The CD155 gene is a member of the immunoglobulin superfamily. We first demonstrate the existence of soluble CD155 (sCD155) isoforms in culture medium conditioned by CD155-expressing cells, in human serum and in cerebrospinal fluid. sCD155 concentration was measured in human serum and cerebrospinal fluid using a specific ELISA. Analysis of conditioned media indicated that sCD155 release does not require protease activity. In order to determine which tissues are responsible for sCD155 expression, we have quantified CD 155 mRNAs in human normal tissues. The highest expression was observed in liver. The CD 155alpha transcript is the most abundant and the proportion of the CD155beta and CD155gamma variants was similar between the tissues. Finally, serum purified sCD155 reduces poliovirus entry mediated by membrane-bound CD155. The high level of CD 155 synthesis in many tissues and the presence of sCD155 in biological fluids suggest the existence of an important role for the protein in cellular function. (C) 2003 Elsevier Inc. All rights reserved.