IL-18 and CD28 use distinct molecular mechanisms to enhance NK cell production of IL-12-induced IFN-gamma.

NK cells play an important role in innate immune resistance, particularly through synthesis of the pro-inflammatory cytokine IFN-gamma. This study compares the abilities of the cytokine IL-18 and the costimulatory cell surface molecule CD28 to enhance IL-12-driven IFN-gamma production by NK cells. Studies with other cytokines (IL-1beta, IL-6, TNF-alpha, IL-15) showed that IL-18 or anti-CD28 treatments were the most efficient inducers of IFN-gamma when combined with IL-12. The ability of IL-18 to enhance IFN-gamma was shown to be dependent on the presence of IL-12. Similarly, although anti-CD28 stimulation alone could enhance IFN-gamma synthesis, this effect was significantly increased in the presence of IL-12. Although neither method of costimulation required de novo protein synthesis for their effects on IFN-gamma mRNA expression, these molecules used distinct mechanisms. Specifically, nuclear run-on analysis revealed that IL-18 in combination with IL-12 enhanced the rate of transcription of the IFN-gamma gene. Conversely, treatment with anti-CD28 plus IL-12 did not significantly up-regulate the rate of transcription of the IFN-gamma gene, but stabilized IFN-gamma mRNA expression within NK cells. These findings illustrate costimulatory pathways that result in potent IFN-gamma responses by NK cells and show that although IL-18 and anti-CD28 can enhance the synthesis of IL-12-driven IFN-gamma, they employ molecular mechanisms that are distinct from one another.