Peripheral lymph nodes (PLN) are critical for immunologic memory formation in response to antigens that penetrate the skin. Blood-borne lymphocytes first encounter such antigens after they home to PLN through a multi-step adhesion process that is normally initiated by L-selectin (CD62L) in high endothelial venules (HEV). Since naive T cells can not enter PLN normally in L-selectin-deficient mice, a delayed type hypersensitivity response to cutaneously applied antigen cannot be mounted. In this study, we report that the administration of activated platelets into the systemic circulation of L-selectin knockout mice restores lymphocyte trafficking to PLN, and reconstitutes T cell-mediated immunity in response to a cutaneous antigen. These effects required platelet-expressed P-selectin that allows activated platelets to transiently form a bridge between lymphocytes and HEV, thereby enabling lymphocytes to undergo subsequent beta2 integrin-dependent firm adhesion. These profound effects of platelet-mediated cell-cell interactions on lymphocyte trafficking and formation of immunologic memory may impact on a variety of autoimmune and inflammatory conditions.

The adhesive mechanisms allowing hematopoietic progenitor cells (HPC) homing to the bone marrow (BM) after BM transplantation are poorly understood. We investigated the role of endothelial selectins and vascular cell adhesion molecule-1 (VCAM-1) in this process. Lethally irradiated recipient mice deficient in both P-and E-selectins (P/E-/-), reconstituted with minimal numbers (

We have used intravital microscopy to study physiologically perfused microvessels in murine bone marrow (BM). BM sinusoids and venules, but not adjacent bone vessels, supported rolling interactions of hematopoietic progenitor cells. Rolling did not involve L-selectin, but was partially reduced in wild-type mice treated with antibodies to P- or E-selectin and in mice that were deficient in these two selectins. Selectin-independent rolling was mediated by alpha4 integrins, which interacted with endothelial vascular cell adhesion molecule (VCAM)-1. Parallel contribution of the endothelial selectins and VCAM-1 is not known to direct blood cell trafficking to other noninflamed tissues. This combination of constitutively expressed adhesion molecules may thus constitute a BM-specific recruitment pathway for progenitor cells analogous to the vascular addressins that direct selective lymphocyte homing to lymphoid organs.

Blood-borne lymphocytes migrate continuously to peripheral lymph nodes (PLN) and other organized lymphoid tissues where they are most likely to encounter their cognate antigen. Lymphocyte homing to PLN is a highly regulated process that occurs exclusively in specialized high endothelial venules (HEV) in the nodal paracortex. Recently, it has become possible to explore this vital aspect of peripheral immune surveillance by intravital microscopy of the subiliac lymph node microcirculation in anesthetized mice. This paper reviews technical and experimental aspects of the new model and summarizes recent advances in our understanding of the molecular mechanisms of lymphocyte homing to PLN which were derived from its use. Both lymphocytes and granulocytes initiate rolling interactions via L-selectin binding to the peripheral node addressin (PNAd) in PLN HEV. Subsequently, a G protein-coupled chemoattractant stimulus activates LFA-1 on rolling lymphocytes, but not on granulocytes. Thus, granulocytes continue to roll through the PLN, whereas LFA-1 activation allows lymphocytes to arrest and emigrate into the extravascular compartment. We have also identified a second homing pathway that allows L-selectin low/(activated/memory) lymphocytes to home to PLN. P-selectin on circulating activated platelets can mediate simultaneous platelet adhesion to PNAd in HEV and to P-selectin glycoprotein ligand (PSGL)-1 on lymphocytes. Through this mechanism, platelets can form a cellular bridge which can effectively substitute for the loss of L-selectin on memory cell subsets.

To characterize the adhesion cascade that directs lymphocyte homing to peripheral lymph nodes (PLNs), we investigated the molecular mechanisms of lymphocyte interactions with the microvasculature of subiliac lymph nodes. We found that endogenous white blood cells and adoptively transferred lymph node lymphocytes (LNCs) tethered and rolled in postcapillary high endothelial venules (HEVs) and to a lesser extent in collecting venules. Similarly, firm arrest occurred nearly exclusively in the paracortical HEVs. Endogenous polymorphonuclear (PMNs) and mononuclear leukocytes (MNLs) attached and rolled in HEVs at similar frequencies, but only MNLs arrested suggesting that the events downstream of primary rolling interactions critically determine the specificity of lymphocyte recruitment. Antibody inhibition studies revealed that L-selectin was responsible for attachment and rolling of LNCs, and that LFA-1 was essential for sticking. LFA-1-dependent arrest was also abolished by pertussis toxin, implicating a requirement for G alpha i--protein-linked signaling. alpha 4 integrins, which play a critical role in lymphocyte homing to Peyer's Patches, made no significant contribution to attachment, rolling, or sticking in resting PLNs. Velocity analysis of interacting LNCs revealed no detectable contribution by LFA-1 to rolling. Taken together, our results suggest that lymphocyte- HEV interactions within PLNs are almost exclusively initiated by L-selectin followed by a G protein-coupled lymphocyte-specific activation event and activation-induced engagement of LFA-1. These events constitute a unique adhesion cascade that dictates the specificity of lymphocyte homing to PLNs.

We report that the cell surface mucin CD43 acts as an anti-adhesin on T lymphocytes. CD43-deficient murine lymphocytes homed significantly more frequently to secondary lymphoid organs than wild-type cells. Intravital microscopy of peripheral lymph node venules revealed that CD43-deficient lymphocytes were twice as likely to tether, roll, and stick than wild-type cells. This effect was due to CD43 interference with the homing receptor, L-selectin, and was most pronounced in venules with low L-selectin ligand density. In vitro, CD43-deficient cells tethered to L-selectin ligands more efficiently and rolled more slowly than wild-type lymphocytes. Thus, CD43 exerts a negative regulatory effect on T cell trafficking by counterbalancing L-selectin-mediated adhesion.