Characterization of a novel high affinity human IL-7 receptor. Expression on T cells and association with IL-7 driven proliferation.

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.