HIGHER AUTOANTIBODY LEVELS AND RECOGNITION OF A LINEAR NH2-TERMINAL EPITOPE IN THE AUTOANTIGEN GAD(65), DISTINGUISH STIFF-MAN SYNDROME FROMINSULIN-DEPENDENT DIABETES-MELLITUS

The smaller form of the GABA-synthesizing enzyme glutamic acid decarbo xylase (GAD(65)) is a major autoantigen in two human diseases that aff ect its principal sites of expression. Thus, destruction of pancreatic beta cells, which results in insulin-dependent diabetes mellitus (IDD M), and impairment of GABA-ergic synaptic transmission in Stiff-Man sy ndrome (SMS) are both characterized by circulating autoantibodies to G AD(65) Anti-GAD(65) autoantibodies in IDDM are predominantly directed to conformational epitopes. Here we report the characterization of hum oral autoimmune responses to GAD(65) in 35 SMS patients, of whom 13 (3 7%) also had IDDM. All SMS patients immunoprecipitated native GAD(65) and the main titers were orders of magnitude higher than in IDDM patie nts. Furthermore, in contrast to the situation in IDDM, autoantibodies in 35 of 35 (100%) of SMS patients recognized denatured GAD(65) on We stern blots. Two major patterns of epitope specificity were identified on Western blots. The first pattern, detected in 25 of 35 SMS patient s (71%), of whom 11 had IDDM (44%), was predominantly reactive with a linear NH2-terminal epitope residing in the first eight amino acids of GAD(65). Nine of nine individuals who were HLA-haplotyped in this gro up carried an IDDM susceptibility haplotype and HLA-DR3, DQw2 was part icularly abundant. The second pattern, detected in 10 of 35 patients ( 29%) of whom two had IDDM (20%), included reactivity with the NH2-term inal epitope plus strong reactivity with one or more additional epitop e(s) residing COOH-terminal to amino acid 101. The second epitope patt ern may represent epitope spreading in the GAD(65) molecule, but may a lso include some cases of epitope recognition associated with IDDM res istant HLA-haplotypes. The principal NH2-terminal linear epitope in GA D(65) distinguishes the reactivity of SMS and IDDM autoantibodies and may be a determinant of pathogenicity for GABA-ergic neurons. The grea ter magnitude and distinct specificity of the humoral response to GAD( 65) in SMS may reflect a biased involvement of the T helper cell type 2 (Th2) subset of CD4(+) T cells and antibody responses, whereas IDDM is likely mediated by the Th1 subset of CD4(+) T cells and cytotoxic T cell responses.