The diabetic antigen glutamic acid decarboxylase (GAD 65) in the human peripheral blood

Glutamic acid decarboxylase (GAD 65) is a diabetes-associated antigen which is generally considered to be strictly intracellular. In order to better understand autoimmunity, this study demonstrates the appearance of GAD 65 in the peripheral human blood and presents implications for the diagnosis and therapy of some autoimmune diseases.

The GAD 65 molecules are detected by their interaction with monoclonal antibodies labeled with dyes in an experimental setup with fluorescence correlation spectroscopy (FCS). These interactions result in changes in Brownian motion measured as fluorescence fluctuations. Sera from 153 patients with diabetes mellitus type 1 and controls were investigated. To enable the representation of the molecule as a model for further discussions, we present structural visualizations of its hydrophobic properties, leading to possible interactions with the cell membrane lipids and epitope locations.

The GAD65 antigen could be measured with a sensitivity of 2.65 microg/ml in 'clean systems' resulting from spiking experiments and human sera. The GAD 65 antigen could be identified in 8 patient sera: 4 children with diabetes mellitus type 1 and 4 adults initially taken as controls but who retrospectively showed signs of autoimmunity.

We conclude that these findings are of significance for the concept of autoimmunity, i.e. in an initial step the immune system is primed by its accessibility to GAD 65. Our experimental results may also be important for the therapy of diabetes mellitus type 1 and other autoimmune diseases by the passive administration of GAD 65 antibodies.