Soluble, secreted tumor necrosis factor receptor polypeptides, polynucleotides encoding the polypeptides, and related compositions and methods are disclosed. The polypeptides comprise one cysteine-rich repeat that is homologous to other tumor necrosis factor receptors, such as transmembrane activator and CAML-interactor (TACI). The polypeptides may be used for detecting ligands, agonists and antagonists. The polypeptides may also be used in methods that modulate B cell activation.