Signaling mechanisms and the activation of leukocyte integrins.

Myeloid cells and lymphocytes primarily circulate in the vascular system but move into the tissues in response to inflammatory signals. Such a mobile lifestyle necessitates the continual making and breaking of cell and cell matrix contacts; the receptors known as the integrins are well suited as mediators of this transient adhesiveness. In general, leukocyte integrins are not constitutively active but become adhesive in response to signaling through other membrane receptors. Candidate receptors for receiving the activating signals are the seven membrane-spanning receptors that are found on all leukocytes. The identity of the signals, however, that are responsible for triggering integrin adhesion in vivo and for promoting directed leukocyte movement into tissues remains incompletely resolved.