Role of Antibodies in Mother-to-Child Transmission of HIV-1

Abstract

Designing an efficacious HIV vaccine remains an elusive goal. Most vaccines provide protection by eliciting protective antibodies. Vaccination studies have provided proof-of-concept that antibodies can protect against viral challenge. However, the efficacy of most promising HIV vaccine trial to date was modest. Therefore, identifying correlates of protection against natural HIV infections in humans may provide valuable information to help inform vaccine design. Mother-to-child transmission (MTCT) is a unique natural setting analogous to vaccination. Infants receive passively-acquired antibodies from their mothers in utero, and therefore have pre-existing HIV-specific antibodies circulating prior to HIV exposure through breastfeeding. In this thesis, we utilized this unique setting of breastfeeding mother-to-child transmission with the goal of identifying humoral correlates of reduced risk of MTCT, with a specific focus on characteristics of these pre-existing HIV-specific antibodies. Anti-HIV antibody activity is determined by the specificity of the antibody for its target region on HIV (its epitope) and by the ability of the antibody to mediate effector functions by activating innate effector cells. Antibody-dependent cellular cytotoxicity (ADCC) is a well-studied effector function targeting infected cells for destruction. ADCC has been correlated with protection in both non-human primate and human vaccination studies. In the setting of MTCT, we previously reported that pre-existing passively-acquired ADCC in infants is associated with improved infant outcome. We hypothesized that pre-existing passively-acquired antibodies that mediate ADCC and/or bind to HIV target specific epitopes conferring protection against MTCT of HIV-1. Throughout this thesis, we investigated the complimentary roles that antibody epitopes and effector function play in risk of MTCT and infant outcome. First, we investigated epitopes of antibodies capable of binding HIV, regardless of their potential to mediate effector function, from a cohort of breastfeeding mother-infant pairs with the goal of identifying epitopes targeted by antibodies associated with reduced risk of MTCT. While we did not observe any correlates of reduced risk of transmission, we did identify a correlate of increased risk. We observed that antibodies capable of binding to the ectodomain region of the HIV envelope transmembrane protein gp41 were correlated with increased MTCT. We hypothesize that the ectodomain of gp41 may be acting as an immune decoy to divert the immune response away from generating protective antibodies. Next, we investigated whether epitopes of ADCC-mediating antibodies affected risk of MTCT and infant outcome. We studied three epitopes commonly targeting by ADCC-mediating antibodies in natural infection and vaccination. Interestingly, ADCC targeting these epitopes was not correlated with reduced risk of MTCT or improved infant outcome. Surprisingly, our results indicate that ADCC targeting these epitopes is correlated with worse infant outcome. Our data suggest that ADCC targeting other epitopes or a polyclonal ADCC response warrant further investigation as potential correlates of protection. Finally, we explored the role and mechanism of ADCC in MTCT in further detail using two approaches. In the first approach, we screened samples from our previous ADCC study in a newly-developed assay designed to detect the potential for antibodies to mediate effector functions in general, not limited to ADCC. Our goals were to understand the mechanism of the association of passively-acquired ADCC with improved infant outcome in more detail as well as to broaden our investigations to include effector functions other than ADCC. In the second approach, we measured ADCC in samples from a separate MTCT cohort with the goal of replicating the results of our initial ADCC study. In both approaches, we observed similar trends to our initial results that ADCC, and potential effector functions in general, are associated with improved infant outcome, although these trends did not reach statistical significance. As a whole, the studies presented here provide information about the role that antibody epitopes and antibody effector functions play in the risk of MTCT and infant outcome. Future studies investigating the mechanisms of the associations described here would have the potential to inform vaccine and therapeutic design.