The Fab Conformations in the Solution Structure of Human Immunoglobulin G4 (IgG4) Restrict Access to Its Fc Region

Abstract

Background: The human IgG4 antibody subclass does not activate complement and forms half-antibodies.Results: Ultracentrifugation and x-ray/neutron scattering together with atomistic modeling revealed asymmetric concentration-dependent IgG4 solution structures.Conclusion: The complement and receptor Fc binding sites are hindered by the Fab regions, explaining loss of activity.Significance: These solution structures clarify IgG4 function and its therapeutic applications.

The Solution Structures of Two Human IgG1 Antibodies Show Conformational Stability and Accommodate Their C1q and FcγR Ligands

“…The successful outcome of our analyses accounted for the reactivity of IgG1 for Fc␥R and C1q. This is in marked contrast to our recent similar analyses for human IgG4, where this binding to the Fc region was most likely sterically hindered by the Fab regions (15). Previously, conformational instabilities were found in IgG4 (15); it is therefore also crucial to identify whether or not IgG1 is also affected by the same instabilities that occur in IgG4.…”

“…for IgG4, which showed a concentration dependence of M2 below 2 mg/ml (14,15). IgG1 19a showed two M1 and M2 peaks at similar values of ϳ4 and ϳ8 nm, respectively (Fig.…”

“…To test a broad range of solution conditions, the six modeled x-ray curves were IgG1 6a at the highest available concentrations of 2, 4, and 2 mg/ml in PBS-50, PBS-137, and PBS-250, respectively, plus IgG1 19a at the highest available concentrations of 1.4, 1.9, and 1.6 mg/ml in PBS-50, PBS-137, and PBS-250, respectively. As previously (15), the occurrence of 4% dimer was assumed to have little effect on the scattering modeling. The modeled neutron curve for IgG1 6a was the highest concentration of 4 mg/ml in PBS-137 at 20°C in heavy water.…”

“…The comparison of IgG1 6a and IgG1 19a with human IgG4 (15) 15). Both data sets indicate that IgG1 is more elongated than IgG4.…”

“…12, A-C). The combination of these IgG1 solution structures with a docking model for the interaction between human IgG1 Fc and the crystal structure of the C1q globular head (42,43) and the crystal structure of the human Fc-Fc␥R receptor (44) shows that the Fc region of human IgG1 is exposed and enables this to react readily with its two major effector ligands, unlike human IgG4 (15).…”