Putative role for interleukin-7 in the maintenance of the recirculating naive CD4+ T-cell pool

Abstract

The capacity of the immune system to respond efficiently to new antigens depends upon a continuous source of naive CD4+ T cells. Such cells exit from the thymus and join the recirculated T-cell pool. Factors present at the sites of naive CD4+ T-cell circulation must be responsible for their survival, since upon removal from their host, naive CD4+ T cells die. However, such factors remain unknown. The presence of the cytokine interleukin-7 (IL-7) in secondary lymphoid organs and the continuous expression of its receptor on naive CD4+ T cells prompted us to examine the possibility that IL-7 might be a survival factor for naive CD4+ T cells. Using naive CD4+ T cells isolated from cord blood we show that IL-7, but not IL-2, can maintain naive CD4+ T-cell viability in vitro for at least 15 days. In addition, we find that IL-7 can induce modest proliferation of naive CD4+ T cells without affecting either their cell surface phenotype or their ability to respond to antigenic stimulation. We also find that after anti-CD3 stimulation, naive CD4+ T cells lose that ability to respond to IL-7. However, if cells are primed with IL-7 prior to antigenic stimulation, their proliferative responses are enhanced. Together, these data suggest a novel and important role for IL-7 in the maintenance and maturation of naive CD4+ T cells, ensuring that they can respond maximally when they first meet antigen in secondary lymphoid tissue.

Figure 1

Naive CD4⁺ T cells were cultured in either medium alone, IL‐2, or IL‐7 for 2, 5, 10 and 15 days. Cells were then harvested, permeabilized, and their DNA was stained using 7AAD. This graph show the percentage of live cells in each culture condition over the 15‐day culture period and is representative of three separate experiments.

Figure 2

Proliferation of naive CD4⁺ T cells to medium, IL‐2, andIL‐7 was assessed by [³H]thymidine incorporation during the last 8 hr of a 5‐day culture. Results show the mean c.p.m. ± SD and are representative of five separate experiments.

Figure 3

Naive CD4⁺ T cells were cultured in IL‐7 for a period of 0 (white bars) or 15 (hatched bars) days prior to stimulation with plate‐immobilized anti‐CD3 in the presence/absence of soluble anti‐CD28 (10 ng/ml). Proliferation was measured 3 days following stimulation by [³H]thymidine incorporation during the last 8 hr of culture. Results show the stimulation indices for triplicate cultures ± SD and are representative of three separate experiments.

Figure 4

Naive CD4⁺ T cells were stimulated with plate immobilized anti‐CD3 for 0–120 hr in the presence/absence of soluble anti‐CD28 (10 ng/ml). Cells were then placed in 96‐well flat‐bottomed plates in the presence/absence of IL‐7 and proliferation was measured 3 days later by [³H]thymidine incorporation. (a) shows proliferative responses to IL‐7 0, 24, 72 and 120 hr after anti‐CD3 stimulation. (b) shows proliferative responses to IL‐7 0, 6, 12, and 18 hr after anti‐CD3 stimulation. (c) shows proliferation to anti‐CD3 or anti‐CD3 plus anti‐CD28 in the presence (filled bars) or absence (open bars) of IL‐7. (d) shows proliferation to medium (open bars) and IL‐7 (filled bars) following 24 hr of stimulation with anti‐CD3 or anti‐CD3 plus anti‐CD28.