Comparative Fitness of Multi-Dideoxynucleoside-Resistant Human Immunodeficiency Virus Type 1 (HIV-1) in an In Vitro Competitive HIV-1 Replication Assay

We examined whether human immunodeficiency virus type 1 (HIV-1) fitness was altered upon the acquisition of a set or subset of five mutations (A62V, V75I, F77L, F116Y, and Q151M) in the pol gene, which confers resistance to multiple dideoxynucleosides (MDR), as well as the zidovudine resistance-associated mutation T215Y, using a competitive HIV-1 replication assay in a setting of an HXB2D genetic background. Target H9 cells were exposed to a 50:50 mixture of paired infectious molecular clones, and HIV-1 in the culture supernatant was transmitted to new cultures every 7 to 10 days. The polymerase-encoding region of the virus was sequenced at various time points, and the relative proportion of the two viral populations was determined. In the absence of drugs, the comparative order for replicative fitness was HIV-1(62/75/77/116/151) > HIV-1(77/116/151) > HIV-1(151) > wild-type HIV-1 (HIV-1(wt)) > HIV-1(75/77/116/151) > HIV-1(151/215) > HIV-1(215). In the presence of zidovudine or didanosine, the order was HIV-1(62/75/77/116/151) > HIV-1(77/116/151) > HIV-1(75/77/116/151) > HIV-1(151) > HIV-1(215). HIV-1(215S(TCC)), a putative intermediate infectious clone for HIV-1(215), replicated comparably to HIV-1(wt), while two putative intermediates for HIV-1(151) [HIV-1(151L(CTG)) and HIV-1(151K(AAG))] replicated much less efficiently than HIV-1(wt) and HIV-1(151), suggesting that for HIV-1(151) to develop, two base substitutions are likely to occur concurrently or within a short interval. These data may illustrate the molecular basis by which HIV-1(151) emerges much less frequently than HIV-1(215). The present data also demonstrate that several MDR HIV-1 variants are more fit than HIV-1(wt) in the absence of drugs and that resistance-associated mutations and drug pressure are critical variates for HIV-1 fitness.