Abstract 1644: Generation and validation of humanized GARP mice for testing novel anti-human GARP antibodies

Abstract GARP is a type I transmembrane protein expressed on regulatory T cells (Tregs) and platelets. GARP expression has been shown to play an important role in the modulation of latent TGF-β1 activity by regulatory T lymphocytes (Tregs), and thus, GARP has emerged as an important regulator of peripheral immune tolerance. While GARP expression is critical for homeostasis, increased GARP expression on Tregs is associated with elevated immunosuppression in the context of tumor biology. Because of these immunomodulatory roles, targeting GARP in the context of oncologic or other diseases is of significant interest. Thus, the development of mouse models for safety and efficacy evaluation of GARP-targeted therapeutics prior to clinical trials is needed. To facilitate assessment of novel anti-human GARP antibodies, Biocytogen has generated a humanized GARP mouse model for in vitro functional validation and in vivo efficacy evaluation assays. Using CRISPR-based gene editing technology, the exons of the mouse Garp gene that encode the extracellular domain were replaced by the human GARP counterpart. mRNA Expression of the human GARP extracellular domain was confirmed in splenocytes isolated from homozygous humanized mice. Expression of human GARP was readily detected in FoxP3+ Treg cells isolated from the spleen of humanized GARP mice as assessed by flow cytometry. Importantly, the frequency of FoxP3+ Tregs and other mature leukocyte subpopulations, including T and B lymphocytes, Natural killer cells, dendritic cells, granulocytes, and monocytes/macrophages, was also similar in the blood and spleen of humanized GARP mice compared to wild-type mice. Subsequent in vivo efficacy assays using the MC38 tumor model demonstrated efficacy of the anti-human GARP antibody in slowing tumor growth in humanized GARP mice. Furthermore, combination of anti-human GARP and anti-mouse PD-1/PD-L1 blockade resulted in a significantly greater tumor inhibition than monotherapies. In summary, GARP humanized mice are a powerful platform for preclinical evaluation of anti-human GARP antibodies for cancer treatment. Citation Format: Ruili Lyu, Huilin Li, Jenna Frame, Yuelei Shen. Generation and validation of humanized GARP mice for testing novel anti-human GARP antibodies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1644.