Specific inhibition of Stat5a/b promotes apoptosis of IL-2-responsive primary and tumor-derived lymphoid cells

Stat5a/b exhibits 96% homology and are required for normal immune function. The present studies examined Stat5a/b function in lymphoid cells by specific and simultaneous disruption of both proteins using novel phosphorothioate-2'-O-methoxyethyl antisense oligodeoxynucleotides (asODN). Efficient delivery was confirmed by the presence of fluorescent TAMRA-labeled ODN in >or=55 and 95% in human primary and tumor cell lines, respectively. Acute asODN administration reduced levels of Stat5a (90%) in 6 h, whereas Stat5b required nearly 48 h to attain the same inhibition, suggesting that the apparent turnover rate for Stat5a was 8-fold higher than that for Stat5b. Expression of the closely related Stat3 protein was unchanged after asODN treatment, however. Molecular ablation of Stat5a/b promoted apoptotic cell death in a significant population of primary PHA-activated T cells (72%) and lymphoid tumor cell line (e.g., YT; 74%) within 24 h, as assessed by 1) visualization of karyolytic nuclear degeneration and other generalized cytoarchitectural alterations, 2) enzymatic detection of TdT-positive DNA degradation, and 3) automated cytometric detection of annexin V translocation. Contrary to findings from Stat5a/b-null mice, cell cycle progression did not appear to be significantly affected. Interestingly, IL-2-insensitive and unprimed T cells and Jurkat cells remained mostly unaffected. Finally, evidence is provided that the cytotoxicity associated with Stat5a/b ablation may derive from activation of caspase-8, an initiator protease that contributes to apoptotic cell commitment. We propose that in lymphoid cells competent to activate Stat5a and Stat5b, both proteins preferentially mediate an antiapoptotic survival influence.     

New insights into the roles of Stat5a/b and Stat3 in T cell development and differentiation

T cell development and differentiation is carefully orchestrated by a series of cytokines. The importance of STAT family proteins in mediating signals by these cytokines is well-known, but new information on the role of STATs in novel aspects of T cell function and T cell subsets continues to accumulate. Recent studies have placed Stat5a/b and Stat3 center stage in T cell development and differentiation. Stat5a/b are indispensable in T regulatory (Treg) cell development and maintenance, and negatively regulate T helper 17 (Th17) cell differentiation. Conversely, Stat3 is essential for Th17 differentiation and inhibits Treg cells. The balance of Treg and Th17 cells is thought to be critical in maintaining immune tolerance, while preserving effective host defense. Therefore, Stat5a/b and Stat3 are emerging to be key players in T cell differentiation and homeostasis.