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 progression in T cells stimulated with an altered peptide ligand (APL) versus wild-type peptide. APL stimulation did not induce proliferation compared to wild-type peptide stimulation. To determine the point at which cell cycle progression is blocked, we have examined molecules responsible for regulating the retinoblastoma tumor suppressor gene product, pRb, which in its active state prevents G₁/S progression. The majority of cells stimulated with an APL did not progress beyond G₁; however, a small population did make the G₁/S transition. These few cells passed the late G₁ restriction point, divided and subsequently arrested at the next G₁ phase. The lack of sustained signaling events following stimulation with an APL failed to induce cyclin E:cdk2 activity, a regulator which hyperphosphorylates and inactivates pRb. Exogenous IL-2 addition did not compensate for the lack of proliferation following APL stimulation. Furthermore, the inability of the cells to enter 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 was not observed, suggesting that instead, decreased cyclin E:cdk complex formation might contribute to the failure to progress from G₁/S. Therefore, while for a majority of cells, wild-type stimulation results in cell cycle progression, APL stimulation is not sufficient to drive cells beyond G₁.