Targeting Platelet–Leukocyte Interactions: Identification of the Integrin Mac-1 Binding Site for the Platelet Counter Receptor Glycoprotein Ibα

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are dependent on the interaction of the leukocyte integrin Mac-1 (αMβ2, CD11b/CD18) and the platelet counter receptor glycoprotein (GP) Ibα. Previous studies have established a central role for the I domain, a stretch of ∼200 amino acids within the αM subunit, in the binding of GP Ibα. This study was undertaken to establish the molecular basis of GP Ibα recognition by αMβ2. The P201–K217 sequence, which spans an exposed loop and amphipathic α4 helix in the three-dimensional structure of the αMI domain, was identified as the binding site for GP Ibα. Mutant cell lines in which the αMI domain segments P201–G207 and R208–K217 were switched to the homologous, but non-GP Ibα binding, αL domain segments failed to support adhesion to GP Ibα. Mutation of amino acid residues within P201–K217, H210–A212, T213–I215, and R216–K217 resulted in the loss of the binding function of the recombinant αMI domains to GP Ibα. Synthetic peptides duplicating the P201–K217, but not scrambled versions, directly bound GP Ibα and inhibited αMβ2-dependent adhesion to GP Ibα and adherent platelets. Finally, grafting critical amino acids within the P201–K217 sequence onto αL, converted αLβ2 into a GP Ibα binding integrin. Thus, the P201–K217 sequence within the αMI domain is necessary and sufficient for GP Ibα binding. These observations provide a molecular target for disrupting leukocyte–platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis

KLF2 Is a Novel Transcriptional Regulator of Endothelial Proinflammatory Activation

The vascular endothelium is a critical regulator of vascular function. Diverse stimuli such as proinflammatory cytokines and hemodynamic forces modulate endothelial phenotype and thereby impact on the development of vascular disease states. Therefore, identification of the regulatory factors that mediate the effects of these stimuli on endothelial function is of considerable interest. Transcriptional profiling studies identified the Kruppel-like factor (KLF)2 as being inhibited by the inflammatory cytokine interleukin-1β and induced by laminar shear stress in cultured human umbilical vein endothelial cells. Overexpression of KLF2 in umbilical vein endothelial cells robustly induced endothelial nitric oxide synthase expression and total enzymatic activity. In addition, KLF2 overexpression potently inhibited the induction of vascular cell adhesion molecule-1 and endothelial adhesion molecule E-selectin in response to various proinflammatory cytokines. Consistent with these observations, in vitro flow assays demonstrate that T cell attachment and rolling are markedly attenuated in endothelial monolayers transduced with KLF2. Finally, our studies implicate recruitment by KLF2 of the transcriptional coactivator cyclic AMP response element–binding protein (CBP/p300) as a unifying mechanism for these various effects. These data implicate KLF2 as a novel regulator of endothelial activation in response to proinflammatory stimuli

T-bet is required for optimal proinflammatory CD4+ T-cell trafficking

Inflammatory responses are controlled by T helper 1 (Th1) lymphocytes. An important function of this polarity is the ability of T cells to traffick appropriately in vivo. This differential trafficking is dependent upon the binding of P-selectin glycoprotein ligand-1 to P- and E-selectin on inflamed endothelium as well as the expression of specific chemokine receptors. Here we show that in the absence of T-box expressed in T cells (T-bet), selective migration of T cells in vivo is completely abrogated and that T-bet regulates the binding of CD4+ T cells to P-selectin. T-bet is also required for the expression of the chemokine receptor CXCR3. Thus, T-bet controls Th1-cell migration to inflammatory sites, which has fundamental consequences for the control of immunologic disease

A Lupus-Associated Mac-1 Variant Has Defects in Integrin Allostery and Interaction with Ligands under Force

Leukocyte CD18 integrins increase their affinity for ligand by transmitting allosteric signals to and from their ligand-binding αI domain. Mechanical forces induce allosteric changes that paradoxically slow dissociation by increasing the integrin/ligand bond lifetimes, referred to as catch bonds. Mac-1 formed catch bonds with its ligands. However, a Mac-1 gene (ITGAM) coding variant (rs1143679, R77H), which is located in the β-propeller domain and is significantly associated with systemic lupus erythematosus risk, exhibits a marked impairment in 2D ligand affinity and affinity maturation under mechanical force. Targeted mutations and activating antibodies reveal that the failure in Mac-1 R77H allostery is rescued by induction of cytoplasmic tail separation and full integrin extension. These findings demonstrate roles for R77, and the β-propeller in which it resides, in force-induced allostery relay and integrin bond stabilization. Defects in these processes may have pathological consequences, as the Mac-1 R77H variant is associated with increased susceptibility to lupus

Focal Adhesion Kinase Regulates Pathogen-Killing Capability and Life Span of Neutrophils via Mediating both Adhesion-Dependent and -Independent Cellular Signals

Various neutrophil functions such as phagocytosis, superoxide production, and survival are regulated by integrin signaling. Despite the essential role of focal adhesion kinase (FAK) in mediating this signaling pathway, its exact function in neutrophils is ill defined. In this study, we investigated the role of FAK in neutrophils using a myeloid-specific conditional FAK knockout mouse. As reported in many other cell types, FAK is required for regulation of focal adhesion dynamics when neutrophils adhere to fibronectin or ICAM-1. Adhesion on VCAM-1-coated surfaces and chemotaxis after adhesion were not altered in FAK null neutrophils. In addition, we observed significant reduction in NADPH oxidase-mediated superoxide production and complement-mediated phagocytosis in FAK null neutrophils. As a result, these neutrophils displayed decreased pathogen killing capability both in vitro and in vivo in a mouse peritonitis model. In adherent cells, the defects associated with FAK deficiency are likely due to suppression of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) signaling and chemoattractant-elicited calcium signaling. Disruption of FAK also reduced chemoattractant-elicited superoxide production in suspended neutrophils in the absence of cell adhesion. This may be solely caused by suppression of PtdIns(3,4,5)P3 signaling in these cells, because the fMLP-elicited calcium signal was not altered. Consistent with decreased PtdIns(3,4,5)P3/Akt signaling in FAK null neutrophils, we also observed accelerated spontaneous death in these cells. Taken together, our results revealed previously unrecognized roles of FAK in neutrophil function and provided a potential therapeutic target for treatment of a variety of infectious and inflammatory diseases