Abstract
Cell cycle re-entry of quiescent T lymphocytes is required for generation of productive T cell responses. Cyclin-dependent kinases (cdk), particularly cdk2, have an essential role in cell cycle re-entry. Cdk2 promotes phosphorylation of Rb and related pocket proteins thereby reversing their ability to sequester E2F transcription factors. Besides Rb, cdk2 phosphorylates Smad2 and Smad3. Smad3 inhibits cell cycle progression from G1 to S phase, and impaired phosphorylation on the cdk-mediated sites renders it more effective in executing this function. In contrast, cdk-mediated phosphorylation of Smad3 reduces Smad3 transcriptional activity and antiproliferative function. Recently, we determined that induction of T cell tolerance resulted in impaired cdk2 activity, leading to reduced levels of Smad3 phosphorylation on cdk-specific sites and increased Smad3 antiproliferative function due to upregulation of p15. We hypothesized that pharmacologic inhibition of cdk2 during antigen-mediated T cell stimulation might provide an effective strategy to control T cell expansion and induce tolerance. (R)-roscovitine (CYC202) is a potent inhibitor of cdk2-cyclin E, which in higher concentrations also inhibits other cdk-cyclin complexes including cdk7, cdk9 and cdk5. It is currently in clinical trials as anticancer drug and recently was shown to induce long-lasting arrest of murine polycystic kidney disease. We examined the effect of roscovitine on T cell responses in vitro and in vivo. We stimulated C57BL/6 T cells with anti-CD3-plus-anti-CD28 mAbs, DO11.10 TCR-transgenic T cells with OVA peptide or C57BL/6 T cells with MHC disparate Balb/c splenocytes. Addition of roscovitine in these cultures resulted in blockade of cell proliferation without induction of apoptosis. Biochemical analysis revealed that roscovitine prevented phosphorylation of cdk2, downregulation of p27, phosphorylation of Rb and synthesis of cyclin A, suggesting an effective G1/S cell cycle block. To determine whether roscovitine could also inhibit clonal expansion of activated T cells in vivo, we employed a mouse model of GvHD. Recipient (C57BL/6 x DBA/2) F1 mice were lethally irradiated and were subsequently infused with bone marrow cells and splenocytes, as source of allogeneic T cells, from parental C57BL/6 donors. Roscovitine or vehicle-control was given at the time of allogeneic BMT and on a trice-weekly basis thereafter for a total of three weeks. Administration of roscovitine protected against acute GvHD resulting in a median survival of 49 days in the roscovitine-treated group compared to 24 days in the control group (p=0.005), and significantly less weight loss. Importantly, roscovitine treatment had no adverse effects on engraftment, resulting in full donor chimerism in the treated mice. To examine whether tolerance had been induced by in vivo treatment with roscovitine, we examined in vitro rechallenge responses. While control C57BL/6 T cells exhibited robust responses when stimulated with (C57BL/6 x DBA/2) F1 splenocytes, responses of T cells isolated from roscovitine-treated recipients against (C57BL/6 x DBA/2) F1 splenocytes were abrogated. These results indicate that roscovitine has direct effects on preventing TCR-mediated clonal expansion in vitro and in vivo and may provide a novel therapeutic approach for control of GvHD.
CD4+ T cells are required to sustain CD8+ cytotoxic T-cell responses during chronic viral infection.
