PT  - JOURNAL ARTICLE
AU  - Saif Huda
AU  - Patrick Waters
AU  - Mark Woodhall
AU  - Maria Isabel Leite
AU  - Leslie Jacobson
AU  - Anna De Rosa
AU  - Michelangelo Maestri
AU  - Roberta Ricciardi
AU  - Jeannine M. Heckmann
AU  - Angelina Maniaol
AU  - Amelia Evoli
AU  - Judy Cossins
AU  - David Hilton-Jones
AU  - Angela Vincent
TI  - IgG-specific cell-based assay detects potentially pathogenic MuSK-Abs in seronegative MG
AID  - 10.1212/NXI.0000000000000357
DP  - 2017 Jul 01
TA  - Neurology - Neuroimmunology Neuroinflammation
PG  - e357
VI  - 4
IP  - 4
4099  - http://nn.neurology.org/content/4/4/e357.short
4100  - http://nn.neurology.org/content/4/4/e357.full
SO  - Neurol Neuroimmunol Neuroinflamm2017 Jul 01; 4
AB  - Objective: To increase the detection of MuSK-Abs using a CBA and test their pathogenicity.Methods: Sera from 69 MuSK-RIA–positive patients with myasthenia gravis (MG) (Definite MuSK-MG), 169 patients negative for MuSK-RIA and AChR-RIA (seronegative MG, SNMG), 35 healthy individuals (healthy controls, HCs), and 16 NMDA receptor-Ab–positive (NMDAR-Ab) disease controls were tested for binding to MuSK on a CBA using different secondary antibodies.Results: Initially, in addition to 18% of SNMG sera, 11% of HC and 19% of NMDAR-Ab sera showed positive binding to MuSK-transfected cells; this low specificity was due to anti-IgG(H+L) detection of IgM bound nonspecifically to MuSK. Using an IgG Fc gamma-specific secondary antibody, MuSK-Abs were detected by CBA in 68/69 (99%) of Definite MuSK-MG, 0/35 HCs, 0/16 NMDAR-Ab, and 14/169 (8%) of SNMG sera, providing increased sensitivity with high specificity. The RIA-negative, CBA-positive MuSK-IgG sera, but not IgM-MuSK–binding sera, reduced agrin-induced AChR clustering in C2C12 myotubes, qualitatively similar to RIA-positive MuSK-Abs.Conclusions: An IgG-specific MuSK-CBA can reliably detect IgG MuSK-Abs and increase sensitivity. In the MuSK-CBA, IgG specificity is essential. The positive sera demonstrated pathogenic potential in the in vitro AChR-clustering assay, although less effective than Definite MuSK-MG sera, and the patients had less severe clinical disease. Use of IgG-specific secondary antibodies may improve the results of other antibody tests.Classification of evidence: This study provides Class III evidence that an IgG-specific MuSK-CBA identifies patients with MG.AChEI=acetylcholinesterase inhibitor; CBA=cell-based assay; CI=confidence interval; DMEM=Dulbecco Modified Eagle Medium; FCS=fetal calf serum; HC=healthy control; HEK=human embryonic kidney; MG=myasthenia gravis; MGFA=Myasthenia Gravis Foundation of America; PSA=Penicillin, Streptomycin, and Amphotericin; RT=room temperature; SNMG=seronegative MG