Background Our lab and others have shown that a proportion of blood and bone marrow specimens from AML patients have an immunosuppressive microenvironment and have hallmarks of immune exhaustion: increased frequencies of regulatory T cells (Tregs)1 and myeloid-derived suppressor cells (MDSCs),2 decreased T-cell proliferation,3 elevated expression of immune checkpoint molecules4 and increased TEx vs. TPEx5-7 populations. Importantly, a subset of these patient samples containing dysfunctional T cells can be rescued by ICB. We have identified the same immune-related features, in a mouse model of AML. BET inhibitors (BETi) havbeen shown to positively affect CD8+ T cells, indirectly via reduction of PD-L1 expression on myeloid cells8,9 and directly by inhibition of chronic TCR activation genes, increasing stem-cell like memory CD8+ T cells.10,11 We hypothesized that BETi may synergize with anti-PD1 therapy in AML through promoting T cell stemness.
Methods Our AML mouse model bears FLT3-ITD and deletion of TET2 restricted to the myeloid lineage (LysM-CRE). For in vitro studies, splenocytes were stimulated with anti-CD3 and either JQ1, anti-PD1 or both and proliferation and differentiation status were assessed by flow cytometry. For in vivo studies, treatment consisted of 2 weeks. s3-ATAC-seq; Cells were prepared as described.12 Libraries were sequenced on a NextSeq 2000. Data was then analyzed and visualized using the ArchR. GSE Accession: GSE205386.
Results We show that inhibitors which target bromodomain and extra-terminal domain (BET) proteins rescue T cell exhaustion. Ex vivo treatment of cells from AML mice and AML patients with BET inhibitors (BETi) reversed CD8+ T cell exhaustion by restoring proliferative capacity and expansion of the more functional TPEx (figure 1). This reversal is enhanced by combined BETi and anti-PD1 treatment. BETi synergized with anti-PD1 in vivo, resulting in the reduction of circulating leukemia cells, enrichment of CD8+ T cells in the bone marrow, and increased expression of Tcf7, Slamf6, and Cxcr5 in CD8+ T cells (figure 2). Finally, we profiled the epigenomes of in vivo JQ1 treated AML-derived CD8+ T cells by single-cell ATAC seq and find that JQ1 increases Tcf7 accessibility specifically in Tex cells, suggesting that BETi likely mechanistically acts by relieving repression of progenitor programs in Tex CD8+ T cells and maintaining a pool of anti-PD1 responsive CD8 T cells (figure 3).
Conclusions Using an AML mouse model that exhibits leukemia-induced immune exhaustion, we demonstrate the pre-clinical efficacy of combining BETi and anti-PD1 therapy in the treatment of AML.
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