Abstract 3126: Tumor microenvironment composition is shaped by tumor cell line-derived xenograft subtype and tumor burden in BRGSF-HIS mice

Abstract Emergence of immunotherapies for treating cancer has been an important step towards a more efficient, selective and safe approach for patients. However, only a fraction of patients can respond to treatments currently available, requiring the identification of additional therapeutic targets. The relevance of preclinical models in the identification of new therapeutic targets and assessment of immunotherapies has vastly improved with mice bearing a human immune system. BRGSF (Balb/C Rag2-/-, IL2Rγ-/-, SIRPαNOD and Flt3-/-) is a highly immunodeficient mouse featuring reduced murine myeloid cells. BRGSF mice reconstituted with human cord blood CD34+ cells (BRGSF-HIS) develop functional lymphoid and myeloid compartments. This engraftment is stable for over a year (Labarthe et al., 2019) and mice do not develop GvHD. Human myeloid compartment can be transiently boosted with exogenous human Flt3L injections. In contrary to other models, which overexpress human cytokines to develop human myeloid cells, Flt3L-treated BRGSF-HIS mice do not show side effects. BRGSF-HIS mice are permissive to mouse and human cancer cell lines engraftment and represent a valuable preclinical model to study cancer development and evaluate novel therapeutics. Here, we show that composition of tumor microenvironment (TME) is tumor burden-dependent. Indeed, BRGSF-HIS mice engrafted with triple negative cancer cell line MDA-MB-231 have a diverse TME, enriched in myeloid cells. The major cell type present in tumors of approximately 200mm3 is CD206+/CD163+ M2-like macrophages, which express high levels of PD-L1 and other immunomodulatory proteins. At this tumor volume, TME is also composed of a small fraction of T cells. However, the frequency of conventional CD4+T cells increase from <5% to 15% of human infiltrate when tumors reach approximately 500 mm3. Interestingly, NK cells are detected at very low levels in tumors of 200 mm3, but increase to 10% in the TME of tumors of 500 mm3. The infiltration of human immune cells in the TME is also tumor type-dependent, as in a colon-derived adenocarcinoma SW480 tumor model, conventional CD4+ T cells are the major cell type present in the TME (40%) of tumors of 200 mm3. Interestingly, these tumors show an infiltrate of γδ T cells as well, ranging from 6% to 8%of human immune cells in tumors of 200 mm3 and 800 mm3, respectively. BRGSF-HIS mice hence represent a valuable tool to investigate immune cell infiltration in the TME, enabling a translatable assessment of mechanism of action of immunotherapies. Citation Format: Florent Creusat, Siham Hedir, Alexis Gonon, Amélie Marguier, Yacine Cherifi, Fabiane Sonego, Gaëlle Martin, Kader Thiam. Tumor microenvironment composition is shaped by tumor cell line-derived xenograft subtype and tumor burden in BRGSF-HIS mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3126.