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Autoimmune arthritis - such as rheumatoid arthritis - affect a significant proportion of the population, which can cause everyday joint pain, decreased mobility and reduced quality of life. Despite having more and more therapeutic options available, there are still a lot of patients who cannot reach remission or low disease activity by current therapies. This causes an urgent need for the development of new treatment options. The Syk tyrosine kinase plays an essential role in B cell receptor, Fc receptor and integrin signaling. It has been shown that the hematopoietic cell-specific deletion of Syk resulted in a complete protection against autoantibody-induced experimental arthritis. This prompted us to test the effect of entospletinib, a second generation, Syk-selective inhibitor, which has a tolerable safety profile according to hematological clinical trials, in experimental autoimmune arthritis. We found that entospletinib dosedependently decreased the macroscopic signs of joint inflammation, while it did not affect the health status of the animals. In line with these findings, local neutrophil accumulation and cytokine levels were reduced compared to the vehicle-treated group, while macrophage accumulation and synovial fibroblast numbers were not significantly altered. Meanwhile, entospletinib dosedependently decreased the cell responses of immune complex- or integrin ligand-activated neutrophils. Overall, we found that selective Syk inhibition by entospletinib reduced the activity of autoantibody-induced experimental arthritis, which seems to be based mainly on the effect of the inhibitor on neutrophil functions. Our data raise the possibility that entospletinib could be a good drug candidate in the treatment of human autoimmune arthritis. [ABSTRACT FROM AUTHOR]

Author(s): Christiane Reinwald [1]; Christine Schauer [1]; Janka Zsófia Csepregi [2]; Deborah Kienhöfer [1]; Daniela Weidner [1]; Marie Malissen [3]; Attila Mocsai [2, 4]; Georg Schett [1]; Martin Herrmann [1]; [...]

Acute graft-versus-host disease (GVHD) considerably limits wider usage of allogeneic hematopoietic cell transplantation (allo-HCT). Antigen-presenting cells and T cells are populations customarily associated with GVHD pathogenesis. Of note, neutrophils are the largest human white blood cell population. The cells cleave chemokines and produce reactive oxygen species, thereby promoting T cell activation (1, 2). Therefore, during an allogeneic immune response, neutrophils could amplify tissue damage caused by conditioning regimens. We analyzed neutrophil infiltration of the mouse ileum after allo-HCT by in vivo myeloperoxidase imaging and found that infiltration levels were dependent on the local microbial flora and were not detectable under germ-free conditions. Physical or genetic depletion of neutrophils reduced GVHD-related mortality. The contribution of neutrophils to GVHD severity required reactive oxygen species (ROS) because selective Cybb (encoding cytochrome b-245, beta polypeptide, also known as NOX2) deficiency in neutrophils impairing ROS production led to lower levels of tissue damage, GVHD-related mortality and effector phenotype T cells. Enhanced survival of Bcl-xL transgenic neutrophils increased GVHD severity. In contrast, when we transferred neutrophils lacking Toll-like receptor-2 (TLR2), TLR3, TLR4, TLR7 and TLR9, which are normally less strongly activated by translocating bacteria, into wild-type C57BL/6 mice, GVHD severity was reduced. In humans, severity of intestinal GVHD strongly correlated with levels of neutrophils present in GVHD lesions. This study describes a new potential role for neutrophils in the pathogenesis of GVHD in both mice and humans., T cells are the main cause of tissue damage in GVHD, but the events leading up to T cell recruitment and activation are not well understood. To identify additional cell [...]

Fungal infections represent a serious threat, particularly in immunocompromised patients (1). Interleukin-1β (IL-1β) is a key pro-inflammatory factor in innate antifungal immunity (2). The mechanism by which the mammalian immune [...]

