CD155-Tigit Pathway Modulation in Dendritic Cell/Acute Myeloid Leukemia Fusion Vaccine Model

Background: We developed a vaccine consisting of primary AML cells fused with autologous dendritic cells (DC) such that a broad array of antigens are presented in the context of DC-mediated costimulation, thereby expanding the anti-AML T cell repertoire. The reestablishment of tumor tolerance by the microenvironment (TME) can limit vaccine efficacy and reversal of TME-mediated immunosuppression has the potential to enhance vaccine immunogenicity. To this end, using an immunocompetent syngeneic AML murine model, we have shown that hypomethylating agents (HMAs) reverse critical aspects of the immunosuppressive TME, and that HMA + fusion vaccination enhances AML-specific immune responses and survival. The TIGIT-CD155 inhibitory checkpoint pathway is a novel pathway fostering tolerance in the innate and adaptive immune arms. Blockade of this pathway reverses tumor-mediated T cell quiescence and promotes DC and NK cell immune surveillance. We aimed to examine if TIGIT-CD155 pathway inhibition +/- HMA enhances vaccine potency in a murine model.

Methods/Results: Using a syngeneic immunocompetent murine AML model, we first investigated the impact of functional TIGIT monoclonal (mAb) blockade +/- decitabine (DAC) +/- vaccine on immune modulation and murine survival. C57BL/6J mice were retro-orbitally inoculated with 5x10⁴ TIB-49 murine AML cells. Animals treated with TIGIT mAb monotherapy had inferior survival as compared to those treated with DAC or DC/AML vaccine monotherapies. Mice treated with vaccine alone, vaccine + TIGIT mAb, and vaccine + DAC + TIGIT mAb had similar survival but each cohort had improved survival compared to untreated animals, suggesting a vaccine-mediated effect not further enhanced by TIGIT mAb. To this end, we sought to investigate if deleting CD155, the native receptor to TIGIT, would impact AML immunogenicity and murine survival. We determined that baseline expression of CD155 in the murine AML cell line, TIB-49, is 72% by Ab staining and flow cytometric analysis. Using CRISPR/cas9 technology, we generated a CD155 knockout TIB-49 cell line. Black mice were retro-orbitally inoculated with 5x10⁴ CD155-deleted or control TIB-49 AML cells. Animals were treated with vaccine, DAC, or both therapies. BLI imaging was performed 1-2/week. Animals inoculated with CD155-deleted AML cells that were treated with both DAC and vaccine show no signs of disease and remain alive 80 days following inoculation as compared to mice treated with vaccine or DAC alone (Fig 1,2).

Conclusions: We demonstrate that CD155-specific deletion from a murine AML cell line confers a survival advantage when mice are exposed to HMA + vaccine as compared to either agent alone. Studies including TCR repertoire analyses are underway to better understand the immunologic underpinnings of this findings as targeting this pathway has promising translational potential.