbs are likely to protect against mother-to-child transmission (MTCT) of HIV. In the absence of antiviral therapy for the mother or infant, HIV-infected mothers can transmit HIV to their babies during pregnancy, delivery, or through breastfeeding; however, only 30-40% of infants in this setting become infected. Therefore, characteristics of maternal HIV-specific antibodies that protect infants against infection could inform HIV vaccine design. The Overbaugh lab (Human Biology) sought to better understand how the mother’s antibody response affects the viruses that are transmitted to infants using samples obtained 20 years ago in a longitudinal HIV study from Kenya, before effective antiretroviral therapies were developed. The group recently published their findings in the Journal of Virology.

Schematic representation of mother-to-child transmission of HIV.
Schematic representation of the role of the maternal antibody repertoire in mother-to-child transmission of HIV. Image provided by Dr. Laura Doepker

It is well-documented that infants who get infected in an MTCT setting only acquire a few of the maternal HIV variants. The authors hypothesized that selection for transmission of a particular maternal HIV variant, and not others, could be partially mediated by maternal nAbs. To test this hypothesis, the investigators selected a mother-child sample pair to characterize the repertoire of maternal nAbs. The mother had been HIV-positive at the time of enrollment in the study, and a blood sample was collected at 31 weeks of pregnancy. The baby was born prematurely at 32-weeks of gestation, but the virus was not detected in the newborn until six weeks of age.  The investigators first isolated memory B-cells –immune cells that produce antibodies– from the sample collected from the mother at 31-weeks of pregnancy. Using a high-throughput functional screen, the investigators cultured and screened individual B-cells for the production of antibodies with the capacity to neutralize an HIV-1 variant. The authors selected an easy-to-neutralize “Tier 1A” HIV-1 variant in an attempt to cast a wide net  to study the highest number of maternal antibodies from this subject. Through this methodology, the group was able to isolate and confirm the neutralizing activity of 39 monoclonal antibodies, which originated from 21 clonal B-cell families.

The investigators then tested the neutralization capacity of both the maternal plasma, which contains a mixture of all antibodies, and individual representatives of the 21 nAb families against several “Tier 1” and “Tier 2” HIV-1 viruses. They found that the plasma more potently neutralized the HIV variants compared to the individual nAbs, suggesting that neutralization activity might be mediated by antibodies that were not isolated in the screen. To determine what regions on Env the plasma was targeting (i.e., epitopes), the investigators tested the neutralization activity of the plasma against a library of Env mutant viruses; however, this screen did not reveal which specific regions are targeted in Env. Nonetheless, removal of particular glycans that normally coat the Env protein to block recognition by the immune system made the viruses more sensitive to neutralization, suggesting the exposure of a critical epitope in Env targeted by nAbs.

Besides protecting against infection by neutralizing HIV, antibodies can “flag” infected cells for elimination by immune cells in a process known as antibody-dependent cell cytotoxicity (ADCC). Previous MTCT studies have reported that ADCC activity correlates with reduced infant mortality in HIV-infected infants. This observation led the group to assess the ADCC functionality of the maternal nAbs. They found that several nAbs mediated ADCC against a “Tier 2” HIV variant. Then, using ELISA and a phage immunoprecipitation method (PhIP), they identified the V3 region in Env as the epitope for these antibodies, suggesting that V3-specific nAbs might have a role in ADCC in MTCT. Dr. Laura Doepker, a postdoctoral fellow in the Overbaugh lab and first author in the study, concluded: “We now know that maternal antibody repertoires can contain diverse V3-specific antibodies that facilitate ADCC. Next, we want to understand how these maternal antibodies impact mother-to-child transmission and the nature of the transmitted virus.”

Finally, the investigators sought to characterize the neutralizing activity of the nAb families against seven HIV variants isolated from the mother just a week before delivery. The maternal plasma neutralized four of the maternal HIV variants, and only one variant was neutralized by individual nAbs, all of which were V3-specific. None of the nAbs neutralized the HIV variant that seeded the infection in the baby, showing that variants transmitted via MTCT are resistant to neutralization at least by the isolated maternal nAbs. Notably, the maternal HIV variant neutralized by the V3-specific nAbs shares its V3 sequence with most maternal and infant HIV variants tested, suggesting that –despite the lack of neutralization– the V3-specific nAbs could bind to these viruses. This led authors to test the binding capacity of V3-specific nAbs against maternal and infant HIV variants using a cell-surface binding assay. Interestingly, they found that all V3-specific nAbs bind to all maternal and infant HIV Envs tested, supporting previous studies that demonstrated that binding is not always sufficient for neutralization. These results provide important insights into how the functionality and specificity of maternal antibody repertoires determine which variants seed infection during MTCT.

This work was supported by the National Institutes of Health and a Faculty Scholar Grant from the Howard Hughes Medical Institute and the Simons Foundation.

UW/Fred Hutch Cancer Consortium member Dr. Julie Overbaugh contributed to this work.

Doepker LE, Simonich CA, Ralph D, et al. Diversity and function of maternal HIV-1-specific antibodies at the time of vertical transmission [published online ahead of print, 2020 Feb 19]. J Virol. 2020;JVI.01594-19. doi:10.1128/JVI.01594-19