Examining the non-linear relationship between monoclonal antiphospholipid antibody sequence, structure and function

Abstract

In the antiphospholipid syndrome (APS), pathogenic antiphospholipid antibodies (aPL) that cause thrombosis or pregnancy morbidity are characterized by binding to anionic phospholipids (PL) and beta2-glycoprotein I (beta(2)GPI). Sequence analysis of human monoclonal aPL has shown that high affinity for these antigens is associated with the presence of three particular amino acids: arginine (Arg), asparagine and lysine in the complementarity determining regions (CDRs) of their heavy and light chains. In vitro expression systems have been used to create variants of the antibodies in which these amino acids have been altered. In general, removal of Arg residues reduces affinity for anionic PL and beta(2)GPI. Arg at different positions in the sequence, however, have different effects on binding affinity and effects on binding are not always mirrored by effects on pathogenicity. This review will focus upon the sequence motifs that have been found to distinguish pathogenic from non-pathogenic aPL, and whether these or other properties may help to identify distinct pathogenic subsets of aPL. In particular, we will focus on our recent work in which we are trying to develop a better understanding of the molecular mechanisms involved in activation of target cells by pathogenic aPL. These studies, together with molecular models of antigen/antibody complexes, help us to understand exactly how pathogenic antibodies interact with antigens. Ultimately, this understanding may aid the design of more powerful diagnostic/prognostic assays and targeted therapeutic agents to block the pathogenic effects of these antibodies.

Use of monoclonal antibodies to dissect specificity and pathogenesis of antiphospholipid antibodies

“…6 Arg and Lys are positively charged and Asn is neutral but can act as a donor or acceptor of hydrogen bonds. Therefore, the presence of Arg, Lys and Asn residues in contact regions of aPL may increase the affinity of binding via electrostatic interactions and hydrogen bonds to negatively charged epitopes on PLs and b 2 GPI.…”

“…We have carried out a systematic analysis of all 57 published human monoclonal aPL sequences and have found a high degree of somatic mutations with an excess of arginine (Arg), lysine (Lys) and asparagine (Asn) residues in the contact sites of aPL in comparison with aspartic acid or glutamic acid. 6 Arg and Lys are positively charged and Asn is neutral but can act as a donor or acceptor of hydrogen bonds. Therefore, the presence of Arg, Lys and Asn residues in contact regions of aPL may increase the affinity of binding via electrostatic interactions and hydrogen bonds to negatively charged epitopes on PLs and b 2 GPI.…”