The adhesion- and degranulation-promoting adaptor protein and its role in the modulation of experimental autoimmune encephalomyelitis.

Adaptor proteins mediate protein-protein interactions in signal transduction cascades. These signaling molecules are organized in multimolecular complexes that translate information from cell surface receptors into cellular responses. The cytosolic adhesion- and degranulation-promoting adaptor protein (ADAP) is expressed in T cells, natural killer cells, myeloid cells, and platelets. Here we summarize the data about the function of ADAP in these cells with respect to their contribution to the pathogenesis of experimental autoimmune encephalomyelitis. We discuss possible mechanisms of strongly attenuated experimental autoimmune encephalomyelitis in ADAP-deficient mice.

T cell-independent modulation of experimental autoimmune encephalomyelitis in ADAP-deficient mice.

The adhesion- and degranulation-promoting adaptor protein (ADAP), expressed in T cells, myeloid cells, and platelets, is known to regulate receptor-mediated inside-out signaling leading to integrin activation and adhesion. In this study, we demonstrate that, upon induction of active experimental autoimmune encephalomyelitis (EAE) by immunization with the myelin oligodendrocyte glycoprotein35-55 peptide, ADAP-deficient mice developed a significantly milder clinical course of EAE and showed markedly less inflammatory infiltrates in the CNS than wild-type mice. Moreover, ADAP-deficient recipients failed to induce EAE after adoptive transfer of myelin oligodendrocyte glycoprotein-specific TCR-transgenic T cells (2D2 T cells). In addition, ex vivo fully activated 2D2 T cells induced significantly less severe EAE in ADAP-deficient recipients. The ameliorated disease in the absence of ADAP was not due to expansion or deletion of a particular T cell subset but rather because of a strong reduction of all inflammatory leukocyte populations invading the CNS. Monitoring the adoptively transferred 2D2 T cells over time demonstrated that they accumulated within the lymph nodes of ADAP-deficient hosts. Importantly, transfer of complete wild-type bone marrow or even bone marrow of 2D2 TCR-transgenic mice was unable to reconstitute EAE in the ADAP-deficient animals, indicating that the milder EAE was dependent on (a) radio-resistant nonhematopoietic cell population(s). Two-photon microscopy of lymph node explants revealed that adoptively transferred lymphocytes accumulated at lymphatic vessels in the lymph nodes of ADAP-deficient mice. Thus, our data identify a T cell-independent mechanism of EAE modulation in ADAP-deficient mice.

The adapter protein ADAP is required for selected dendritic cell functions.

BACKGROUND: The cytosolic adaptor protein ADAP (adhesion and degranulation promoting adapter protein) is expressed by T cells, natural killer cells, myeloid cells and platelets. ADAP is involved in T-cell-receptor-mediated inside-out signaling, which leads to integrin activation, adhesion and reorganization of the actin cytoskeleton. However, little is known about the role of ADAP in myeloid cells. In the present study, we analyzed the function of ADAP in bone-marrow-derived dendritic cells (BMDCs) from ADAP-deficient mice. RESULTS: ADAP-deficient BMDCs showed almost normal levels of antigen uptake, adhesion, maturation, migration from the periphery to the draining lymph nodes, antigen-specific T-cell activation, and production of the proinflammatory cytokines IL-6 and TNF-∝. Furthermore, we provide evidence that the activation of signaling pathways after lipopolysaccharide (LPS) stimulation are not affected by the loss of ADAP. In contrast, ADAP-deficient BMDCs showed defects in CD11c-mediated cellular responses, with significantly diminished production of IL-6, TNF-∝ and IL-10. Actin polymerization was enhanced after CD11c integrin stimulation. CONCLUSIONS: In summary, we propose that the adapter molecule ADAP is critical for selected CD11c integrin-mediated functions of dendritic cells.

CCR7-mediated LFA-1 functions in T cells are regulated by 2 independent ADAP/SKAP55 modules.

The ß2-integrin lymphocyte function-associated antigen-1 (LFA-1) plays a crucial role within the immune system. It regulates the interaction between T cells and antigen-presenting cells and facilitates T-cell adhesion to the endothelium, a process that is important for lymphocyte extravasation and homing. Signals mediated via the T-cell receptor and the chemokine receptor CCR7 activate LFA-1 through processes known as inside-out signaling. The molecular mechanisms underlying inside-out signaling are not completely understood. Here, we have assessed the role of the ADAP/SKAP55 module for CCR7-mediated signaling. We show that loss of the module delays homing and reduces intranodal T-cell motility in vivo. This is probably because of a defect in CCR7-mediated adhesion that affects both affinity and avidity regulation of LFA-1. Further analysis of how the ADAP/SKAP55 module regulates CCR7-induced integrin activation revealed that 2 independent pools of the module are expressed in T cells. One pool interacts with a RAPL/Mst1 complex, whereas the other pool is linked to a RIAM/Mst1/Kindlin-3 complex. Importantly, both the RAPL/Mst1 and the RIAM/Mst1/Kindlin-3 complexes require ADAP/SKAP55 for binding to LFA-1 upon CCR7 stimulation. Hence, 2 independent ADAP/SKAP55 modules are essential components of the signaling machinery that regulates affinity and avidity of LFA-1 in response to CCR7.

Expression of SKAP-HOM in DCs is required for an optimal immune response in vivo.

