Differential Effect of E-Selectin Antibodies on Neutrophil Rolling and Recruitment to Inflammatory Sites

The Role of Adhesion Molecules in Human Leukocyte Attachment to Porcine Vascular Endothelium: Implications for Xenotransplantation

Many obstacles still prevent successful xenotransplantation of porcine donor organs. When hyperacute rejection is averted, transplanted pig organs are subject to acute vascular and cellular rejection. In autologous systems, leukocyte recruitment into inflamed tissues involves selectins, integrins, and Ig family members. To determine whether these mechanisms allow human leukocytes to effectively enter porcine grafts, the pathways by which human leukocytes adhere to TNF-α-stimulated porcine aortic endothelium were examined under static and physiologic flow conditions. L-selectin and E-selectin had overlapping functions in neutrophil capture and rolling, whereas Ab blockade of E-selectin and the β2 integrins inhibited firm arrest of rolling neutrophils. Combined blockade of selectins and β2 integrins resulted in negligible human neutrophil attachment to pig endothelium. Lymphocyte attachment to porcine endothelium was primarily L-selectin mediated, whereas β2 integrin and VCAM-1/very late Ag-4 (VLA-4) interactions promoted static adhesion. Concurrent β2 integrin, VLA-4, VCAM-1, and L-selectin blockade completely inhibited lymphocyte attachment. Thus, interactions between leukocyte-endothelial cell adhesion receptor pairs remained remarkably intact across the human-porcine species barrier. Moreover, disrupting the adhesion cascade may impair the ability of human leukocytes to infiltrate a transplanted porcine organ during rejection.

Importance of E-selectin for firm leukocyte adhesion in vivo

Leukocyte adhesion under flow is preferentially mediated by the selectins. In this study we used intravital microscopy to investigate whether E-selectin may promote firm leukocyte adhesion in vivo. E-Selectin is expressed by endothelial cells activated with tumor necrosis factor-alpha (TNF-alpha) and causes slow leukocyte rolling. Microinjection of formyl-methionyl-leucyl-phenylalanine (fMLP) or macrophage inflammatory protein-2 (MIP-2) next to a venule of the TNF-alpha-treated mouse cremaster muscle significantly increased the number of adherent leukocytes. In gene-targeted mice homozygous for a null mutation in the E-selectin gene or in wild-type mice treated with an E-selectin monoclonal antibody (mAb), this response was significantly attenuated (by >80%). No such defect was seen in intercellular adhesion molecule-1 (ICAM-1)-deficient mice. E-Selectin-null mice showed more rapid leukocyte rolling than wild-type or ICAM-1-deficient mice, resulting in significantly shortened leukocyte transit times through venules. Topical application of fMLP onto the whole cremaster muscle generated the same number of adherent leukocytes in wild-type and E-selectin-deficient mice. We conclude that slow leukocyte rolling through E-selectin results in long transit times, which are essential for efficient leukocyte adhesion in response to a local chemotactic stimulus.

Sialyl Lewisx (sLex) and an sLexMimetic, CGP69669A, Disrupt E-Selectin–Dependent Leukocyte Rolling In Vivo

Leukocyte rolling is the earliest observable event in their recruitment from the circulation to inflamed tissue. This rolling is mediated largely by interaction between the selectin family of adhesion molecules and their glycosylated ligands. Although the nature of these ligands and their interaction with the selectins is not fully understood, it is accepted that expression of fucosylated sialylated glycans such as sialyl Lewisx (sLex) is required for function. Despite findings that sLex inhibits binding of leukocytes to E-selectin in vitro, and has beneficial effects in inflammatory disease models, inhibition of E-selectin–dependent leukocyte rolling in vivo has not been described. Functional overlap between the selectins has been noted and reduction of rolling by E-selectin antibodies only occurs if P-selectin is absent or blocked. We demonstrate that leukocyte rolling velocity in tumor necrosis factor alpha (TNFα)-stimulated mouse cremaster is increased following treatment with either sLex or the sLex-mimetic CGP69669A and that rolling is dramatically reduced if CGP69669A is applied in the presence of anti–P-selectin antibody. These effects are characteristic of E-selectin antagonism. In contrast, surgically stimulated (L- or P-selectin–dependent) rolling is unaffected by either sLex or CGP69669A. Our data demonstrate that CGP69669A is an effective and selective antagonist of E-selectin in vivo.

