Notch ligand Delta-like 4 induces epigenetic regulation of Treg cell differentiation and function in viral infection.

Notch ligand Delta-like ligand 4 (DLL4) has been shown to regulate CD4 T-cell differentiation, including regulatory T cells (T). Epigenetic alterations, which include histone modifications, are critical in cell differentiation decisions. Recent genome-wide studies demonstrated that T have increased trimethylation on histone H3 at lysine 4 (H3K4me3) around the T master transcription factor, Foxp3 loci. Here we report that DLL4 dynamically increased H3K4 methylation around the Foxp3 locus that was dependent upon upregulated SET and MYDN domain containing protein 3 (SMYD3). DLL4 promoted Smyd3 through the canonical Notch pathway in iT differentiation. DLL4 inhibition during pulmonary respiratory syncytial virus (RSV) infection decreased Smyd3 expression and Foxp3 expression in T leading to increased Il17a. On the other hand, DLL4 supported Il10 expression in vitro and in vivo, which was also partially dependent upon SMYD3. Using genome-wide unbiased mRNA sequencing, novel sets of DLL4- and Smyd3-dependent differentially expressed genes were discovered, including lymphocyte-activation gene 3 (Lag3), a checkpoint inhibitor that has been identified for modulating Th cell activation. Together, our data demonstrate a novel mechanism of DLL4/Notch-induced Smyd3 epigenetic pathways that maintain regulatory CD4 T cells in viral infections.