Abstract PR001: Cytokine-linked stromal remodeling in transformed follicular lymphoma impacts macrophage function and can be detected non-invasively

Introduction: Despite transformation of indolent follicular lymphoma (FL) into aggressive lymphoma (transformed FL, tFL) remaining the main cause of disease-related mortality, the underlying mechanisms of transformation remain incompletely understood. With recent evidence suggesting that non-malignant populations in the microenvironment contribute to the process of transformation, this study aimed to dissect the role of lymphoma microenvironment interactions in FL transformation. Methods: Comprehensive proteomic (CODEX) and transcriptomic profiling, including bulk and single cell RNA-sequencing alongside spatial transcriptomics (GeoMx, Visium HD), was applied to 95 FL and tFL nodal samples. Plasma proteomic profiling (Somascan) was applied to plasma from FL (n=6) and tFL (n=7) samples. In vitro coculture assays involving primary macrophages and fibroblast, FL and tFL cell lines and primary patient-derived samples were used for functional validation. Results: Across all methods, FL associated lymph-node-organizing follicular dendritic cells and CCL21+ fibroblast reticular cells were lost at tFL and replaced by emerging CCL21- fibroblasts. These CCL21- fibroblasts colocalized with tFL-enriched GPNMB+SPP1+ macrophages forming macrophage-fibroblast niches in tFL. To determine the functional impact of these niches, macrophages differentiated from peripheral blood monocytes from healthy donors and an FL patient were exposed to fibroblast conditioned media (CM) from two fibroblast cell lines in vitro. Exposure to fibroblast CM led to increased levels of macrophage SPP1 (p0. 05), determined by flow cytometry, and impaired phagocytic capacity of macrophages against FL/tFL cell lines (adj. p 0. 01) and autologous primary FL cells, measured through live cell imaging, providing evidence for a pro-tumoral function of these macrophage-fibroblast niches. Stromal remodeling at transformation was accompanied by a loss of stromal organizing cytokines CCL21, CXCL13 and CCL19 in transcriptomic data of tFL lymph nodes, resulting in distinct cytokine profiles in FL and tFL. Through plasma profiling of 86 soluble fibroblast and macrophage-associated proteins in 13 plasma samples (FL=6, tFL=7) from 11 patients, we demonstrate that these distinct nodal cytokine profiles of FL and tFL can be detected non-invasively in the plasma. tFL plasma samples were characterized by lower CCL19 (p=0. 027), CXCL13 (p=0. 049) and CCL21 (p=0. 145) compared to FL. Comparing plasma proteomic data from an independent dataset of untreated FL patients (n=18) and healthy donors (n=3), these cytokines were significantly (p0. 01) higher in indolent FL as compared to healthy controls. Conclusion: The study identifies and demonstrates the functional impact of cytokine linked microenvironmental remodeling at FL transformation, including the creation of pro-tumoral niches of colocalizing GPNMB+SPP1+ macrophages and CCL21- fibroblasts, providing potential for future novel therapeutic strategies targeting these niches and for non-invasive diagnostics. Citation Format: Laura Beckmann, Nicholas J. Haradhvala, Stephanie Pei Tung Yiu, Sam Sadigh, Zachary Leibowitz, Erik Landolsi, Santiago Rivero, Stephanie Deng, Sabrina Hu, Mingzeng Zhang, Daniel Filip, Amber Pospistle, Yao Yu Yeo, Katie Maurer, Johan Gustafsson, Kai Stewart, Vignesh Shanmugam, McKayla Van Orden, Wesley Lu, Shuqiang Li, Kenneth J. Livak, Huaying Qiu Qiu, Ankit Basak, Alex K. Shalek, Brian P. Danysh, Mikaela McDonough, Satyen Gohil, Yue Ren, Yinglu Zhou, Donna Neuberg, Svitlana Tyekucheva, Scott J. Rodig, Eric Jacobsen, Marek Mraz, Mark A. Murakami, Habibe Kurt, Catherine J. Wu, Gad Getz, Sizun Jiang, Erin M. Parry. Cytokine-linked stromal remodeling in transformed follicular lymphoma impacts macrophage function and can be detected non-invasively abstract. In: Proceedings of the Fifth AACR International Meeting on Advances in Malignant Lymphoma: From Discovery to Clinical Impact; 2026 Jun 24-27; Philadelphia, PA. Philadelphia (PA): AACR; Blood Cancer Discov 2026;7 (3Suppl): Abstract nr PR001.