On the other hand, the binding of integrin extracelluar domains to ligands or other agonists (stimulatory antibody, PMA, Mg2+ or Mn2+), and physiological force exerted on the bond, could initiate conformational change of the integrin, which then sends biochemical and mechanical signalling into the cell to regulate multiple cellular functions; this is termed ‘outside-in’ signalling.12,13 In T cells, integrin bidirectional signals lead to the formation of the immunological synapse, stabilization of T-cell–APC contact to facilitate T-cell activation, proliferation and cytokine secretion (e.g. interleukin-2, interferon-γ).19–21 In macrophages, integrin activation induces cytoskeletal rearrangement during the

process of phagocytosis, cytokine mRNA stabilization (e.g. interleukin-1β) and cell differentiation.22 Integrin signalling also enhances neutrophil

degranulation and activation of NADPH oxidase, leading to production of reactive oxygen species,23 or induces see more polarization of cytolytic granules in natural killer cells or cytolytic T lymphocytes.24 In the following discussion, we will describe those key effectors involved in integrin bidirectional signalling pathways, with particular attention to the signalling molecules in T lymphocytes. After the TCR/CD3 complex is engaged with the MHC–peptide complex, Src kinase (lymphocyte-specific protein tyrosine kinase; LCK) is phosphorylated and activated, leading to phosphorylation Apoptosis inhibitor of immunoreceptor tyrosine-based activation motifs on the TCRξ/CD3 chains. Kinase ζ-associated protein of molecular weight 70 000 (ZAP-70) is recruited to the TCR/CD3 complex GBA3 and is phosphorylated by LCK. Activated ZAP-70 then phosphorylates a number of downstream adaptors, including linker for activation of T cells (LAT) and Src homology

2 (SH2) domain-containing leucocyte protein of molecular weight 76 000 (SLP-76) (Fig. 1). Elevated levels of LCK in cloned cytolytic T cells markedly increase cytolytic activity and enhance LFA-1 expression levels with increased cell binding to the ligand intercellular adhesion molecule 1 (ICAM-1).25 In LCK-deficient Jurkat cells (i.e. JCaM1.6 cells) or in Src kinase inhibitor PP2-treated Jurkat cells, CD3 ligation-induced adhesion to ICAM-1 is dramatically reduced.26 These studies suggest that LCK is a positive regulator for integrin activation. Similarly, ZAP-70-deficient Jurkat cells fail in TCR-induced integrin β1-mediated adhesion and the kinase activity of ZAP-70 required for LAT phosphorylation is crucial for integrin activation.27 This fits with the defective integrin activation and adhesion in LAT-deficient Jurkat cells. Further, LAT is associated directly or indirectly with a number of key signalling proteins, including phosphatidylinositol 3-kinase, the inducible T-cell kinase (ITK), SLP-76, and phospholipase C-γ1 (Fig. 1). These kinases, adaptors or enzymes have been implicated to play critical roles in TCR-induced ‘inside-out’ signalling for integrin activation.