INSIDE-OUT AND OUTSIDE-IN MECHANISMS IN LEUKOCYTE ADHESION

Leukocytes alternate between a non adherent phenotype and a firmly adh erent phenotype during transit from the bloodstream and interstitial f luids through lymphoid areas and peripheral tissue. Strong cell-cell i nteractions control immune surveillance and specialized effector funct ions by various leukocyte subsets. Such interactions are mediated by l eukocyte adhesion receptors, which encompass a heterogenous family of transmembrane glycoproteins, some of which are selectively expressed b y cells of lymphoid and myeloid origin. The role of any given adhesion receptor at any point in time depends upon a variety of factors, incl uding the immediate microenvironment and the state of cell activation. In lymphoid and myeloid cells, adhesion receptors are subject to fine development control, such that their function, besides providing adhe sion, extends to costimulating cell locomotion and proliferation in an antigen-dependent or indipendent fashion. The molecular basis of the multiple functions of leuckocyte adhesion receptors are the subject of this review.