Molecular dissection of an immunodominant epitope in Kv1.2-exclusive autoimmunity.

Introduction: Subgroups of autoantibodies directed against voltage-gated potassium channel (Kv) complex components have been associated with immunotherapy-responsive clinical syndromes. The high prevalence and the role of autoantibodies directly binding Kv remain, however, controversial. Our objective was to determine Kv autoantibody binding requirements and to clarify their contribution to the observed immune response. Methods: Binding epitopes were studied in sera (n = 36) and cerebrospinal fluid (CSF) (n = 12) from a patient cohort positive for Kv1.2 but negative for 32 common neurological autoantigens and controls (sera n = 18 and CSF n = 5) by phospho and deep mutational scans. Autoantibody specificity and contribution to the observed immune response were resolved on recombinant cells, cerebellum slices, and nerve fibers. Results: 83% of the patients (30/36) within the studied cohort shared one out of the two major binding epitopes with Kv1.2-3 reactivity. Eleven percent (4/36) of the serum samples showed no binding. Fingerprinting resolved close to identical sequence requirements for both shared epitopes. Kv autoantibody response is directed against juxtaparanodal regions in peripheral nerves and the axon initial segment in central nervous system neurons and exclusively mediated by the shared epitopes. Discussion: Systematic mapping revealed two shared autoimmune responses, with one dominant Kv1.2-3 autoantibody epitope being unexpectedly prevalent. The conservation of the molecular binding requirements among these patients indicates a uniform autoantibody repertoire with monospecific reactivity. The enhanced sensitivity of the epitope-based (10/12) compared with that of the cell-based detection (7/12) highlights its use for detection. The determined immunodominant epitope is also the primary immune response visible in tissue, suggesting a diagnostic significance and a specific value for routine screening.

Autoantibodies against "rods and rings"-related IMPDH2 in hepatitis C genotype 1 and DAA therapy in a "real life" cohort.

Autoantibodies against inosine-5'-monophosphate-dehydrogenase-2 (IMPDH2; "rods and rings" pattern) develop in chronic hepatitis C (CHC) patients under treatment with peg-interferon (IFN) and ribavirin (RBV), an inhibitor of IMPDH2. We investigated the influence of the alternative therapy with direct-acting antivirals (DAA)/ribavirin on anti-IMPDH2 autoantibody generation and the use of anti-IMPDH2 development as a marker for therapy outcome (sustained virologic response, SVR). We analyzed a "real life" cohort of 104 unselected CHC genotype 1 (GT1) patients treated with IFN/first-generation DAA/RBV prospectively compared to a historic cohort of 59 IFN/RBV-treated CHC GT1 patients. First-generation DAA were boceprevir (BOC) or telaprevir (TPR). Serum autoantibodies were tested by indirect immunofluorescence (IFA) using recombinant IMPDH2 expressing HEK293 cells and native HEp2-cells as substrates. 64/163 (39%) CHC patients turned anti-IMPDH2 positive during therapy, but only 43/163 (26%) showed also "rods and rings" structures. 99/163 (61%) were tested as anti-IMPDH2 negative. 53/104 (51%) CHC patients undergoing IFN/DAA/RBV therapy were anti-IMPDH2 positive and 38/104 (37%) were in parallel anti-"rods and rings" positive. HCV clearance/SVR rate after IFN/DAA/RBV therapy and anti-IMPDH2 status were not significantly dependent. CHC GT1 patients treated with IFN/first-generation DAA/RBV developed anti-IMPDH2 autoantibodies comparable to previous studies including patients under IFN/RBV therapy. Anti-IMPDH2 titers show no use as a marker for therapy outcome in CHC GT1 patients.

Routine detection of serum antidesmocollin autoantibodies is only useful in patients with atypical pemphigus.

Autoantibodies against the 3 desmocollin (Dsc; Dsc1-Dsc3) isoforms have been described in different pemphigus variants. Here, we developed state-of-the-art detection systems for serum anti-Dsc1, Dsc2 and Dsc1 IgG and IgA. These assays were applied in 5 different cohorts including pemphigus vulgaris (PV) patients with compatible direct immunofluorescence (IF) microscopy but no reactivity against desmogleins 1 and 3 (n = 24) and sera from patients with autoimmune blistering diseases with positive direct IF microscopy taken at the time of diagnosis (n = 749). We found that detection of anti-Dsc serum reactivity is not helpful in the routine diagnosis of PV, pemphigus foliaceus and paraneoplastic pemphigus but may be valuable in pemphigus vegetans.

Intracellular and non-neuronal targets of voltage-gated potassium channel complex antibodies.

