Abstract 6072: Characterization of the immunological responses in a mouse model expressing humanized FcγR/FcRn

Abstract Therapeutic antibodies have revolutionized cancer treatment by leveraging immune mechanisms such as Fc-effector functions, which depend on interactions between IgG and Fcγ receptors (FcγRs). However, preclinical evaluation of antibody pharmacokinetics (PK) and pharmacodynamics (PD) remains challenging due to species-specific differences in FcγR and FcRn expression and function. We previously reported a FcγR humanized mouse model that expresses a human-like pattern of FcγRs (including FcγRI, FcγRIIA, FcγRIIB, FcγRIIIA, and FcγRIIIB - genO-hFcγR). These receptors are functional and enable accurate evaluation of Fc-effector functions such as ADCC and B-cell depletion. The model demonstrated Fc-dependent activity of therapeutic IgG, allowing differentiation between antibodies with regular versus enhanced FcγR binding, and supports ranking of Fc-engineered antibodies in preclinical studies. Herein we describe a model expressing human FcRn in addition to humanized FcγRI, FcγRIIA, FcγRIIB, FcγRIIIA, and FcγRIIIB. As the FcRn is involved in IgG recycling and transport, this model was developed to enhance the translatability of PK studies by enabling human FcRn-mediated IgG recycling, while maintaining PD assessment of Fc-engineered therapeutic antibodies. We now aim to characterize the immunological competence of the humanized genO-FcγR/FcRn model in the context of inflammation and humoral immunity. Specifically, we investigated whether the immune responses of these mice are comparable to those of wild-type (WT) mice when exposed to defined immunological challenges. We assessed cytokine profiles and cell activation following lipopolysaccharide (LPS) stimulation. Upon LPS challenge, genO-hFcγR/hFcRn mice secreted inflammatory cytokines at levels comparable to WT controls. Additionally, CD25 and CD69 were upregulated on NK and T cells. Similarly, treatment with anti-mouse CD3 antibody triggered a rapid immune response characterized by elevated production of multiple cytokines in both genO-hFcγR/hFcRn and WT mice. Finally, to evaluate humoral responses, we performed a T-cell dependent antibody response assay using Keyhole Limpet Hemocyanin (KLH) as immunogen. genO-hFcγR/hFcRn mice successfully produced antigen-specific IgG and IgM antibodies in response to treatment with KLH, confirming functional cooperation between T and B cells, as observed in WT mice. In summary, these results demonstrate that genO-hFcγR/hFcRn mice are immunologically competent and respond physiologically to immune challenges. This model is a robust and reliable tool for studying Fc-engineered therapeutic antibodies in a context that preserves native immune functionality. The genO-hFcγR/hFcRn model is also being improved to enable tolerability to hIgG1 antibodies, and flexibility of therapeutics testing through expression of human immune checkpoints. Citation Format: Angela Pappalardo, Julie Donaghey, Danielle Huggins, Patrick Kirby, Fabiane Sonego, Gaëlle H. Martin, Kader Thiam. Characterization of the immunological responses in a mouse model expressing humanized FcγR/FcRn 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 6072.