Beta-2-glycoprotein specificity of human anti-phospholipid antibody resides on the light chain: a novel mechanism for acquisition of cross-reactivity by an autoantibody

We have recently shown that the anti-cardiolipin activity of human anti-phospholipid antibody UK4 (lambda) resides on its heavy chain. We now show that UK4 possesses strong reactivity to the plasma-protein β2-Glycoprotein I (β2-GPI) also. Utilizing chain shuffling experiments involving an unrelated anti-p185 antibody 4D5 (kappa) with no reactivity to β2-GPI, we now demonstrate that both the constructs possessing the auto-antibody-derived light chain exhibited significant binding to β2-GPI. However, the construct possessing UK4 heavy chain in association with 4D5 light chain, exhibited no anti-β2-GPI activity. Furthermore, there was a low increase (≅10%) in the binding of UK4 to cardiolipin in the presence of β2-GPI. The results demonstrate that anti-β2-GPI activity resides on UK4 light chain and, importantly, this activity could be transferred to a novel antibody construct via the light chain alone. Computer-generated models of the three-dimensional structures of UK4 and its hybrids, suggest predominant interaction of UK4 light chain with domain IV of β2-GPI. Molecular docking experiments highlight a number of potential sites on β2-GPI for interaction of UK4 and indicate as to how β2-GPI recognition may occur primarily via the autoantibody light chain. The study provides first demonstration of the occurrence of anti-phospholipid and anti-β2-GPI activities separately on heavy and light chains of an autoantibody. The possible mechanisms that such antibodies may employ to recognise their antigens, are discussed.