Molecular Mechanisms Underlying IL-4-Induced Leukocyte Recruitment In Vivo: A Critical Role for the α4 Integrin

IL-4 is known to induce recruitment of eosinophils and mononuclear leukocytes. In vitro this occurs in part by selective expression of VCAM-1, the ligand for the α4 integrin. The objective of this study was to determine the molecular mechanisms that underlie IL-4-induced leukocyte recruitment in vivo. Mice received an intrascrotal injection of IL-4 (100 ng). Twenty-four hours later, leukocyte rolling, adhesion, and emigration in cremasteric postcapillary venules were examined via intravital microscopy, and expression of VCAM-1 and P- and E-selectin was quantitated using a radiolabeled mAb technique. IL-4 increased VCAM-1 expression, but P-selectin and E-selectin remained at constitutive levels. IL-4 induced significant increases in leukocyte adhesion and emigration, with 50% of the emigrated cells being eosinophils and the remainder being mononuclear leukocytes. Leukocyte rolling in IL-4-treated mice was >95% inhibitable using an anti-P-selectin Ab. However, IL-4-induced leukocyte recruitment was unaltered in mice treated chronically with P-selectin Ab or mice deficient in either P-selectin or P- and E-selectin, suggesting that the residual rolling supported all of the IL-4-induced recruitment. In IL-4-treated mice following P-selectin blockade, tethering and rolling were not dependent on L-selectin, but were abolished by α4 integrin blockade. These findings show that the α4 integrin can initiate leukocyte-endothelial cell interactions in the absence of selectins under shear conditions in vivo, and that the absence of selectins does not affect recruitment of eosinophils and mononuclear cells to IL-4-treated tissue.

Acceleration and Increased Severity of Collagen-Induced Arthritis in P-Selectin Mutant Mice

P-selectin plays an important role in leukocyte adherence to microvascular endothelium and is expressed in synovial tissue from patients with rheumatoid arthritis (RA). However, the contribution of P-selectin to the initiation and chronicity of joint inflammation is not well understood. In these studies, collagen-induced arthritis (CIA) was induced in P-selectin mutant (−/−) mice to explore the role of P-selectin in the development of joint inflammation. Surprisingly, CIA onset was accelerated and severity was increased in P-selectin mutant mice, compared with wild-type mice (+/+). Increased levels of anti-type II collagen IgG were detected in both nonarthritic and arthritic P-selectin mutant mice from days 14–91. In addition, splenocytes isolated from immunized and nonimmunized P-selectin mutant mice produced significantly less IL-2 and IL-4, but significantly higher levels of IL-10 and IL-5 than splenocytes from wild-type mice. These observations show that P-selectin-mediated leukocyte rolling is not required for the development of murine CIA and that P-selectin expression exerts a controlling effect on the development of Ag-driven inflammatory joint disease, possibly by mediating the recruitment and/or trafficking of specific leukocyte subtypes into lymphoid tissue or inflammatory foci.

Characterization of Eosinophil Adhesion to TNF-α-Activated Endothelium Under Flow Conditions: α4 Integrins Mediate Initial Attachment, and E-Selectin Mediates Rolling

The multistep model of leukocyte adhesion reveals that selectins mediate rolling interactions and that integrins mediate firm adhesion processes. In this study, the interaction between eosinophils and TNF-α-activated HUVEC (second or third passage) was studied under flow conditions (0.8 and 3.2 dynes/cm2). Especially the role of α4 integrins on eosinophils and E-selectin on HUVEC was studied. Inhibition of the integrin α4 chain on eosinophils reduced the number of firmly adhered resting eosinophils to TNF-α-stimulated endothelium by 43% whereas the percentage rolling cells increased 2.2-fold compared with untreated control eosinophils. Blocking of E-selectin on the endothelium reduced the number of adherent eosinophils by only 23% and 16%. In this situation, however, hardly any rolling adhesion was observed, and the few rolling cells showed a low rolling velocity. Blocking both α4 integrin on eosinophils and E-selectin on HUVEC reduced the number of adhered eosinophils by 95%. P-selectin did not significantly participate in eosinophil adhesion to TNF-α-activated HUVEC. Inhibition of both α4 integrins and β2 integrins on eosinophils resulted in a reduction of adhered cells by 65% and a 3-fold increase in percentage rolling cells. Taken together, these results clearly show that resting eosinophils preferentially use constitutively active α4 integrins (α4β1, α4β7) for the first attachment to TNF-α-activated HUVEC. In addition, α4 integrins and E-selectin work synergistically in eosinophil adherence to TNF-α-activated HUVEC. Although E-selectin is important for eosinophil rolling under these conditions, P-selectin plays only a minor role.

