Role of the Human Immunodeficiency Virus Type 1 Envelope Gene in Viral Fitness

Abstract

A human host offers a variety of microenvironments to the infecting human immunodeficiency virus type 1 (HIV-1), resulting in various selective pressures, most of them directed against the envelope (env) gene. Therefore, it seems evident that the replicative capacity of the virus is largely related to viral entry. In this study we have used growth competition experiments and TaqMan real-time PCR detection to measure the fitness of subtype B HIV-1 primary isolates and autologous env-recombinant viruses in order to analyze the contribution of wild-type env sequences to overall HIV-1 fitness. A significant correlation was observed between fitness values obtained for wild-type HIV-1 isolates and those for the corresponding env-recombinant viruses (r ‫؍‬ 0.93; P ‫؍‬ 0.002). Our results suggest that the env gene, which is linked to a myriad of viral characteristics (e.g., entry into the host cell, transmission, coreceptor usage, and tropism), plays a major role in fitness of wild-type HIV-1. In addition, this new recombinant assay may be useful for measuring the contribution of HIV-1 env to fitness in viruses resistant to novel antiretroviral entry inhibitors.

Clinical Significance of Human Immunodeficiency Virus Type 1 Replication Fitness

“…Using virus isolates without drug resistance mutations in pol, overall fitness in a wholevirus assay correlated most closely with that of a recombinant-virus assay in which the env gene was amplified from the patient specimen (142). In contrast, in one isolate with drug resistance mutations, the pol gene appeared to make a major contribution to viral fitness (142).…”

“…This information is interesting from a pathogenic perspective but also provides potentially important data for designing a clinically relevant recombinant virus assay to assess the fitness of HIV-1 isolates in different patient populations. The relative fitnesses of whole-virus isolates of HIV-1 obtained from untreated patients and recombinant viruses containing the corresponding env sequences were very similar in a multiple-cycle growth competition assay in PBMCs, suggesting that env has a dominant influence on the replication fitness of HIV-1 in untreated patients (116,142). In one of these studies, which evaluated two different strains of HIV-1, the relative fitnesses of the whole virus and its corresponding env-recombinant virus also correlated with affinity for CD4 and CCR5 receptors (116).…”

“…Some examples of measuring virus directly include quantitation of a viral protein, such as p24, by enzyme-linked immunosorbent assay (106,156,163); measurement of reverse transcriptase activity (164, 175); and quantitation of proviral DNA (116). In addition, in growth competition assays, the relative amounts of test and reference strains can be quantified by sequence analysis of proviral DNA or viral RNA (65,69,78,86,117,154,155,169,174), heteroduplex tracking assays (HTAs) (140), or allele-specific real-time PCR (33,142,176). It should be noted that the latter methods do not provide information on the extent of viral replication.…”

“…Real-time PCR has also been used to quantify the relative amounts of test and reference strains in a growth competition assay by allele-specific amplification of viral nucleic acid at a codon where the two variants differ (33,142,176). Real-time PCR has an advantage over other molecular assays used to quantify the relative proportion of the test and reference strains because of its high throughput and wide linear range of detection.…”

“…In one of these studies, which evaluated two different strains of HIV-1, the relative fitnesses of the whole virus and its corresponding env-recombinant virus also correlated with affinity for CD4 and CCR5 receptors (116). It is interesting that in the one treatmentexperienced patient assessed, the correlation between the replication of whole virus and that of the env-recombinant virus was poor, suggesting that resistance mutations in protease and reverse transcriptase have a greater influence on replication fitness than env during treatment failure (142). Another study found that differences in LTRs in different HIV-1 subtypes were associated with differences in replication fitness that were cell type specific, suggesting that noncoding regions of the viral genome may also influence replication fitness (168,169).…”