Stat5 activation is uniquely associated with cytokine signaling in peripheral T cells

The activation and subsequent proliferation of peripheral T cells requires the engagement of the T cell and a cytokine receptor, typically the IL-2 or IL-4 receptors. Critical to understanding the regulation of peripheral T c ells is the knowledge of the unique contributions of each receptor to full T cell activation and cell cycle progression. Mice deficient in Stat5a and Stat5b have demonstrated the essential role that these highly related prote ins play in cell cycle progression following peripheral T cell activation. Here we demonstrate that activation of the Stat5 proteins by tyrosine phosp horylation is uniquely contributed by cytokine receptor signaling and speci fically does not occur through the T cell receptor complex.