PROJECT SUMMARY/ABSTRACT Antiretroviral therapy-based pre-exposure prophylaxis (PrEP) is a highly effective biomedical HIV prevention strategy. However, many individuals at high risk of HIV acquisition are not receiving PrEP due to multiple issues, including potential drug toxicity, adherence and cost. As such, there is a need to develop additional safe and effective biomedical HIV prevention strategies. Major prevention strategies currently in development include the provision or induction of anti-viral neutralizing antibodies by passive immunization or vaccination, respectively. The utility of neutralizing antibodies for preventing HIV infection has been validated in nonhuman primates. Passively transferred antibodies protect nonhuman primates from oral, vaginal, rectal and intravenous challenges with simian human immunodeficiency virus (SHIV). Antibody conferred protection is primarily due to blockade of the viral entry process but can also involve the elimination of infected cells or virions via antibody Fc dependent functions. An often overlooked issue in preclinical nonhuman primate studies of neutralizing antibody conferred protection from SHIV challenge is that human HIV infections are a consequence of exposure to infectious bodily fluids containing both cell-free and cell-associated virus. The viruses that initiate new HIV infections are termed transmitted founder (TF) viruses. Preliminary data assessed in vitro neutralization of cell- free and cell-to-cell spread of a panel of SHIVs with envelopes from TF HIVs using the PGT121 anti-HIV neutralizing antibody. For several of these viruses, PGT121 fully neutralized cell-free spread but did not neutralize cell-to-cell spread. The proposed research will assess if distinct neutralization of cell-free and cell-to- cell viral spread impacts the ability of a neutralizing antibody to prevent infection following in vivo viral exposure. This work will use one of the TF SHIVs, SHIVBG505, to perform cell-free or cell-associated challenges in rhesus macaques infused with PGT121, an isotype control or a version of PGT121 with abrogated/diminished Fc dependent functions. The central hypotheses are: (I) PGT121 will prevent infection following cell-free SHIVBG505 challenge; (II) PGT121 will not prevent infection following cell-associated SHIVBG505 challenge; and (III) Fc dependent functions will contribute to the outcomes of cell-free or cell-associated SHIVBG505 challenges. These hypotheses will be evaluated across three specific aims: (I) assess if PGT121 protects rhesus macaques from challenge with cell-free SHIVBG505; (II) assess if PGT121 protects rhesus macaques from challenge with cell- associated SHIVBG505; and (III) determine the impact of Fc dependent functions on the outcome of cell-free or cell-associated SHIVBG505 challenges in PGT121 infused rhesus macaques. Generated data will contribute to the development of antibody-based HIV prevention tools, by providing a refined definition of the char...