The Function Of Dotll In Innate Immune Responses And The Underlying Epigenetic Mechanism

The immune system consists of innate immune system and adaptive immune system.As the first line for host to defense against the invasion of pathogens,the innate immune cells express many kinds of pattern recognition receptors(PRRs)which can recognize the pathogen associated molecular patterns(PAMPs)of the invading pathogens,and then activate the innate signaling to induce proinflammatory cytokines and type I interferons(IFNs)production for the elimination of the invading pathogens.However,overproduction of proinflammatory cytokines and type I interferons(IFNs)during the innate immune response may lead to immunological injuries and autoimmune inflammatory diseases.So,the innate immune response must be tightly regulated.A balance between the positive and negative regulation of innate immune activation is necessary for pathogen clearance and autoimmune-damage avoidance.There are lots of elements that participate in the regulation of innate immunity,including posttranslational modifications(PTMs)and epigenetic regulation.The cytoplasmic processes of innate immune regulation are always associated with PTMs of innate immune sensors and downstream signaling molecules that have been widely investigated.It has been proved that the activation of innate immunity leads to nuclear reprogramming and the changes of epigenetic modifiers expression.Thus,more attention has been paid to the nuclear transcriptional regulation of proinflammatory cytokines,chemokines and type Ⅰ interferons.Epigenetic modification is one of the most important elements functioned in the regulation of gene transcription.However,the role of epigenetic modification in innate immune regulation is still unclear,which needs further investigation.Different epigenetic modifications have different roles in the regulation of gene expression.DNA methylation is always involved in gene repression,while histone acethylation is associated with the transcriptional activation.However,histone methylation is out of the ordinary.It associates with both of the transcription activation and repression due to different modification locations and statuses(mono-,di-or trimethylation).For instance,H3K4me3,H3K36me3 and H4K20me1 are usually defined as gene active mark,whereas H3K9me3,H3K27me3,andH4K20me3 can inhibit gene transcription.It is well known that lysine-specific methyltransferases(KMTs)and demethylases(KDMs)are responsible for the dynamic regulation of histone methylation.The regulatory mechanisms of epigenetic modifiers have been gradually revealed in different biological processes,such as embryonic development,physical senescence,tumorigenesis and so on.What’s more,it has been uncovered recently that there is a tight connection between histone methylation and innate immune regulation.H3K79 is similar with H3K4,H3K36 whose methylation is connected with gene transcriptional activation.H3K79 modification is essential for the regulation of embryonic development,somatic reprogramming,DNA damage response and cellular differentiation,nevertheless,the role of H3K79 methylation in innate immune regulation is still unrevealed.Dot1l is the only confirmed H3K79 methyltransferase that can catalyze the methylation modification of H3K79,whereas the demethylase of H3K79 is still undefined.In recent years,most studies about Dot1 l are concentrated on its role in tumorigenesis.It has been proved that Dot1 l is involved in the pathogenesis of leukemia,breast cancer,pancreatic adenocarcinoma and so on.As a histone modifier,it may have more extensive functions besides tumorigenesis and cell proliferation.Dot1 l is widely expressed in immune cells.However,the role of Dot1 l in the immune regulation remains unclear.Here,we found that H3K79me2/3 modification at Il6 and Ifnb1 promoter was increased upon innate stimuli in mouse peritoneal macrophages.Then,we stimulated mouse peritoneal macrophages with Poly(I:C),LPS or VSV for different time,and found that the expression of Dot1 l was increased in macrophages upon innate stimuli.In human monocytic cell line THP1,innate stimuli also induced the upregulation of Dot1 l expression.Furthermore,silencing of Dot1 l inhibited the production of IL-6 and IFN-β in mouse peritoneal macrophages or THP1 cells stimulated with TLR ligands or infected with RNA virus.Mechanistically,silencing of Dot1 l had no effect on the activation of NF-κB、MAPK and IRF3 signal pathways in response to innate stimuli.We further found that Dot1 l recruitment to Il6 and Ifnb1 promoter was increased in mouse peritoneal macrophages upon innate activation.Silencing of Dot1 l could inhibit H3K79me2/3 modification at Il6 and Ifnb1 promoter in macrophages stimulated with TLR ligands or infected with RNA virus.In conclusion,Dot1 l promotes TLRs or RLRs-triggered production of IL-6 and IFN-β in macrophages.Dot1l-mediated H3K79me2/3 modification enhances the innate stimuli-induced transcription of Il6 and Ifnb1.Our results demonstrate that Dot1 l acts as an epigenetic modifiers in the regulation of innate immunity.