BLyS BINDS TO B CELLS WITH HIGH AFFINITY AND INDUCES ACTIVATION OF THE TRANSCRIPTION FACTORS NF-κB AND ELF-1

Abstract B lymphocyte stimulator (BLyS) is a novel member of the TNF family of proteins expressed by myeloid cells as membrane-bound and soluble forms. BLyS was shown to act specifically on B cells, inducing proliferation and immunoglobulin production both in vitro and in vivo. The present study was undertaken to characterize binding of radiolabeled BLyS to its cognate receptor on human B lymphocytes and examine intracellular events initiated by BLyS binding. Similar to other TNF family members, BLyS is present in solution as a homotrimer as determined by gel filtration chromatography and light scattering analysis. BLyS binding to B cells is specific as other TNF family members tested did not compete for 125 I-BLyS binding. Analysis of equilibrium binding of 125 I-labeled BLyS to purified human tonsillar B cells demonstrated saturable binding. Scatchard analysis of the binding data revealed a single class of high-affinity binding on human B cells with approximately 2600 binding sites per cell and an apparent dissociation constant (K D ) of about 0.1 nM. In addition we report that BLyS binding to B cells results in the activation of NF-κB and the Ets family transcription factor, ELF-1, and in the induction of mRNA for Polo-like kinase (PLK).