P-Selectin Binding Promotes the Adhesion of Monocytes to VCAM-1 Under Flow Conditions

This study examined the adhesive interaction of peripheral blood monocytes with VCAM-1 and analyzed the effect of P-selectin binding to monocytes on the adhesive interaction with VCAM-1 under flow conditions. P-selectin glycoprotein ligand-1 is expressed on most monocytes. Furthermore, most monocytes bind soluble P-selectin derived from platelets. P-selectin binding to monocytes did not alter the amount of expression of α4 integrin on monocytes. However, the mean channel fluorescence value for binding Cy2-conjugated soluble VCAM-1 to P-selectin-bound monocytes was slightly more than that for binding Cy2-conjugated soluble VCAM-1 to untreated monocytes. Under flow conditions, the number of P-selectin-bound monocytes bound to VCAM-1 was much higher than that of untreated monocytes bound to VCAM-1. These bindings were abolished by pretreatment of untreated monocytes and P-selectin-bound monocytes with anti-VCAM-1 mAb or anti-α4 integrin mAb. Furthermore, P-selectin binding to monocytes increased shear resistance and thus increased the adhesive strength of monocytes to VCAM-1. These findings indicate that P-selectin binding to monocytes enhances the adhesive interaction of monocytes with VCAM-1. It is suggested that P-selectin glycoprotein ligand-1/P-selectin interaction and α4 integrin/VCAM-1 interaction can act sequentially in the adhesion cascade that regulates monocyte trafficking to inflammatory and atherosclerotic lesion.

Mice Lacking Two or All Three Selectins Demonstrate Overlapping and Distinct Functions for Each Selectin

Selectins support the capture and rolling of leukocytes in venules at sites of inflammation and in lymphocyte homing. Gene-targeted mice with null mutations at the L-, E-, or P-selectin locus develop normally and show mild (E−/−) to moderate (P−/−, L−/−) defects in inflammatory cell recruitment. Mice lacking both P- and E-selectin (E/P−/−) have severe neutrophilia and spontaneous skin infections that limit their life span. Other combinations of selectin deficiency have not been investigated. We have generated novel mice lacking L- and P-selectin (L/P−/−), L- and E-selectin (L/E−/−), or all three selectins (E/L/P−/−) by bone marrow transplantation. L/P−/− mice (only E-selectin present) show an absence of leukocyte rolling after trauma and severely reduced rolling (by ∼90%) in inflammation induced by TNF-α. Residual rolling in L/P−/− mice was very slow (3.6 ± 0.2 μm/s after TNF-α). L/E−/− mice (only P-selectin present) showed rolling similar to that of L−/− at increased velocities (15.1 ± 0.3 μm/s). The number of adherent leukocytes after 2 or 6 h of TNF-α treatment was not significantly reduced in L/E−/− or L/P−/− mice. E/L/P−/− mice showed very little rolling after TNF-α, all of which was blocked by mAb to α4 integrin. Adherent and emigrated neutrophils were significantly reduced at 6 h after TNF-α. We conclude that any one of the selectins can support some neutrophil recruitment but eliminating all three selectins significantly impairs neutrophil recruitment.

Mechanisms of Selective Leukocyte Recruitment from Whole Blood on Cytokine-Activated Endothelial Cells Under Flow Conditions

Selective recruitment of eosinophils to sites of allergic and parasitic inflammation involves specific adhesion and activation signals expressed on or presented by stimulated endothelial cells. Here we examined leukocyte recruitment on cytokine-activated HUVEC under flow conditions. We perfused whole blood through a flow chamber to examine mechanisms of selective leukocyte recruitment. Although there was substantial recruitment of leukocytes on TNF-α-stimulated HUVEC, we found no selective accumulation of any particular leukocyte subpopulations. In contrast, fewer leukocytes were recruited to IL-4-stimulated HUVEC, but the recruitment was selective for eosinophils. We examined the role of adhesion molecules in these interactions and found that eosinophil recruitment was completely blocked with an α4 integrin mAb at the shear rates examined. A significant number of neutrophils were also recruited to IL-4-stimulated HUVEC, and these interactions required P-selectin and P-selectin glycoprotein ligand-1. Thus, whole blood perfusion over cytokine-activated endothelium revealed that IL-4-stimulated HUVEC support selective recruitment of eosinophils, whereas TNF-α-stimulated HUVEC lack selectivity for any leukocyte subclass.

