Immunoglobulin aggregation leading to Russell body formation is prevented by the antibody light chain.

Russell bodies (RBs) are intracellular inclusions filled with protein aggregates. In diverse lymphoid disorders these occur as immunoglobulin (Ig) deposits, accumulating in abnormal plasma or Mott cells. In heavy-chain deposition disease truncated antibody heavy-chains (HCs) are found, which bear a resemblance to diverse polypeptides produced in Ig lightchain (LC)‐deficient (L/) mice. In L/ animals, the known functions of LC, providing part of the antigen-binding site of an antibody and securing progression of B-cell development, may not be required. Here, we show a novel function of LC in preventing antibody aggregation. L/ mice produce truncated HC naturally, constant region (C) and C lack CH1, and C is without CH 1o r C H1 and CH2. Most plasma cells found in these mice are CD138 Mott cells, filled with RBs, formed by aggregation of HCs of different isotypes. The importance of LC in preventing HC aggregation is evident in knock-in mice, expressing C without CH1 and CH2, which only develop an abundance of RBs when LC is absent. These results reveal that preventing antibody aggregation is a major function of LC, important for understanding the physiology of heavy-chain deposition disease, and in general recognizing the mechanisms, which initiate protein conformational diseases. (Blood. 2010;115:282-288)