Hypomethylating Agent Leads to Enhanced Immunogenicity of a Dendritic Cell/Acute Myeloid Leukemia Fusion Vaccine and Prolonged Survival in an Immunocompetent Mouse Model

Immunotherapy for acute myeloid leukemia (AML) holds great promise as a modality to overcome tolerance and elicit memory responses needed for durable remissions. Our group pioneered a novel personalized vaccine that targets a broad array of leukemic antigens (Ags) by fusing whole patient-derived AML cells to autologous dendritic cells (DCs). In a clinical trial of AML patients in chemotherapy-induced remission, vaccination induced an expansion of tumor-specific T cells and resulted in prolonged remissions in a subset of patients. Enhancing response to vaccination relies on increasing Ag presentation and overcoming factors in the microenvironment that contribute to tolerance.

While the mechanism of action of hypomethylating agents (HMAs) is multifactorial, HMAs are increasingly recognized as exhibiting immunomodulatory functions thought to contribute to therapeutic efficacy. We have previously shown that the HMA SGI-110 augments AML Ag processing and increases AML susceptibility to T cell-mediated killing. In the present study, we aimed to elucidate the effect of HMAs on enhancing Ag presentation and modulating the AML microenvironment as well as to investigate the impact of HMA on the immunogenicity of a DC/AML vaccine.

We first investigated the ability of SGI-110 to reverse critical aspects of the AML immunosuppressive milieu and enhance vaccine efficacy. C57BL/6J mice were retro-orbitally inoculated with 1x10⁵ syngeneic TIB-49 AML cells and treated with low-dose SGI-110 (1mg/kg) to assess immunologic effects without preventing engraftment. AML engraftment in the spleen and bone marrow (BM) was similarly observed in treated and control animals. However, SGI-110-treated animals showed decreased PD-1 expression on CD4 T cell splenocytes (mean 3.1% vs. 6.3%; n=4; p=0.01). SGI-110 therapy was also associated with decreased myeloid-derived suppressor cell (MDSC) burden in the BM (n=5; p=0.01) and spleen (n=4; p=0.03) compared to control.Consistent with these findings, treated mice demonstrated increased AML-specific immunity via increased T cells isolated from the BM or spleen that expressed intracellular IFNϒ following ex-vivo exposure to TIB-49 lysate (mean 3% vs. 0.5%; n=5; p=0.02).We then examined the potential synergy of SGI-110 + vaccine in this model. Mice treated with SGI-110 + vaccine showed a significant increase in BM and spleen CD4 (n=5; p=0.05) and CD8 (n=5; p=0.02) T cell IFNϒ in response to tumor lysate as compared to vaccine or SGI-110 alone.

To further elucidate the mechanism behind the immune activating effects of HMAs, we investigated the effect of HMA on the endogenous retroviral (ERV) pathway. Activation of this pathway results in increased IFN ß expression leading to increased MHC HLA Class I expression and subsequently, immune stimulation in a solid tumor model (Chiappinelli et al. Cell . 2015). MOLM-14 cells were exposed to low-dose HMA daily X 3 days and analyzed for ERV-3 expression using western blotting (WB). The results showed significant upregulation of ERV-3 expression after treatment. Due to activation of the ERV pathway, increased IFN ß expression was observed with WB analysis. Indeed, analysis of an HLA-A2 positive human AML cell line, THP1, showed significant HLA-A2 upregulation after HMA exposure. Most importantly, HLA-A2 upregulation led to a significant rise in tumor Ag presentation, detected by measuring the binding capacity of a unique T-cell receptor-like antibody using flow cytometric analysis in THP-1 cells.

Given these effects, we sought to examine whether treatment with vaccine + SGI-110 would enhance survival in this model. Mice were treated with: vehicle, SGI-110 X 5 days, vaccine, or both agents. Monotherapy with either agent led to a survival advantage compared to untreated mice (p<0.05). Notably, the combination arm further prolonged survival as compared to untreated mice (p<0.05).

In conclusion, SGI-110 modulates critical factors contributing to the immunosuppressive milieu in AML by decreasing T cell PD-1 expression and promoting MDSC expansion, resulting in enhanced anti-AML immunity. Moreover, SGI-110 augments the immune response to an autologous DC/AML vaccine in a murine model resulting in a survival benefit. In addition, HMA enhances Ag presentation, in part due to activation of the ERV pathway. The combination of a novel personalized DC/AML fusion vaccine and HMA holds great potential and a clinical trial is being initiated.