OBJECTIVES: Autoantibodies against the extracellular domains of the voltage-gated potassium channel (VGKC) complex proteins, leucine-rich glioma-inactivated 1 (LGI1) and contactin-associated protein-2 (CASPR2), are found in patients with limbic encephalitis, faciobrachial dystonic seizures, Morvan's syndrome and neuromyotonia. However, in routine testing, VGKC complex antibodies without LGI1 or CASPR2 reactivities (double-negative) are more common than LGI1 or CASPR2 specificities. Therefore, the target(s) and clinical associations of double-negative antibodies need to be determined. METHODS: Sera (n=1131) from several clinically defined cohorts were tested for IgG radioimmunoprecipitation of radioiodinated α-dendrotoxin (125I-αDTX)-labelled VGKC complexes from mammalian brain extracts. Positive samples were systematically tested for live hippocampal neuron reactivity, IgG precipitation of 125I-αDTX and 125I-αDTX-labelled Kv1 subunits, and by cell-based assays which expressed Kv1 subunits, LGI1 and CASPR2. RESULTS: VGKC complex antibodies were found in 162 of 1131 (14%) sera. 90 of these (56%) had antibodies targeting the extracellular domains of LGI1 or CASPR2. Of the remaining 72 double-negative sera, 10 (14%) immunoprecipitated 125I-αDTX itself, and 27 (38%) bound to solubilised co-expressed Kv1.1/1.2/1.6 subunits and/or Kv1.2 subunits alone, at levels proportionate to VGKC complex antibody levels (r=0.57, p=0.0017). The sera with LGI1 and CASPR2 antibodies immunoprecipitated neither preparation. None of the 27 Kv1-precipitating samples bound live hippocampal neurons or Kv1 extracellular domains, but 16 (59%) bound to permeabilised Kv1-expressing human embryonic kidney 293T cells. These intracellular Kv1 antibodies mainly associated with non-immune disease aetiologies, poor longitudinal clinical-serological correlations and a limited immunotherapy response. CONCLUSIONS: Double-negative VGKC complex antibodies are often directed against cytosolic epitopes of Kv1 subunits and occasionally against non-mammalian αDTX. These antibodies should no longer be classified as neuronal-surface antibodies. They consequently lack pathogenic potential and do not in themselves support the use of immunotherapies.

Neurochondrin is a neuronal target antigen in autoimmune cerebellar degeneration.

OBJECTIVE: To report on a novel neuronal target antigen in 3 patients with autoimmune cerebellar degeneration. METHODS: Three patients with subacute to chronic cerebellar ataxia and controls underwent detailed clinical and neuropsychological assessment together with quantitative high-resolution structural MRI. Sera and CSF were subjected to comprehensive autoantibody screening by indirect immunofluorescence assay (IFA) and immunoblot. Immunoprecipitation with lysates of hippocampus and cerebellum combined with mass spectrometric analysis was used to identify the autoantigen, which was verified by recombinant expression in HEK293 cells and use in several immunoassays. Multiparameter flow cytometry was performed on peripheral blood and CSF, and peripheral blood was subjected to T-cell receptor spectratyping. RESULTS: Patients presented with a subacute to chronic cerebellar and brainstem syndrome. MRI was consistent with cortical and cerebellar gray matter atrophy associated with subsequent neuroaxonal degeneration. IFA screening revealed strong immunoglobulin G1 reactivity in sera and CSF with hippocampal and cerebellar molecular and granular layers, but not with a panel of 30 recombinantly expressed established neural autoantigens. Neurochondrin was subsequently identified as the target antigen, verified by IFA and immunoblot with HEK293 cells expressing human neurochondrin as well as the ability of recombinant neurochondrin to neutralize the autoantibodies' tissue reaction. Immune phenotyping revealed intrathecal accumulation and activation of B and T cells during the acute but not chronic phase of the disease. T-cell receptor spectratyping suggested an antigen-specific T-cell response accompanying the formation of antineurochondrin autoantibodies. No such neurochondrin reactivity was found in control cohorts of various neural autoantibody-associated neurologic syndromes, relapsing-remitting multiple sclerosis, cerebellar type of multiple system atrophy, hereditary cerebellar ataxias, other neurologic disorders, or healthy donors. CONCLUSION: Neurochondrin is a neuronal target antigen in autoimmune cerebellar degeneration.

Autoantibodies against glutamate receptor δ2 after allogenic stem cell transplantation.

OBJECTIVE: To report on a Caucasian patient who developed steroid-responsive transverse myelitis, graft vs host disease of the gut, and anti-GluRδ2 after allogenic stem cell transplantation. METHODS: Histoimmunoprecipitation (HIP) with the patient's serum and cryosections of rat and porcine cerebellum followed by mass spectrometry was used to identify the autoantigen. Correct identification was verified by indirect immunofluorescence using recombinant GluRδ2 expressed in HEK293 cells. RESULTS: The patient's serum produced a granular staining of the cerebellar molecular layer (immunoglobulin G1 and immunoglobulin G3; endpoint titer: 1:1,000) but did not react with other CNS tissues or 28 established recombinant neural autoantigens. HIP revealed a unique protein band at ∼110 kDa that was identified as GluRδ2. The patient's serum also stained GluRδ2 transfected but not mock-transfected HEK293 cells. Control sera from 38 patients with multiple sclerosis, 85 patients with other neural autoantibodies, and 205 healthy blood donors were negative for anti-GluRδ2. Preadsorption with lysate from HEK293-GluRδ2 neutralized the patient's tissue reaction whereas control lysate had no effect. In addition to anti-GluRδ2, the patient's serum contained immunoglobulin G autoantibodies against the pancreatic glycoprotein CUZD1, which are known to be markers of Crohn disease. CONCLUSIONS: In the present case, the development of anti-GluRδ2 was associated with transverse myelitis, which was supposedly triggered by the stem cell transplantation. Similar to encephalitis in conjunction with anti-GluRδ2 reported in a few Japanese patients, the patient's neurologic symptoms ameliorated after steroid therapy.