Everything about Hiv Tropism totally explained
HIV tropism refers to the
cell type that the
human immunodeficiency virus (HIV) infects and replicates in. HIV tropism of a patient's virus is measured by the
Trofile assay.
HIV can infect a variety of
cells such as
CD4+ helper T-cells and
macrophages that express the
CD4 molecule on their surface. HIV-1 entry to macrophages and T helper cells is mediated not only through interaction of the virion envelope glycoproteins (
gp120) with the CD4 molecule on the target cells but also with its
chemokine coreceptors.
Macrophage (M-tropic) strains of HIV-1, or non-syncitia-inducing strains (NSI) use the beta-chemokine receptor
CCR5 for entry and are thus able to replicate in macrophages and CD4+ T-cells . The normal
ligands for this receptor,
RANTES, macrophage inflammatory protein (MIP)-1-beta and MIP-1-alpha, are able to suppress HIV-1 infection
in vitro. This CCR5 coreceptor is used by almost all primary HIV-1 isolates regardless of viral genetic subtype.
T-tropic isolates, or
syncitia-inducing (SI) strains replicate in primary CD4+ T-cells as well as in macrophages and use the alpha-chemokine receptor,
CXCR4, for entry . The alpha-chemokine,
SDF-1, a ligand for
CXCR4, suppresses replication of T-tropic HIV-1 isolates. It does this by down regulating the expression of CXCR4 on the surface of these cells.
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 coreceptor alone doesn't explain viral tropism, as not all
R5 viruses are able to use CCR5 on macrophages for a productive infection .
HIV can also infect a subtype of
dendritic cells, MDC-1, which probably constitute a major reservoir that maintains infection when T helper cell numbers have declined to extremely low levels.
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