Sialyl Lewisx (sLex) and an sLexMimetic, CGP69669A, Disrupt E-Selectin–Dependent Leukocyte Rolling In Vivo

Leukocyte rolling is the earliest observable event in their recruitment from the circulation to inflamed tissue. This rolling is mediated largely by interaction between the selectin family of adhesion molecules and their glycosylated ligands. Although the nature of these ligands and their interaction with the selectins is not fully understood, it is accepted that expression of fucosylated sialylated glycans such as sialyl Lewisx (sLex) is required for function. Despite findings that sLex inhibits binding of leukocytes to E-selectin in vitro, and has beneficial effects in inflammatory disease models, inhibition of E-selectin–dependent leukocyte rolling in vivo has not been described. Functional overlap between the selectins has been noted and reduction of rolling by E-selectin antibodies only occurs if P-selectin is absent or blocked. We demonstrate that leukocyte rolling velocity in tumor necrosis factor alpha (TNFα)-stimulated mouse cremaster is increased following treatment with either sLex or the sLex-mimetic CGP69669A and that rolling is dramatically reduced if CGP69669A is applied in the presence of anti–P-selectin antibody. These effects are characteristic of E-selectin antagonism. In contrast, surgically stimulated (L- or P-selectin–dependent) rolling is unaffected by either sLex or CGP69669A. Our data demonstrate that CGP69669A is an effective and selective antagonist of E-selectin in vivo.

Role of primary and secondary capture for leukocyte accumulation in vivo

Leukocyte accumulation during inflammation depends on the concerted action of selectin and integrin adhesion molecules, which promote capture, rolling, and arrest of these cells on activated endothelium. In addition to interacting with endothelial cells, leukocytes can also adhere to already adherent leukocytes through an L-selectin-dependent mechanism. Initiation of adhesion through this mechanism has been called nucleation and leads to characteristic geometric patterns (ie, clusters and strings) of adherent leukocytes in flow chambers. We have used intravital microscopy of tumor necrosis factor-alpha (TNF-alpha)-treated mouse cremaster muscles to quantitatively investigate the potential role of leukocyte-leukocyte adhesion in initiating and maintaining the leukocyte clusters that are commonly observed in inflamed venules. Our data show that in TNF-alpha-treated venules with diameters between 23 and 108 microm, leukocyte adhesion occurs in clusters that are 19 to 50 microm long and 8 to 44 microm wide. They are almost entirely made up of slow-rolling leukocytes. Of all leukocytes recruited into a cluster (100%), the majority enter the cluster rolling along the endothelium and sharply reduce their velocity in the absence (59%) or presence (15%) of other leukocytes in proximity (one cell diameter). Some of the rolling leukocytes (17%) pass through the cluster without reducing their velocity. Recruitment of leukocytes from the free flow regime into a cluster is a rare event and accounts for only 7 (1.2%) of 476 leukocytes arriving in the cluster. However, of the leukocytes captured from the free flow, 6 initiated contact with a slow-rolling leukocyte rather than making direct contact with the endothelium. Our data show that leukocyte-leukocyte interactions can occur in vivo but are not important for cluster formation. This is confirmed by the observation of normal cluster formation in L-selectin-deficient mice, in which leukocyte-leukocyte interactions under flow are abolished. We conclude that leukocyte-mediated nucleation contributes little to leukocyte recruitment during inflammation in vivo. Cluster formation appears to be dominated by areas of endothelium with a higher expression of E-selectin, because cluster formation is greatly reduced in E-selectin-deficient mice.