Abstract B044: Spatial transcriptomics reveals a distinct B cell signature in the dormant prostate cancer bone microenvironment
Authors: Nasr Mostafa M. , Bishop Ryan T. , du Bois Haley , Li Tao , Frieling Jeremy S. , Lynch Conor C. 
Published: November 2024

Abstract
Background & Objective: Prostate cancer (PCa) cells frequently disseminate to bone where they can remain dormant for extended periods. The precise cellular and molecular mechanisms responsible remain largely unknown. The advent of spatial transcriptomics provides a unique opportunity to study key bone microenvironment-dormant PCa cell interactions in situ. Here, we developed a novel immunocompetent in vivo model of PCa dormancy in bone and employed the technology to address this major gap in knowledge. Methods: We established an in vitro protocol to induce dormancy in multiple PCa cell lines, including the C57BL/6 murine PCa cell line, RM1. Isogenic active and dormant PCA cells underwent bulk RNA Seq. We used an intra- iliac artery (IIA) approach to allow niche-specific seeding of dormant PCa within the bone. Spatial transcriptomics (10X Visium) was performed on tissue sections and R packages (Seurat & CellChat) were used for data analysis. Results: We validated in vitro PCa dormancy via cell cycle analysis, membrane dye retention and ERK/p38 ratio. RNASeq analysis of dormant PCa cells (vs. active) revealed upregulation of immunomodulatory genes such as, GDF15, BDNF, A2M, VGF, NLRP1, and CD55. C57BL/6 immunocompetent mice were inoculated with active or dormant RM1 cells via IIA. Majority of active RM1 mice reached endpoint within 15 days. In contrast, mice bearing dormant RM1 (D-RM1) had extended survival and were removed from study after 30 days. Histological analysis revealed solitary D-RM1 (panCK+/Ki67-/CC3-) on endosteal surfaces compared to macrometastases (high Ki67+) in the active RM1 group. Using IIA PBS injected tumor naïve bones as a control, we applied spatial transcriptomics (10X Visium) to interrogate the microenvironment surrounding active and dormant RM1 PCa cells. Bioinformatic analyses demonstrated higher expression of B cell markers (CD19, CD79a, CD79b, Ighm, and Pax5) and enhanced B cell activation/antigen phagocytosis signature in the D- RM1 group compared to other groups. We validated the presence of CD19 B-cells in proximity to the D-RM1 cells in subsequent tissue sections. In contrast, B cells were located less frequently and to the periphery of active RM1 metastases. Inference analysis using CellChat also identified a significant and distinct signaling pattern between CD55 and CD97 in the B cell enriched clusters in D-RM1 group. CD55/CD97 interaction is a noted mediator of B cell homing. These data suggest a potential role for distinct populations of B cells during the establishment of PCa dormancy in bone. Conclusion: Spatial transcriptomic analysis of the dormant PCa bone microenvironment reveals an enrichment of CD19+ B cells with a distinct signaling pattern. Given their known roles in immune tolerance we posit that infiltrating B cells may play a role in protecting and sustaining newly disseminated dormant PCa cells from immune mediated elimination.
Citation Format: Mostafa M. Nasr, Ryan T. Bishop, Haley du Bois, Tao Li, Jeremy S. Frieling, Conor C. Lynch. Spatial transcriptomics reveals a distinct B cell signature in the dormant prostate cancer bone microenvironment [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr B044.