Abstract 73: Blockade of p38 MAPK reduces the tumor-induced immune suppressive microenvironment in metastatic breast cancer

Abstract The ability of CD8+ T cells to mount an anti-tumor immune response is compromised by immune suppression in the tumor microenvironment (TME). Tumor Associated Macrophages (TAMs) and Myeloid Derived Suppressor Cells (MDSCs) are a major part of this immune suppressive network. Targeting these populations remains challenging. Previously, we have reported that pharmacological and genetic blockade of p38 MAPK impeded the expansion and mobilization of monocytic and granulocytic MDSCs in mouse mammary carcinoma models. We also found that blockade of p38 or depletion of MDSCs reduced tumor growth and metastasis while enhancing the levels of CD8+ T cells in the primary tumors. In the present study, we asked whether CD8+ T cells contribute to the anti-metastatic activity of p38 inhibitor (p38i) and how p38 blockade affects the functional status of T cells and MDSCs. By using the mouse mammary carcinoma 4T1 model, we found that depletion of CD8+ T cells negated the effects of p38i on tumor growth and metastasis, indicating that CD8+ T cells contribute to the anti-tumor and anti-metastatic effects of p38 blockade. Next, we examined whether p38i exhibits a direct effect on T cells. The results of the T cell proliferation in vitro assays revealed that p38 blockade did not have a direct impact on T cell proliferation in response to αCD3/αCD28 stimulation. To determine the effect of p38 blockade on T cells in vivo, we performed single cell RNA-seq on the 4T1 tumor models treated with p38i and the 4T1 model with p38α (Mapk14) knockout (p38ko). This study revealed that p38 blockade by p38i or by inactivation of p38 in tumor cells decreased the amount of exhausted T cells and increased Th1 cells in the TME, indicating a positive effect on T cell functions. Furthermore, we observed a significant decrease in inflammatory signaling in granulocytes and monocytes upon p38 blockade. Our previous study showed that p38i did not affect generation of MDSCs in vitro in response to G-CSF & GM-CSF. To determine whether p38i alters MDSCs in vivo, we assessed MDSC gene signature in monocytic and granulocytic MDSCs isolated from spleens of tumor-bearing mice subjected to p38 blockade. This work revealed that the MDSC gene signature was reduced in both p38i and p38ko groups compared to tumor bearing mice treated with vehicle-control. These results indicated a reduction in the MDSC generation in the in vivo model. Our study revealed that blockade of p38 reduces tumor induced immune suppression and may enhance anti-tumor immune response in metastatic breast cancer. Citation Format: Priyanka Rajan, Justin Zonneville, Robert Zollo, Mackenzie Honikel, Sofija Raudins, Sean Colligan, Brian Morreale, Mohammed Alruwaili, Mohammed Alqarni, Scott Olejniczak, Joseph Barbi, Scott Abrams, Andrei Bakin. Blockade of p38 MAPK reduces the tumor-induced immune suppressive microenvironment in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 73.