Graft-Versus-Host Disease Impairs the Histone Methyltransferase Dot1l-Regulated Reconstitution of Plasmacytoid Dendritic Cells in Mice Undergoing Allo-HSCT

Plasmacytoid dendritic cells (pDCs) derived either from adoptive transfer from the donor graft or stem cell reconstitution can attenuate and prevent graft-versus-host disease (GVHD) in both pre-clinical and clinical settings. However, the reconstitution of donor pDCs is severely impaired during GVHD via an unknown mechanism. Here we demonstrate that the histone methyltransferase Dot1l, which specifically catalyzes methylation of histone H3 at lysine 79 (H3K79me), is critical for regulating the commitment and differentiation of pDCs from hematopoietic stem cells (HSCs) and we observed its function was severely impaired in GVHD mice. We have previously demonstrated that Flt3L-induced DCs can program allogeneic T cells to reduce their GVHD toxicity (Blood 2016). Using this platform, we explored candidate histone methyltransferase(s) that affected pDC development. We found the inhibitor specific to Dot1l dramatically decreased the frequency and number of pDCs in cultures compared to other chemical probes that inhibit Ezh2, MLL1, G9a and Jmjd3, respectively. Under steady-state condition, pDCs develop from HSCs through successive steps of lineage commitment and differentiation: multiple potent progenitors (MPP) → macrophage and DC progenitors (MDP) → common DC progenitors (CDP). Upon culturing in the presence of Flt3L+SCF, MPP produced 2-fold and 5-fold more pDCs than MDP and HSCs, respectively, over 6 days' incubation. For this reason, we focused on evaluating the effect of Dot1l deletion on pDC development from MPP and CDP, which represent the early and later stage of pDC progenitors, respectively. To examine the specific role of Dot1l in DC development, we crossed Dot1l conditional knock mice (Dot1lf/f) to ER-Cre B6 mice to generate ER-Cre.Dot1lf/f B6 mice. We administered tamoxifen to ER-Cre.Dot1lf/f B6 mice at day 0 and day +1 to delete Dot1l, highly purified MPP and CDP, and cultured them in the presence of Flt3L+SCF. Deletion of Dot1l led to significant decrease of (3- to 5-fold) pDCs from MPP but marginally decreased CDP production of pDCs. These results suggest that Dot1l is required for the initial commitment and differentiation of MPP into pDCs or the expansion of selected pDCs. To test it, we added Dot1l inhibitor SGC to the cultures containing wild-type (WT) mouse-derived MPP, CDP and pDCs. Inhibiting Dot1l with SGC decreased both the frequency (3-fold) and number (5-fold) of pDCs in the MPP culture compared to control, however, it did not affect th