Epitope spreading and a varying but not disease-specific GAD65 antibody response in Type I diabetes. The Childhood Diabetes in Finland Study Group.
OBJECTIVE
The aim of this study was to analyse the conformational and linear epitope profiles of glutamic acid decarboxylase antibody (GAD65-ab)-positive sera to find disease-specific epitope profiles and to study, whether GAD65-ab epitope recognition changes or spreads during the prediabetic period and, thus, can provide markers to differentiate early from later stages of progression to diabetes.


METHODS
Sera from subjects before (n = 21), at onset (n = 44), or at increased risk of Type I (insulin-dependent) diabetes mellitus (n = 20) and from patients with stiff-man syndrome (SMS, n = 18) or polyendocrine autoimmune syndrome (PAS, n = 21) were analysed for conformational and linear GAD65 epitope recognition by an immunohistochemical blocking test based on human monoclonal GAD65-ab (MICA 1-10) and western blotting of a GAD65 epitope-cDNA-library.


RESULTS
A redundant reactivity of many GAD65-ab positive sera to three major conformational (EP-1, EP-2, EP-3) and two dominant linear epitope clusters (amino acid 1-124 and 535-585) was observed in diabetes, polyendocrine autoimmune syndrome and stiff-man syndrome and no disease-specific epitopes or epitope-profiles were detected. Epitope recognition broadened with higher titres and with the vulnerability of patients to acquire additional autoimmune diseases apart from diabetes. Low GAD65-ab serum titres (< 1200 arbitrary units) and EP-1 recognition in the absence of EP-2 binding characterised the early immune response. Changing epitope profiles combined stable recognition of EP-1 with gain or loss of reactivity to C-terminal epitopes during follow-up.


CONCLUSIONS
A maturing autoantibody response, which could spread from EP-1-recognition to other regions of GAD65, resulted in titre-related rather than disease-specific epitope profiles which were not sufficient to predict whether GAD65-ab positive subjects will progress to Type I diabetes, autoimmune polyendocrine syndrome or stiff-man syndrome.
Söhnlein
P
P
Dept of Orthopaedic Surgery, University of Heidelberg, Germany.
Müller
M
M
Syren
K
K
Hartmann
U
U
Böhm
B O
BO
Meinck
H M
HM
Knip
M
M
Akerblom
H K
HK
Richter
W
W
D000293:Adolescent; D000328:Adult; D001327:Autoimmune Diseases / Q000097:blood / Q000276:immunology; D002648:Child; D002675:Child, Preschool; D003922:Diabetes Mellitus, Type 1 / Q000097:blood / Q000276:immunology*; D000939:Epitopes / Q000276:immunology; D005260:Female; D005968:Glutamate Decarboxylase / Q000737:chemistry / Q000276:immunology*; D006801:Humans; D007223:Infant; D007527:Isoenzymes / Q000737:chemistry / Q000276:immunology*; D008297:Male; D008875:Middle Aged; D011236:Prediabetic State / Q000097:blood / Q000276:immunology*; D012307:Risk Factors; D016750:Stiff-Person Syndrome / Q000097:blood / Q000276:immunology
D000939:Epitopes; D007527:Isoenzymes; D005968:Glutamate Decarboxylase; C401141:glutamate decarboxylase 2