SKAP-55, SKAP-55-related and ADAP adaptors modulate integrin-mediated immune-cell adhesion

Abstract

Integrin adhesion is essential for aspects of immune function, including antigen presentation and migration in lymph nodes, germinal centers and sites of inflammation. Antigen receptors on B and T cells generate 'inside-out' signals for increased integrin clustering and adhesion. Although upstream components of B-cell-receptor or T-cell-receptor signaling are needed, the identity of key downstream effectors that mediate integrin adhesion is only just emerging. New candidates include immune-cell-specific adaptor proteins ADAP, SKAP-55 and SKAP-55-related (SKAP-55R). SKAP-55 has recently been identified as an effector in T cells in SKAP-55-deficient mice, whereas SKAP-55R is needed for B-cell adhesion. ADAP is required for SKAP-55 and SKAP-55R protein stability. SKAP-55 and SKAP-55R have unexpectedly specialized roles in T- and B-cell adhesion of the immune system.

Figure 1

Roles of integrin-mediated adhesion in T-cell function. (a) Integrin-mediated T-cell transmigration through blood vessels. Selectins expressed on T cells weakly interact with the vascular endothelium to induce rolling and tethering of T cells. Chemokines or TCR signaling activate LFA-1 and α4 integrins. This increases the avidity of integrins to enable them to firmly bind to the ligands, ICAMs or VCAMs, such that T cells are arrested at this stage. These T cells then migrate along the blood vessel or transmigrate through the endothelium, and are mediated via adhesion molecules, such as CD31 [also known as PECAM-1 (platelet endothelial cell-adhesion molecule-1)], CD44 (homing function and Indian-blood-group system) and JAM1 (junctional adhesion-molecule 1). (b) Integrins mediate T-cell-antigen-presenting cell (APC) conjugate formation and the immunological-synapse (IS) formation. In antigen presentation, initially weak contacts mediated by integrins are followed by ligation of the TCR complex that induces ‘inside-out’ signals, which fully activate the high avidity of LFA-1. In the contact area of the T-cell-APC conjugate, receptors are rearranged into signaling clusters (termed microclusters) and eventually form mature immunological synapses, which are also termed supermolecular activation clusters (SMACs). TCR–CD3, CD4 and 8, CD28 or protein kinase C θ (PKCθ) are translocated to the center of the contact area to form the cSMAC, while a ring of LFA-1 and talin forms the pSMAC.

Figure 2

TCR-mediated ‘inside-out’ signaling events that lead to increased LFA-1 adhesion. After TCR is linked to the peptide-MHC complex, CD4- or CD8-associated LCK activates ZAP-70, which, in turn, phosphorylates key tyrosines on LAT, leading to the recruitment of phospholipase Cγ1 (PLCγ1) and GADS that binds constitutively in a complex with SLP-76. ITK is recruited to the complex for the phosphorylation of PLCγ1 and the mobilization of intracellular calcium. The binding of the SLP-76 SH2 domain to ADAP provides a link to SKAP-55 as an effector for LFA-1 clustering. ADAP itself functions as a chaperone for SKAP-55 and SKAP-55R degradation and binds to VASP and dynein, which could regulate the actin cytoskeleton (not shown). Further downstream, SKAP-55 might interact with Rap1-binding protein Riam or RapL and promote Rap1 translocation to the plasma membrane to regulate LFA-1 clustering. In B cells, BCR engagement might initiate the activation of SYK kinase that,in turn, phosphorylates the SLP-76 homolog SLP-65. The manner by which SLP-65 recruits SKAP-55R and further downstream molecules to activate integrins is not yet known.

Figure 3

Structures of ADAP, SKAP-55 and SKAP-55R and their binding partners. The molecular-domain structure of ADAP, SKAP-55 and SKAP-55R. ADAP has a proline-rich domain, multiple tyrosine sites, two putative nuclear localization sites, two SH3 domains and an EVH1-domain binding site. SKAP-55 and SKAP-55R contain a PH domain, a C-terminal SH3 domain and three tyrosine-based residues, whereas the N-terminal region in SKAP-55R also has a well-defined coiled-coil domain. Abbreviations: EVH1, Ena (enabled)–VASP (vasodilator-stimulated phospho protein) homology-1; NLS, nuclear localization sites; PH, pleckstrin homology domain; PPP, proline-rich region; SH3, Src homology 3 domain; YYY, tyrosine phosphorylation motifs.