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 progressionin T cells stimulated with an altered peptide ligand (APL) versus wild-typepeptide. APL stimulation did not induce proliferation compared to wild-typepeptide stimulation. To determine the point at which cell cycle progressionis blocked, we have examined molecules responsible for regulating theretinoblastoma tumor suppressor gene product, pRb, which in its activestate prevents G1/S progression. The majority of cells stimulated with anAPL did not progress beyond G1; however, a small population did make theG1/S transition. These few cells passed the late G1 restriction point,divided and subsequently arrested at the next G1 phase. The lack ofsustained signaling events following stimulation with an APL failed toinduce cyclin E:cdk2 activity, a regulator which hyper-phosphorylates andinactivates pRb. Exogenous IL-2 addition did not compensate for the lack ofproliferation following APL stimulation. Furthermore, the inability of thecells to enter S phase during partial T cell activation cannot be accountedfor by p27Kip1 inhibition of cyclin E:cdk2 complexes. Upon APL stimulation,an increase in association of p27Kip1 with cyclin E:cdk2 complex was notobserved, suggesting that instead, decreased cyclin E:cdk complex formationmight contribute to the failure to progress from G1/S. Therefore, while fora majority of cells, wild-type stimulation results in cell cycleprogression, APL stimulation is not sufficient to drive cells beyond G1.