Like interleukin (IL)-2 and IL-4, IL-7 can act as a growth factor for activated T lymphocytes. Upon screening a panel of growth factor-dependent T cell lines, we found that only the cell line CT6 responded to IL-7, indeed as vigorously as to IL-2. Obviously, these findings challenge the validity of previous results on IL-2 production obtained using the CT6 cell line. However, they also demonstrate a novel and sensitive system for the bioassay of IL-7. The ability of the surveyed T cell lines to proliferate to IL-7 corresponded with the expression of IL-7 receptors (IL-7R) on the cell surface. The murine IL-7R on CT6 was shown to bind IL-7 with dual affinity and was visualized as an affinity cross-linked complex of 93 kDa. This IL-7R appears similar to that seen on murine splenic T cells and on 70Z/3, the pre-B cell line from which the murine IL-7R was cloned.

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We have recently shown that activation of T cells causes structural changes in the interleukin-7 receptor (IL-7R) (Foxwell et al. Int. Immunol. 1992, 4: 277). Unactivated cells expressed a receptor characterized as a cross-linked protein of 107-kDa whereas activated cells had reduced levels of this 107-kDa complex and now express a major cross-linked product of 93 kDa. These changes in receptor expression were concomitant with the acquisition of IL-7 growth responsiveness by activated T cells. In this study, the effect of the potent immunosuppressive agents cyclosporin A and FK506 on the activation-induced responsiveness to IL-7-driven proliferation and the concomitant changes in receptor structure have been investigated. Cyclosporin A and FK506 suppressed the expression of the 93-kDa complex and the loss of the 107-kDa complex on activated cells. The presence of exogenous IL-7 inhibited the effects of the drugs on IL-7R structure, allowing expression of the 93-kDa complex. Expression of the 93-kDa complex could also be induced either by ionomycin or phorbol esters. As observed for other T cell activation parameters, only those which induced a calcium signal (ionomycin) but not protein kinase C (phorbol esters) were sensitive to the drugs. In all studies, the expression of the 93-kDa complex correlated with the ability of cells to proliferate to IL-7, and thus these results further support the hypothesis that the 93-kDa form of the IL-7R is required to transmit the cytokine's growth signal. Moreover, these data suggest that activation-induced transcriptional events are required for the expression of the 93-kDa complex and the down-regulation of the 107-kDa complex. As reported for IL-2R and IL-4R, our data also show that the expression of another T cell growth factor receptor is sensitive to the effects of cyclosporin A and FK506. These observations also have important implications for reported cyclosporin A effects on the thymus where IL-7 can act as a growth factor for thymocytes.

Although both unstimulated and activated human T cells express high affinity IL-7R, only activated T cells can proliferate to IL-7. This responsiveness may occur as a direct result of changes in the structure of the IL-7R during T-cell activation. We have previously demonstrated such changes by affinity cross-linking studies, and have shown that unstimulated human T cells express a single IL-7R of 90 kDa, whereas activated T cells express an additional 76-kDa IL-7 binding protein. In this study the origin and function of the p90 and p76 molecules have been investigated. To determine the role of each of these receptors in IL-7 driven proliferation, IL-7R expression and proliferative capacity were monitored during mitogenic stimulation. These analyses showed that the ability of PBMC to proliferate to IL-7 correlated with expression of the p76 IL-7R, and not with expression of the p90 IL-7R. IL-7-driven proliferation is mediated via high affinity IL-7R, and accordingly, Scatchard analysis revealed that, like the p90 IL-7R, the p76 IL-7R bound IL-7 with dual (high; Kd 38 pM and low; Kd 360 pM) affinity. Deglycosylation studies showed that the p90 and p76 IL-7R are not simply differently glycosylated isoforms of a single receptor. In agreement, mAb to the previously cloned IL-7R were found to stain unstimulated T cells that express only the p90 IL-7R but not T-cell clones that express predominantly the p76 IL-7R. These antibodies also immunoprecipitated the cloned IL-7R as a 90-kDa species from both 125-I-surface-labeled resting and activated T cells, but were unable to precipitate the 76-kDa IL-7R. In addition, PCR analysis of p76-expressing cells could not detect splicing of the extracellular domain of the cloned IL-7R, thereby excluding the possibility that the p76 IL-7R is a previously undescribed splice variant of the cloned IL-7R. These data demonstrate that the p90 IL-7R is the T-cell homologue of the cloned IL-7R, and imply that the p90 and p76 IL-7R have different extracellular domains. Taken together these data suggest that the 76-kDa receptor is a novel high affinity IL-7R that may be necessary for IL-7 driven proliferation in human T cells.