T cell activation is regulated by a balance between phosphorylation and dephosphorylation that is under the control of kinases and phosphatases. Here, we examined the role of a non-receptor-type protein tyrosine phosphatase, PTP-PEST, using retrovirus-mediated gene transduction into murine T cells. Based on observations of vector markers (GFP or Thy1.1), exogenous PTP-PEST-positive CD4(+) T cells appeared within 2 days after gene transduction; the percentage of PIP-PEST-positive cells tended to decrease during a resting period in the presence of IL-2 over the next 2 days. These vector markers also showed much lower expression intensities, compared with control cells, suggesting a correlation between the percent reduction and the low marker expression intensity. A catalytically inactive PIP-PEST mutant also showed the same tendency, and stepwise deletion mutants gradually lost their ability to induce the above phenomenon. On the other hand, these PIP-PEST-transduced cells did not have an apoptotic phenotype. No difference in the total cell numbers was found in the wells of a culture plate containing VEC- and PIP-PEST-transduced T cells. Moreover, serine/threonine kinase Akt, but not the anti-apoptotic molecules Bc1-2 and Bcl-XL, reversed the phenotype induced by PTP-PEST. We discuss the novel mechanism by which Ala interferes with PIP-PEST. (C) 2014 Elsevier Inc. All rights reserved.