MOLECULAR-BASIS FOR THE LACK OF T-CELL PROLIFERATION INDUCED BY AN ALTERED PEPTIDE LIGAND

In this report, we explore the mechanisms underlying cell cycle progre ssion in T cells stimulated with an altered peptide ligand (APL) versu s wild-type peptide. APL stimulation did not induce proliferation comp ared to wild-type peptide stimulation. To determine the point at which cell cycle progression is blocked, we have examined molecules respons ible for regulating the retinoblastoma tumor suppressor gene product, pRb, which in its active state prevents G(1)/S progression. The majori ty of cells stimulated with an APL did not progress beyond G(1); howev er, a small population did make the G(1)/S transition. These few cells passed the late G(1) restriction point, divided and subsequently arre sted at the next G(1) phase. The lack of sustained signaling events fo llowing stimulation with an APL failed to induce cyclin E:cdk2 activit y, a regulator which hyper-phosphorylates and inactivates pRb, Exogeno us IL-2 addition did not compensate for the lack of proliferation foll owing APL stimulation. Furthermore, the inability of the cells to ente r S phase during partial T cell activation cannot be accounted for by p27(Kip1) inhibition of cyclin E:cdk2 complexes. Upon APL stimulation, an increase in association of p27(Kip1) with cyclin E:cdk2 complex wa s not observed, suggesting that instead, decreased cyclin E:cdk comple x formation might contribute to the failure to progress from G(1)/S. T herefore, while for a majority of cells, wild-type stimulation results in cell cycle progression, APL stimulation is not sufficient to drive cells beyond G(1).