ANALYSIS OF THE BINDING-SITE ON INTERCELLULAR-ADHESION MOLECULE-3 FOR THE LEUKOCYTE INTEGRIN LYMPHOCYTE FUNCTION-ASSOCIATED ANTIGEN-1

Intercellular adhesion molecule 3 (ICAM-3, CD50) is a member of the immunoglobulin superfamily and is a constitutively expressed ligand for the leukocyte integrin LFA-1 (CD11a/CD18). ICAM-3 is expressed at high levels by all resting leukocyte populations and antigen presenting cells and is a major ligand for LFA-1 in the resting immune system. ICAM-3 is a signal transducer and may play a hey role in initiating immune responses. Mutant ICAM-3 Fc-chimeric proteins were quantitatively analyzed for their ability to bind COS cells expressing human LFA-1. The LFA-1-binding site on ICAM-3 is located in the N-terminal 2 Ig domains. Domains 3-5 do not significantly contribute to adhesion. The binding site has been further resolved by rational targeting of. 14 point mutations throughout domains 1 and 2, coupled with modeling studies. Within domain 1 a cluster of residues (Glu(37), Leu(66), Ser(68), and Gln(75)), that are predicted to lie on the CC'FG face of the Ig fold, play a dominant role in LFA-1 binding.