P27(Kip1) regulates T cell proliferation

Our studies addressed the mechanism by which serum acts in conjunction with T cell receptor (TCR) agonists to promote the proliferation of primary spl enic T cells. When added to resting splenocytes, TCR agonists initiated G(0 )/G(1) traverse and activated cyclin D3-cdk6 complexes in a serum-independe nt manner. On the other hand, both TCR agonists and 10% serum were required for the activation of cyclin E-cdk2 and cyclin A-cdk2 complexes and the en try of cells into S phase. Serum facilitated cdk2 activation by maximizing the extent and extending the duration of the TCR-initiated down-regulation of the cdk2 inhibitor, p27(Kip1). Although p27(Kip1) levels were reduced (a lbeit submaximally) in cells stimulated in serum-deficient medium, nearly a ll of the cdk2 complexes in these cells contained p27(Kip1). I, contrast, i n cells receiving TCR agonist and 10% serum, little if any p27(Kip1) was pr esent in cyclin-cdk2 complexes. Unlike wild-type splenocytes, p27(Kip1)-nul l splenocytes did not require serum for cdk2 activation or S phase entry wh ereas loss of the related cdk2 inhibitor, p21(Cip1), did not override the s erum dependence of these responses. We also found that cdk2 activation was both necessary and sufficient for maximal expression of cdk2 protein. These studies provide a mechanistic basis for the serum dependence of T cell mit ogenesis.