Long-term exposure to superantigen induces p27Kip1 and Bcl-2 expression in effector memory CD4+ T cells

The long-term exposure of mice to superantigen SEA using a mini-osmotic pump (SEA pump) induced a long-lasting expansion of Vβ3⁺CD4⁺ T cells with T helper (Th) 2 cell-type properties. Removal of the SEA pump 10 days after pump implantation did not significantly alter the level of Vβ3⁺CD4⁺ T cell expansion/maintenance. Furthermore, CFSE-labeled CD4⁺ T cells failed to divide when transferred to post-implantation day 15 mice. Thus, CD4⁺ T cells appeared to survive for at least 30 days in the absence of a sufficient amount of antigen to trigger cell division. STAT6 deficient mice, in which Th2 cell development is largely impaired, also exhibited a protracted cell expansion, similar to that observed in normal mice, suggesting that the Th2 cell property is dispensable for the maintenance of Vβ3⁺CD4⁺ T cell expansion. The expanded CD4⁺ T cells on post-implantation day 26 were arrested in the G₀/G₁ phase of the cell cycle and showed a lower level of cell division upon restimulation. The Cdk inhibitor p27^Kip1 was highly expressed, and Cdk2 was downregulated. Moreover, the CD4⁺ T cells were resistant to in vitro apoptosis induction in parallel with their level of Bcl-2 expression. Collectively, the Vβ3⁺CD4⁺ T cells appeared to develop into long-lived memory T cells with cell cycle arrest upon long-term exposure to SEA.