INVOLVEMENT OF CD31 IN LYMPHOCYTE-MEDIATED IMMUNE-RESPONSES - IMPORTANCE OF THE MEMBRANE-PROXIMAL IMMUNOGLOBULIN DOMAIN AND IDENTIFICATION OF AN INHIBITING CD31 PEPTIDE

CD31 (PECAM-1) is an immunoglobulin gene superfamily cell adhesion mol ecule found on vascular endothelium, platelets, and leukocytes. Lympho cyte expression of CD31 is most closely associated with the CD45RA(+)C D8(+) naive T phenotype. CD31 has recently been shown to play a role i n leukocyte egress to inflammatory sites. The mechanism of CD31 adhesi on remains under investigation. Several investigators have reported ev idence for a heterotypic ligand. We have previously shown that CD31 is phosphorylated with cell activation, which suggests a possible role f or CD31 in cell activation events. We therefore studied the effects of CD31 antibodies on in vitro assays of lymphocyte activation. One CD31 antibody, LYP21, inhibited the mixed lymphocyte reaction (MLR) in a s pecific and dose-dependent fashion. An LYP21 epitope was localized to the sixth Ig domain of CD31. This peptide and a scrambled control pept ide were synthesized and used to study effects of this epitope on lymp hocyte activation. The CD31 peptide strongly inhibited the MLR. Becaus e CD31 is expressed on both stimulator and responder populations, stim ulator peripheral blood leukocytes and responder lymphocyte population s were separately incubated with CD31 peptide or control peptide and t hen washed before mixing. The CD31 peptide inhibited the MLR equally w hen either stimulator or responder cells were preincubated with the CD 31 peptide. We further sorted responder cells into CD31-high and CD31- low populations and separately incubated these subsets with peptides. The CD31 peptide strongly inhibited MLRs, regardless of level of respo nder-cell CD31 expression. Examination of MLR reactions involving the CD31 peptide showed dispersed small aggregates of cells, rather than t he single large aggregate observed in control MLRs. The CD31 peptide d id not affect activation of lymphocytes by phorbol myristate acetate ( PMA) and ionomycin. These results suggest that a surface CD31-ligand i nteraction may have a functional role in alloimmune lymphocyte activat ion and identify a functionally important domain of CD31. (C) 1995 by The American Society of Hematology.