Cytoplasmic Islet Cell Antibodies Recognize Distinct Islet Antigens in IDDM But Not in Stiff Man Syndrome

Abstract

Cytoplasmic islet cell antibodies are well-established predictive markers of IDDM. Although target molecules of ICA have been suggested to be gangliosides, human monoclonal ICA of the immunoglobulin G class (MICA 1xyd6) produced from a patient with newly diagnosed IDDM recognized glutamate decarboxylase as a target antigen. Here we analyzed the possible heterogeneity of target antigens of ICA by subtracting the GAD-specific ICA staining from total ICA staining of sera. This was achieved 1) by preabsorption of ICA+ sera with recombinant GAD65 and/or GAD67 expressed in a baculovirus system and 2) by ICA analysis of sera on mouse pancreas, as GAD antibodies do not stain mouse islets in the immunofluorescence test. We show that 24 of 25 sera from newly diagnosed patients with IDDM recognize islet antigens besides GAD. In contrast, GAD was the only islet antigen recognized by ICA from 7 sera from patients with stiff man syndrome. Two of these sera, however, recognized antigens besides GAD in Purkinje cells. In patients with IDDM, non-GAD ICA were diverse. One group, found in 64% of the sera, stained human and mouse islets, whereas the other group of non-GAD ICA was human specific. Therefore, mouse islets distinguish two groups of non-GAD ICA and lack additional target epitopes of ICA besides GAD. Longitudinal analysis of 6 sera from nondiabetic ICA+ individuals revealed that mouse-reactive ICA may appear closer to clinical onset of IDDM in some individuals. Mouse-reactive ICAs, however, remained absent in 36% of the patients at diagnosis of IDDM.

Immunological heterogeneity in Type I diabetes: presence of distinct autoantibody patterns in patients with acute onset and slowly progressive disease

“…Our data confirm recent findings that IA-2 and GAD are the dominant target of ICA in patients with classical features of Type I diabetes [31,32], but also support the hypothesis that this approach is not sufficient to identify all patients with Fig. 1A,B.…”

Stiff-person syndrome (SPS) and anti-GAD-related CNS degenerations: Protean additions to the autoimmune central neuropathies

“…In SPS levels of anti-GAD in serum are usually highly raised, are reactive with GAD65 but almost equally so with GAD67 [7,37], can be detected by immunoblotting using purified GAD from rat brain [38], are inhibitory for the enzymatic activity of GAD [7], and react by immunofluorescence (IFL) with cells in pancreatic islets, whether or not there is coexisting diabetes, but also on sections from different regions of mammalian brain [39,40] (Fig. 2).…”

“…In a provocative study, Viannelo et al [41] used IFL on cultured hippocampal neurons, on which GABAergic synapses are well represented, and staining patterns differed according to different neurological disorders, SPS, ataxia or epilepsy; the tentative suggestion was that differences in GAD epitope recognition differentiated clinical phenotypes. In T1D on the other hand anti-GAD are usually at low to moderate levels and mostly specific for GAD65, react predominantly with highly conformational epitopes and so are rarely detected by immunoblotting [39,40,42], seldom inhibitory for the enzymatic activity of GAD [43] and, although reactive with pancreatic islets, do not react by IFL with brain [38]. However since this latter statement is based on early (1990s) reports and on [22,23] an unpublished communication to us from a neuroimmunology diagnostic laboratory, a systematic study is needed of the reactivity with brain of anti-GAD in classical T1D.…”

COOH-Terminal Clustering of Autoantibody and T-Cell Determinants on the Structure of GAD65 Provide Insights Into the Molecular Basis of Autoreactivity

“…Affinity maturation of an ongoing antibody response is associated with both "epitope focusing" to provide for selection of higher affinity antibodies and also "epitope spreading" to enable the immune response to encompass wider regions of the antigenic molecule. In type 1 diabetes specifically, there is evidence that the autoantibody response may first involve the COOH-terminal domain, with later spreading to the PLP and NH 2 -terminal domains (3,26). Notably, four of five major T-cell epitopes restricted by the high-risk HLA allele DRB1*0401 could be localized on the structure of GAD65 within the same region as the immunodominant B-cell epitope regions ctc1 and ctc2, and the fifth epitope was close to E264, which defines the M6 epitope in the PLP domain.…”