The clinical implications of reduced viral fitness
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- Barbour, J.D. & Grant, R.M. Curr Infect Dis Rep (2004) 6: 151. doi:10.1007/s11908-996-0012-x
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Viral fitness, defined as the extent of viral adaptation to the host environment, arises from tissue tropism, immune system evasion, drug resistance, and viral replication capacity. The fitness of wild-type and drug-resistant HIV-1 varies widely, associating with plasma viremia, CD4+ T-cell count, and clinical progression. HIV-1 fitness may be measured in competitive culture assays, single cycle assays, or single cycle assays based on a subgenomic fragment of HIV-1, which has been standardized as the replication capacity assay (pol RC). During virologic failure of antiretroviral therapy, CD4 T-cell counts remain elevated while pol RC declines and remains durably lower because of drug-selected changes in the gag and pol genes. CD4 T-cell sparing also is observed among patients without evidence of drug resistance who carry a low pol RC virus. Reduced HIV-1 replication capacity and virulence may occur because of drug resistance or viral escape from host immune responses.