MuSK myasthenia gravis monoclonal antibodies: Valency dictates pathogenicity

Neurol Neuroimmunol Neuroinflamm. 2019 Feb 21;6(3):e547. doi: 10.1212/NXI.0000000000000547. eCollection 2019 May.


Objective: To isolate and characterize muscle-specific kinase (MuSK) monoclonal antibodies from patients with MuSK myasthenia gravis (MG) on a genetic and functional level.

Methods: We generated recombinant MuSK antibodies from patient-derived clonal MuSK-specific B cells and produced monovalent Fab fragments from them. Both the antibodies and Fab fragments were tested for their effects on neural agrin-induced MuSK phosphorylation and acetylcholine receptor (AChR) clustering in myotube cultures.

Results: The isolated MuSK monoclonal antibody sequences included IgG1, IgG3, and IgG4 that had undergone high levels of affinity maturation, consistent with antigenic selection. We confirmed their specificity for the MuSK Ig-like 1 domain and binding to neuromuscular junctions. Monovalent MuSK Fab, mimicking functionally monovalent MuSK MG patient Fab-arm exchanged serum IgG4, abolished agrin-induced MuSK phosphorylation and AChR clustering. Surprisingly, bivalent monospecific MuSK antibodies instead activated MuSK phosphorylation and partially induced AChR clustering, independent of agrin.

Conclusions: Patient-derived MuSK antibodies can act either as MuSK agonist or MuSK antagonist, depending on the number of MuSK binding sites. Functional monovalency, induced by Fab-arm exchange in patient serum, makes MuSK IgG4 antibodies pathogenic.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Antibodies, Monoclonal / immunology*
  • Antibodies, Monoclonal / isolation & purification
  • Autoantibodies / immunology*
  • Autoantibodies / isolation & purification
  • Cells, Cultured
  • Humans
  • Muscle Fibers, Skeletal
  • Myasthenia Gravis / immunology*
  • Receptor Protein-Tyrosine Kinases / agonists
  • Receptor Protein-Tyrosine Kinases / antagonists & inhibitors
  • Receptor Protein-Tyrosine Kinases / immunology*
  • Receptors, Cholinergic / immunology*
  • Recombinant Proteins


  • Antibodies, Monoclonal
  • Autoantibodies
  • Receptors, Cholinergic
  • Recombinant Proteins
  • MUSK protein, human
  • Receptor Protein-Tyrosine Kinases