A new antigenic epitope localized within human kappa light chains specific for rheumatoid arthritis and systemic lupus erythematosus

Unravelling autoimmune pathogenesis by screening random peptide libraries with human sera

The incidence of autoimmunity is increasing worldwide. The long preclinical period of autoimmune disorders is characterised by an enhanced exposure over time of autoreactive T cells to an increased number of autoantigenic determinants and autoantibodies production. The discovery of novel autoimmune-disease related epitopes is a task that remains extremely challenging in order to establish predictive and preventive strategies of the disease onset. In this Opinion article we highlight the contribution of screening combinatorial random peptide libraries with patients sera in unravelling the etiopathogenesis of autoimmunity.

Peptide mimics of a major lupus epitope of SmB/B'

One of the most distinguishing features of systemic lupus erythematosus is the presence of high concentrations of autoantibodies that recognize a limited number of self-antigens. Even though many lupus autoantigens have been identified, the inciting triggers of these abnormal immune responses are not fully understood. One mechanism that could generate these autoantibodies is a normal immune response toward a foreign epitope that mimics a common antigenic target of an autoantigen. Antibody generated toward the foreign epitope could also bind the autoantigen. This "cross-reactivity" would result in the presentation of the autoantigen to the immune system. Under autoimmune-prone conditions, tolerance toward the native protein is broken and an autoimmune response is initiated. Previously, it was suggested that Epstein-Barr virus might use such a mechanism to initiate an autoimmune response. Cross-reactive epitopes may have a similar amino acid sequence or a similar tertiary structure that is independent of amino acid sequence. A major, and likely initial, target of the lupus anti-SmB' response is a repeated, proline-rich sequence, PPPGMRPP. To identify potential cross-reactive targets, we used affinity-purified autoantibodies specific for PPPGMRPP to screen a random heptapeptide phage display library. Eighty-five clones were isolated and sequenced with eleven distinct sequence motifs being identified. Two of these motifs were homologous to the SmB' epitope, while the other nine were not. Interestingly, one of the peptide motifs that mimicked the SmB' epitope is identical to a peptide sequence found in the Epstein-Barr virus major DNA binding protein.