Characterisation of antibodies from MuSK-MG patients.

<p>(A) MuSK antibody titres of 14 MuSK MG patients, 30 healthy controls and 5 AChR MG patients as determined by RIA. Cut-off for MuSK antibody positivity was the mean titre of pooled healthy controls + 3x standard deviation. (B) Example of IgG subclass profiles from 3 MuSK patients. Patient plasma was incubated with HEK293 cells expressing MuSK, or to mock-transfected HEK293 cells. A subclass-specific secondary antibody was added, followed by a fluorescent tertiary antibody, and the amount of binding (mean fluorescence intensity, mfi) was measured by flow cytometry. Δmfi is the mfi obtained with cells expressing MuSK minus the mfi with mock-transfected cells. (C) Correlation between IgG subclasses, measured by flow cytometry as for (B), and MuSK antibody titres measured by RIA of 14 MuSK patient plasmas. Statistic analysis: linear regression followed by Pearson correlation (Gaussian distribution was determined by D’Agostino and Pearson test). (D) IgG1-3 and IgG4 subclasses were purified from MuSK-MG patients 9 and 12. The purity of each of these two subclass groups was analysed by flow cytometry as for (B). The IgG4 fraction for both patients contains some contamination of IgG2, but the IgG1-3 fraction is devoid of IgG4 for both patients.