INSIDE-OUT AND OUTSIDE-IN MECHANISMS IN LEUKOCYTE ADHESION

Leukocytes alternate between a non adherent phenotype and a firmly adherent phenotype during transit from the bloodstream and interstitial fluids through lymphoid areas and peripheral tissue. Strong cell-cell interactions control immune surveillance and specialized effector functions by various leukocyte subsets. Such interactions are mediated by leukocyte adhesion receptors, which encompass a heterogenous family of transmembrane glycoproteins, some of which are selectively expressed by cells of lymphoid and myeloid origin. The role of any given adhesion receptor at any point in time depends upon a variety of factors, including 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 adhesion, 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.