Naive CD4+ T cells can be sensitized with IL-7.

We addressed the question whether it is possible to lower the threshold for naive T cells to respond to antigens. Purified adult and cord blood derived CD4+ CD45RA+ naive T cells were incubated in the presence of various cytokines for two days ("primed T cells"), after which the cytokines were removed by extensive washing. Primed and unprimed cells were activated with solid phase-coupled anti-CD3 and soluble anti-CD28 monoclonal antibodies (MoAb). Naive T cells, primed with interleukin(IL)-7 proliferated more vigorously than unprimed cells. Primed cells required 6 h for antigenic stimulation, whereas unprimed cells required 20 h. The priming also shifted the threshold of naive T cells in order to stimulate the antigen concentration to a lower level. The addition of IL-10 almost completely abrogated the enhancing effect of IL-7 on naive T cells. Other cytokines (IL-1, IL-2, IL-6, IL-12, interferon (IFN)-gamma, tumour necrosis factor (TNF)-alpha had less effect on the cell proliferation. However, priming of naive T cells with IL-7 had no impact on the proliferation to allogeneic immature or mature dendritic cells (DC). We conclude that the antigen-independent activation of naive T cells with IL-7 prior to antigen stimulation sensitizes cells, and may be of help in trying to stimulate immune responses against weak antigens. This approach, however, does not enhance proliferative responses stimulated by DC.