The NH2‐terminal domain of rat CD2 binds rat CD48 with a low affinity and binding does not require glycosylation of CD2

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Abstract
CD2, CD48 and CD58 are structurally similar cell adhesion‐molecules forming a subset of the immunoglobulin superfamily (IgSF). In humans CD58 is a ligand for CD2 while in mice CD2 binds CD48. We constructed a soluble chimeric molecule comprising the extracellular portion of rat CD48 and domains 3 and 4 of rat CD4 (sCD48‐CD4) and used it to examine whether CD2 is a ligand for CD48 in rats. sCD48‐CD4‐coated polystyrene Dynabeads™ formed rosettes on rat CD2‐transfected COS‐7 cells, and this rosetting was blocked by anti‐CD2 (OX34) and anti‐CD48 (OX45) monoclonal antibodies. We used sucrose‐gradient ultracentri‐fugation to show that sCD48‐CD4 binds, in solution, to soluble forms of rat CD2 including the single NH2‐terminal IgSF domain of rat CD2 expressed in bacteria. The upper limit of the affinity of the rat CD48‐CD2 interaction is 4 × 105 M−1, lower than the published affinity of human CD2 for CD58. These results show that rat CD48 binds CD2 on its NH2‐terminal IgSF domain with a low affinity and that binding is independent of glycosylation.