Guide: Humanized FcγR mouse models for the assessment of therapeutic antibodies – practical considerations for translational research

Key messages (summary)

• Fcγ receptors (FcγRs) are critical determinants of antibody efficacy and safety, especially for IgG-based therapeutics.
• Species differences between murine and human FcγRs significantly impact antibody binding, effector function, and PK/PD interpretation.
• Wild-type mice often provide misleading results for Fc-mediated functions (e.g., ADCC, ADCP).
• Humanized FcγR mouse models enable translationally relevant evaluation of: Effector function, Fc-engineering strategies, and safety (e.g., cytokine release, immune activation)
• Model selection directly impacts data interpretation and candidate ranking.
• Using an inappropriate model can lead to: False efficacy signals, under- or over-estimation of toxicity, and incorrect Fc-optimization decisions

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Introduction/background

Therapeutic antibodies rely not only on target binding via Fab regions, but also on Fc-mediated engagement of immune cells.

Fc-FcγR interactions regulate several mechanisms, such as antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP).

Implication: Accurate modelling of FcγR biology is essential for predicting clinical performance.

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What are Fcγ receptors?

Fcγ receptors (FcγRs) are cell surface receptors expressed on immune cells that bind the Fc region of IgG antibodies.

Main human FcγR

• Activating receptors: FcγRI (CD64), FcγRIIA (CD32A), FcγRIIIA (CD16A)
• Inhibitory receptor: FcγRIIB (CD32B)

In humans, FcγRs are expressed as follows and drive key effector functions:

• NK cells → ADCC (primarily FcγRIIIA)
• Macrophages → phagocytosis (FcγRI, FcγRIIA)
• Dendritic cells → antigen presentation
• B cells → immune regulation (FcγRIIB)

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What is their role in the immune response?

FcγRs link humoral immunity to cellular effector functions.

Engagement of FcγRs leads to:

• Target cell killing (ADCC)
• Phagocytosis (ADCP)
• Cytokine secretion
• Immune complex clearance

Implication for therapeutics: FcγR engagement is often required for efficacy (e.g., oncology antibodies), but can also drive toxicity (e.g. systemic inflammation)

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What are the differences between mouse and human FcγR?

Major translational gaps

• Different receptor repertoire (Van Damme et al., 2026): Humans: FcγRI, IIA, IIB, IIIA, IIIB vs Mice: FcγRI, IIB, III, IV (non-orthologous mapping)
• Different binding affinities: Human IgG subclasses interact differently with mouse FcγRs
• Different expression patterns: Cell-specific expression does not fully overlap, e.g., human NK cells strongly drive ADCC via FcγRIIIa whereas mouse NK cells are less effective, and mouse macrophages rely on FcγRIV (absent in humans), leading to engagement of different effector mechanisms for the same antibody
• Different signaling balance: Activating vs inhibitory receptor ratios differ

‍Key consequence: Murine FcγR biology does NOT accurately predict human Fc-FcγR interactions, leading to poor translational relevance (Van Damme et al., 2026; Nimmerjahn & Ravetch, 2008; Bruhns, 2012).

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How to choose the correct model for a study

Ask yourself these questions first:

• Does my antibody rely on Fc-effector functions (ADCC / ADCP)?
• Is Fc engineering (e.g., glycoengineering, mutations) part of the strategy?
• Do I need to assess efficacy, safety, or both?
• Is this for early screening or decision-grade data?
• Do I need human-like immune cell engagement?

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Comparison of humanized FcγR mouse models

Takeaway: Models with physiological expression of the complete human FcγR repertoire provide the highest translational value.

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Practical examples

What happens if I test an Fc-engineered antibody in WT mice?

Observed effects:

• Reduced or absent Fc-dependent activity

Consequence:

• False negative efficacy
• Misleading ranking of Fc variants

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What model should I use to evaluate Fc optimization?

Preferred approach

Humanized FcγR Knockin models with:

• Full receptor repertoire
• Physiological regulation

Why:

• Captures binding differences between Fc variants
• Captures functional effects (ADCC/ADCP)
• Captures safety-relevant activation

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How to design an ADCC study in mice

Use models expressing human FcγRIIIa (CD16A)

Ensure:

• Presence of functional NK cells
• Correct FcγR expression levels

Select appropriate:

• Tumor model (syngeneic vs xenograft)
• Antibody dose range

Avoid:

• WT mice for human IgG1 ADCC evaluation

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What endpoints should be measured?

