Pathogenic potential of IgG binding to water channel extracellular domain in neuromyelitis optica.
BACKGROUND
Autoantibody specific for the aquaporin-4 astrocytic water channel is restricted to serum and CSF of patients with neuromyelitis optica (NMO) and related CNS inflammatory demyelinating disorders (relapsing optic neuritis and longitudinally extensive transverse myelitis). NMO-typical lesions are distinct from MS-typical lesions. Aquaporin-4 is lost selectively at vasculocentric sites of edema/inflammation coinciding with focal deposits of immunoglobulins (Ig) G, M, and terminal complement products, with and without myelin loss. Evidence for antigen-specific autoantibody pathogenicity is lacking.


METHODS
We used confocal microscopy and flow cytometry to evaluate the selectivity and immunopathological consequences of Ig binding to surface epitopes of living target cells expressing aquaporin-4 fused at its cytoplasmic N-terminus with GFP. We tested serum, IgG-enriched and IgG-depleted serum fractions, and CSF from patients with NMO, neurologic control patients, and healthy subjects. We also analyzed aquaporin-4 immunoreactivity in myelinated adult mouse optic nerves and spinal cord, and plasma cell Ig isotypes in archived brain tissue from an NMO patient.


RESULTS
Serum IgG from patients with NMO binds to the extracellular domain of aquaporin-4; it is predominantly IgG(1), and it initiates two potentially competing outcomes, aquaporin-4 endocytosis/degradation and complement activation. Serum and CSF lack aquaporin-4-specific IgM, and plasma cells in CNS lesions of NMO contain only IgG. Paranodal astrocytic endfeet highly express aquaporin-4.


CONCLUSIONS
NMO patients' serum IgG has a selective pathologic effect on cell membranes expressing aquaporin-4. IgG targeting astrocytic processes around nodes of Ranvier could initiate demyelination.
Hinson
S R
SR
Department of Laboratory Medicine and Pathology, Mayo Clinic College of Medicine, Rochester, MN 55905, USA.
Pittock
S J
SJ
Lucchinetti
C F
CF
Roemer
S F
SF
Fryer
J P
JP
Kryzer
T J
TJ
Lennon
V A
VA
D000818:Animals; D051401:Aquaporin 4 / Q000276:immunology / Q000378:metabolism*; D001666:Binding Sites, Antibody; D002460:Cell Line; D004705:Endocytosis / Q000276:immunology; D045604:Extracellular Fluid / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism*; D051379:Mice; D009471:Neuromyelitis Optica / Q000276:immunology* / Q000378:metabolism* / Q000473:pathology; D011485:Protein Binding / Q000276:immunology
C496030:AQP4 protein, human; D051401:Aquaporin 4; D007074:Immunoglobulin G