STAP-2 negatively regulates BCR-mediated B cell activation by recruiting tyrosine-protein kinase CSK to LYN.

Although signal-transducing adaptor protein-2 (STAP-2) acts in certain immune responses, its role in B cell receptor (BCR)-mediated signals remains unknown. In this study, we have revealed that BCR-mediated signals, cytokine production and antibody production were increased in STAP-2 knockout (KO) mice compared with wild-type (WT) mice. Phosphorylation of tyrosine-protein kinase LYN Y508 was reduced in STAP-2 KO B cells after BCR stimulation. Mechanistic analysis revealed that STAP-2 directly binds to LYN, dependently of STAP-2 Y250 phosphorylation by LYN. Furthermore, phosphorylation of STAP-2 enhanced interactions between LYN and tyrosine-protein kinase CSK, resulting in enhanced CSK-mediated LYN Y508 phosphorylation. These results suggest that STAP-2 is crucial for controlling BCR-mediated signals and antibody production by enhanced CSK-mediated feedback regulation of LYN.

STAP-2 positively regulates FcεRI-mediated basophil activation and basophil-dependent allergic inflammatory reactions.

Basophils are an important cell type in the regulation of Th2 immune responses. Recently, we revealed that signal-transducing adaptor protein-2 (STAP-2) negatively regulates mast cell activation via FcεRI. However, the role of STAP-2 in basophil maturation and activation remained unclear. In this study, we demonstrated the normal development of basophils in STAP-2-deficient (STAP-2-/-) mice. We also demonstrated in vitro normal basophil differentiation and FcεRI expression in STAP-2-/- mice, suggesting that STAP-2 is dispensable for basophil maturation. Using bone marrow-derived cultured basophils (BMBs), we showed that degranulation and cytokine production of STAP-2-/- BMBs were lower than those of wild-type (WT) BMBs upon stimulation with IgE/Ag. In accordance with the reduction of degranulation and cytokine production, phosphorylation of several signal molecules such as Lyn, PLC-γ2 and Erk was reduced in STAP-2-/- BMBs after stimulation via FcεRI. Finally, it was observed that IgE-dependent chronic allergic inflammation of STAP-2-/- mice was significantly inhibited compared with WT mice. Taken together, we conclude that STAP-2 is an adaptor molecule that positively regulates FcεRI-mediated basophil activation and basophil-dependent allergic inflammatory reactions.

A Novel α9 Integrin Ligand, XCL1/Lymphotactin, Is Involved in the Development of Murine Models of Autoimmune Diseases.

The integrin α9ß1 is a key receptor involved in the development of autoimmune diseases. However, the detailed mechanism for the association of α9ß1 integrin with its ligands remains unclear. In this study, we introduce XCL1/lymphotactin, a member of the chemokine family, as a novel ligand for α9 integrin. Using α9 integrin-overexpressing NIH3T3 cells and endogenously α9 integrin-expressing human rhabdomyosarcoma cells, the interaction between XCL1 and α9 integrin was confirmed by pull-down assays. XCL1 enhanced α9 integrin-dependent cell migration of these cells, thus acting on α9 integrin as a chemoattractant. We also analyzed the in vivo function of XCL1 in the development of anti-type II collagen Ab-induced inflammatory arthritis (CAIA) in BALB/c mice and experimental autoimmune encephalomyelitis in C57BL/6 mice, because α9 integrin is involved in these autoimmune disease models. In CAIA, recombinant XCL1 aggravated the disease and this exacerbation was inhibited by an anti-α9 integrin Ab. An XCL1-neutralizing Ab produced in this study also ameliorated CAIA. Furthermore, the XCL1-neutralizing Ab abrogated the disease progression in experimental autoimmune encephalomyelitis. Therefore, to our knowledge this study provides the first in vitro and in vivo evidence that the interaction between XCL1 and α9 integrin has an important role for autoimmune diseases.

Anti-IL-17A blocking antibody reduces cyclosporin A-induced relapse in experimental autoimmune encephalomyelitis mice.

Cyclosporin A (CsA) is effective at reducing pathogenic immune responses, but upon withdrawal of CsA the immune response often "rebounds" resulting in a relapse or exacerbation of disease. The mechanisms, cells and cytokines involved in the relapse or exacerbation after CsA withdrawal are unknown. We hypothesized that CsA withdrawal induces IL-17 production that could be responsible for relapse, and examined the effect of anti-IL-17A antibody on relapse induced after CsA withdrawal in mouse experimental autoimmune encephalomyelitis (EAE). CsA treatment markedly decreased the EAE disease score during the first episode, but augmented disease severity after CsA withdrawal, compared to untreated mice. After discontinuation of CsA the production of IL-17A was increased and the severity of relapse in EAE was reduced by treatment with anti-IL-17A antibody. These results suggest that the resumption of T cell immune responses after CsA withdrawal leads to a burst of IL-17A production that is at least partially responsible for relapse in EAE mice.