Bruhns2009 - FcγR Specificity and Affinity for IgG Subclasses
Full citation: Bruhns, P., Iannascoli, B., England, P., Mancardi, D.A., Fernandez, N., Jorieux, S., & Daëron, M. (2009). Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses. Blood, 113(16), 3716–3725. https://doi.org/10.1182/blood-2008-09-179754
Raw file: [[raw/bruhns2009.pdf]]
Summary
Bruhns et al. undertook a systematic investigation of the binding specificity and affinity of all six human FcγRs — FcγRI (CD64), FcγRIIA, FcγRIIB, FcγRIIC (CD32), FcγRIIIA, and FcγRIIIB (CD16) — and all known polymorphic variants, for all four IgG subclasses. Prior literature was incomplete, heterogeneous, and sometimes discrepant, with studies using cell lines expressing multiple FcγRs, different ligand preparations, and inconsistent methodologies. This study used CHO-K1 transfectants expressing single, FLAG-tagged FcγRs (sorted to equivalent surface expression) and measured affinity by surface plasmon resonance (SPR) on soluble ectodomains — providing the most comprehensive and internally consistent binding dataset available at publication.
The study established a full affinity hierarchy across all hFcγR–IgG subclass combinations (see affinity table below), identified specific residues determining FcγRIII subclass selectivity, and demonstrated that low-fucosylation of IgG1 monoclonal antibodies enhances binding to FcγRIII (but not FcγRII) by 2–2.7-fold. A central and unexpected finding was that the inhibitory receptor FcγRIIB has markedly lower affinity for IgG1, IgG2, and IgG3 than all activating hFcγRs — with ~35-fold lower affinity for IgG1 than FcγRIIA, making FcγRIIB dependent on co-engagement with activating receptors by the same immune complex to function.
This paper is not a dengue study. It is included in this wiki as the authoritative affinity reference for the FcγR biology that underlies dengue-specific findings: the FcγRIIa H131/R131 polymorphism (Garcia2009, Garcia2010), immune complex accumulation in post-dengue autoimmunity, and ADE via FcγR-mediated viral uptake.
Study Design
- Type: In vitro mechanistic/biochemical study
- Sample size: Not applicable (cell-based binding assays; SPR measurements)
- Setting: Institut Pasteur and Inserm, Paris, France; in vitro
- Experimental systems: CHO-K1 transfectants expressing FLAG-tagged FcγRs; HEK-293T cells producing soluble ectodomain-3xFLAG fusion proteins; BIAcore 2000 SPR biosensor
- FcγRs studied: FcγRI; FcγRIIA-H131 and -R131; FcγRIIB; FcγRIIC; FcγRIIIA-F158 and -V158; FcγRIIIB-NA1, -NA2, -SH
- IgG preparations: Polyclonal human IgG1, IgG2, IgG3, IgG4 (ultracentrifuged monomers + F(ab’)₂-complexed immune complexes); monoclonal chimeric anti-NIP IgG; therapeutic mAbs anti-CD20, anti-RhD, anti-HLA-DR (CHO and YB2/0 produced)
Key Findings
1. IgG Subclass Binding Specificity
- IgG1 and IgG3 bind all hFcγRs
- IgG2 binds FcγRIIA-H131 (high affinity), FcγRIIA-R131 (lower), and FcγRIIIA-V158 (low affinity); does NOT bind FcγRI or FcγRIIIB as ICs
- IgG4 binds FcγRI, FcγRIIA (both variants), FcγRIIB, FcγRIIC, and FcγRIIIA-V158; does NOT bind FcγRIIIB as ICs
- No human FcγR is truly specific for a single IgG subclass — all display subclass selectivity through markedly different affinities
2. Affinity Hierarchy by SPR (KA ×10⁵ M⁻¹)
| Receptor | IgG1 | IgG2 | IgG3 | IgG4 |
|---|---|---|---|---|
| FcγRI | 650±280 | n.m. | 610±320 | 340±190 |
| FcγRIIA-H131 | 52±12 | 4.5±1.0 | 9.1±1.3 | 1.7±0.4 |
| FcγRIIA-R131 | 35±5 | 1.0±0.0 | 8.9±0.1 | 2.1±0.2 |
| FcγRIIB/C | 1.2±0.1 | 0.3±0.0 | 1.7±0.2 | 2.0±0.4 |
| FcγRIIIA-V158 | 20±2 | 0.7±0.1 | 98±15 | 2.5±0.2 |
| FcγRIIIA-F158 | 11.7±1.9 | 0.3±0.0 | 77±11 | 2.0±0.4 |
| FcγRIIIB-NA1/NA2/SH | ~2.0–2.2 | n.m. | ~11 | n.m. |
n.m. = not measurable (no detectable affinity). All values ×10⁵ M⁻¹.
