1. ¹Tumor Immunology Programme, German Cancer Research Center, Heidelberg, Germany
2. ²Kaplan Medical Center, Rehovot, Israel
3. ³Department for Human Genetics, University of Heidelberg, Heidelberg, Germany
4. ⁴Center for Pharmacology, University of Frankfurt, Frankfurt, Germany

Correspondence: P Altevogt, Tumor Immunology Programme, D010, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg D-69120, Germany. E-mail: P.Altevogt@dkfz.de

Received 14 March 2007; Revised 25 June 2007; Accepted 26 June 2007; Published online 15 August 2007.

Abstract

Exosomes are small membrane vesicles that are secreted from a variety of cell types into various body fluids including the blood and urine. These vesicles are thought to play a role in cell–cell interactions. CD24 is a small but extensively glycosylated protein linked to the cell surface by means of a glycosyl-phosphatidylinositol anchor. In this study we found that CD24 is present in membrane vesicles characterized as exosomes that were isolated from the urine of normal individuals. CD24 was expressed by both tubule cells and podocytes and treatment of the latter with a cholesterol-extracting agent, but not with a calcium ionophore, caused the release of CD24-containing exosomes. Using CD24 as a marker, we found exosomes in the urine of newborn infants and in the amniotic fluid of pregnant women with similar findings made in mice. Interestingly, studies with CD24 knockout mice showed that the exosomes are released from the fetus but not from the mother; however, exosome release was similar from both the knockout and the wild-type mice. This indicates that CD24 is not essential for exosome formation or release but may be a convenient exosome marker. Our studies suggest that exosomal secretion from the embryonic kidney could play a biological role at the fetal–maternal interphase.