The cytosolic adaptor molecule SKAP-HOM, similar to the T cell-specific homologue SKAP55, interacts directly with ADAP, and both molecules are involved in inside-out signaling. Previous studies have shown that in the absence of SKAP-HOM, antigen receptor-triggered integrin-mediated adhesion is impaired severely in B cells but not in T cells. In addition, loss of SKAP-HOM results in a less severe clinical course of EAE. DCs are the most potent APCs and express SKAP-HOM. However, the role of SKAP-HOM in DCs remains unknown. Here, we assessed whether the reduced severity of EAE observed in SKAP-HOM-deficient mice is at least partially a result of an impaired cooperation between APCs and T cells. We demonstrate that migration of LC in vivo and the spontaneous motility of BMDCs in vitro are increased in the absence of SKAP-HOM. In contrast, triggering of the integrin results in a drastic decrease of DC motility and in enhanced actin polymerization in SKAP-HOM-deficient DCs. Furthermore, the antigen-dependent conjugate formed between wild-type T cells and SKAP-HOM(-/-) DCs is delayed in comparison with wild-type DCs. Strikingly, fewer antigen-specific T cells are induced by immunization with SKAP-HOM(-/-) BMDCs as compared with wild-type BMDCs in vivo. Thus, these findings suggest that SKAP-HOM expression in DCs is required for the induction of an optimal immune response.

Strain-specific susceptibility for neurodegeneration in a rat model of autoimmune optic neuritis.

Heterogeneity in clinical disease course and histopathology complicates the treatment of multiple sclerosis. We detected important differences in neurodegeneration in various subtypes of myelin oligodendrocyte glycoprotein (MOG)-induced optic neuritis. Dark Agouti (DA) rats showed a significantly higher survival of retinal ganglion cells in comparison to Brown Norway rats. After surgical transection of the optic nerve neuronal loss was similar in both rat strains. We identified an increased expression of interleukin 1beta and glial cell line-derived neurotrophic factor in DA rats as the possible mechanism of the observed endogenous neuroprotection in MOG-induced optic neuritis.

TGF-beta1-mediated control of central nervous system inflammation and autoimmunity through the inhibitory receptor CD26.

The T cell marker CD26/dipeptidyl peptidase (DP) IV is associated with an effector phenotype and markedly elevated in the human CNS disorder multiple sclerosis. However, little is known about the in vivo role of CD26/DP IV in health and disease, and the underlying mechanism of its function in CNS inflammation. To directly address the role of CD26/DP IV in vivo, we examined Th1 immune responses and susceptibility to experimental autoimmune encephalomyelitis in CD26(-/-) mice. We show that gene deletion of CD26 in mice leads to deregulation of Th1 immune responses. Although production of IFN-gamma and TNF-alpha by pathogenic T cells in response to myelin Ag was enhanced in CD26(-/-) mice, production of the immunosuppressive cytokine TGF-beta1 was diminished in vivo and in vitro. In contrast to the reduction in TGF-beta1 production, responsiveness to external TGF-beta1 was normal in T cells from CD26(-/-) mice, excluding alterations in TGF-beta1 sensitivity as a mechanism causing the loss of immune regulation. Natural ligands of CD26/DP IV induced TGF-beta1 production in T cells from wild-type mice. However, natural ligands of CD26/DP IV failed to elicit TGF-beta1 production in T cells from CD26(-/-) mice. The striking functional deregulation of Th1 immunity was also seen in vivo. Thus, clinical experimental autoimmune encephalomyelitis scores were significantly increased in CD26(-/-) mice immunized with peptide from myelin oligodendrocyte glycoprotein. These results identify CD26/DP IV as a nonredundant inhibitory receptor controlling T cell activation and Th1-mediated autoimmunity, and may have important therapeutic implications for the treatment of autoimmune CNS disease.

The ADAP/SKAP55 signaling module regulates T-cell receptor-mediated integrin activation through plasma membrane targeting of Rap1.

Adhesion of T cells after stimulation of the T-cell receptor (TCR) is mediated via signaling processes that have collectively been termed inside-out signaling. The molecular basis for inside-out signaling is not yet completely understood. Here, we show that a signaling module comprising the cytosolic adapter proteins ADAP and SKAP55 is involved in TCR-mediated inside-out signaling and, moreover, that the interaction between ADAP and SKAP55 is mandatory for integrin activation. Disruption of the ADAP/SKAP55 module leads to displacement of the small GTPase Rap1 from the plasma membrane without influencing its GTPase activity. These findings suggest that the ADAP/SKAP55 complex serves to recruit activated Rap1 to the plasma membrane. In line with this hypothesis is the finding that membrane targeting of the ADAP/SKAP55 module induces T-cell adhesion in the absence of TCR-mediated stimuli. However, it appears as if the ADAP/SKAP55 module can exert its signaling function outside of the classical raft fraction of the cell membrane.

The role of adaptor proteins in lymphocyte activation.

Research within the last 10 years has provided compelling evidence that adaptor proteins regulate the major pathways of lymphocyte activation. Based upon their differential subcellular localization, transmembrane adaptors and cytosolic adaptors can be distinguished. Here we review some of the most recent findings about both types of adaptor proteins which have facilitated our understanding how immunoreceptors control lymphocyte activation and differentiation.