Chemoattractants induce a rapid and transient upregulation of monocyte alpha4 integrin affinity for vascular cell adhesion molecule 1 which mediates arrest: an early step in the process of emigration.

Chemoattractants and chemokines induce arrest of rolling monocytes during emigration from blood into tissues. In this study, we demonstrated that alpha4 integrin affinity for vascular cell adhesion molecule (VCAM)-1 was upregulated rapidly and transiently by chemoattractants and stromal cell-derived factor (SDF)-1alpha and mediated monocyte arrest. alpha4 integrin affinity changes were detected and blocked using soluble VCAM-1/Fc (sVCAM-1/Fc). In a flow cytometry assay, markedly increased sVCAM-1/Fc binding to human blood monocytes or U937 cells transfected with formyl peptide (FP) receptor was detected 30 s after FP or SDF-1alpha treatment and declined after 2 min. In a parallel plate flow chamber assay, FP, C5a, platelet-activating factor, or SDF-1alpha coimmobilized with VCAM-1 induced leukocyte arrest, which was blocked by inclusion of sVCAM-1/Fc but not soluble nonimmune immunoglobulin G in the assay buffer.

Positive regulation of T cell activation and integrin adhesion by the adapter Fyb/Slap.

The molecular adapter Fyb/Slap regulates signaling downstream of the T cell receptor (TCR), but whether it plays a positive or negative role is controversial. We demonstrate that Fyb/Slap-deficient T cells exhibit defective proliferation and cytokine production in response to TCR stimulation. Fyb/Slap is also required in vivo for T cell-dependent immune responses. Functionally, Fyb/Slap has no apparent role in the activation of known TCR signaling pathways, F-actin polymerization, or TCR clustering. Rather, Fyb/Slap regulates TCR-induced integrin clustering and adhesion. Thus, Fyb/Slap is the first molecular adapter to be identified that couples TCR stimulation to the avidity modulation of integrins governing T cell adhesion.

Apoptotic cells are present in the CNS throughout acute and chronic-progressive EAE in the absence of clinical recovery.

Experimental allergic encephalomyelitis (EAE) is an autoimmune, demyelinating disorder of the central nervous system induced in susceptible animals as a model for the human disease multiple sclerosis. Antibodies against the leukocyte adhesion molecule alpha4 integrin have been shown to prevent and reverse acute and chronic EAE of the guinea pig. The results presented in this paper implicate apoptosis as the mechanism of reversal of EAE following treatment with anti-alpha4 integrin antibody. Apoptotic cells were observed in the central nervous system (CNS) throughout chronic-progressive EAE of the guinea pig in the absence of clinical recovery. Many of the apoptotic cells were identified as T cells using immunohistochemistry. Similarly, apoptotic cells were present in the CNS of animals during anti-alpha4 integrin-mediated recovery from acute and chronic disease. Therefore, anti-alpha4 integrin-mediated recovery from EAE is due to the prevention of the influx of new inflammatory cells into the CNS that are required to replace those undergoing apoptosis.

Alpha 4 beta 1 integrin/VCAM-1 interaction activates alpha L beta 2 integrin-mediated adhesion to ICAM-1 in human T cells.

Modulation of integrin affinity and/or avidity provides a regulatory mechanism by which leukocyte adhesion to endothelium is strengthened or weakened at different stages of emigration. In this study, we demonstrate that binding of high-affinity alpha 4 beta 1 integrins to VCAM-1 strengthens alpha L beta 2 integrin-mediated adhesion. The strength of adhesion of Jurkat cells, a human leukemia T cell line, or MnCl2-treated peripheral blood T cells to immobilized chimeric human VCAM-1/Fc, ICAM-1/Fc, or both was quantified using parallel plate flow chamber leukocyte detachment assays in which shear stress was increased incrementally (0.5-30 dynes/cm2). The strength of adhesion to VCAM-1 plus ICAM-1, or to a 40-kDa fragment of fibronectin containing the CS-1 exon plus ICAM-1, was greater than the sum of adhesion to each molecule alone. Treatment of Jurkat or blood T cells with soluble cross-linked VCAM-1/Fc or HP2/1, a mAb to alpha 4, significantly increased adhesion to ICAM-1. These treatments induced clustering of alpha L beta 2 integrins, but not the high-affinity beta 2 integrin epitope recognized by mAb 24. Up-regulated adhesion to ICAM-1 was abolished by cytochalasin D, an inhibitor of cytoskeletal rearrangement. Taken together, our data suggest that the binding of VCAM-1 or fibronectin to alpha 4 beta 1 integrins initiates a signaling pathway that increases beta 2 integrin avidity but not affinity. A role for the cytoskeleton is implicated in this process.

The NF-kappa B signal transduction pathway in aortic endothelial cells is primed for activation in regions predisposed to atherosclerotic lesion formation.

Atherosclerotic lesions form at distinct sites in the arterial tree, suggesting that hemodynamic forces influence the initiation of atherogenesis. If NF-kappaB plays a role in atherogenesis, then the activation of this signal transduction pathway in arterial endothelium should show topographic variation. The expression of NF-kappaB/IkappaB components and NF-kappaB activation was evaluated by specific antibody staining, en face confocal microscopy, and image analysis of endothelium in regions of mouse proximal aorta with high and low probability (HP and LP) for atherosclerotic lesion development. In control C57BL/6 mice, expression levels of p65, IkappaBalpha, and IkappaBbeta were 5- to 18-fold higher in the HP region, yet NF-kappaB was activated in a minority of endothelial cells. This suggested that NF-kappaB signal transduction was primed for activation in HP regions on encountering an activation stimulus. Lipopolysaccharide treatment or feeding low-density lipoprotein receptor knockout mice an atherogenic diet resulted in NF-kappaB activation and up-regulated expression of NF-kappaB-inducible genes predominantly in HP region endothelium. Preferential regional activation of endothelial NF-kappaB by systemic stimuli, including hypercholesterolemia, may contribute to the localization of atherosclerotic lesions at sites with high steady-state expression levels of NF-kappaB/IkappaB components.