Background:   HAART suppresses viral loads to undetectable levels and increases peripheral CD4 T-cell counts in most HIV patients, however, peripheral CD4⁺ T cells remain depleted in some patients despite having reduced viral loads. Interleukin-7 (IL-7) is a T-cell homeostatic cytokine involved in production and maintenance of peripheral T cells. To determine the utility of IL-7 as an immunotherapeutic, we assessed the effect of HAART treatment on the ability of peripheral CD4 cells to proliferate in response to IL-7 ex vivo.

Methods:  Peripheral blood mononuclear cells (PBMC) were isolated from whole blood and stimulated 6 days with IL-7 (10 ng/mL) and anti-CD3 antibody (5 µg/mL) or IL-7 alone (100 ng/mL). CD4⁺ cells were phenotypically assessed via flow cytometry (CD28, CD95, and CD127) and proliferation was assessed using a CFSE dilution assay. IL-7 signaling was assessed by Western blot probing for phosphorylated STAT5 after IL-7 or interferon-alpha (IFN-α) (100 µ/mL) and IL-7 treatment.

Results:  IL-7 (100 ng/mL) stimulation ex vivo induced efficient CD4⁺ T-cell proliferation from uninfected patients, on average 34.6% of the CD4 T cells in culture proliferated. However CD4⁺ T cells from HIV⁺ patients did not proliferate as robustly (16.7%). HAART-treated HIV⁺ patients exhibited an increased proliferative capacity (27.4%).  Similarly, low dose IL-7 (10 ng/mL) and anti-CD3 treatment resulted in decreased responsiveness of CD4⁺ T cells from HIV⁺ patients compared to uninfected and HAART-treated patients.  The absence of any significant alteration of IL-7R surface expression in the 3 cohorts indicated that reduced IL-7 responsiveness was not the result of decreased receptor availability. We hypothesized that other cytokines may be impacting IL-7 responsiveness and undertook an assessment of the antiviral cytokine interferon-alpha (IFN-α). Pretreatment of CD4⁺ T cells with IFN-α influenced neither CD4⁺ T-cell proliferation nor STAT5 activation in the IL-7-treated cultures. Therefore, the presence of IFN-α would not be predicted to abrogate the potential benefits of IL-7 immune therapy in vivo, however, the exact cause of the decreased IL-7 responsiveness in HIV⁺ untreated patients remains unknown.

Conclusions:  HAART treatment increases the functional efficacy of IL-7 in HIV⁺ patients. These findings suggest that IL-7 has the potential to be an effective immunotherapeutic to increase CD4⁺ T-cell levels if administered to patients who are virologically responsive to HAART.