PROJECT SUMMARY One major objective facing the HIV research community is to facilitate drug-free viral remission in People Living with HIV (PLWHIV). The host immune responses that can achieve this post-treatment control (PTC) are poorly understood because PTC is rarely observed in PLWHIV and the commonly studied SIV-infected rhesus macaques. CD8+ T cells are essential for control of HIV replication in the presence and absence of ART, but no studies have identified that CD8+ T cells can promote ART- free viral remission or delay the time to viral rebound after ART interruption. We recently identified a Mauritian cynomolgus macaque (MCM) model for sustained control of SIV replication after stopping ART. MCMs who shared a non-protective MHC haplotype called M3 began receiving ART 14 days after SIV infection and stopped receiving ART eight months later. They suppressed SIV replication for more than five months after ART interruption. In vivo depletion of CD8+ cells led to prompt viral rebound, indicating a vital role for CD8+ cells in post-treatment viral control (PTC). Our current proposal aims to unravel the function and type of CD8+ cells responsible for PTC in MCMs. This is the second competitive renewal of an ongoing R01 awarded to our lab. The first grant cycle explored the role of CD8+ T cells targeting invariant SIV epitopes in the control of live attenuated SIV. The second cycle focused on whether the IL-15 superagonist N-803 enhanced CD8+ T cell function to suppress SIV replication. During cycle 2, we serendipitously discovered that MCMs frequently become PTCs when ART is initiated two weeks post-infection, even without additional therapeutic interventions. We are now proposing to use tools developed during both previous grant cycles to test the hypothesis that MHC class I restricted CD8+ T cells are required for PTC through a mechanism that is independent of host MHC class I genetics. We will determine whether the M3 MHC haplotype is necessary for PTC in MCMs. We will use in vivo immune depletion studies and carefully engineered viruses with point mutations in viral peptides restricted by M3 MHC class I alleles to characterize the specific CD8+ population(s) required for PTC. Upon concluding this study, we will know whether PTC is universal among MCMs who begin receiving ART two weeks post-infection and if PTC depends on MHC class I restricted CD8+ T cells. Successful completion of this study will identify whether PTC in MCMs is independent of MHC genotype and if PTC relies upon virus-specific cytotoxic CD8+ T cells. This information could revolutionize our understanding of the specific types and functions of CD8+ cells that mediate PTC.