PROJECT SUMMARY Human Immunodeficiency Virus type 1 (HIV-1) infects a number of different immune cell types with diverse replication strategies and clinical outcomes. Amongst these, we have a good understanding of infection in T- Cells, depletion of which results in Acquired Immune Deficiency Syndrome (AIDS). Moreover, AIDS is now well controlled using combination anti-retroviral therapy (cART), meaning the duration and quality of life has greatly increased for many people living with HIV-1. However, an estimated 50% of HIV-1 patients in the US still develop HIV-1-asssociated dementia (HAND) and other neurocognitive disorders. This is because beyond T- Cells, HIV-1 also enters the central nervous system (CNS) and establishes a reservoir of infection in brain- resident macrophages and microglia that is more difficult to treat. In addition, virus replication strategies in macrophage and microglia differ from those in T-Cells, particularly at late stages, and remain less well understood. What is clear is that HIV-1 replication in these cells results in secretion of a complex mixture of host cytokines and other toxic products, as well as viral proteins, each contributing to a neurotoxic environment that remains intensely studied yet challenging to treat. Understanding how HIV-1 replicates in brain-resident immune cells and the host factors involved is therefore of high significance and has the potential to guide future drug design. Our prior work and new preliminary data reveal that Amyloid Precursor Protein (APP) is highly expressed in macrophages and microglia and plays a pivotal role in both regulating HIV-1 infection and production of neurotoxic compounds by infected cells. APP is a transmembrane protein that is processed through a non-amyloidogenic (non-toxic) pathway at the plasma membrane or an amyloidogenic pathway within endolysosomes and multivesicular bodies (MVBs), which produces amyloid proteins that aggregate and form plaques associated with neurodegenerative conditions such as Alzheimer’s Disease as well as HAND. Our data shows that amyloidogenic APP processing and associated formation of a cleavage product termed C99 functions to restrict HIV-1 replication in macrophages and microglia as well as a number of cell line models. Specifically, C99 competes with the HIV-1 Gag polyprotein for access to and control of MVB sorting, suppressing the production and release of new infectious virions. Understanding how APP and C99 function in more detail will therefore provide important new insights into macrophage/microglia-specific HIV-1 replication strategies and restrictions. To evade APP activity, Gag competes to direct MVBs to exocytic pathways that enable virus release but also enhance APP processing and secretion of amyloids. As such, the battle between APP and Gag determines the overall level of virus replication and amyloid production in infected cells, which we will explore in Aim 1. Our data also shows that novel compounds termed γ-secr...