The inhibitory roles of nitric oxide (NO) in T cell proliferation have been observed and studied extensively over the last two decades. Despite efforts, the fundamental pathway by which NO exerts its inhibitory actions remains to be elucidated although recent evidence suggests that the transcription factor Wilms tumor I (WT1) may be important. WT1 has been linked to numerous developmental pathways in particular nephrogenesis. Due to its roles in development and cell proliferation, polymorphisms within the WT1 gene can result in malignancies such as leukemia and Wilms tumor. WT1 functions as a transcriptional regulator and its activity is controlled through phosphorylation by protein kinase A (PKA). PKA-dependent WT1 phosphorylation results in translocation of WT1 from the nucleus to the cytosol, a process that interferes with WT1 transcriptional activities. In the current study we demonstrate that WT is expressed in human lymphocytes. Using the proliferative compound PHA we induced T cell proliferation and growth correlated with an increase in the expression of WT1 measured by RT-PCR, flow cytometry and immunoblot. Co-stimulation with the NO donor SNOG at concentrations of 0, 100, 300 and 600 mu M reduced in a concentration dependent way the PHA-induced upregulation of WT1 that correlated with a reduction in T cell proliferation. We conclude that WT1 might be an important component of the NO-dependent regulation of T lymphocyte proliferation and potential function. (C) 2007 Elsevier Inc. All rights reserved.