HUMAN CELL-ADHESION MOLECULE CD2 BINDS CD58 (LFA-3) WITH A VERY-LOW AFFINITY AND AN EXTREMELY FAST DISSOCIATION RATE BUT DOES NOT BIND CD48OR CD59

CD2 is a T lymphocyte cell-adhesion molecule (CAM) belonging to the im munoglobulin superfamily (IgSF) which mediates transient adhesion of T cells to antigen-presenting cells and target cells. Reported ligands for human CD2 include the structurally-related IgSF CAMs CD58 (LFA-3) and CD48 as well as, more controversially, the unrelated cell-surface glycoprotein CD59. Using surface plasmon resonance technology, which a voids several pitfalls of conventional binding assays, we recently rep orted that rat CD2 binds rat CD48 with a very low affinity (K-d 60-90 mu M) and dissociates rapidly (k(off) greater than or equal to 6 s(-1) ) [van der Merwe, P. A., Brown, M. H., Davis, S. J., and Barclay, A. N . (1993) EMBO J. 12, 4945-4954]. In contrast, a study using convention al equilibrium binding methods reported a much higher affinity (K-d 0. 4 mu M) for human CD2 binding CD58 which suggested that the weak bindi ng of rat CD2 to CD48 may not represent a typical CAM interaction. In the present study we have used surface plasmon resonance to obtain def initive affinity and kinetic data on the interactions of a soluble, re combinant form of human CD2 with soluble forms of CD58, CD48, and CD59 . Binding of CD2 to CD58 was readily detected but we were unable to de tect any direct interaction between CD2 and either CD59 or CD48 under conditions in which very low affinity interactions (K-d similar to 0.5 mM) would have been detected. In contrast to previous reports we foun d that human CD2 bound CD58 with a very low affinity (K-d 9-22 mu M) a nd dissociated with an extremely fast dissociation rate constant (k(of f) greater than or equal to 4 s(-1)). The association rate constant (k (on)) could not be measured directly but was calculated to be greater than or equal to 400 000 M(-1) s(-1). Taken together, these results pr ovide conclusive evidence that CAM interactions can have very low affi nities and extremely fast dissociation rate constants.