POS1140 120 Cross-species cellular mapping and humanization of Fc receptors to advance antibody modeling

Background: Fc receptors mediate the effector functions of antibodies. Immunoglobulin G (IgG), the predominant antibody in circulation and in clinical use, engages diverse Fc gamma (Fcγ) receptors differentially expressed by a myriad of cells. However, the exact expression of patterns of these Fcγ receptors across species, cell types, tissues, and disease contexts remain unclear. Objectives: We aimed to provide a comprehensive overview of Fcγ receptor and FcRn expression at the transcriptomic and proteomic levels in humans, macaques, and mice, and analyze their underlying gene regulatory networks. In addition, we intended to bridge interspecies differences and advance preclinical development of antibodies through the design of novel humanized mice. Methods: Fcγ receptor and FcRn expression was evaluated in multiple species and models at the proteomic, transcriptomic, and gene regulatory level using flow cytometry, single-cell transcriptomics and the integration of large-scale prior knowledge models. Results: We reveal crucial species-specific differences in both Fcγ receptor diversity and cell-specific expression that compromise the translation of mouse and macaque preclinical models for antibody research. To mitigate this issue, we generated a new mouse model in which humanized Fcγ receptors (FcγRI/CD64, FcγRIIA/CD32A, FcγRIIB/CD32B, FcγRIIIA/CD16A, and FcγRIIIB/CD16B) expressed under control of their relevant human promotors replace their murine counterparts. To further improve human antibody pharmacokinetics, human FcRn also replaces its murine ortholog. We comprehensively mapped the expression of the knock-in receptors and found that humanization led to a more faithful cell-specific Fcγ receptor expression. We validated the functionality of these knock-in humanized receptors through antibody-dependent antigen presentation, anaphylaxis, cytotoxicity, and pharmacokinetic assays, and reveal how cytokines profoundly impact cell type-specific expression of Fcγ receptors. Conclusion: This cross-species Fcγ receptor atlas and humanized mouse model with refined Fcγ receptor expression and antibody pharmacokinetics is expected to improve the preclinical assessment of antibody-based therapeutics. REFERENCES: NIL . Acknowledgements: NIL . Disclosure of Interests: Karel F.A. Van Damme: None declared, Dorine Sichien Argenx, Katrien Van der Borght: None declared, Justine Van Moorleghem: None declared, Elisabeth De Leeuw: None declared, Dirk Elewaut not related to this project, Ariane Morel Innate Pharma, Eric Vivier Innate Pharma, Fabiane Sonego genOway, Kader Thiam genOway, Sofie Voet argenx, Bianca Balbino argenx, Bart Lambrecht not related to this project. © The Authors 2025. This abstract is an open access article published in Annals of Rheumatic Diseases under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Neither EULAR nor the publisher make any representation as to the accuracy of the content. The authors are solely responsible for the content in their abstract including accuracy of the facts, statements, results, conclusion, citing resources etc.