The use of nanomaterials has raised safety concerns, as their small size facilitates accumulation in and interaction with biological tissues. Here we show that exposure of endothelial cells to TiO 2 nanomaterials causes endothelial cell leakiness. This effect is caused by the physical interaction between TiO 2 nanomaterials and endothelial cells' adherens junction protein VE-cadherin. As a result, VE-cadherin is phosphorylated at intracellular residues (Y658 and Y731), and the interaction between VE-cadherin and p120 as well as b-catenin is lost. The resulting signalling cascade promotes actin remodelling, as well as internalization and degradation of VE-cadherin. We show that injections of TiO 2 nanomaterials cause leakiness of subcutaneous blood vessels in mice and, in a melanoma-lung metastasis mouse model, increase the number of pulmonary metastases. Our findings uncover a novel non-receptor-mediated mechanism by which nanomaterials trigger intracellular signalling cascades via specific interaction with VE-cadherin, resulting in nanomaterial-induced endothelial cell leakiness.

The CXCR4 antagonist plerixafor augments frequency of circulating neutrophils via release from the lung and prevents neutrophil homing to the bone marrow.

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Integrin undergoes different activation states by changing its quaternary conformation. The integrin ␤ hybrid domain acts as a lever for the transmission of activation signal. Integrins represent a large family of type I heterodimeric (␣ and ␤ subunits) membrane proteins capable of bidirectional signal transduction serving cell growth, differentiation, and apoptosis (1). The ␤ I-like domain is flanked by the hybrid and PSI 1 (for plexins, semaphorins, and integrins) domains (Fig. 1A) (2, 3). The hybrid domain has been shown to be important in the propagation of the activation signal from one end of the ␤ subunit to the other (2, 4-6). Recently, superimposed structural coordinates of liganded-open ␣ IIb ␤ 3 headpiece with that of unliganded-closed ␣ V ␤ 3 revealed an ϳ10-Å shift and a concomitant rotation of the hybrid domain relative to the last helix of the ␤ I-like domain upon ligand binding (7). mAbs that attenuate this swing-out motion of the integrin ␣ 5 ␤ 1 hybrid domain prevent effective allosteric activation of the ␤ I-like domain (5, 6).Collective observations from electron microscopy images and crystal structures of integrin ␣ IIb ␤ 3 , ␣ V ␤ 3 , and ␣ 5 ␤ 1 suggest that integrin may undergo at least three activation states depicted by different quaternary conformations (3, 7-13). A bent integrin with the hybrid domain in close proximity of the integrin epidermal growth factor (I-EGF) 3 and 4 domains represents the resting state. The extended integrin with the hybrid domain distally separated from I-EGF 3 and 4 but orientated toward the ␣ subunit ␤-propeller depicts a low affinity state, whereas the extended integrin with a swing-out hybrid domain away from the ␣ subunit ␤-propeller represents a high affinity state.The conceptualization of different integrin affinity states also derives from earlier functional studies. Observations were made on integrins having different ligand binding properties under different cellular or extracellular conditions. Resting platelet integrin ␣ IIb ␤ 3 binds immobilized fibrinogen, but binding to soluble fibrinogen, fibronectin, or von Willebrand factor requires platelet activation by agonist (14 -16). Divalent cation Mn 2ϩ activates integrin ␣ 4 ␤ 1 to bind VCAM-1, whereas adhesion to fibronectin-derived CS-1 peptide requires additional activating mAb (17). Real time analysis of integrin ␣ 4 ␤ 1 binding to fluorescent-conjugated ligand mimetic via chemokine receptor activation on leukocytes also suggests integrin acquiring multiple affinity states (18). For the integrin ␣ L ␤ 2 , ligand ICAM-1 exhibits higher affinity for purified ␣ L ␤ 2 from T cells than . Prior exposure of ␣ L ␤ 2 to ICAM-1 increased its binding to . Soluble ICAM (sICAM)-3 binds to ␣ L ␤ 2 with 9-fold lower affinity than sICAM-1 (21). We also reported the requirement of two ␣ L ␤ 2 -activating mAbs, KIM185 and KIM127, for their adhesion to ICAM-3 as compared with ICAM-1 (22). Crystal structures of engineered intermediate affinity ␣ L I-domain (L161C/F299C) and high affinity ␣ L I-domain (K287C/K294C...

LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) are members of the β2 integrins involved in leukocyte function during immune and inflammatory responses. We aimed to determine a minimized β2 subunit that forms functional LFA-1 and Mac-1. Using a series of truncated β2 variants, we showed that the subregion Q23-D300 of the β2 subunit is sufficient to combine with the αL and αM subunits intracellularly. However, only the β2 variants terminating after Q444 promote cell surface expression of LFA-1 and Mac-1. Thus, the major cysteine-rich region and the three highly conserved cysteine residues at positions 445, 447, and 449 of the β2 subunit are not required for LFA-1 and Mac-1 surface expression. The surface-expressed LFA-1 variants are constitutively active with respect to ICAM-1 adhesion and these variants express the activation reporter epitope of the mAb 24. In contrast, surface-expressed Mac-1, both the wild type and variants, require 0.5 mM MnCl2 for adhesion to denatured BSA. These results suggest that the role of the β2 subunit in LFA-1- and Mac-1-mediated adhesion may be different.