Switch from R5 to X4-tropic HIV occurs even in patients on HAART

Patients successfully treated with highly active antiretroviral therapy (HAART) often show a switch from CCR5 to CXCR4-using virus in cellular reservoirs, according to a study presented in the 1^st April edition of the Journal of Acquired Immune Deficiency Syndromes.

Although similar switches have previously been observed in patients not taking antiretroviral therapy, this is the first study to show a similar effect in patients whose HIV is controlled. This may go some way to explain the poor CD4 cell count gains observed in some patients with undetectable viral loads.

In addition to the CD4 receptor on the surface of white blood cells, HIV also binds to a co-receptor in order to fuse with and infect the target cell. Early in HIV infection, 'R5-tropic’ variants use the CCR5 co-receptor. However, a switch to use of the CXCR4 receptor often occurs later in HIV infection and is associated with more rapid disease progression and loss of CD4 T-cells. HIV that uses the CXCR4 co-receptor is termed ‘X4-tropic’.

Investigators from Toulouse, France, wished to ascertain whether a similar switch from ‘R5-tropic’ to ‘X4-tropic’ virus occurs in patients whose HIV levels are fully suppressed by a HAART regimen. They carried out a genetic analysis of the HIV found in 32 patients with undetectable viral loads for five years of therapy and used this to predict which co-receptor the virus would use. Genetic analyses were carried out on blood samples before the start of therapy and after 30 and 60 months of treatment.

“We have shown for the first time that an R5 to X4 switch in the majority virus population can occur despite effective HAART, suggesting that potent antiretroviral therapy produces the conditions necessary for the gradual emergence of X4 variants in cellular reservoirs,” conclude the investigators. “Impaired CD4⁺ T-cell restoration in these patients suggests that this emergence of X4 variants in patients on HAART is clinically relevant.”

All of the patients involved in this study were treated with AZT (zidovudine, Retrovir) or d4T (stavudine, Zerit) plus 3TC (lamivudine, Epivir) and either indinavir (Crixivan) or ritonavir (Norvir). At the start of the study, the median CD4 cell count was 40 cells/mm³ and viral load was 100,000 copies/ml.

After six months of HAART, all of the patients had a viral load below 200 copies/ml. However, after the first two years of the study, a highly sensitive viral load method became available, and all of the patients in the study were found to have viral loads below 20 copies/ml for the remaining three years.

Before the start of HAART, HIV extracted from the patients’ plasma was analysed for co-receptor usage by carrying out a genetic sequence analysis of the third variable region (V3) of the envelope gene env. This corresponds to the portion of the molecule on the surface of HIV that binds to the co-receptors.

As viral loads were undetectable at months 30 and 60 there was insufficient HIV in the blood to repeat this analysis. Consequently the researchers extracted proviral DNA from peripheral blood mononuclear cells (PBMCs) to assess co-receptor use. This ‘reservoir’ of archived HIV genetic material is found within the nucleus of resting white blood cells and is responsible for replenishing the viral population when antiretroviral therapy is stopped. The investigators checked that genetic analysis of genetic material from the PBMCs gave similar results to HIV in the blood by comparing findings from the analysis of PBMCs and HIV from the blood in seven patients, finding comparable results.

The study’s investigators were interested in finding out whether the predominant HIV variants in these reservoirs also switched to X4-tropic strains, fuelled by low level viral replication despite levels of HIV below the limit of detection.

At baseline, 23 of the patients had predominantly R5-tropic virus in the blood. Of these, eleven (48%) showed a progressive emergence of X4 variants in cellular reservoirs over the course of the study, with none showing a reversion from X4 to R5-tropic variants. The remaining twelve retained R5-tropic variants throughout the study.

In contrast, all of the nine patients who harboured X4 virus at baseline maintained the X4 variant over the five years, with none switching to R5-tropic variants.

When they compared the CD4 cell counts of the patients, the investigators found there were no differences in baseline CD4 cell count between the three groups at baseline. However, the patients who had X4-tropic virus, either throughout the study or following a switch from R5-tropic variants, had lower CD4 cell counts by the end of the five-year follow-up period (p = 0.02).

The researchers also carried out a detailed ‘clonal’ analysis of the variants found in the PMBCs of eight patients. In three of five patients who switched from R5 to X4 variants, small numbers of X4-tropic variants were seen at baseline, which expanded and became dominant over the five years of the study. In contrast, two of three patients who retained X4 variants across the study had small numbers of R5-tropic variants at baseline, but these did not become dominant over the course of therapy.

“The general assumption is that an accumulation of mutations in V3 is required to achieve the transition from R5 to X4 variants and that these mutations occur only during active virus replication in the absence of antiretroviral therapy,” write the investigators. “Nevertheless, we observed in the present study an R5 to X4 switch of the predominant virus population in the setting of effective HAART."

The investigators speculate that the emergence of X4 variants could have two causes. Firstly, X4-tropic variants could replicate preferentially at low levels during HAART, infecting PBMCs and forming cellular reservoirs. Alternatively, cells already containing X4-tropic variants could be more long-lasting than those containing R5-tropic variants, leading to their dominance after five years of suppressive anti-HIV therapy.