THU479 FOXA1 Loss Remodels The Tumor Immune Microenvironment In Late-stage Prostate Cancer

Abstract Disclosure: L.T. Brea: None. H. Shi: None. V. Keo: None. W. Xie: None. X. Lu: None. G. Gritsina: None. X. Wang: None. S.H. Park: None. J. Zhao: None. J. Yu: None. Background: Castration resistant prostate cancer (CRPC) has shown a poor response to immune checkpoint inhibitors due to its immunosuppressive nature. CRPC is infiltrated by tumor-associated macrophages (TAMs) and T regulatory cells (Tregs), which induce an immunosuppressive tumor immune microenvironment (TIME) and promote tumor progression. A deeper understanding of tumor intrinsic mechanisms shaping the TIME in prostate cancer (PCa) is needed to harness immunotherapies for PCa patients. Previous studies have found that epithelial transcription factor FOXA1 is downregulated in CRPC. In accordance, we have reported that FOXA1 loss induces tumor cell invasion and macrophage recruitment in vitro through HIF1A-CCL2 signaling. However, the extent to which FOXA1 loss regulates the TIME in PCa remains unclear, largely due to the lack of immune-proficient mouse models for the study of FOXA1 function in the context of PCa. Methods: To examine how FOXA1 loss regulates the PCa TIME, we generated a novel genetically engineered mouse model (GEMM) with prostate-specific deletion of Pten and Foxa1 (PbCre4:Ptenf/fFoxa1f/f) to compare with the established Pten-null PCa mouse model (PbCre4:Ptenf/f). We performed immunohistochemistry analyses on the GEMM prostate tumor tissues to characterize histopathology changes and assess tumor immune infiltration. We also performed spatial transcriptomics and scRNA-seq analyses of the GEMMs to comprehensively evaluate the effect of Foxa1 loss on downstream pathways and the surrounding TIME. Finally, we performed bioinformatics analysis of PCa patient datasets to validate the clinical relevance. Results: We found PbCre4:Ptenf/fFoxa1f/f mouse prostate tumors exhibited a more aggressive histopathologic tumor phenotype compared to age-matched PbCre4:Ptenf/f. Notably, immunohistochemistry staining and spatial transcriptomic analysis of the GEMMs revealed striking remodeling of the TIME upon Foxa1 loss, with marked increases in tumor infiltration by Tregs and TAMs. Moreover, CIBERSORTx analysis of patient datasets revealed FOXA1 expression negatively correlates with Treg and M2-macrophage infiltration. We also found pro-tumorigenic inflammatory cytokine signaling genes, linked to the recruitment of immunosuppressive cells, were upregulated in PbCre4:Ptenf/fFoxa1f/f prostate tumors. Furthermore, we confirmed inflammatory pathways were upregulated among FOXA1-low patient samples. Additional studies are ongoing to further elucidate the mechanisms by which FOXA1 regulates inflammatory cytokine signaling and the TIME in PCa.Conclusion: This study presents a novel GEMM for the study of FOXA1 function in an immunocompetent setting. Our data supports FOXA1 as a critical tumor intrinsic regulator of the TIME. This will have important implications on the design of immunotherapeutic approaches for late-stage PCa often with FOXA1 loss. Presentation: Thursday, June 15, 2023