CARDIOLIPIN BINDING A LIGHT-CHAIN FROM LUPUS-PRONE MICE

Autoantibodies in systemic lupus erythematosus react with multiple epi topes on highly conserved molecules such as nucleic acids, cytoskeleta l proteins, phospholipids, and phospholipid-binding proteins. Analysis of the heavy-and light-chain variable sequences (VH and VL) has shown that a restricted set of V genes gives rise to these autoantibodies. Several monoclonal antibodies were developed from a strain of mouse pr one to lupus (F1 male NZW x BXSB). Two of these antibodies, A1.72 and A1.84, reacted directly with cardiolipin and their VH and VL sequences were analyzed. Surprisingly, these two antibodies had identical light -chain variable sequences despite having substantially different heavy -chain variable sequences. This VL sequence, VL 72/84 was 97% identica l with the germ-line sequences with only four single nucleotide substi tutions. When this VL sequence was shuffled with the VH sequence of ot her monoclonal antibodies and expressed as single chain variable fragm ent (scFv) in Escherichia coli, it imparted cardiolipin-binding activi ty to the hybrids. Furthermore, the VL 72/84 sequence, when expressed alone without any VH sequence, also bound to cardiolipin. The antibodi es and their recombinant fragments were immunoaffinity-purified on car diolipin liposomes. The dissociation constant of the light chain for c ardiolipin was similar to the intact molecule (21 +/- 0.01 vs 20 +/- 0 .03 nM). These studies demonstrate that the VL sequence alone, in the absence of any other immunoglobulin domains, can mediate cardiolipin b inding, raising the possibility that antigen specificity of certain an tibodies may exclusively reside in their light-chain sequences.