ABSTRACT Despite the significant public health impact of highly active antiretroviral therapy and pre-exposure prophylaxis on the HIV-1 pandemic, fundamental questions remain about the biological mechanisms that define the risk of sexually acquiring HIV-1. Notably, there is no explanation why, after accounting for genetic variants that directly modify HIV-1 entry and replication (foremost CCR5Δ-32), individuals with documented high exposure to HIV- 1 exhibit diverse outcomes ranging from rapid infection to prolonged natural resistance to HIV-1 infection. A better understanding of these phenotypes could lead to novel interventions to reduce HIV-1 acquisition. We previously identified functional variants (missense/nonsense, untranslated region, or splice site) in host genes that are strongly associated with altered risk of heterosexually acquired HIV-1 infection. Notably, genetic variants in CD101 had the strongest association with increased HIV-1 risk, while variants in AXL had substantially reduced risk. Both genes are thought to regulate host inflammatory responses. CD101 regulates T cell activation and is expressed on many immune cell types. We recently reported that circulating T cells and monocytes from donors with these CD101 variants, compared to donors without functional CD101 variants, show higher overall cellular activation, but with reduced expression of certain HIV-protective interferon stimulated genes. In contrast, AXL is a receptor tyrosine kinase thought to act through myeloid cells to inhibit cellular immune activation, possibly through inhibition of toll-like receptor-mediated innate immune responses. Our overarching hypothesis is that host variation in CD101 and AXL genotypes contributes to altered inflammatory homeostasis, thereby influencing the risk of HIV-1 infection. Specifically, we hypothesize that CD101 variants confer a higher risk of HIV-1 infection by increasing the abundance and activation of HIV-1 susceptible T cells, while also compromising their innate antiviral defenses. Conversely, the absence of these variants, possibly augmented by reduced inflammation conferred by AXL variants, lowers HIV-1 infection risk. We propose to evaluate our hypothesis in three aims: in Aim 1, we use genomic data from a cohort with well- characterized HIV-1 acquisition phenotypes to replicate in a second high HIV-risk African cohort that AXL variants balance the impact of CD101 variants on HIV-1 susceptibility. In Aim 2, we test the epidemiological and population generalizability of our hypothesis by quantifying the association of CD101 and AXL variation with susceptibility to HIV-1 infection in two US populations for which genomic data already exist: one that includes predominantly white men who have sex with men; and a second, ethnically diverse cohort recruited across the entire US. In Aim 3, we apply molecular techniques to blood and genital mucosal samples archived in a unique biospecimen repository to directly link the presence ...