Varying Roles of E-Selectin and P-Selectin in Different Microvascular Beds in Response to Antigen

Expression of E-selectin and P-selectin is critical in the effector phase of leukocyte recruitment in response to Ag. Whether their relative roles differ between tissues in response to the same Ag is unknown. In this study, a type I hypersensitivity response was elicited in C57BL/6 mice by systemic sensitization with OVA. Following local Ag challenge, endothelial selectin expression was examined in the skin and cremaster muscle microvasculature using a dual-radiolabeled mAb technique. Next, the dermal and muscle microcirculations were visualized using intravital microscopy to establish roles for P-selectin and/or E-selectin. In untreated mice, leukocyte recruitment in both skin and skeletal muscle was mediated entirely by P-selectin. Following Ag challenge, leukocyte rolling flux and adhesion were dramatically increased and leukocyte rolling velocity was unchanged in muscle. Only P-selectin expression increased in muscle, and leukocyte recruitment was entirely dependent upon this selectin. In contrast, in Ag-challenged skin, leukocyte rolling flux did not increase, but rolling velocity dropped profoundly. In skin, only E-selectin expression increased, and blockade of either E-selectin or P-selectin had minimal effect on either rolling flux or rolling velocity. Blockade of both selectins reduced rolling flux by 80% and increased rolling velocity sevenfold. These data highlight striking differences in expression of the endothelial selectins in separate microvascular beds in response to the same stimulus and demonstrate that these differences underlie very different patterns of leukocyte recruitment. The data underscore the importance of studying individual microvascular beds to understand tissue-specific leukocyte recruitment in vivo.

High Affinity Very Late Antigen-4 Subsets Expressed on T Cells Are Mandatory for Spontaneous Adhesion Strengthening But Not for Rolling on VCAM-1 in Shear Flow

The very late Ag-4 (VLA-4) integrin supports both rolling and firm adhesion of leukocytes on VCAM-1 under shear flow. The molecular basis for the unique ability of a single adhesion molecule to mediate these versatile adhesive processes was investigated. VLA-4 occurs in multiple activation states, with different affinities to ligand. In this study we tested how these states regulate VLA-4 adhesiveness under shear flow in Jurkat T cells and PBL. VLA-4 on nonstimulated Jurkat cells supported rolling and spontaneous arrest on VCAM-1, whereas a Jurkat activation mutant with reduced VLA-4 affinity failed to spontaneously arrest after tethering to or during rolling on VCAM-1. The contribution of VLA-4 affinity for ligand to rolling and spontaneous arrests on immobilized VCAM-1 was dissected using soluble VLA-4 ligands, which selectively block high affinity states. VLA-4 saturation with ligand completely blocked spontaneous adhesion strengthening post-tethering to VCAM-1, but did not impair rolling on the endothelial ligand. High affinity VLA-4 was found to comprise a small subset of VLA-4 on resting Jurkat cells and PBL. This subset is essential for firm adhesion but not for tethering or rolling adhesions on VCAM-1. Interestingly, low and high affinity VLA-4 states were found to mediate similar initial tethering to ligand. High affinity VLA-4, constitutively expressed on circulating T cells, may control their early adhesion strengthening on VCAM-1-expressing endothelium before exposure to vascular chemokines and activation of additional integrins.

