Epigenetic regulation in trained and tolerized macrophages

Abstract

Innate immune memory is the phenomenon that can either enhance (trained) or suppress (tolerized) immune response by innate immune cells during the second encounter of the stimuli such as pathogens. Previous studies reported that epigenetic regulations critically regulate both types of innate immune memory. However, a complete understanding of the underlying mechanisms is still lacking. In this study, we performed a screening of an epigenetics compound library to identify inhibitors that affect ß-glucan (BG)-trained or LPS-tolerized macrophages. Among 181 compounds tested, various inhibitors targeting Aurora kinase, histone methyltransferase PRMT and EZH2, histone demethylase LSD1 and JMJD2, histone deacetylase 6 and sirtuin-1, and PARP showed inhibitory activity against LPS tolerance by promoting effects on TNFα production. The effect of LSD1 in LPS tolerance was validated in this study. Inhibition or silencing of LSD1 results in suppressed LPS tolerance by enhance cytokines production. However, no change was observed in H3K4me3 level associated with the Tnf promoter upon LSD1 inhibition. In trained immunity, we identified an inhibitor of O6-methylguanine DNA methyltransferase (MGMT), a DNA repair enzyme of the lesion triggered by alkylating agent, as a novel regulator of trained immunity in macrophages. MGMT expression significantly increased after the priming with BG, and inhibition or silencing of MGMT expression during the priming resulted in increased trained immunity. To further investigate the role of MGMT in trained immunity, mice with myeloid specific deletion of MGMT were generated (LysM-crecre/+; MGMTfl/fl mice). However, targeted deletion of MGMT expression resulted in a decrease in proinflammatory cytokines production in trained immunity both in vitro and in vivo. Depletion of MGMT in trained macrophages resulted in decrease downstream signaling molecules of TLR4 receptor, such as p38 and SAPK/JNK. In addition, signaling downstream of dectin-1 receptor, such as mTOR, and glycolytic function was also damped. These results highlight the unexpected role of MGMT in regulation of trained immunity beyond its role in DNA repair. Based on transcriptomic data, MGMT may regulate trained immunity through farnesoid X receptor (Nr1h4) and PR domain zinc figure protein 5 (Prdm5). Taken together, this screening approach indicates that innate immune memory is regulated by numerous mechanisms and the novel target proteins identified from this research has the potential to regulate innate immune memory, which could be used as a target of therapeutic agents for several diseases.