Abstract 3371: genO-BRGSF-HIS mice: A humanized mouse model for assessment of Treg-targeting therapies

Abstract Therapies targeting regulatory T cells (Tregs) have emerged as a promising strategy to overcome immune suppression and enhance anti-tumor immunity. Tregs maintain immune tolerance under physiological conditions, but within the tumor microenvironment they promote immune evasion and resistance to immunotherapy. Selective depletion or functional modulation of Tregs can restore effector T-cell activity and improve responses to checkpoint inhibitors and other immunotherapies. However, the development of Treg-targeting approaches requires preclinical models that accurately recapitulate human Treg biology, including phenotype, activation status, and distribution in peripheral tissues and tumors. Conventional mouse models often fail to reflect these human-specific features, limiting their predictive value for safety and efficacy. Humanized models that support the development and long-term engraftment of functional Tregs and mimic human immune-tumor interactions are therefore essential to bridge the translational gap and guide clinical development of next-generation immunotherapies. genO-BRGSF-HIS mice, engrafted with human hematopoietic stem cells, enable long-term reconstitution of diverse human T-cell subsets, including CD4+, CD8+, γδ T cells, and Tregs. Among emerging targets expressed by Treg, CCR8, a chemokine receptor expressed on Tregs, has gained attention for its potential in selective Treg depletion to enhance anti-tumor immunity. Importantly, CCR8 expression differs between species—restricted to tumor-infiltrating Tregs in mice but present on both infiltrating and peripheral Tregs in humans. We assessed CCR8 expression in genO-BRGSF-HIS mice and found a pattern consistent with human biology. Treatment of naïve genO-BRGSF-HIS mice with a CCR8-depleting antibody resulted in Treg depletion in spleen and blood. While efficacy of CCR8-targeting compounds remains to be investigated in genO-BRGSF-HIS mice, it is key to first investigate whether Treg cells are recruited into the tumor microenvironment (TME). Thus, we evaluated human immune cell infiltration in the TME of cell derived xenografts. Tumor infiltration by human immune cells varied by tumor type and burden, with Tregs (CD4+FoxP3+CD127-) displaying dynamic activation profiles, including PD-1, GARP, and TIM-3 expression in the TME of A549 lung carcinoma xenografts. Notably, subsets resembling induced Tregs (iTregs) were identified, suggesting adaptive regulatory mechanisms within the TME. These findings support the use of genO-BRGSF-HIS mice as a translational platform for investigating Treg biology and evaluating therapeutic strategies in oncology. Citation Format: Gaëlle H. Martin, Florent Creusat, Siham Hedir, Amélie Marguier, Fabiane Sonego, Kader Thiam. genO-BRGSF-HIS mice: A humanized mouse model for assessment of Treg-targeting therapies abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3371.