Viral phenotype and fitness

The fitness is a parameter defining the replicative adaptation of an organism to its environment. Recently, the HIV research field has shown renewed interest in the replicative capacity of human immunodeficiency virus type 1 (HIV-1) due to the potential impact of ex vivo viral fitness on population size, drug resistance, and disease progression. In the absence of antiretroviral therapy, strains containing drug-resistance mutations have a reduced fitness compared to the wild-type (wt) quasispecies within the population. Therefore, selective pressure of drug therapy leads to dramatic shifts in the quasispecies distribution and fitness of those mutants with decreased sensitivity to the respective antiretrovirals. Even if the presence of drug resistance mutations can decrease the viral fitness and explain a sustained CD4+T cell count in presence of high viral load, the current treatment guidelines advocate a switch in antiretroviral treatment regimens following the emergence of primary drug resistant mutations and possibly prior to selection of compensatory changes.