High-dose IL-2 and IL-15 enhance the in vitro priming of naive CD4+ T cells for IFN-gamma but have differential effects on priming for IL-4.

Cytokines play an important role for the in vitro differentiation of naive CD4+ T cells into IL-4- or IFN-gamma-producing cells. The presence of both IL-4 and IL-2 is required to prime cells for IL-4 in vitro. Using purified CD4+/LECAM-1high T cells from TCR transgenic mice as naive responder cells, the role of IL-15 was studied to see if it functioned similarly to IL-2 with regard to IL-4 priming. Purified CD4+ T cells cultured in the presence of IL-4 and anti-IL-2 failed to prime cells for IL-4 production. Addition of IL-15 to these same cultures could not restore priming for IL-4, suggesting that IL-2 and IL-15 may have different functional properties during the in vitro differentiation of IL-4-producing cells. The role of IL-15 in priming for IFN-gamma was also examined. The addition of high doses of IL-15 to priming cultures resulted in a striking increase in the amount of IFN-gamma produced following restimulation. Similarly, addition of a relatively high dose of IL-2 also produced a significant enhancement of IFN-gamma production; however, as previously reported, the presence of IL-12 in priming cultures induced the greatest increase in IFN-gamma production, leaving it as the predominant controller of Th1 differentiation in physiologic situations. Finally, IL-15 was shown to increase proliferation of activated CD4+ T cell blasts but not of naive CD4+ T cells. Moreover, cultures containing both IL-12 and IL-15 showed greater proliferation than either cytokine alone, suggesting an additive effect between these cytokines.