ABSTRACT: Project 2 Despite much progress in treatment and care, mother-to-child-transmission (MTCT) of HIV-1 remains an important problem; yearly, over 250,000 new pediatric HIV-1 infections occur. Thus, additional measures are needed to stop the global pediatric HIV-1 epidemic. In this proposal, we will test a synergistic passive-active immunization strategy by passively protecting the newborn through maternal immunization that results in passive antibody transfer in utero and in breast milk, and by actively immunizing the infant to protect against HIV-1 acquisition once maternal antibodies wane. Project 2's goal in this effort is to develop a pediatric HIV-1 vaccine that induces HIV Env antibody responses that can protect against oral SHIV acquisition. Based on our own studies demonstrating immunogenicity and partial efficacy of MVA-SIV vaccines in neonatal macaques, and on the partial efficacy of the human RV144 human HIV-1 vaccine trial, we hypothesize that an MVA-based SHIV vaccine that will be complemented with DNA-SHIV prime and HIV Env booster immunizations will enhance protective pediatric HIV Env antibody responses. In Aim 1, we will optimize a pediatric HIV-1 vaccine regimen by testing different prime/boost modalities and intervals, and systemic and mucosal routes of immunization. The pediatric vaccine with the highest frequencies and strongest functional HIV Env systemic and mucosal antibody responses will be evaluated for efficacy against a repeated low-dose oral SHIV- 1157ipd3N4 challenge regimen to reflect the continuous exposure of human infants to HIV-1 in breast milk (Aim 2). Importantly, our challenge virus contains a clade C HIV Env from an HIV-1 infected Zambian infant ensuring that our pediatric HIV-1 vaccine will have high translational potential for future use in human infants. Pediatric vaccine efficacy will be explored in the context of maternal vaccination and SHIV-infection. With support from Core 2, we will perform a very thorough analysis of pediatric anti-HIV Env antibody responses that will define (i) the importance of neutralizing versus binding antibodies in vaccine-mediated protection, (ii) the role of follicular T helper cell development in the antibody response to vaccination, (iii) whether infant NK cell function can support ADCC, (iv) whether pediatric anti-HIV Env antibody responses can protect against oral SHIV acquisition, and (v) whether mucosal anti-HIV Env antibodies contribute to vaccine-mediated protection. We will also determine whether vaccine interference by maternal vaccine- or SHIV infection- induced antibodies alters pediatric vaccine responses and efficacy (Aim 3). We expect that passively transferred maternal antibodies will protect against early SHIV exposure when the infant's immune system is still inexperienced, and active immunization of the infant with an optimized pediatric vaccine prime-boost regimen will result in the induction of immune responses that can protect against later oral SHIV challenges. Thus, if successful, our innovative combined maternal-infant HIV-1 vaccine strategy could provide protection throughout the breastfeeding period and end the global pediatric HIV-1 epidemic.