Background: Studies in a variety of systems indicate that a substantial proportion of cells within an antigen-specific CD8⁺ T-lymphocyte population can appear functionally compromised. The degree of such dysfunction can vary considerably, both quantitatively and qualitatively. However, the factors that determine these differences are poorly understood. Combination antiretroviral drug therapy (ART) suppresses HIV-1 replication and provides a model in which virus levels are manipulated during a persistent infection in humans. We exploited this model to examine the impact of variable plasma virus load (pVL) on the functionality of HIV-specific CD8⁺ T-lymphocyte populations both in individuals undergoing structured treatment interruptions and in individuals who were intermittently compliant with ART.

Methods: The functional status of HIV-specific CD8⁺ T-lymphocytes was assessed using ELISpot methodology and intracellular cytokine staining (ICS) for interferon (IFN)-g. Fluorochrome-labelled peptide-major histocompatibility complex (pMHC) class I tetramer staining and clonotypic quantitative PCR for the TCRB locus of previously defined HIV-specific T-cell clones were used to detect the physical presence of CD8⁺ T-lymphocytes expressing cognate TCRs irrespective of their phenotypic or functional properties.

Results: The proportion of HIV-specific CD8⁺ T-lymphocytes capable of mounting an effector response to antigen challenge directly ex vivo was related to the kinetics of virus exposure. Specifically: 1) after prolonged suppression of pVL with ART, physical and functional measures of HIV-specific CD8⁺ T-lymphocyte frequencies approximated; and 2) the percentage of functionally responsive cells in the HIV-specific CD8⁺ T-lymphocyte populations declined substantially when therapy was discontinued and pVL recrudesced in the same individuals.

Conclusions: The proportion of CD8⁺ T-lymphocytes that exhibit an impaired functional phenotype directly ex vivo in populations expressing TCRs specific for individual HIV-derived antigens is related to pVL. These findings have implications for the interpretation of quantitative data generated by methods that rely on functional readouts.