CXCR4, (a CXC chemokine Receptor), also called fusin, is an alpha-chemokine receptor specific for stromal-derived-factor-1 (SDF-1 also called CXCL12), a molecule endowed with potent chemotactic activity for lymphocytes. This receptor is one of several chemokine receptors that HIV isolates can use to infect CD4+ T cells. Traditionally, HIV isolates that use CXCR4 are known as T-cell tropic isolates. Typically these viruses are found late in infection. It is unclear whether the emergence of CXCR4-using HIV is a consequence or a cause of immunodeficiency.

Besides, CXCR4 is upregulated during the implantation window in natural and HRT cycles in the endometrium, producing, in presence of a human blastocyst, a surface polarization of the CXCR4 receptors suggesting that this receptor is implicated in the adhesion phase of human implantation.

CXCR4's ligand SDF-1 is known to be important in hematopoietic stem cell homing to the bone marrow and in hematopoietic stem cell quiescence. Unusually for chemokines, SDF-1 and CXCR4 are a relatively "monogamous" ligand-receptor pair (other chemokines tend to use several different chemokine receptors in a fairly "promiscuous" manner). Because the interaction between SDF-1 and CXCR4 plays an important role in holding hematopoietic stem cells in the bone marrow, drugs that block the CXCR4 receptor appear to be capable of "mobilizing" hematopoietic stem cells into the bloodstream as peripheral blood stem cells. Peripheral blood stem cell mobilization is very important in hematopoietic stem cell transplantation (as a recent alternative to transplantation of surgically-harvested bone marrow) and is currently performed using drugs such as G-CSF. G-CSF is a growth factor for neutrophils (a common type of white blood cells), and may act by increasing the activity of neutrophil-derived proteases such as neutrophil elastase in the bone marrow leading to proteolytic degradation of SDF-1. AMD3100 is an investigational drug, not yet in routine clinical use, which directly blocks the CXCR4 receptor: it appears to be a very efficient inducer of hematopoietic stem cell mobilization in animal and human studies.

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