5-Azacytidine-Mediated Modulation of the Immune Microenvironment in Murine Acute Myeloid Leukemia

SIMPLE SUMMARY: Leukemic growth results in selective silencing of gene transcripts involved in immune detection, differentiation, and apoptosis. Hypomethylating agents (HMAs), such as 5-azacytidine, have been shown to reprogram the genetic landscape in various cancers to restore both transgene and endogenous transcript expression. In this study, we show that 5-azacytidine treatment re-establishes both luciferase (transgene) and immune-related (endogenous) transcript expression in murine leukemia models, highlighting the broad applicability of HMAs in rescuing tumor suppressor and tumor antigen gene expression and enhancing cancer susceptibility to immunity. In addition, the immune microenvironment undergoes alterations after 5-azacytidine treatment, giving additional insight into the varied mechanisms behind 5-azacytidine-induced tumor growth control and possible combinatorial approaches to better control leukemia. ABSTRACT: Cancer cells accumulate epigenetic modifications that allow escape from intrinsic and extrinsic surveillance mechanisms. In the case of acute myeloid leukemias (AML) and myelodysplastic syndromes, agents that disrupt chromatin structure, namely hypomethylating agents (HMAs), have shown tremendous promise as an alternate, milder treatment option for older, clinically non-fit patients. HMAs reprogram the epigenetic landscape in tumor cells through the reversal of DNA hypermethylation. Therapeutic effects resulting from these epigenetic changes are incredibly effective, sometimes resulting in complete remissions, but are frequently lost due to primary or acquired resistance. In this study, we describe syngeneic murine leukemias that are responsive to the HMA 5-azacytidine (5-Aza), as determined by augmented expression of a transduced luciferase reporter. We also found that 5-Aza treatment re-established immune-related transcript expression, suppressed leukemic burden and extended survival in leukemia-challenged mice. The effects of 5-Aza treatment were short-lived, and analysis of the immune microenvironment reveals possible mechanisms of resistance, such as simultaneous increase in immune checkpoint protein expression. This represents a model system that is highly responsive to HMAs and recapitulates major therapeutic outcomes observed in human leukemia (relapse) and may serve as a pre-clinical tool for studying acquired resistance and novel treatment combinations.