The importance of the light chain for the epitope specificity of human anti-U1 small nuclear RNA autoantibodies present in systemic lupus erythematosus patients

Abs to U1 RNA are frequently found in patients suffering from systemic lupu s erythematosus overlap syndromes and Ab titers correlate with disease acti vity. We describe the isolation of the first human anti-U1 RNA autoantibodi es from a combinatorial IgG library made from the bone marrow of a systemic lupus erythematosus patient. With the use of phage display technology, two anti-U1 RNA single-chain variable fragment (scFv) Abs were selected, Both high affinity anti-U1 RNA Ab fragments (K-d similar to 1 nM) recognize stem IT of U1 RNA and mere derived from the same heavy chain gene (VH3-11) and the same lambda (3r) light chain gene although somatic mutiations, predomin antly present in the complementarity-determining regions, are different. Ex periments, in which the heavy chain genes of both anti-U1 RNA scFvs were re shuffled with the original light chain repertoire of the patient resulted, alter selection on stern loop II, in a large number of RNA-binding Ab fragm ents. All these stem loop II-specific RNA binding clones used a similar, bu t not identical, 3r lambda light chain. When scFvs were selected from the r eshuffled libraries by stem loop IV, representing the other autoantigenic s ite of U1 RNA, most selected Ab clones did react with stem loop TV, but no longer with stem loop II. The stem loop IV-reactive Ab clones contained dif ferent, not 3r-related, light chains. These results point to a major role f or the light chain in determining the sequence specificity of these disease -related anti-U1 RNA Abs, The possibility that secondary light chain rearra ngements are involved in this autoimnaune response is discussed.