Abstract 251: Inhibition of stromal p38MAPKalpha triggers innate-adaptive anti-tumor immunity in metastatic breast cancer

Abstract Metastases are the main cause of death in breast cancer patients. Among breast metastatic sites, bone is the most prevalent site that can also seed secondary metastases. Moreover, once tumor cells reach the bone, the disease is considered incurable and treatments are only palliative. For this reason, new therapies that limit metastatic growth are desperately needed. We previously showed that pharmacological inhibition of p38MAPKα (p38i) limited visceral and bone metastases in several clinically relevant models of breast cancer metastasis. Indeed, we found that this approach had no impact on tumor cells but instead limited the supporting nature of the stromal compartment. These findings led us to ask how p38i impacted the stromal compartment. Using a novel strategy to segregate tumor-infiltrating from non-tumor-infiltrating stromal cells in the bone metastatic site, we carried out single cell RNA sequencing (scRNA-Seq) to investigate how p38i altered immune and non-immune tumor stroma. Our analyses revealed that p38i treatment shifts tumor-associated macrophages in the bone towards a tumor-suppressive phenotype with increased signatures for IFNγ signaling and adaptive immunity. Importantly, the gene signature we identified in bone macrophages under p38i treatment correlated with better survival among patients with luminal B breast tumors. Given the importance of the p38i gene signature in patient samples, we next depleted macrophages and also blocked IFNγ and found both approaches reversed the p38i anti-tumor effect. Because T cells are an important source of IFNγ in vivo, we next assessed whether T cells were required by p38i to limit tumor growth. Depletion of CD4+ T cells, but not CD8+ T cells, blunted the p38i anti-tumor effect. Although p38i did not increase CD4+ T cell infiltration, p38i increased the percentage of activated CD4+ T cells infiltrating bone metastases. The requirement for CD4+ T cells in our model and the lack of evident OX40 ligand in our scRNA-Seq data led us to ask if p38i could be used in combination with immunotherapy to further limit metastases. Indeed, combination of p38i and agonist anti-OX40 (agOX-40) immunotherapy synergistically reduced metastatic growth and increased overall survival in our mouse models. Finally, because our work revealed no obvious role for CD8+ T cells in this model, we next asked if introduction of a CD8+ T cell antigen could further increase the anti-tumor effects of p38i. We thus modified PyMT-BO1 cells to express ovalbumin, which contains a strong CD8 antigen and combined it with p38i and agOX40. This combination led to a greater than 90% cure rate of mice with metastatic lesions. Although the mechanisms by which inhibition of p38MAPKα shapes the metastatic tumor microenvironment are still under investigation, our findings indicate that metastatic breast cancer patients may benefit from immunotherapy when carried out in combination with p38i. Citation Format: Douglas Vendas Faget, Xianmin Luo, Qihao Ren, Jiayu Ye, Thomas Cole, Zhangting Yao, Bhavna Murali, Xinming Su, Yalin Xu, Joseph B. Monahan, Katherine N. Weilbaecher, David G. DeNardo, Sheila A. Stewart. Inhibition of stromal p38MAPKalpha triggers innate-adaptive anti-tumor immunity in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 251.