Efficacy endpoints

• Tumor growth inhibition
• Target cell depletion
• Survival

Mechanistic endpoints

• NK cell activation markers
• Phagocytosis assays
• Receptor occupancy

Safety endpoints

• Cytokine profiling
• Immune cell activation status
• Off-target effects

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What are the limitations of FcγR mouse models?

Even humanized models:

• May not fully recapitulate human immune complexity
• Can differ in cell distribution or activation thresholds

HIS models:

• High variability
• Xenogeneic artefacts

Tumor models:

• Lack of fully human tumor microenvironment

Conclusion: FcγR models improve predictivity but must be interpreted within context.

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FAQ

What is the best FcγR model for antibody studies?

The most appropriate FcγR mouse model is one that expresses the full set of human Fcγ receptors under physiological regulation such as the genO-hFcγR model, as this enables accurate recapitulation of Fc-mediated immune engagement and supports translationally relevant assessment of antibody efficacy and safety (Van Damme et al., 2026, Nimmerjahn & Ravetch, 2008; Bruhns, 2012).

When do I need a humanized FcγR mouse model?

A humanized FcγR mouse model is required when evaluating therapeutic antibodies whose mechanism of action depends on Fc-mediated effector functions, particularly in oncology or immunology settings, where accurate prediction of ADCC, ADCP, or cytokine responses is critical.

What are the differences between FcγR humanized mouse models?

FcγR humanized mouse models differ in the number of receptors humanized, their expression patterns, and whether they are expressed under endogenous regulatory control, with models recapitulating the full human FcγR repertoire, such as the genO-hFcγR model, generally providing superior translational relevance compared to partial or non-physiological systems.

How are FcγR models used to evaluate therapeutic antibodies?

Humanized FcγR models are used to assess antibody efficacy, mechanism of action, and safety by enabling in vivo engagement of immune effector cells through human Fc receptors, thereby providing insight into ADCC, ADCP, and cytokine-mediated effects.

How are FcγR models used in ADCC studies?

FcγR models allow evaluation of ADCC by supporting interaction between antibody-opsonized target cells and FcγR-expressing effector cells such as NK cells, enabling quantification of tumor cell killing and immune activation in a human-relevant context.

How do Fcγ receptors influence antibody efficacy?

Fcγ receptors influence antibody efficacy by determining the strength and quality of effector cell engagement, with activating receptors promoting cytotoxicity and phagocytosis, while inhibitory receptors dampen immune responses, thereby shaping the overall therapeutic outcome (Nimmerjahn & Ravetch, 2008).

How do Fcγ receptors influence antibody safety?

Fcγ receptors influence antibody safety by mediating immune activation and cytokine release, which can contribute to systemic inflammation or adverse events when excessive or non-specific Fc engagement occurs.

How do Fc engineering modifications affect FcγR interactions?

Fc engineering modifications, such as glycoengineering or point mutations, alter the affinity of antibodies for specific Fcγ receptors, enabling tuning of effector functions to enhance efficacy or reduce toxicity.

How do FcγR polymorphisms impact antibody function?

FcγR polymorphisms can significantly influence antibody binding affinity and downstream effector functions, contributing to variability in patient responses to antibody therapies.

Can FcγR models be combined with FcRn or other models?

FcγR models can be combined with FcRn humanized models or other immune-humanized systems to simultaneously assess antibody effector function and pharmacokinetics, providing a more comprehensive evaluation of therapeutic antibodies.

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References

1. Bruhns, P. (2012). Properties of mouse and humanIgG receptors and their contribution to disease models. Blood, 119(24),5640–5649.
2. Nimmerjahn, F., & Ravetch, J. V. (2008). Fcγreceptors as regulators of immune responses. Nature Reviews Immunology,8, 34–47.
3. Van Damme, KFA et al. (2026). Cross-species cellular mapping and humanization of Fcγ receptors to advance antibody modeling. Science Immunology, 11, eady7328

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