How SAMHD1 regulates HIV-1 innate immunity and viral gene expression

Abstract The lack of clear mechanisms by which host proteins regulate HIV-1 innate immunity in macrophages is a significant knowledge barrier to a functional cure for HIV-1 persistent infection. In this revised R01 proposal, we seek to investigate non-canonical functions of the cellular protein sterile alpha motif and HD domain-containing protein 1 (SAMHD1) in regulating innate immune responses during HIV-1 infection of primary macrophages. Since SAMHD1 was discovered as an HIV-1 restriction factor in 2011, extensive studies have revealed the mechanisms underlying SAMHD1-mediated restriction of HIV-1 replication in myeloid cells and resting CD4+ T cells. In 2018, we discovered a novel function of SAMHD1 in suppressing the innate immune response to viral infections and inflammation in macrophages. However, the underlying mechanisms of action remain unclear, representing a significant knowledge gap given the importance of macrophages in HIV-1 persistence. Our long-term goal is to define the functional roles and molecular mechanisms of cellular proteins, such as SAMHD1, in regulating innate immunity during persistent HIV-1 infection of macrophages. Our overall objective is to reveal how SAMHD1 inhibits the type I interferon (IFN-I) and NF-κB pathways in primary macrophages, acting as a multifaceted repressor of innate immune signaling induced by viral infection. IFN-I induction plays a key role in antiviral innate immunity; however, HIV-1 infection does not induce a strong IFN-I response. NF-κB is critical for HIV-1 gene transcription and immune activation, and transcriptional inhibition of viral gene expression is the main mechanism of HIV-1 latency. Thus, our central hypothesis is that SAMHD1 suppresses innate immunity to HIV-1 infection in primary macrophages by inhibiting IFN-I induction and NF-κB activation. Our three specific aims are: Aim 1. Define how SAMHD1 and IRF7 interaction inhibits IFN-I induction during HIV-1 infection; Aim 2. Examine SAMHD1 interaction with MAVS and effects on anti-HIV-1 innate immune responses; Aim 3. Investigate the mechanisms by which SAMHD1 inhibits NF-kB activation during HIV-1 infection. Our interdisciplinary studies will fill knowledge gaps to fundamentally enhance our mechanistic understanding of SAMHD1 and associated proteins in regulating HIV-1 infection, viral gene expression, immune activation, and anti-HIV-1 innate immune responses. Thus, our proposed studies of how SAMHD1 regulates immune activation and HIV-1 infection in macrophages will provide new knowledge studying viral persistence.