Quantitation of HIV-1 Entry Cofactor Expression

Infection of CD4+ T-lymphocytes by HIV-1 is initiated by binding of the virus to the CD4 receptor on the lymphocyte surface, followed by fusion of the virus with the cell membrane (1 ). However, expression of CD4 on certain non-human cells does not render them permissive for HIV-1 entry, suggesting that an additional entry cofactor is required (2 ,3 ). The identity of this cofactor remained elusive until Berger and colleagues reported that the α-chemokine receptor fusin/CXCR4 functions as the entry cofactor for T-cell line-tropic isolates of HIV-1 (4 ). Subsequent progress in the field has been rapid. To date, three β-chemokine receptors (CCR2b, CCR3, and CCR5) have also been identified as HIV-1 entry cofactors (5 ).