Different mixtures of R5, X4, and R5/X4 virus strains may be present in an HIV-infected patient. In these cases, the virus population

is described as being mixed tropic. Currently, Apoptosis inhibitor phenotypic tropism assays cannot differentiate between dual-tropic and mixed-tropic (collectively referred to as D/M) virus populations [1]. Throughout infection, R5 virus is most commonly detected. CXCR4-using variants are more likely to be detected in patients with advanced disease and low CD4 T-cell counts, as either R5/X4 or mixed populations of R5 and X4 strains [2-7]. The detection of exclusively X4 virus in clinical samples is rare. There is a strong relationship between the CD4 T-cell count and the likelihood of the detection of CXCR4-using virus; levels range from around 10% in patients Wortmannin nmr with CD4 T-cell counts above 350 cells/μL to up to 50% at CD4 T-cell

counts less than 200 cells/μL. A higher prevalence (40–50%) of CXCR4-using viruses is also seen in treatment-experienced patients, but this is reflective of low nadir CD4 T-cell counts more than of treatment per se, and is almost entirely attributable to an increase in R5/X4 and mixed populations. The emergence of CXCR4-using virus is associated with disease progression, but whether the emergence is a cause or a consequence of HIV disease progression has been the subject of debate. The prevailing opinion is that CXCR4-using strains emerge as a result of immunological deterioration, CD4 T-cell depletion and disease progression. The HIV-1 subtype is a further factor influencing preferential Resminostat HIV-1 co-receptor use [8, 9]. Virological failure of a CCR5 antagonist is often but not universally associated with a tropism shift, that is, emergence of pre-existing CXCR4-using virus (up to 63% in clinical trials) [10]. In about one-third of patients who retain R5 virus at failure, the R5 virus shows phenotypic resistance to the antagonist [11-14]. Clinical trials of CCR5 antagonists have confirmed the specificity of the antiviral

effect for R5 virus [15-22]. As these agents only inhibit the replication of R5 variants, a tropism test is essential prior to CCR5 antagonist use in order to exclude patients harbouring X4 or R5/X4 variants in whom no significant virological response to treatment is anticipated (reviewed in [23]). HIV-1 tropism may be determined phenotypically, by assessing the ability of a recombinant virus containing patient-derived envelope sequences to infect CCR5 or CXCR4 reporter cell lines that also express CD4. It may also be inferred genotypically from the sequence of the gp120 V3-loop. Both methods have advantages and drawbacks [23]. Among phenotypic methods, the original Trofile assay (Monogram Biosciences, San Francisco, CA) was used to screen patients for inclusion in clinical trials of CCR5 antagonists [1, 15-21].