Anti-idiotypic antibodies to anti-PDC-E2 in primary biliary cirrhosis and normal subjects

Protective role of anti-idiotypic antibodies in autoimmunity--lessons for type 1 diabetes

Circulating autoantibodies to beta cell antigens are present in the majority of patients with Type 1 diabetes. These autoantibodies can be detected before and at time of clinical diagnosis of disease. Although the role of autoantibodies in the pathogenesis of the disease is debated, their presence indicates a dysregulation of the humoral immune response. Mechanisms regulating autoantibodies in Type 1 diabetes are not well understood. In contrast, in other autoimmune diseases there is acceptance that autoantibodies are regulated not only by antigen but also by other antibodies that bind to the antigen-binding site of these autoantibodies (anti-idiotypic antibodies). The proposed purpose of this network is to maintain an equilibrium between autoantibodies and their anti-idiotypic antibodies, preventing autoimmunity, while allowing a robust response to exogenous antigen. Anti-idiotypic antibodies regulate both autoantibody binding and their levels by a) neutralizing autoantibodies, and b) inhibiting the secretion of autoantibodies. Because it has been proposed that the B lymphocytes that produce autoantibodies function as autoantigen presenting cells, inhibiting their binding to autoantigen by anti-idiotypic antibodies may prevent development of autoimmune disease. This hypothesis is supported by the presence of anti-idiotypic antibodies in healthy individuals and in patients in remission from autoimmune diseases, and by the lack of anti-idiotypic antibodies during active disease. We recently reported the presence of autoantibodies to glutamate decarboxylase in the majority of healthy individuals, where their binding to autoantigen is prevented by anti-idiotypic antibodies. These anti-idiotypic antibodies are absent at clinical diagnosis of Type 1 diabetes, revealing the presence of autoantibodies. Type 1 diabetes (T1D) is an autoimmune disease characterized by the dysfunction and destruction of insulin-producing beta cells by autoreactive T cells. Although much progress has been made towards understanding the respective roles of effector and regulatory T cells in this beta cell destruction, the development of autoantibodies to beta cell proteins is widely considered simply a by-product of the autoimmune destruction of the beta cells, rather than having an active role in the pathogenesis. This view is starting to change based on increasing recognition that autoantibodies can have defined roles in other autoimmune diseases, and the emergence of new data on their role in T1D. This exploration of the role of autoantibodies in autoimmune disease has been spurred, in part, by increasing recognition that development of autoimmune diseases is influenced by regulatory antibodies (anti-idiotypic antibodies) directed against the unique binding site of autoantibodies. This review provides an overview of the development and function of these anti-idiotypic antibodies, and present evidence supporting their role in the development of autoimmune diseases. Finally, we conclude this review with a model of the events that may cause loss of anti-idiotypic antibodies and the implications for the development of T1D.

The autoimmunity of primary biliary cirrhosis and the clonal selection theory

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease in which an immune-mediated injury targets the small intrahepatic bile ducts. PBC is further characterized by highly specific serum antimitochondrial autoantibodies (AMA) and autoreactive T cells, a striking female predominance, a strong genetic susceptibility, and a plethora of candidate environmental factors to trigger the disease onset. For these reasons PBC appears ideal to represent the developments of the clonal selection theory over the past decades. First, a sufficiently potent autoimmunogenic stimulus in PBC would require the coexistence of numerous pre-existing conditions (mostly genetic, as recently illustrated by genome-wide association studies and animal models) to perpetuate the destruction of the biliary epithelium by the immune system via the persistence of forbidden clones. Second, the proposed modifications of mitochondrial autoantigens caused by infectious agents and/or xenobiotics well illustrate the possibility that peculiar changes in the antigen structure and flexibility may contribute to tolerance breakdown. Third, the unique apoptotic features demonstrated for cholangiocytes are the ideal setting for the development of mitochondrial autoantigen presentation to the immune system through macrophages and AMA thus turning the non traditional mitochondrial antigen into a traditional one. This article will review the current knowledge on PBC etiology and pathogenesis in light of the clonal selection theory developments.

The lack of anti-idiotypic antibodies, not the presence of the corresponding autoantibodies to glutamate decarboxylase, defines type 1 diabetes

Autoantibodies to glutamate decarboxylase 65 (GAD65Ab) are commonly believed to be a major characteristic for type 1 diabetes (T1D). We investigated the presence of GAD65Ab in healthy individuals (n = 238) and first-degree relatives (FDRs) of T1D patients (n = 27) who tested negative for GAD65Ab in conventional RIAs. Sera were applied to affinity columns coated with GAD65-specific mAbs to absorb anti-idiotypic antibodies (anti-Ids). The absorbed sera were analyzed for binding to GAD65 by RIAs. Both healthy individuals and FDRs present GAD65Ab that are inhibited by anti-Id, masking them in conventional detection methods. The presence of GAD65Ab-specific anti-Ids was confirmed by competitive ELISA. Remarkably, T1D patients (n = 54) and Stiff Person Syndrome patients (n = 8) show a specific lack of anti-Ids to disease-associated GAD65Ab epitopes. Purified anti-Ids from healthy individuals and FDRs inhibited the binding of GAD65Ab from T1D patients to GAD65. We conclude that masked GAD65Ab are present in the healthy population and that a lack of particular anti-Ids, rather than GAD65Ab per se, is a characteristic of T1D. The lack of these inhibitory antibodies may contribute to T cell activation by GAD65Ab.

Amino acids critical for binding of autoantibody to an immunodominant conformational epitope of the pyruvate dehydrogenase complex subunit E2: Identification by phage display and site-directed mutagenesis

The E2 subunit of the mitochondrial multienzyme pyruvate dehydrogenase complex (PDC-E2) is the major autoantigen in the liver disease, primary biliary cirrhosis (PBC). An epitope region which has been localized to amino acids 91-227 is believed to include the residue K173 to which is attached the lipoyl cofactor. We investigated structural features of this epitope region by screening random peptide phage-displayed libraries and identified prevalent phagotopes that contained likely contact amino acids in separate regions of the linear sequence, H132M133, and F178, V180. These were confirmed by site-directed alanine mutagenesis singly or in combination of the HM and FV residues in wild-type (wt) PDC-E2, and by immunization of rabbits with phage that expressed peptides MHLNTPP or FVLPWRI. The lipoyl lysine K173 also was mutated. Reactivities of mutants and wild-type (wt) PDC-E2, compared by ELISA using 12 PBC sera, showed decremental reactivity of mutant versus wt PDC-E2 (normalized to 100%): wt PDC-E2 (100%)>>PDC-E2(F178A,V180A) (mean+/-S.D., 59+/-17%)>PDC-E2(M133A) (50+/-13%)>PDC-E2(H132A) (36+/-13%)>PDC-E2(H132A,M133A) (28+/-8%)>PDC-E2(H132A,M133A,F178V,M180A) (18+/-13%). Notably PDC-E2(K173A) retained full reactivity (93+/-21%). Rabbits immunized with phage peptides generated antibodies reactive with entire PDC-E2. Our data convincingly validate phage library technology for defining spatially disparate contact residues for conformational epitopes. Ensuing data could be generally applicable to search for occult extrinsic agents as initiators of autoimmunity.