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HIV tropism
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<p>HIV tropism refers to the <a href="/facts/Cell_type/1boyDWXb">cell type</a> in which the human immunodeficiency virus (<a href="/facts/HIV/ulY2rgzu">HIV</a>) infects and replicates. HIV <a href="/facts/Tissue_tropism/3CQs377B">tropism</a> of a patient's virus is measured by the <a href="/facts/Trofile_assay/NwcrdgF2">Trofile assay</a>. </p><p>HIV can infect a variety of <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> such as <a href="/facts/T_helper_cell/nhwAYkDj">CD4+ helper T-cells</a> and <a href="/facts/Macrophage/z7hkQu3o">macrophages</a> that express the <a href="/facts/CD4/l3krxz97">CD4</a> molecule on their surface. HIV-1 entry to macrophages and T helper cells is mediated not only through interaction of the virion envelope glycoproteins (<a href="/facts/Gp120/WMsnFax4">gp120</a>) with the CD4 molecule on the target cells but also with its <a href="/facts/Chemokine/YrySj9eD">chemokine</a> coreceptors. </p><p>Macrophage (M-tropic) strains of HIV-1, or non-syncitia-inducing strains (NSI) use the beta-chemokine receptor <a href="/facts/CCR5/Ya83LIg8">CCR5</a> for entry and are thus able to replicate in macrophages and CD4+ T-cells. These strains are now called R5 viruses. The normal <a href="/facts/Ligand/6etB3qeW">ligands</a> for this receptor—<a href="/facts/RANTES/bfcYCrNH">RANTES</a>, <a href="/facts/Macrophage_inflammatory_protein/RPAp5XY2">macrophage inflammatory protein</a> (MIP)-1β and MIP-1α—are able to suppress HIV-1 infection <i>in vitro</i>. This CCR5 coreceptor is used by almost all primary HIV-1 isolates regardless of viral genetic subtype. </p><p>T-tropic isolates, or <a href="/facts/Syncitia/cplhv2rx">syncitia</a>-inducing (SI) strains replicate in primary CD4+ T-cells as well as in macrophages and use the alpha-chemokine receptor, <a href="/facts/CXCR4/4v3deXHf">CXCR4</a>, for entry. These strains are now called X4 viruses. The alpha-chemokine <a href="/facts/Stromal_cell-derived_factor-1/K1IrAmDP">SDF-1</a>, a ligand for CXCR4, suppresses replication of T-tropic HIV-1 isolates. It does this by downregulating the expression of CXCR4 on the surface of these cells. </p><p>Viruses that use only the CCR5 receptor are termed R5, those that only use CXCR4 are termed X4, and those that use both, X4R5. However, the use of a coreceptor alone does not explain viral tropism, as not all R5 viruses are able to use CCR5 on macrophages for a productive infection. </p>