Project Summary ART initiation (ARTi) is a unique clinical juncture in which virus replication and host immune responses are in flux and treatment of a substantial component of people living with HIV (PLWH) is possible. In both preclinical studies and recent clinical trials, infusion of broadly neutralizing antibodies (bnAbs) at ARTi has shown exciting preliminary results, but key questions about the mechanisms of action, the requisite bnAb properties, and the extent of clinical impact remain. Further, it is unclear if these benefits can be extended to the >35 million PLWH currently on suppressive ART, by using bnAbs after treatment interruption and ART re-initiation. Several clinical trials are planned or ongoing, but the complexity of human research limit the ability to definitively elucidate key mechanisms. Our scientific premise is that our molecularly defined, mixed bnAb-sensitive and resistant, barcoded transmitted/founder (TF) SHIV/NHP model system is uniquely poised to determine the extent and durability of bnAb activity at ARTi/re-initiation and decipher the mechanistic role of neutralization potency and effector function on reservoir dynamics, durable immune responses, and virus control. Our group has generated a body of work demonstrating that TF SHIVs reproduce key features of HIV-1 immunopathogenesis. We have expanded the model to incorporate genetic barcoding and virus inocula containing defined mixtures of bnAb-sensitive and resistant viruses. In this system, each animal is infected with a precise ratio of TF SHIVs encoding wildtype (WT; bnAb-sensitive) and escape mutant (EM; bnAb resistant) viruses, which have similar replication kinetics but markedly different sensitivities to V3-glycan bnAbs. Because WT and EM viruses have unique barcodes, we can track bnAb-sensitive vs. resistant virus clonotypes over time and across tissues through high-throughput sequencing, allowing for statistically powerful within-animal comparisons, as well as comparisons across treatment arms. Here, we will leverage this novel NHP system to determine the effects of bnAbs at ARTi and re-initiation and dissect the roles of bnAbs’ neutralizing and effector functions through a coordinated NHP experiment comparing 4 treatment groups: (i) ART alone, (ii) ART + bnAb, (iii) ART + bnAb with disabled effector function, and (iv) ART + bnAb with enhanced effector function. We will then determine if similar effects can be seen with use of bnAbs at ART re-initiation in animals already on suppressive ART who underwent ATI. This strategy allows us to define bnAb’s clinical impact and test our overall hypothesis that both the neutralizing potency and effector function of bnAbs at ARTi are essential to activity on the reservoir, host immunity, and induction of virus control. Successful completion of this project would have substantial significance to the HIV cure field, as it rigorously tests promising roles for bnAbs in HIV cure strategies that could guide the clinical d...