text: IL-10 Production Is Maintained by High TCR Signal Strength and IL-12
  We next investigated whether repeated strong TCR activation is a compensatory signal for IL-12-induced STAT4 signaling in the induction of IL-10 in Th1 cells. For this, CD4+ Tcells were differentiated for 2 consecutive weeks with high antigen doses in the presence or absence of IL-12 throughout (Figures 4A-4D). High antigen dose and IL-12 cooperated to induce maximal IL-10 production (Figures 4A and 4B), given that this combination resulted in the highest numbers of IL-10-producing Th1 cells. Repeated high antigen dose stimulation in the absence of exogenously added IL-12 resulted in the production of IL-10 by Th1 cells, suggesting that repeated strong TCR triggering may overcome the need for IL-12 for IL-10 induction (Figures 4C and 4D). However, IL-10 induction under these conditions was abrogated when IL-12p40-deficient DCs were used as APCs (Figure4E). Thus, IL-12 is essential during both primary and secondary antigenic stimulation for production of IL-10 by Th1 cells. 
  To determine the requirements for stability of the IL-10-producing Th1 cells, we differentiated CD4+ Tcells for 1 week with high antigen doses with or without IL-12 (Figures 4A and 4C), washed them, and then restimulated them for an additional week with a low antigen dose, in the absence or presence of IL-12 (Figure4F). Th1 cells induced in the first week to produce IL-10 by culture with high antigen doses and IL-12 lost their ability to express IL-10 when recultured with low doses of OVA, which could be compensated for, to some extent, by addition of IL-12 to the secondary cultures (Figure4F), again suggesting that antigen dose and IL-12 signals cooperate for the induction of IL-10. Finally, DO11.10 CD4+ cells that were exposed to low doses of antigen and IL-12 during the primary differentiation phase produced high amounts of IFN-gamma but little IL-10, but they could be induced to produce IL-10 when both high antigen dose and IL-12 were present during the recall phase (FigureS4). Thus, high antigen dose and IL-12 are required for sustaining the induction of IL-10 production by Th1 cells. 
  