Novel Roles for Arginine Modifying Enzymes in Immune Regulation

Arginine modifying enzymes are important transcriptional regulators involved in many cellular processes. Cytokine expression by immune cells is a highly regulated process, since cytokine imbalance is associated with severe pathological consequences including autoimmunity, autoinflammation and asthma. Th2 mediated IL-4 expression is a tightly regulated process. The NFAT interacting protein NIP45 is an essential regulator of IL-4 expression by Th2 cells. Furthermore, NIP45-mediated IL-4 expression is enhanced by arginine methylation of NIP45. Here, I demonstrate that arginine deimination of NIP45 by PAD4 negatively regulates IL-4 expression. PAD4 can deiminate NIP45 both in vitro and in vivo. Overexpression of PAD4 can suppress NIP45-mediated IL-4 expression. Interestingly, the suppressive activity of PAD4 is independent of its catalytic activity. Furthermore, Th2 cells from PAD4-deficient mice that I generated for this thesis display elevated IL-4 secretion. Taken together, I have established that PAD4 is a negative regulator of IL-4 expression. Although PAD4 can deiminate NIP45, the catalytic activity of PAD4 is not required for suppression of NIP45-mediated IL-4 secretion. CARM1 coactivates NFkB-mediated transcription of several proinflammatory genes including IL-6, TNFa, and IP-10 in MEFs. In contrast, I demonstrate that CARM1 is a negative regulator of NFkB-mediated proinflammatory cytokine secretion in macrophages. CARM1-deficient BMDMs secrete significantly more IL-6 and TNFa upon LPS stimulation than CARM1-sufficient BMDMs, without exhibiting a difference in IL-10 secretion. In addition, CARM1 is reported to coactivate glucocorticoid-receptor mediated transcription. Here, I show that glucocorticoid mediated trans-repression of NFkB in BMDMs is not regulated by CARM1. In conclusion, the results from this thesis reveal novel roles for arginine modifying enzymes in cytokine expression by Th2 cells and macrophages. PAD4 is a negative regulator of NIP45-mediated IL-4 secretion in Th2 cells. In addition, the generation of the PAD4 conditional knockout strain will allow a more detailed analysis of PAD4 in immune regulation. Furthermore, CARM1 is a negative regulator of NFkB-mediated proinflammatory cytokine secretion in macrophages, revealing cell-type specific differences for CARM1 function in macrophages and MEFs.