Novel immune response to gluten in individuals with schizophrenia

Abstract

A link between celiac disease and schizophrenia has been postulated for several years, based primarily on reports of elevated levels of antibody to gliadin in patients. We sought to examine the proposed connection between schizophrenia and celiac disease by characterizing the molecular specificity and mechanism of the anti-gliadin immune response in a subset of individuals with schizophrenia. Blood samples from individuals with schizophrenia and elevated anti-gliadin antibody titer were examined for celiac disease-associated biomarkers, including antibodies to transglutaminase 2 (TG2) enzyme and deamidated gliadin peptides, as well as the HLA-DQ2 and -DQ8 MHC genes. The anti-gliadin antibody response was further characterized through examination of reactivity towards chromatographically separated gluten proteins. Target proteins of interest were identified by peptide mass mapping. In contrast to celiac disease patients, an association between the anti-gliadin immune response and anti-TG2 antibody or HLA-DQ2 and -DQ8 markers was not found in individuals with schizophrenia. In addition, the majority of individuals with schizophrenia and anti-gliadin antibody did not exhibit antibody reactivity to deamidated gliadin peptides. Further characterization of the antibody specificity revealed preferential reactivity towards different gluten proteins in the schizophrenia and celiac disease groups. These findings indicate that the anti-gliadin immune response in schizophrenia has a different antigenic specificity from that in celiac disease and is independent of the action of transglutaminase enzyme and HLA-DQ2/DQ8. Meanwhile, the presence of elevated levels of antibodies to specific gluten proteins points to shared immunologic abnormalities in a subset of schizophrenia patients. Further characterization and understanding of the immune response to gluten in schizophrenia may provide novel insights into the etiopathogenesis of specific disease phenotypes.

Figure 1

Comparison of mean antibody levels in celiac disease and gluten-sensitive schizophrenia patients. A) Antibodies to native gliadin. Individuals with schizophrenia that were positive for anti-gliadin antibodies had lower levels of IgG (P<0.05) and IgA (P<0.005) antibodies to gliadin than celiac disease patients. B) Antibodies to a deamidated (glutamine-glutamate substituted) trimer of a fusion gliadin peptide containing PLQPEQPFP and PEQLPQFEE sequences in the same patients as in panel A. Individuals with schizophrenia who were positive for anti-gliadin antibodies had significantly lower mean IgG and IgA antibody reactivity to deamidated gliadin than celiac disease patients (P<0.001). C) IgA antibodies to recombinant human TG2. Levels were significantly lower in the schizophrenia group than the celiac disease group (P<0.001). Dotted lines indicate established cutoff values for positivity. Error bars represent the standard error of the mean.

Figure 2

A) Chromatogram of gluten proteins eluted from the BioGel P-100 size exclusion column, following acetic acid extraction. B) SDS-PAGE profile (Coomassie-stained) of fractions 1–6 of gluten proteins collected from the column.

Figure 3

Pattern of antibody reactivity to gluten proteins of the Cheyenne cultivar in individuals with schizophrenia and controls. A) SDS-PAGE profile of fractions 1–6 of gluten proteins. B–E) Reactivity of serum antibodies from gliadin-immunized rabbits (B), a representative healthy control (C), representative patients 1–4 with celiac disease (D), and representative patients 1–4 with schizophrenia and elevated anti-gliadin antibody (E) towards transferred gluten proteins from panel A. Detection of bound antibodies was by the ECL system and autoradiography film. Arrows in panels A and E point to the ~33 kDa doublet to which antibody reactivity was found in schizophrenia patients.

Figure 4

Characterization of the specificity of serum antibody to the ~33 kDa doublet. Protein bands were individually excised, extracted, and re-analyzed by immunoblotting with the serum of representative individuals with schizophrenia in order to identify the target protein. Lane 1) Protein doublet, containing upper and lower bands; lane 2) upper protein band; lane 3) lower protein band. Patient immune reactivity was found to be directed primarily at the upper band.