Project Summary/Abstract Despite the remarkable success of the combined antiretroviral therapy (cART) to control HIV-1 infection, viral reservoirs persist indefinitely under treatment. These persisting viral populations constitute the principal burden for an effective HIV-1 cure, as they lead to a rapid rebound in viremia when treatment fails or after analytic treatment interruption (ATI). Although persisting viral populations in tissue comprise most of the reservoir, the majority of the reservoir studies do not include tissue samples due to the difficulties to obtain tissue samples containing active foci of replication, the site of the rebounding virus. Therefore, we are not able to properly study these viral populations and their main characteristics remain unknown. Understanding the nature and properties of rebound should bring us a step closer to identifying the reservoir associated with persistence and rapid rebound after ATI. Our previous studies defining the early foci of rebound after ATI revealed an unexpected result as all detected SIV infected cells has a myeloid morphology and stained negative for CD3 and other T cell markers. Those studies were focused on the early reservoir, established when cART is initiated 4 days post challenge. Under these conditions, there are no virus specific cell mediated or humoral immune responses. However, at his point is clear that rebounding virus populations are filtered by immune responses in play when cART was initiated. Based on critical consideration and better model infection in humans, this project will determine the impact of innate and adaptive immune pressure impacts the milieu of the emerging foci in tissues leveraging our PET/CT guided necropsy and resection surgery workflow allowing the study of viral rebound within tissue during the eclipse phase. We intend to use our ability to localize SIV active viremia sites in infected macaques, to perform a detailed identification of the cells and anatomical compartments that support rebound of persistent SIV. We will study the effect of the innate and adaptive immune system and disrupt myeloid cell populations to determine the impact on the kinetics, magnitude, and viral population dynamics during rebound. The proposed project is scientifically relevant in its innovative approach and methodologies as it will allow to determine the main properties and dynamics the eclipse phase of rebound of persistent viral population after ATI. Given the importance of persistent viral populations in rebound. A better understanding of rebound virus population dynamics after ATI is key to developing successful strategies to cure HIV-1.