Abstract LB-278: Vaccination with CD47-/- whole tumor cell vaccines creates a homogenous tumor microenvironment with distinct suppressor cell phenotypes

Cancer recurrence and metastasis accounts for approximately 90% of cancer-related deaths, despite which effective prophylactic measures are unavailable. Vaccination is a powerful tool for generating tumor-specific responses by exposing masked tumors to the immune system. Using CD47, a cell-surface protein that interacts with SIRP-α on macrophages as the target, we developed whole-cell cancer vaccines for the subcutaneous mouse melanoma. We immunized C57BL/6 mice using non-replicating CD47-/- B16F10 cells and challenged the mice with B16F10 tumors a week later. We showed that the vaccines effectively suppressed tumor growth: 33% of vaccinated mice remained tumor-free 90 days post tumor challenge, and the rest showed 2-fold reduced tumor growth rates and 5-fold reduced tumor sizes. Immunophenotyping showed a dramatic increase in activated effector cell subsets and M1-type macrophages aided by a significant reduction in the tumor-associated macrophage and myeloid-derived suppressor cell compartments. Additionally, we found that CD47 was down regulated in the vaccinated mice significantly. It has been emphasized in previous research that common characteristics of tumors that have been irreversible thus far like metastatic potential, downregulation of MHC genes, and overexpression of evasion markers, can be attributed to the specific suppressive phenotypes of antigen presenting cells in the tumor microenvironment (TME). In the tumors that did escape after vaccination, the TME was analyzed for immune phenotypes at three different stages of tumor growth - small (200-300mm3), medium (500-600mm3), and large (800-900mm3) tumor volumes. In comparison with the vehicle (PBS) vaccinated tumors, we found that in the CD47-/- B16F10 vaccinated tumors, the immune cells infiltrating the tumors maintain significantly homogenous levels throughout the different tumor growth stages. This was true especially for the suppressive phenotypes like the myeloid derived suppressor cells (MDSCs), the tumor-associated macrophages (TAMs), and the PD-1+ exhausted T cell subsets. Additionally, we performed deeper mechanistic analyses of the various TAM subsets infiltrating all the tumors and found that the CD47-/- B16F10 vaccinated tumors had significantly less TAMs, except for one specific subset - Ly6Chi MHC-IIlow TAMs. These TAMs are known to infiltrate in hypoxic conditions and we found corresponding increase in neo-vasculature around these tumors. These TAMs were also found to express high levels of Ly6G, the neutrophil lineage marker. Overall, our preliminary characterization showed that vaccination with syngeneic CD47-/- tumor cells orchestrates the levels and enhances distinct subtypes of tumor infiltrating suppressive lymphocytes, opening up avenues for further combinatorial therapy regimes and aiding the process of turning “cold” tumors “hot”. Citation Format: Subhadra Jayaraman Rukmini, Huanjing Bi, Puloma Sen, Benjamin Everhart, Busra Buyuk, Sha Jin, Kaiming Ye. Vaccination with CD47-/- whole tumor cell vaccines creates a homogenous tumor microenvironment with distinct suppressor cell phenotypes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-278.