CD5 tyrosine phosphatase controls common gamma-chain cytokine-mediated STAT and extracellular signal-related kinase phosphorylation in activated human lymphoblasts: Inhibition of proliferation without induction of apoptosis

The objective of this study was to test whether CD45 signals can influence signaling processes in activated human lymphoblasts. To this end, we genera ted lymphoblasts which proliferate in response to common gamma -chain cytok ines, but readily undergo apoptosis after cytokine withdrawal. In experimen ts with the CD45R0 mAb UCHL-1, but not control CD45 mAbs, we found signific ant inhibition of proliferation. Interestingly, the pan-CD45 mAb GAP8.3, wh ich is most effective in inhibition of OKT-3-mediated proliferation in quie scent lymphocytes, was ineffective in lymphoblasts. Addition of CD3 mAb OKT -3 had no influence on IL-2-mediated proliferation (with or without UCHL-1) . In contrast, after addition of OKT-3 to IL-4- and IL-7-stimulated prolife ration assays, UCHL-1 signals could not significantly alter cellular prolif eration. We did not find induction of apoptosis following CD45R0 signaling. In Western blots using mAbs detecting phosphorylated STAT-3, STAT-5, STAT- 6, or extracellular signal-related kinase 1/2, we found that CD45R0 signali ng could effectively diminish phosphorylation of these intracellular signal ing components. Using RT-PCR, we found that CD45R0 signaling inhibited IL-2 mRNA production without major influence on IL-13, IL-5, or IFN-gamma mRNA levels. Costimulation with OKT-3 and IL-2 optimally induced secretion of IF N-gamma, TNF-alpha, and IL-5, which was not decreased by CD45 signals. In c onclusion, we illustrate that CD45R0 signals control early cytokine recepto r-associated signaling processes and mRNA and DNA synthesis in activated hu man lymphoblasts. Furthermore, we show the existence of CD45 epitopes (GAP8 .3), which are active and critical for signaling in quiescent lymphocytes, but are nonfunctional in activated human lymphoblasts.