Identification of a BET Bromodomain Inhibitor That Enables Treg Function: A Combinatorial Strategy to Inhibit GVHD

Targeting bromodomain and extra-terminal (BET) proteins has recently provided a useful approach for limiting production of pro-inflammatory cytokines. These epigenetic readers regulate transcription of genes involved in inflammation and cancer. Development of BET inhibitors (BETi) has generated significant interest for their therapeutic potential. Because inflammatory signals and donor T cells promote graft-versus-host disease (GVHD), regulating both pathways could be beneficial for abrogating this disorder. The objective of the present study was to identify a BETi which did not interfere with expansion / function of CD4 + FoxP3 + regulatory T cells (Treg) in vivo to enable their combined application following aHSCT to ameliorate GVHD. We have reported that Tregs can be markedly expanded with increased functional capacity by targeting TNFRSF25 and CD25 with TL1A-Ig and low dose IL-2, respectively (Wolf, et al 2017; Copsel et al. 2018). Here, mice were concomitantly treated over 7 days with TL1A-Ig+IL-2 together with BETi. We found that the BETi EP11313 did not decrease frequency/numbers or phenotype of expanded Tregs (Figure 1) as well as effector molecules (IL-10 and TGF-β). However, BETi JQ1 interfered with Treg proliferation and altered their phenotype. Notably, in Treg expanded mice, EP11313 diminished tnfa and ifng but not il-2 RNA levels. Phospho-STAT5 expression in Tregs was not affected by EP11313 supporting the notion that Treg IL-2 signaling remained intact. MHC-mismatched aHSCT (B6→BALB/c) was performed using in vivo expanded donor Tregs in the presence or absence of EP11313 short-term treatment in the recipient. Early post-transplant, improvement in the splenic and LN CD4/CD8 ratio along with fewer effector cells and high Treg levels in aHSCT recipients treated with expanded Tregs+EP11313 was detected. Interestingly, this group exhibited a significant diminution of GVHD clinical score early post-transplant with less skin involvement. Moreover, following transplant of low numbers of highly purified expanded Tregs, improved clinical GVHD scores were observed in EP11313 treated recipients (Figure 2). In total, we conclude that use of this novel combinatorial strategy can suppress pre-clinical GVHD. We propose that because BETi have demonstrated the ability to kill tumor cells, these compounds may provide additional anti-tumor activity together with GVL following allogeneic HSCT.