Important Contributions of P-Selectin Glycoprotein Ligand-1-Mediated Secondary Capture to Human Monocyte Adhesion to P-Selectin, E-Selectin, and TNF-α-Activated Endothelium Under Flow In Vitro

In this study, an in vitro flow model and a blocking mAb to P-selectin glycoprotein ligand-1 (PSGL-1) were used to define the role of PSGL-1 in monocyte attachment and rolling on E- and P-selectin and in attachment and accumulation on 6-h TNF-α-activated HUVEC. KPL1, an adhesion-blocking mAb directed against the tyrosine sulfate motif of PSGL-1, abolished monocyte-adhesive interactions with P-selectin, but only partially blocked monocyte interaction with E-selectin. Further analysis showed that on E-selectin, KPL1 blocked only secondary (i.e., monocyte/monocyte) interactions, but did not block primary (i.e., monocyte/E-selectin) interactions, with secondary adhesion accounting for 90% of the total adhesive interactions on either E- or P-selectin. On cytokine-activated HUVEC, monocytes initially attached and formed linear strings of adherent cells, which involved both primary and secondary adhesion. PSGL-1 or L-selectin mAb reduced string formation, and the combination of PSGL-1 and L-selectin mAb prevented monocyte strings and inhibited 86% of accumulation. Monocyte attachment and rolling on purified adherent monocytes were also critically dependent on PSGL-1 on the adherent monocytes. These studies document that secondary interactions between monocytes, mediated by PSGL-1, are crucial for monocyte initial attachment, rolling, and accumulation on activated endothelium under laminar shear flow.

The Roles of L-Selectin, β7 Integrins, and P-Selectin in Leukocyte Rolling and Adhesion in High Endothelial Venules of Peyer’s Patches

Lymphocyte trafficking into Peyer’s patches requires β7 integrins and L-selectin. Here, we use intravital microscopy to examine leukocyte rolling and adhesion in Peyer’s patch high endothelial venules (HEV) of wild-type, L-selectin-deficient (L−/−), β7 integrin-deficient (β7−/−), and β7/L−/− mice. Although the leukocyte rolling flux fraction was reduced by 70%, Peyer’s patches in L−/− mice were of normal size and cellularity. In β7−/− mice, the rolling flux fraction was normal, but the number of adherent leukocytes in HEV was greatly reduced. The median leukocyte rolling velocity was reduced in L−/− mice and increased in β7−/− mice, suggesting that β7 integrins and L-selectin mediate rolling in Peyer’s patch HEV at different velocities. β7/L−/− exhibited both a low rolling flux fraction and low adhesion and had severely reduced Peyer’s patch size and cellularity. The residual rolling in these mice was completely blocked by a P-selectin mAb. A significant P-selectin component was also detected in the other genotypes. Twenty-six percent of B and T lymphocytes isolated from Peyer’s patches of wild-type mice expressed functional ligands for P-selectin, and this fraction was increased to 57% in β7/L−/− mice. Peyer’s patch HEV were found to express P-selectin under the conditions of intravital microscopy, but not in situ. Our data suggest a novel P-selectin dependent mechanism of lymphocyte homing to Peyer’s patches. In situ, β7 integrins and L-selectin account for all lymphocyte homing to Peyer’s patches, but P-selectin-dependent rolling, as induced by minimal trauma, may support trafficking of effector T lymphocytes to Peyer’s patches.

IL-12 Promotes the Adhesion of NK Cells to Endothelial Selectins Under Flow Conditions

This study examined the adhesive interactions of peripheral blood NK cells with P- and E-selectin and analyzed the effect of IL-12 on the binding of NK cells to these selectins. P-selectin glycoprotein ligand-1 (PSGL-1) is expressed on most resting and IL-12-activated NK cells. However, the percentage of resting NK cells bound to P-selectin-IgG was 15%, and that of activated NK cells bound to P-selectin-IgG was 65%. Furthermore, the number of IL-12-activated NK cells bound to P-selectin-transfected Chinese hamster ovary cells was significantly higher than that of resting NK cells under flow conditions. These interactions were abolished by the incubation of these NK cells with anti-PSGL-1 (PL-1) mAb. Thus, PSGL-1/P-selectin interaction is important in the binding of resting and activated NK cells to P-selectin. NK cells express sialyl-Lewisx (sLex) structure recognized by anti-sLex mAb (KM-93), and IL-12 activation of NK cells increased the mean fluorescence intensity of KM-93-reactive NK cells. Adhesion of IL-12-activated NK cells to E-selectin-transfected Chinese hamster ovary cells was stronger than that of resting NK cells under flow conditions. These interactions were reduced markedly by incubation with anti-sLex mAb. Thus, sLex is the major ligand of resting and activated NK cells for E-selectin. These findings indicate that IL-12 stimulation of NK cells promotes their adhesion activity to endothelial selectins.