Interleukin-7 signalling is sufficient to phenotypically and functionally prime human CD4⁺ naïve T cells

Interleukin-7 (IL-7) is produced by bone marrow and lymphoid stromal cells and is involved in the synthesis, survival and homeostasis of T cells. These attributes are the basis for current strategies to utilize IL-7 as an immune modulator for several clinical conditions to replenish depleted T-cell numbers. Because we had previously determined that IL-7 can induce potent human immunodeficiency virus replication in the otherwise non-permissive CD4 ⁺ naïve T-cell compartment, we evaluated here the impact of IL-7 on the phenotype and functional potential of naïve CD4⁺ T cells in an attempt to understand the mechanism of this induction. We demonstrate that IL-7 mediated the up-regulation of CD25, CD95 and human leucocyte antigen-DR, while it did not alter the expression of CD45RO, CD69, CD40, or CD154. Examination of the cytokine profile of IL-7-treated naïve T cells using a Type1/Type2 Proteome Array indicated a remarkable IL-7-mediated induction of interferon-γ production, while the other cytokines evaluated (IL-2, IL-12, tumour necrosis factor-α, IL-4, IL-5, IL-10 and IL-13) were not affected. Intracellular staining of IL-7-treated naïve T cells for interferon-γ verified the Proteome data. IL-7 did not induce cell cycle proliferation of naïve CD4⁺ T cells, as evaluated by 7-AAD/pyronin immunostaining and carboxyfluorescein diacetate succinimidyl ester dye tracking. IL-7 treatment of naïve CD4⁺ T cells induced their ability to prime monocytes, as was indicated by induction of CD80 and CD86 expression on monocytes cocultured with IL-7-treated naïve CD4⁺ T cells. Collectively, these data indicate that IL-7 signalling is sufficient to phenotypically and functionally prime human CD4⁺ naïve T cells independent of antigen stimulation.