PECAM-1 Polymorphism Affects Monocyte Adhesion to Endothelial Cells Reyna S. Goodman, 1,5 Christopher M. Kirton, 2 Gertie J. Oostingh, 3 Michael P. Scho ¨n, 3 Michael R. Clark, 2 J. Andrew Bradley, 4 and Craig J. Taylor 1 Background. Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) plays an important role in leukocyte- endothelial cell adhesion and transmigration. Single nucleotide polymorphisms of PECAM-1 encoding amino acid substitutions at positions 98 leucine/valine (L/V), 536 serine/asparagine (S/N), and 643 arginine/glycine (R/G) occur in strong genetic linkage resulting in two common haplotypes (LSR and VNG). These PECAM-1 polymorphisms are associated with graft-versus-host disease after hematopoietic stem cell transplantation and with cardiovascular disease, but whether they influence PECAM-1 function is unknown. Methods. We examined the effect of homozygous and heterozygous expression of the PECAM-1 LSR and VNG genotypes on the adhesive interactions of peripheral blood monocytes and activated endothelial cell monolayers under shear stress in a flow-based cell adhesion assay. Results. There was no difference in monocyte adhesion between the two homozygous genotypes of PECAM-1 but when monocytes expressed both alleles in heterozygous form, firm adhesion of monocytes to endothelial cells was markedly increased. PECAM-1 polymorphism expressed in homozygous or heterozygous form by endothelial cells did not influence monocyte adhesion. Conclusions. This is, to our knowledge, the first demonstration that PECAM-1 genotype can alter the level of monocyte binding to endothelial cells and a demonstration that heterozygous expression of a polymorphic protein may lead to altered function. Keywords: PECAM-1, CD31, Polymorphism, Cell adhesion. (Transplantation 2008;85: 471–477) P latelet endothelial cell adhesion molecule-1 (PECAM-1/ CD31) is a member of the immunoglobulin superfamily of cell adhesion molecules. Although best known for its role in cell adhesion and leukocyte transmigration across capillary endothelium (1, 2), PECAM-1 is also important for the main- tenance of vascular endothelial integrity and participates as a regulatory molecule in a wide range of events including an- giogenesis, apoptosis and cell signaling (3–5). PECAM-1 is a 130-kD type 1 transmembrane glycoprotein consisting of six extracellular immunoglobulin-like domains, a transmem- brane region, and cytoplasmic tail that carries two immuno- receptor tyrosine inhibitory motifs (6, 7). It is expressed on the surface of various hematopoietic cells, including platelets, monocytes, neutrophils, and naı ¨ve T and B lymphocytes, and is a major constituent of endothelial intercellular junctions (8, 9). PECAM-1 mediates adhesive interactions between ad- jacent cells by direct homophilic binding, whereby the mem- brane distal N-terminal domains of PECAM-1 molecules bind in an antiparallel manner (10). Such binding facilitates firm leukocyte adherence to vascular endothelium, particu- larly at the site of intercellular junctions, and enables leuko- cyte transmigration across the endothelium into underlying interstitial tissues. PECAM-1 is encoded by a 75-kb gene that resides on the long arm of chromosome 17, 17q23 (11). A number of gene polymorphisms of human PECAM-1 have been identified, giv- ing rise to several allelic isoforms of the glycoprotein molecule. Single nucleotide polymorphisms have been described that en- code amino acid substitutions in the PECAM-1 molecule at po- sitions 98 (leucine L/valine V) located in domain 1, 536 (serine S/asparagine N) in domain 6, and 643 (arginine R/ glycine G) in the cytoplasmic tail. These polymorphisms are held in strong genetic linkage resulting in two different haplotypes (LSR and VNG) that each have a frequency of approximately 0.5 in the white population (12, 13). There are no published reports on any possible function altering effects of such gene polymorphisms on PECAM-1 dependent cell ad- hesion. However, single nucleotide gene polymorphisms in PECAM-1 may be associated with progression of athero- sclerosis and the development of cardiovascular disease (14, 15). In addition, PECAM-1 polymorphisms have been reported to influence the incidence of graft-versus-host dis- ease (GvHD) after hematopoietic stem cell transplantation (HSCT) (16 –23). Although it has been suggested that any influence on GvHD might be attributable to minor histocom- patibility antigen differences between PECAM-1 polymor- phisms, our previously reported findings do not support this explanation (24). We observed that the incidence of GvHD This work was supported by the Cambridge University Hospitals Foundation Trust Biomedical Research Centre and a grant from the Addenbrooke’s Charitable Trust. 1 Tissue Typing Laboratory, Cambridge University Hospitals NHS Founda- tion Trust, Addenbrooke’s Hospital, Cambridge, United Kingdom. 2 Department of Pathology, Molecular and Therapeutic Immunology, Uni- versity of Cambridge, Cambridge, United Kingdom. 3 Rudolf Virchow Center, DFG Research Center for Experimental Biomedi- cine and Department of Dermatology, Bayerische Julius Maximilians University, Wu ¨ rzburg, Germany. 4 Department of Surgery, University of Cambridge, Addenbrooke’s Hospital, Cambridge, United Kingdom. 5 Address correspondence to: Reyna S. Goodman, Tissue Typing Laboratory (Box 209), Cambridge University Hospitals NHS Foundation Trust, Addenbrooke’s Hospital, Hills Road, Cambridge. CB2 0QQ, United Kingdom. E-mail: reyna.goodman@addenbrookes.nhs.uk Received 12 October 2007. Accepted 7 November 2007. Copyright © 2008 by Lippincott Williams & Wilkins ISSN 0041-1337/08/8503-471 DOI: 10.1097/TP.0b013e3181622d65 Transplantation • Volume 85, Number 3, February 15, 2008 471