The CD45 protein tyrosine phosphatase is required for the completion of the activation program leading to lymphokine production in the Jurkat human T cell line.

Stimulation of the T cell antigen receptor, TCR-CD3, induces tyrosine phosphorylation of specific cellular proteins through activation of a tyrosine kinase. The possible regulatory role of the CD45 protein tyrosine phosphatase in this process was explored by studying the functional properties of cellular variants of the Jurkat T cell line which have been selected to have normal levels of the TCR-CD3 complex, but low or negative expression of CD45. These variants had less than 20% of the normal membrane tyrosine phosphatase activity. Triggering the TCR-CD3 receptor on the CD45 variants with anti-CD3 mAb induced the activation of a tyrosine kinase. Tyrosine phosphorylation of cellular substrates as well as of the CD3 zeta chain was qualitatively comparable to normal cells although the extent of stimulation was lower. No differences were observed between the variants and the normal cells in the duration of the tyrosine phosphorylation signal. The increase in intracellular calcium concentration following receptor stimulation was also less efficient, suggesting that CD45 is necessary for optimal generation of the second messengers of the activation. The CD45 deficient cells secreted highly reduced levels of lymphokines (IL-2, IL-3 or GM-CSF) after activation by anti-CD3 mAb combined with the phorbol ester TPA. This impaired lymphokines production is related to the absence of CD45 since a CD45+ revertant subclone, isolated from one CD45- clone, produced normal levels of cytokines upon activation via CD3, while CD45- subclones were unable to secrete cytokines following activation via CD3. However, upon activation with Ca2+ ionophore and PMA, all CD45- (sub)clones secreted cytokines at levels comparable to those produced by CD45+ cells. These results show that CD45 is required for cytokine production after activation via the TCR-CD3 complex.