IgG4 autoantibodies against muscle-specific kinase undergo Fab-arm exchange in myasthenia gravis patients

Inga Koneczny, Jo A. A. Stevens, Anna De Rosa, Saif Huda, Maartje G. Huijbers, Abhishek Saxena, Michelangelo Maestri, Konstantinos Lazaridis, Paraskevi Zisimopoulou, Socrates Tzartos, Jan Verschuuren, Silvere M. van der Maarel, Philip van Damme, Marc H. De Baets, Peter C. Molenaar, Angela Vincent, Roberta Ricciardi, Pilar Martinez-Martinez*, Mario Losen*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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Abstract

Autoimmunity mediated by IgG4 subclass autoantibodies is an expanding field of research. Due to their structural characteristics a key feature of IgG4 antibodies is the ability to exchange Fab-arms with other, unrelated, IgG4 molecules, making the IgG4 molecule potentially monovalent for the specific antigen. However, whether those disease-associated antigen-specific IgG4 are mono- or divalent for their antigens is unknown. Myasthenia gravis (MG) with antibodies to muscle specific kinase (MuSK-MG) is a well recognized disease in which the predominant pathogenic IgG4 antibody binds to extracellular epitopes on MuSK at the neuromuscular junction; this inhibits a pathway that clusters the acetylcholine (neurotransmitter) receptors and leads to failure of neuromuscular transmission. In vitro Fab-arm exchange-inducing conditions were applied to MuSK antibodies in sera, purified IgG4 and IgG1-3 sub-fractions. Solid-phase cross-linking assays were established to determine the extent of pre-existing and inducible Fab-arm exchange. Functional effects of the resulting populations of IgG4 antibodies were determined by measuring inhibition of agrin-induced AChR clustering in C2C12 cells. To confirm the results, kappa/kappa, lambda/lambda and hybrid kappa/lambda IgG4s were isolated and tested for MuSK antibodies. At least fifty percent of patients had IgG4, but not IgG1-3, MuSK antibodies that could undergo Fab-arm exchange in vitro under reducing conditions. Also MuSK antibodies were found in vivo that were divalent (monospecific for MuSK). Fab-arm exchange with normal human IgG4 did not prevent the inhibitory effect of serum derived MuSK antibodies on AChR clustering in C2C12 mouse myotubes. The results suggest that a considerable proportion of MuSK IgG4 could already be Fab-arm exchanged in vivo. This was confirmed by isolating endogenous IgG4 MuSK antibodies containing both kappa and lambda light chains, i.e. hybrid IgG4 molecules. These new findings demonstrate that Fab-arm exchanged antibodies are pathogenic. (C) 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license.

Original languageEnglish
Pages (from-to)104-115
Number of pages12
JournalJournal of Autoimmunity
Volume77
DOIs
Publication statusPublished - Feb 2017

Keywords

  • Fab-arm exchange
  • IgG4
  • MuSK
  • Myasthenia gravis
  • Autoimmunity
  • THROMBOTIC THROMBOCYTOPENIC PURPURA
  • HUMAN-IMMUNOGLOBULIN-G
  • PEMPHIGUS-VULGARIS
  • ACQUIRED NEUROMYOTONIA
  • ACETYLCHOLINE-RECEPTOR
  • NEUROMUSCULAR-JUNCTION
  • SUBCLASS DISTRIBUTION
  • LIMBIC ENCEPHALITIS
  • COMPLEMENT ACTIVATION
  • IMMUNE-COMPLEXES

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