Osteoclasts, the only bone-resorbing cells, are central to the pathogenesis of osteoporosis, yet their development and regulation are incompletely understood. Multiple receptors of the immune system use a common signaling paradigm whereby phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) within receptor-associated adapter proteins recruit the Syk tyrosine kinase. Here we demonstrate that a similar mechanism is required for development of functional osteoclasts. Mice lacking two ITAM-bearing adapters, DAP12 and the Fc receptor [gamma]-chain (FcR[gamma]), are severely osteopetrotic. [DAP12.sup-/-][FcR[gamma].sup.-/-] bone marrow cells fail to differentiate into multinucleated osteoclasts or resorb bone in vitro and show impaired phosphorylation of the Syk tyrosine kinase, [syk.sup.-/-] progenitors are similarly defective in osteoclast development and bone resorption. Intact SH2-domains of Syk, introduced by retroviral transduction, are required for functional reconstitution of [syk.sup.-/-] osteoclasts, whereas intact ITAM-domains on DAP12 are required for reconstitution of [DAP12.sup.-/-][FcR[gamma].sup.-/-] cells. These data indicate that recruitment of Syk to phosphorylated ITAMs is critical for osteodastogenesis. Although DAP12 appears to be primarily responsible for osteoclast differentiation in cultures directly stimulated with macrophage-colony stimulating factor and receptor activator of NF-[kappa]B ligand cytokines, DAP12 and FcR[gamma] have overlapping roles in supporting osteoclast development in osteoblast-osteoclast cocultures, which mirrors their overlapping functions in vivo. These results provide new insight into the biology of osteoclasts and suggest novel therapeutic targets in diseases of bony remodeling.

Abstract Background During inflammation, β2-integrins mediate leukocyte adhesion to the endothelium accompanied by the activation of the spleen tyrosine kinase Syk. Results We investigated leukocyte adhesion and rolling in cremaster muscle venules before and during stimulation with fMLP using mice with a Syk-/- hematopoietic system. In unstimulated venules, Syk-/- leukocytes adhered less efficiently than control leukocytes while rolling was similar between Syk-/- and control leukocytes. During fMLP-superfusion, control mice showed significantly increased adhesion accompanied by reduced rolling. For Syk-/- leukocytes, an increase in adhesion with a concomitant decrease in rolling was only observed during the first three minutes during fMLP stimulation, but not at later time points. We also investigated leukocyte spreading against the vessel wall during fMLP stimulation and found a significant impairment of spreading for Syk-/- leukocytes. Additional in vitro experiments revealed that the adhesion and spreading defect seen in Syk-/- chimeric mice was due to compromised β2-integrin-mediated outside-in signaling. Conclusion We provide substantial evidence for an important role of Syk in mediating β2-integrin dependent outside-in signaling leading to sustained leukocyte adhesion and spreading during the inflammatory response in vivo.

Background: During inflammation, β2-integrins mediate leukocyte adhesion to the endothelium accompanied by the activation of the spleen tyrosine kinase Syk. Results: We investigated leukocyte adhesion and rolling in cremaster muscle venules before and during stimulation with fMLP using mice with a Syk-/- hematopoietic system. In unstimulated venules, Syk-/- leukocytes adhered less efficiently than control leukocytes while rolling was similar between Syk-/- and control leukocytes. During fMLP-superfusion, control mice showed significantly increased adhesion accompanied by reduced rolling. For Syk-/- leukocytes, an increase in adhesion with a concomitant decrease in rolling was only observed during the first three minutes during fMLP stimulation, but not at later time points. We also investigated leukocyte spreading against the vessel wall during fMLP stimulation and found a significant impairment of spreading for Syk-/- leukocytes. Additional in vitro experiments revealed that the adhesion and spreading defect seen in Syk-/- chimeric mice was due to compromised β2-integrin-mediated outside-in signaling. Conclusion: We provide substantial evidence for an important role of Syk in mediating β2-integrin dependent outside-in signaling leading to sustained leukocyte adhesion and spreading during the inflammatory response in vivo. [ABSTRACT FROM AUTHOR]