Human immunodeficiency virus (HIV) enters target cells by forming a ternary complex between the viral envelope protein gp120 and two cellular receptor proteins: CD4 and a chemokine receptor [[1–6], reviewed in ]. HIV viral strains have been described which use a wide range of different chemokine receptors, although the majority use either CCR5 (R5 strains), CXCR4 (X4 strains) or both of these receptors. Consistent with a requirement for chemokine receptors as cofactors for viral entry, the chemokine ligands have been reported to reduce HIV infectivity in vitro [8–10]. Furthermore, mutations in the gene encoding CCR5, such as the CCR5-Δ 32 allele, provide some protection against HIV infection in vivo [11–13]. Consequently, agents which block HIV interaction with chemokine receptors are candidate antiviral therapies which can be used in conjunction with protease inhibitors and reverse transcriptase inhibitors to attenuate a third phase of the virus life-cycle: cell entry [7, 10, 14, 15], in the same way as the novel fusion inhibitor enfuvrtide 
Interestingly, the HIV gp120 protein which interacts with the chemokine co-receptor primarily through its V3 loop can induce leukocyte chemotaxis, demonstrating that some intracellular signals are generated through the the virus:receptor interaction [17, 18]. This signalling occurs even though the site of the gp120 interaction with the chemokine receptors appears to be only partially overlapping with the natural ligand binding site [14, 19–22].
It has been proposed that this chemotactic signalling might play a role during HIV infection in vivo, possibly by recruiting susceptible T-cells to sites of viral replication . In other retroviruses envelope/receptor interactions are known to be mitogenic  and this may facilitate nuclear translocation and integration of the provirus. In HIV, however, it is not known whether the ability to productively engage the chemokine receptors in this way plays any direct role in acute viral entry and subsequent productive infection of the target cell. Guntermann and colleagues showed that pertussis toxin (which blocks Gi-mediated signalling through chemokine receptors) block cellular infection with HIV in vitro . Montes et al. obtained similar results, and also showed that the MEK inhibitor U0126 could block both chemokine-receptor-induced ERK activity and HIV infection in vitro . However, neither pertussis toxin nor MEK inhibition are specific for chemokine signalling pathways: Gi and ERKs participate in other intracellular signalling pathways, so it is possible that HIV infection was inhibited because of blockade of downstream pathways not initiated through productive occupancy of the chemokine receptors.
Recently, we have described a new class of chemokine inhibitors, termed Broad Spectrum Chemokine Inhibitors (BSCIs) which block chemokine-induced chemotaxis in a range of leukocytes, irrespective of the chemokine used [26, 27]. These BSCIs are highly selective for chemokines, however, and have no effect on chemotaxis induced by a range of other chemoattractants such as TGF-β, fMLP or C5a. Importantly, the molecular target of the BSCIs is not the chemokine receptors themselves: BSCIs do not bind to chemokine receptors, do not affect chemokine receptor levels on the cell surface, and do not interefere with the binding of chemokine ligands to the receptors . Instead, they are thought to specifically inhibit intracellular signals required for chemokine-induced migration but not for migration induced by non-chemokine pathways , although their molecular target has not yet been published. As a result, members of the BSCI family have been shown to be potentially useful new anti-inflammatory agents in a wide range of diseases .
BSCIs provide an ideal tool to probe the importance of chemokine-induced intracellular signalling in HIV infection. Since the effects of these compounds are apparently selective for chemokine receptor-induced signals, if BSCIs interefere with cellular infection by HIV in vitro this will indicate that productive signalling by the chemokine co-receptor is likely to be important for successful infection. In the present study, we have investigated whether the first BSCI to be described, termed Peptide 3 , affects gp120 binding to chemokine receptors or cellular infection by HIV in vitro.