The Glutamate Decarboxylase and 38KD Autoantigens in Type 1 Diabetes: Aspects of Structure and Epitope Recognition

Abstract

A major question regarding the autoimmune destruction of pancreatic @-cells is the nature of the primary autoantigen. The definition of such an antigen includes that it elicits 0cell destruction. If that destruction is mediated by CD8 + cytotoxic T-cells then this antigen would be the initial target. If p-cells are indirectly destroyed following homing of CD4’ T-cells to the islets of Langerhans, then the primary antigen can be defined as the initiating antigen or a protein crossreacting with the initiating antigen, which can be from a virus. Those criteria again include that the primary antigen is &cell specific, since other tissues are not destroyed, or that the protein is only visible or accessible to the immune system in the &cells, i.e. if it is expressed in other tissues then it is invisible to or protected from the immune system. If tolerance is absent or lost toward the primary antigen, autoimmunity and destruction can result. We know however from transgenic models of @cell autoimmunity that even in the absence of tolerance to a 0-cell antigen there can be absence of autoimmunity and destruction until a triggering event such as an infection with a virus containing the same or a crossreacting antigen (1,2). If active tolerance to the primary autoantigen is maintained there should be no autoimmunity and no disease. The smaller form of the enzyme glutamic acid decarbosylase, GAD,,, as well as a 38kD 0-cell protein are two potential primary autoantigens in type 1 diabetes. Both proteins have been identified using circulating islet cell autoantibodies in type 1 diabetic sera to probe extracts of islet cell proteins. This approach reveals whether candidate autoantibodies are of sufficient affinity and specificity to bind their target antigen and form immuno complexes in the presence of an excess of other islet cell proteins. Both GAD65 and 38kD antibodies are IgG antibodies and both are present at the very early stages of &cell destruction. GADss was first detected as a protein which was immunoprecipitated by IgG antibodies in about 80% of newly diagnosed type 1 diabetic patients but not present in healthy individuals (3). This molecule defined as a 64kD protein doublet (a and 0) with an isoelectric point of 6.7 (4) was identified as GAD,, (5,6). A second molecule of Mr 65kD, which coprecipitated with the 64kD antigen in cytosol but not membrane fractions of rat islets was identified as the larger form of glutamic acid decarboxylase, GAD67 (5,6). GAD,, and GAD,, are encoded for by two non-allelic genes that may have developed from a common ancestral gene. GAD,, and GAD,, are highly diverse in the first 95 amino acids but share an extensive homology in the remainder of the molecule (7). GAD,, is clearly a dominant autoantigen in type 1 diabetes whereas GAD,, seems to play only a secondary role. The 38kD protein was first sporadically detected in immunoprecipitates with type 1 diabetic sera in rare preparations of human and rat islet cell proteins. Recently however an improved extraction method has resulted in consistant detection of the 38kD antigen in immunoprecipitates from all islet cell preparations with a subgroup of diabetic sera (8).