ICAM

(= intercellular adhesion molecule)

ICAM-1 is the glycoprotein ligand (80-155 kD) for LFA-1 (CD11a/CD18: b2 -integrin) and to a lesser extent Mac-1 (CD11b/CD18). ICAM-1 is expressed on the luminal surface of endothelial cells and is upregulated in response to IL1 or TNF treatment. ICAM-1 is also expressed on various haematopoietic cells and is upregulated on activated T- and B-cells. It is a member of the immunoglobulin superfamily and has five C2 domains. Not only is it an important ligand for leucocyte adhesion, but it is also the site to which rhinovirus binds and to which Plasmodium falciparum -infected erythrocytes adhere. ICAM-2 (55-68 kD) is constitutively expressed on endothelium and on resting lymphocytes and monocytes and is not upregulated by inflammatory cytokines. It has only two Ig superfamily domains. CD50 has now been identified as ICAM-3 (120 kD) and plays a role in the early stages of the immune response; it is also a ligand for LFA-1. Cross-linking of ICAM-3 seems to induce an increase in intracellular calcium and triggers activation of tyrosine kinases.