Replicative capacity results complicated by minority wild-type virus

The unique co-transfection step inherent to single-cycle HIV resistance assays, can result in even relatively small amounts of wild-type virus (WT) within a viral population significantly affecting the apparent replication capacity (RC) and phenotype of mutant strains, conclude researchers from Abbott Laboratories in Illinois, USA.

The replication capacity and susceptibility of plasma isolates from patients who are off therapy or not adherent to treatment, in which wild-type virus may expand to significant levels, should be interpreted with caution, they say in an abstract presented to the Mexico resistance meeting.

In a typical single-cycle HIV phenotypic assay - the most common test for RC and phenotypic resistance - the ‘pool’ of DNA generated from patient plasma is transfected into target cells in order to capture and preserve the protease and reverse transcriptase sequence heterogeneity of the plasma virus. However, co-transfection of different viral variants into the same cell might provide the opportunity for genetic recombination or complementation, or both.

Hongmei Mo and colleagues found that four mutant constructs with different genotypes derived from dual protease inhibitor-experienced subjects receiving lopinavir/ritonavir (LPV/r) therapy displayed <5% RC when transfected alone. Co-transfection of as little as 9% of the WT clone increased the RC of the mutant clones to up to 14%. Co-transfection of a higher proportion of the WT clone further enhanced the RC of the mutants to 31–81%.

The LPV susceptibility of four mutant clones with sufficient RC for phenotypic evaluation when transfected alone ranged from 44- to 302-fold, compared to the WT clone. Incremental cotransfection of 9–50% of the WT clone decreased the LPV IC50 of the mutant clones by up to 96%, compared to WT.

Ref: Mo H, Lu L, Kempf D et al. The impact of minor populations of wild-type HIV on the replication capacity and phenotype of mutant variants in a single-cycle HIV resistance assay. Abstract 85.

	HIV Treatment Bulletin Vol 4 No7 August / September 2003