* Laboratory for Experimental Immunology and Department of Immunobiology, CLB, and
Department of Cell Biology and Histology, Academic Medical Center, University of Amsterdam, The Netherlands; and
Sir William Dunn School of Pathology, University of Oxford, United Kingdom
Correspondence: Dr. Jörg Hamann, Laboratory for Experimental Immunology, G1-106, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands. E-mail: j.hamann{at}amc.uva.nl.
The EGF-TM7 family is a group of class B seven-span transmembrane (TM7) receptors expressed predominantly by cells of the immune system. Family members CD97, EMR1, EMR2, EMR3, and ETL are characterized by an extended extracellular region with a variable number of N-terminal epidermal growth factor (EGF)-like domains coupled to a TM7 domain by a stalk. The EGF domain region of the recently identified EMR2 differs from that of CD97 in only 6 out of 236 amino acids. Although small, this difference has been shown to alter ligand specificity. To analyze the structure and cellular distribution of EMR2, a specific monoclonal antibody (2A1) was generated. Use of 2A1 has demonstrated EMR2, like CD97, to be expressed as a heterodimeric receptor consisting of an extracellular 
part and a TM7/cytoplasmic ß part. Analysis of EMR2 expression on primary blood leukocytes, on hematopoietic cells lines, and in situ revealed a myeloid-restricted profile. Highest expression levels were detected on the more mature CD16+ blood monocytes, on macrophages, and on BDCA-3+ myeloid DC, whereas little if any expression was found on granulocytes. Unlike CD97, no expression was observed on resting or activated lymphocytes. Different expression patterns and the inability of EMR2 to interact with the CD97 ligand CD55 indicate that the molecular twins EMR2 and CD97 likely have nonredundant functions.
Key Words: CD97 • myeloid lineage • proteolytic processing
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