Key values for dengue-relevant interpretation:
- FcγRIIA-H131 vs. R131 for IgG2: 4.5 vs. 1.0 — H131 binds IgG2 4.5× more efficiently
- FcγRIIA-H131 vs. R131 for IgG1: 52 vs. 35 — H131 binds IgG1 ~1.5× more efficiently (not the ~10× difference sometimes assumed)
- FcγRIIA-H131 vs. R131 for IgG3: 9.1 vs. 8.9 — essentially identical
- FcγRIIB vs. FcγRIIA-H131 for IgG1: 1.2 vs. 52 — FcγRIIB is ~43× lower affinity for IgG1
- FcγRIIIA-V158 for IgG3: KA ~9.8×10⁶ M⁻¹ — crosses the high-/low-affinity threshold (~9×10⁶ M⁻¹), enabling binding of monomeric IgG3
3. The Inhibitory Receptor FcγRIIB is Structurally Weak
FcγRIIB has the lowest affinity for IgG1, IgG2, and IgG3 of all hFcγRs. The affinity hierarchy for IgG1 from highest to lowest is: FcγRI >> FcγRIIA > FcγRIIIA > FcγRIIIB >> FcγRIIB. FcγRIIB’s inhibitory signalling (ITIM) therefore cannot operate independently — it requires co-engagement with activating FcγRs by the same immune complex. Activating receptors effectively recruit FcγRIIB into the complex, enabling inhibitory signalling. The implication: whenever activating FcγR engagement exceeds inhibitory FcγRIIB engagement in a leukocyte, the net result is inflammatory activation.
4. FcγRIIIA V158 vs. F158: Qualitative, Not Just Quantitative, Difference
- FcγRIIIA-V158 can bind monomeric IgG3 (KA ~9.8×10⁶ M⁻¹); FcγRIIIA-F158 cannot
- The V158 substitution moderately increases affinity for all IgG subclasses (~1.7-2.5× for IgG1 and IgG2)
- FcγRIIIA binds IgG3 with ~5× higher affinity than IgG1 (V158: 98 vs 20 ×10⁵ M⁻¹) — IgG3 is the preferred ligand
- FcγRIIIB polymorphisms (NA1/NA2/SH) do NOT significantly affect IgG affinity
5. IgG Fucosylation Affects FcγRIII but not FcγRII
Low-fucosylation (YB2/0-produced mAbs) enhanced affinity for FcγRIIIA by 2.2–2.4× and for FcγRIIIB by 1.6–2.7×. Fucosylation did not affect affinity for FcγRIIA or FcγRIIB. This explains why low-fucosylated therapeutic IgG1 has enhanced ADCC (FcγRIIIA-dependent) without affecting the inhibitory pathway (FcγRIIB).
