New roles for an old cytokine: characterizing how exposure to Il-12 alters human CD4 And CD8 T cell responses

CD4 and CD8 T cells are constantly exposed to inflammatory signals that influence diverse functional outcomes during infections and certain autoimmune disorders. One of the signals controlling CD4 and CD8 T cell functions is the inflammatory cytokine IL-12. Previous studies have focused on how IL-12 regulates CD4 and CD8 T cell functions when present during or after the activation of the T cell receptor (TCR). However, based on murine studies, we have only recently begun to appreciate that exposure to inflammatory signals, driven in part by IL-12, could alter how CD4 and CD8 T cells respond to TCR stimulation. Although intriguing, these studies have left several questions unanswered. Does IL-12 similarly regulate the function of human T cells? If so, what is the exact molecular mechanism by which IL-12 mediates these effects? To address these critical questions, we examined how IL-12 pretreatment altered human CD4 and CD8 T cell responses to subsequent TCR stimulation.

In CHAPTER III, we examined how prior exposure to IL-12 alters the responses of human CD4 T cells to subsequent TCR stimulation. Some of our key findings were that IL-12 pretreatment increased the production of IFN-γ, TNF-α, IL-13, IL-4 and IL-10 after TCR stimulation, suggesting that prior exposure to IL-12 potentiates the TCR-induced release of a range of cytokines. Based on the intracellular staining and mRNA expression data, we concluded that the IL-12-mediated increased production of a range of cytokines was a consequence of at least two separate mechanisms, increased mRNA expression for IFN-γ and increased release of TNF-α, IL-13, IL-4 and IL-10. In CHAPTER IV, we explored the mechanisms by which IL-12 pretreatment altered human CD4 T cell responses to TCR stimulation. We observed that IL-12 pretreatment increased the phosphorylation of AKT, P38 and LCK following TCR stimulation without altering other TCR signaling molecules, suggesting that this potentially mediates the increase in transcription of cytokines. In addition, the IL-12-mediated enhancement of cytokines that were not transcriptionally regulated was partially driven by increased oxidative metabolism. Collectively our results uncovered a novel function of IL-12 in regulating human CD4 T cell responses; specifically, it enhanced the release of a range of cytokines potentially by altering TCR signaling pathways and by enhancing oxidative metabolism. Then, in CHAPTER V, we examined the effects of IL-12 pretreatment in altering the responses of human CD8 T cells to subsequent TCR stimulation. Our key finding was that pretreatment of human CD8 T cells with IL-12 resulted in increased IFN-γ and TNF-α cytokine mRNA and protein production following subsequent TCR challenge. Mechanistically, prior exposure to IL-12 increased the TCR induced activation of select MAPKs and AKT without altering the activation of more proximal TCR signaling molecules. Together our results suggest that prior exposure to IL-12 potentiates human CD8 T cell responses to TCR stimulation possibly by altering the activation of TCR signaling pathways.

In the end, our results increase our understanding of the physiologic properties of human CD4 and CD8 T cell and provide mechanistic insight into novel functions for IL-12. Our results also provide insights into potential avenues to improve the current uses of IL-12 in therapeutics.