IgG4 breaking the rules

Abstract

SUMMARYImmunoglobulin G4 (IgG4) antibodies have been known for some time to be functionally monovalent. Recently, the structural basis for this monovalency has been elucidated: the in vivo exchange of IgG half-molecules (one H-plus one L-chain) among IgG4. This process results in bispeci®c antibodies that in most situations will behave as functionally monovalent antibodies. The structural basis for the abnormal behaviour of IgG4 seems to be largely the result of a single amino acid change relative to human IgG1: the change of a proline in core hinge of IgG1 to serine. This results in a marked shift in the equilibrium between interchain disulphide bridges and intrachain disulphide bridges, which for IgG4 results in 25±75% absence of a covalent interaction between the H-chains. Because of strong non-covalent interactions between the CH3 domains (and possibly also between the CH1 domain and the trans-CH2 domain) IgG4 is a stable four-chain molecule and does not easily exchange halfmolecules under standard physiological conditions in vitro. We postulate that the exchange is catalysed in vivo by protein disulphide isomerase (PDI) and/or FcRn (the major histocompatibility complex (MHC)-related Fc receptor) during transit of IgG4 in the endosomal pathway in endothelial cells. Because IgG4 is predominantly expressed under conditions of chronic antigen exposure, the biological relevance of this exchange of half-molecules is that it generates antibodies that are unable to form large immune complexes and therefore have a low potential for inducing immune in¯ammation. In contrast to monovalent immunoglobulin fragments, these scrambled immunoglobulins have a normal half-life. The signi®cance of the ensuing bispeci®city needs further evaluation, because this will be relevant only in situations where high IgG4 responses are found to two unrelated antigens that happen to be present in the body at the same time and place. In this context the signi®cance of IgG4 autoreactivity might have to be re-evaluated. The main function of IgG4, however, is presumably to interfere with immune in¯ammation induced by complement-®xing antibodies, or, in the case of helminth infection or allergy, by IgE antibodies.

Engineering an Improved IgG4 Molecule with Reduced Disulfide Bond Heterogeneity and Increased Fab Domain Thermal Stability

“…IgG4 has also been shown to be liable to undergo Fab arm exchange, and the presence of HCLC after running IgG4 on SDS-PAGE has been well described (15,17,18,20). The hinge flexibility and its tendency to form intra-DSBs can be limited by the introduction of S241P to the core hinge (20,21,30).…”

“…Comparisons between IgG1 and IgG4 have also consistently shown that IgG4 molecules have lower Fab domain thermal stability compared with IgG1. IgG4 molecules are unique compared with other human IgG isotypes in that they can form functionally monovalent bispecific molecules through a mechanism called "Fab arm" or "halfmolecule" exchange (15)(16)(17)(18)(19). The interhinge DSBs at positions 239 and 242 have been shown to be liable to form intrahinge DSBs generating half-molecules that can covalently reassociate with an IgG4 half-molecule of the same or a different variable region (20,21).…”

IgG subclass distribution of anti‐ADAMTS13 antibodies in patients with acquired thrombotic thrombocytopenic purpura

“…However, only large immune complexes can crosslink FccRs, thereby activating innate immune effector cells [23]. Functional monovalency might decrease the pathologic potential of IgG 4 antibodies and might, during contact with antigen, result in only small and relatively harmless immune complexes with a low potential for induction of immune activation [24]. Although anti-ADAMTS13 IgG 4 antibodies have yet to be shown to be functionally monovalent and to interfere with complement activation in vivo, it is tempting to speculate that IgG 4 autoantibodies, to some extent, act as ÔprotectiveÕ antibodies in TTP patients, inducing a milder and treatable form of TTP, whereas IgG 1 (and IgA) anti-ADAMTS13 antibodies might have higher pathogenic potential, particularly in the absence of IgG 4 .…”

Switch-mediated activation and retargeting of CAR-T cells for B-cell malignancies

“…To test whether this notion could be translated to the IgG4 sCAR design, a serine-to-proline mutation [S228P relative to the IgG4 molecule (33)] was incorporated in the IgG4 hinge to enhance interchain sCAR disulfide formation (IgG4m) (SI Appendix, Fig. S6A).…”