Background:  Intermittent administration of interleukin-2 (IL-2) in combination with ART leads to preferential increase in naïve CD4⁺CD25⁺ T cells that express high levels of forkhead transcription factor P3 (foxP3). These cells are thought to play a role in the maintenance of a state of low turnover and sustained expansion of the CD4⁺ T-cell pool seen after IL-2 therapy. The goal of this study was to determine whether the increase in foxP3 expression associated with CD4⁺CD25⁺ T cells seen in the setting of IL-2 therapy is maintained following HAART interruption.

Methods:  Immunophenotypic analysis was performed on cryopreserved peripheral blood mononuclear cells (PBMC) from 26 HIV⁺ patients on long-term intermittent IL-2 therapy who underwent HAART interruption immediately after an IL-2 cycle. Samples were taken from before and 1 month after an IL-2 cycle in the setting of continuous HAART (IL-2/on) as well as before and 1 month after an IL-2 cycle in the setting of HAART interruption (IL-2/off). Samples from 15 patients were also tested for foxP3 by real-time polymerase chain reaction (RT-PCR). The number of copies of foxP3 was adjusted for the amount of RNA present in PBMC, as well as for the proportion of cells staining CD4⁺CD25⁺. Paired testing was used for comparisons.

Results:  There were significant increases in CD4⁺ T cell counts following both the IL-2/on cycle (+487 cells/mL; p <0.001) and the IL-2/off cycle (+207 cells/mL; p <0.001); this change was significantly greater following the IL-2/on cycle (p = 0.02). HIV RNA significantly increased following the IL-2/off cycle only (p <0.001). The increase in memory CD4⁺ T cells expressing high levels of CD25 (regulatory T cells) was less in the IL-2/on cycle (+0.13%) compared with IL-2/off (0.53%; p = <0.001). The increase in naïve T cells expressing CD25 was also less for the IL-2/on cycle (+11.1%) compared with IL-2/off (+19.9; p = 0.04). However, foxP3 expression in PBMC, corrected for percentage of CD4⁺CD25⁺ T cells, increased following the IL-2/on cycle (+44.0%) but decreased following the IL-2/off cycle (–25.4%, p = 0.003). There was a significant correlation between foxP3 expression in PBMC and percentage of CD25 expression on naïve CD4⁺ T cells (R = 0.48, p <0.001).

Conclusions:  Despite increases in CD4⁺ T cells and increases in expression of CD25 on both naïve and memory CD4⁺ T cells, the amount of foxP3 expression was lower following IL-2 therapy and HAART interruption. This suggests that viremia can blunt the preferential expansion of CD4⁺CD25⁺ T cells expressing high levels of foxP3.