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Junctional adhesion molecule-A (JAM-A) is a transmembrane protein expressed at tight junctions of endothelial and epithelial cells and on the surface of platelets and leukocytes. The role of JAM-A in leukocyte transmigration in vivo was directly investigated by intravital microscopy using both a JAM-A-neutralizing monoclonal antibody (mAb) (BV-11) and JAM-A-deficient (knockout [KO]) mice. Leukocyte transmigration (but not adhesion) through mouse cremasteric venules as stimulated by interleukin 1beta (IL-1beta) or ischemia/reperfusion (I/R) injury was significantly reduced in wild-type mice treated with BV-11 and in JAM-A KO animals. In contrast, JAM-A blockade/genetic deletion had no effect on responses elicited by leukotriene B(4) (LTB(4)) or platelet-activating factor (PAF). Furthermore, using a leukocyte transfer method and mice deficient in endothelial-cell JAM-A, evidence was obtained for the involvement of endothelial-cell JAM-A in leukocyte transmigration mediated by IL-1beta. Investigation of the functional relationship between JAM-A and PECAM-1 (CD31) determined that dual blockade/deletion of these proteins does not lead to an inhibitory effect greater than that seen with blockade/deletion of either molecule alone. The latter appeared to be due to the fact that JAM-A and PECAM-1 can act sequentially to mediate leukocyte migration through venular walls in vivo.

Effective immune responses require the directed migration of leukocytes from the vasculature to the site of injury or infection. How immune cells “find” their site of extravasation remains largely obscure. Here, we identified a previously unrecognized role of platelets as pathfinders guiding leukocytes to their exit points in the microvasculature: upon onset of inflammation, circulating platelets were found to immediately adhere at distinct sites in venular microvessels enabling these cellular blood components to capture neutrophils and, in turn, inflammatory monocytes via CD40-CD40L-dependent interactions. In this cellular crosstalk, ligation of PSGL-1 by P-selectin leads to ERK1/2 MAPK-dependent conformational changes of leukocyte integrins, which promote the successive extravasation of neutrophils and monocytes to the perivascular tissue. Conversely, blockade of this cellular partnership resulted in misguided, inefficient leukocyte responses. Our experimental data uncover a platelet-directed, spatiotemporally organized, multicellular crosstalk that is essential for effective trafficking of leukocytes to the site of inflammation. [ABSTRACT FROM AUTHOR]