6. H131R Mutation: A Specific Effect on IgG2 and IgG4
- H131R decreases affinity for IgG2 (4.5→1.0 ×10⁵ M⁻¹; 4.5× reduction)
- H131R increases affinity for IgG4 (1.7→2.1 ×10⁵ M⁻¹; modest increase)
- H131R has minimal effect on IgG1 and IgG3 affinity
Methods Used
- Surface Plasmon Resonance (BIAcore 2000; KA determined from steady-state SPR at equilibrium)
- Flow cytometry (indirect immunofluorescence on CHO transfectants)
- CHO-K1 transfection / stable cell line selection
- Site-directed mutagenesis (FcγRIIIA-F158 generated from V158 template)
- SDS-PAGE + Western blot (for ectodomain purity and glycosylation verification)
- PNGase F deglycosylation
Entities Mentioned
- FcγRIIa Receptor (primary focus: H131/R131 affinity data for all IgG subclasses)
- NS1 Protein (indirect: NS1-TLR4 activation engages FcγR-bearing cells)
Concepts Addressed
- Antibody-Dependent Enhancement (FcγRIIA and FcγRIIIA affinity hierarchy; which receptor mediates ADE most efficiently; V158 vs. F158 in secondary dengue IgG1 binding)
- Autoimmunity in Dengue (FcγRIIB weakness → IC clearance failure → IC accumulation → autoimmune activation)
- Infection-Triggered Autoimmunity (FcγRIIB co-engagement model; IC-mediated inflammation)
- Cross-Reactive Antibodies (IgG subclass preferences for FcγRs; which subclasses drive ADE vs. IC-mediated inflammation)
Relevance & Notes
Grounding the FcγRIIa polymorphism story in dengue: Garcia2009 and Garcia2010 show FcγRIIa-HH genotype (homozygous H131) is associated with worse dengue outcomes and post-dengue autoimmune sequelae. The conventional explanation was that HH has lower IgG1 affinity → poorer IC clearance → IC accumulation. Bruhns2009 refutes the affinity premise: H131 (HH) actually binds IgG1 slightly more efficiently than R131 (RR), and binds IgG3 identically. The principal H131/R131 difference is for IgG2 (H131 4.5× better). This repositions the dengue FcγRIIa story: whatever makes HH worse, it is not lower IgG1/IgG3 affinity. Possibilities include (a) stronger activating FcγR signaling via higher IgG1 affinity → more inflammatory activation; (b) excess IgG2-IC capture in H131 individuals if IgG2 anti-dengue antibodies are abundant; or (c) the relevant mechanism is FcγRIIB weakness, not FcγRIIa variant affinity per se.
Grounding IC persistence in dengue autoimmunity: Garcia2009 finds IC accumulation in post-dengue patients correlated with IgG titers, suggesting ongoing antibody production driving IC formation. Bruhns2009 explains why these ICs are pro-inflammatory: activating FcγRs (especially FcγRIIA, KA ~52 ×10⁵ for IgG1) readily capture them, while inhibitory FcγRIIB (KA ~1.2 ×10⁵ for IgG1) is too weak to provide effective negative feedback independently. The net result is sustained activating FcγR signaling without effective inhibitory termination — a plausible molecular basis for chronic IC-driven inflammation in the subset of patients with high IgG titers.
IgG3 and ADCC in dengue: FcγRIIIA-V158 has ~5× higher affinity for IgG3 than for IgG1, and can bind monomeric IgG3 (high-affinity behavior). If early dengue responses are IgG3-dominant, V158 homozygous individuals may have substantially stronger ADCC and FcγRIIIA-mediated cellular responses than F158 homozygous individuals — potentially explaining why FcγRIIIA genotype is associated with anti-CD20 therapeutic efficacy and, by extension, may modulate dengue clearance efficiency.
Questions Raised
- What is the IgG subclass distribution of anti-dengue antibodies in primary vs. secondary infection? If IgG2 is substantial in secondary responses, the H131/R131 IgG2 affinity difference (4.5×) becomes the dominant pharmacological variable.
- Does FcγRIIA-H131 signal more strongly per IC than FcγRIIA-R131 — not just bind more avidly? Receptor signaling capacity, not just affinity, may determine IC clearance vs. IC-driven inflammation outcome.
- Is there a measurable difference in FcγRIIIA-mediated ADCC against DENV-infected cells between V158/V158 and F158/F158 donors using patient-derived anti-dengue IgG3?