B lymphocyte stimulator (BLyS) is a member of the tumor necrosis factor superfamily of cytokines. When the 152 amino acids of the C-terminus are associated into a homotrimer, this protein exhibits the ability to stimulate B cell proliferation and differentiation. Since numerous potential therapeutic indications have been identified for BLyS and other BLyS-derived products, large quantities of the protein are needed to further basic research and clinical trials. In this work, we have developed a high yield recombinant expression system that utilizes Escherichia coli as the host organism. Recombinant soluble BLyS (rsBLyS) production was achieved through the use of the phoA promoter system. This expression system, coupled to a semi-defined fermentation process,. resulted in final purified yields of 435mg/L of property folded, trimeric.. biologically active rsBLyS. This level of production is an 11-fold increase in volumetric yields compared to the process currently being used for clinical production. Furthermore. the increased rsBLyS production obtained from this process enabled the development of a conventional purification scheme that eliminated the use of a BLyS-affinity resin. (C) 2004 Elsevier Inc. All rights reserved.