The ability to enter monocyte-derived macrophage (MDM) in vitro is commonly used to define macrophage-tropic HIV-1 despite the fact that viruses vary continuously in their ability to enter MDMs in vitro, and MDMs vary in their ability to support HIV-1 entry (Joseph et al., 2014; Peters et al., 2006). This makes it difficult to distinguish viruses that are adapted to replicating in macrophage from those that are adapted to replicating in T cells. We use the Affinofile cell line ( Johnston et al., 2009) to assay for macrophage tropism by capitalizing on the fact that macrophage-tropic HIV-1 has an enhanced ability to enter cells expressing low levels of CD4 (Joseph et al., 2014; Peters et al., 2006; Duenas-Decamp et al., 2009; Dunfee et al., 2006; Gorry et al., 2002; Martin-Garcia et al., 2006; Peters et al., 2004) and Affinofile cells can be induced to express a wide range of CD4 densities (Johnston et al., 2009). We induce Affinofile cells to express either high or low CD4, infect those cells with pseudotyped reporter virus, and quantify percent infectivity at low CD4 relative to infectivity at high CD4. Macrophage-tropic viruses have an enhanced ability to infect at low CD4. Using this approach we have found that macrophage-tropic strains of HIV-1 are relatively rare and that most HIV-1 variants require high levels of CD4 to enter cells, a phenotype we have referred to as R5 T cell-tropic.
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