Antibody-dependent pathogenicity is suggested in multiple sclerosis (MS) by intrathecal immunoglobulin production, IgG and complement deposition in the most common immunopathological lesion subtype (pattern II), and by a recent report that 47% of MS patients’ sera contain a glial potassium-channel-specific-IgG(inwardly-rectifying, Kir4.1). Our study’s aims were to determine, in MS serum and CSF, the frequency and specificity of Kir4.1-binding-IgG and, in demyelinating MS lesions, whether Kir4.1-immunoreactivity is retained or lost.
We tested by ELISA(Kir4.1-peptide 83–120) sera from 286 clinically and geographically diverse MS patients (229 population-based and 57 clinic-based),99 healthy controls and 109 disease controls, and cerebrospinal fluid [CSF] from 25 MS and 22 controls. CSFs and clinic-based MS-subset serum (50)were tested on functional Kir4.1-expressing cells, using methodologies validated for detecting clinically-pertinent neural plasma membrane-reactive autoantibodies: immunofluorescence and immunoprecipitation (solubilized recombinant human Kir4.1). We evaluated Kir4.1-immunoreactivity in brain from 15 archival histopathologically-confirmed MS cases(22 plaques: 8 early active, 8 inactive, 6 remyelinated; 13 periplaque regions)and compared 3 non-neurological cases (8 normal-appearing white/gray matter regions).
Kir4.1-peptide-ELISA reactivity was rare and did not differ significantly for 286 MS or 208 control sera (both 1%); no CSF was positive. IgGin 0/50 clinic-based MS sera immunoprecipitated Kir4.1, but control Kir4.1-specific-IgG did. By immunofluorescence,1/50 MS sera yielded faint plasmalemmal staining on both Kir4.1-expressing and non-expressing cells; 16/50 bound faintly to intracellular components. In all cases, IgG binding was quenched by absorption with liver powder or non-transfected cell lysates. Control Kir4.1-specific-IgG binding was quenched only by Kir4.1 protein-containing lysates. IgG in 0/25 MS CSFs bound to Kir4.1-transfected cells, live or fixed. Glial Kir4.1-immunoreactivity was increased relative to baseline normal brain expression (3 controls) in early active and remyelinated MS lesions, and in periplaque white matter (15 patients).
We did not find Kir4.1-specific-IgG in MS sera or CSF, nor Kir4.1 loss from glial cells in active demyelinating MS lesions. Serological testing for Kir4.1-IgG is unlikely to aid MS diagnosis. The target antigen of MS remains elusive.
The National Institutes of Health, the National Multiple Sclerosis Society and the Mayo Clinic Robert and Arlene Kogod Center on Aging.