Elsevier

Autoimmunity Reviews

Volume 3, Issues 7–8, November 2004, Pages 487-492
Autoimmunity Reviews

Autoimmune epitopes: autoepitopes

https://doi.org/10.1016/j.autrev.2004.07.011Get rights and content

Abstract

The identity of reactants for autoantibodies has been successively refined from whole cellular organelles (immunofluorescence), identified molecules (immunoblot; gene expression libraries), epitope regions (truncated cDNAs; peptide scanning) to contact residues, as described here. Most autoantibodies react with conformational epitopes, in which amino acids distant in the linear sequence come into contiguity by protein folding. Identification of contact sites with the antibody paratope requires particular technologies, crystallography, or antibody screening of phage-displayed random peptide libraries. The latter is illustrated by our studies on the autoepitope for anti-PDC-E2 (AMA) in primary biliary cirrhosis (PBC), anti-GAD65 in type 1 diabetes, and anti-C1 of type II collagen in collagen-induced arthritis. More precise definition of the structure of conformational autoepitopes could (a) clarify controversial aspects of autoimmunity including epitope mimicry, epitope spreading, and molecular spatial relationships between B and T cell autoepitopes, and (b) impact on novel diagnostic and therapeutic (vaccine) molecules.

Introduction

The many fascinating aspects to autoimmune epitopes include the key question on why these are so uniquely selected in autoimmunity from among the myriads of possible molecular configurations expressed in the organism. The answer could well reveal the solution to genesis of autoimmunity. Questions of interest are whether autoimmune epitopes constitute just a few dominant or multiple disparate sequences on an autoantigen, the frequency and significance of spreading (recruitment) of epitopes, relationships between epitope locations for B and T lymphocytes, and whether molecular specification of epitopes can provide support for epitope mimicry as a trigger for autoimmunity. In this synoptic review, we consider the analysis of autoepitopes in three selected diseases, primary biliary cirrhosis (PBC), autoimmune Type 1 diabetes mellitus, and rheumatoid arthritis (RA).

Section snippets

Historical background

Burnet [1] in his classic 1957 monograph on clonal selection, in recounting his version of “The Facts of Immunity”, depicts a representative antigenic molecule showing the entire surface occupied by potential antigenic determinants (Fig. 1). Just a few of these, A–D, are specified as being “active” in a capacity to elicit a corresponding antibody population a–d, with the remainder being “inactive” by reason of their identity with a self motif. Burnet cited Kabat in proposing that an antigenic

Mapping of autoepitopes

The question that then arose was the precise location on the autoantigen of the reactant, i.e., the actual autoepitope. Various strategies were developed based on the supposition that an antibody epitope could be defined in terms of a linear amino acid sequence. The techniques included expression of proteins from an enzymatically truncated cDNA that encoded the autoantigen [7], use of cDNAs to create hybrid molecules or swap mutants at specified locations [8], molecular genetic insertion of

Pyruvate dehydrogenase complex E2: autoantigen for PBC

As described above, the traditional AMA (anti-M2) in the PBC sera reacts selectively with E2 subunits of one or more of the 2-oxo-acid dehydrogenase complexes (2-OADC), most often pyruvate dehydrogenase complex E2 (PDC-E2). The autoepitope for PDC-E2 was initially localized on the basis of reactivity across species, and homology among the E2 subunits of the other reactive 2-OADC enzymes, to a short sequence of highly conserved amino acids that form the attachment site of the lipoyl cofactor [5]

Glutamic acid decarboxylase (GAD) 65 kDa—autoantigen for Type 1 diabetes

The 65-kDa isoform of glutamic acid decarboxylase (GAD65) is a major autoantigen in Type 1 diabetes. Antibody epitopes on GAD65 initially were localized by expression of truncated proteins from deletion mutants of cDNARs or by creation of swap mutants in which sequences of GAD65 were replaced by homologous sequences of the nonautoantigenic isoform, GAD67 [8]. Two main epitope regions of GAD65 for diabetes sera, and for various monoclonal islet cell antibodies (MICAs), were located, one in the

Type II collagen (CII) : autoantigen for inflammatory arthritis

CII is a credible candidate as the primary autoantigen for rheumatoid arthritis by reason of its high representation in articular cartilage, the presence particularly in early RA of autoantibodies to CII, and the resemblance to RA of the experimental model, collagen-induced arthritis (CIA). Mice with CIA have been a source of numerous mAbs that react with native but not denatured CII, certain of which readily transfer arthritis to naïve recipients. These mAbs have been mapped to particular

T cell autoepitopes

Identification of T-cell autoepitopes is in some respects simpler, but in others more complex, than for antibody autoepitopes; simpler because such epitopes are short linear peptides that are accommodated within the antigen-binding groove of MHC molecules, and more complex because of (a) the relatively low representation of reactive T cells in blood versus the affected tissue [21], (b) the polymorphic nature of MHC molecules resulting in differing selection, for any given autoimmune disease, of

Conclusions

Knowledge of autoepitopes is crucial for the understanding of autoimmunity including interactions between T and B lymphocytes. Screening of phage-displayed peptide libraries now allows for precision in mapping of antibody epitopes, including likely contact residues in a conformational epitope region. Rewards could include insights into autoimmune induction and mechanisms of injury, and improved diagnostics and vaccine-type immunotherapies.

Acknowledgement

We thank Duncan Crombie for designing Fig. 1.

Take-home messages

  • Autoantibodies mostly engage conformational epitopes, wherein spatially disparate motifs are brought into contiguity by protein folding.

  • Current techniques for defining a reactive molecular sequence (autoepitope) on an autoantigenic molecule do not identify critical contact residues for an autoantibody or an autoimmune T cell receptor.

  • Phage display provides a novel technique that can be used to identify such critical contact residues

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