Trimolecular complexes of λ light chain dimers in serum of a patient with multiple myeloma

Background: Patients with multiple myeloma often have Bence Jones proteins composed of free monoclonal light chains of the κ or λ type in their urine. Usually, these light chains exist as monomeric or dimeric forms, but rarely, larger molecules, such as tetramers, have been reported in the serum. Methods and Results: We report the presence of trimeric complexes of λ light chain dimers in a patient who was diagnosed with a free λ light chain multiple myeloma 2 years earlier and subsequently underwent a stem cell transplant. Recently, the patient presented with a large serum M-spike (23 g/L) by protein electrophoresis. The spike consisted of monoclonal λ light chains without a heavy chain. The urine contained only 8 mg of λ light chain in a 24-h specimen. Quantitative analysis of the serum and urinary free light chains (FLCs) indicated the probability of larger aggregates of FLCs. Size-exclusion chromatography, electrophoresis, analytical ultracentrifugation, and mass spectrometric studies of the serum revealed almost exclusively the presence of trimolecular aggregates of λ light chain dimers without other multimeric species. Conclusion: Monoclonal λ light chains may present as hexameric aggregates that cannot be cleared by renal excretion.