BACKGROUND: Cast nephropathy in multiple myeloma patients is caused by heterotypic aggregation of Tamm-Horsfall glycoprotein (THP) with monoclonal light chains (Bence Jones protein, BJP). Co-aggregation of these proteins is triggered by the binding of BJP to a specific peptide portion of THP. Defining those factors that alter the interaction between BJP and THP may help to understand further the pathogenesis of cast nephropathy and serve to decrease the morbidity and mortality of cast nephropathy. We hypothesized that pH, calcium, furosemide, and the carbohydrate moiety of THP all modulate the protein-protein interaction between BJP and THP. EXPERIMENTAL DESIGN: Binding affinity and aggregation rate of human THP with two human nephrotoxic BJP were tested at pH 8.5, 7.4, and 5.0. The effects of calcium, furosemide, and free sialic acid were also evaluated. Binding and aggregation of BJP with THP purified from normal volunteers treated with oral colchicine were observed. The carbohydrate components of THP were also analyzed. RESULTS: An acidic environment increased initial binding rate and produced a parallel increase in the aggregation rate of THP with BJP. Calcium and furosemide enhanced aggregation rates without interfering with binding. Colchicine treatment decreased the amount of sialic acid linked α(2-6) to galactose (NeuAcα2-6Gal) and sialic acid linked α(2-3) to galactose (NeuAcα2-3Gal) on THP and thereby decreased the aggregation rate with BJP without altering binding. Addition of free sialic acid did not alter binding but did decrease aggregation rates of the two proteins. CONCLUSIONS: Environmental conditions modulate BJP-THP interactions and may be responsible for inducing cast nephropathy in multiple myeloma. Modification of carbohydrate components of THP or use of oligosaccharides may decrease or prevent cast nephropathy.