Blocking the cell cycle of Th cells activated for the first time by antigen, costimulation, and cytokine-inducing signals, we could show that for the instruction to memorize expression of interleukin-4 (IL-4) and IL-10, Th cells have to enter the S phase of the first cell cycle after their primary activation. This points to a critical role of epigenetic DNA modifications during DNA replication in the induction of cytokine memory. For the induction of IL-4 memory, concomitant signaling through the T cell receptor and the IL-4 receptor is required. The signals are induced independently of progression into the S phase, can be stored for at least one day, and commit the cell for IL-4 memory when entering the initial S phase.

Induction of IL-4 memory can also occur independent of IL-4 receptor signaling, as can be shown in signal transducer and activator of transcription 6 (Stat6)-deficient Th cells. Using the cellular affinity matrix technology, we have isolated viable IL-4-secreting and non-secreting Th cells from Stat6-deficient mice. In contrast to the IL-4-negative T cells, the IL-4-positive T cells stably expressed high levels of the Th2-specific transcription factor GATA-3. Accordingly, retrovirally introduced expression of exogenous GATA-3 in Stat6-deficient Th cells induced their differentiation into IL-4-secreting Th2 cells. Furthermore, it induced expression of endogenous GATA-3, thus revealing an autoregulatory loop controling GATA-3 expression levels and stabilizing Th2 differentiation and memory.

This epigenetic and transcriptional stability may explain why it proved so difficult to redirect Th cell cytokine expression. Changing epigenetic modifications of cytokine genes and modulating GATA-3 expression levels may serve as future targets for redirecting an established cytokine memory in chronic immune reactions and immunopathology.

References
Richter, A., M. Löhning, and A. Radbruch. 1999. Instruction for cytokine expression in T helper lymphocytes in relation to proliferation and cell cycle progression. J. Exp. Med. 190:1439-1450. Löhning, M., W. Ouyang, Z. Gao, M. Assenmacher, S. Ranganath, A. Radbruch, and K.M. Murphy. 2000. Stat6-independent GATA-3 autoactivation directs IL-4-independent Th2 development and commitment. Immunity 12:27-37.