Posible papel de Porphyromonas gingivalis en el desarrollo de la artritis reumatoide / Possible role of Porphyromonas gingivalis in the develpment of rheumatoid arthritis

Varios estudios han sugerido una asociación entre la periodontitissevera, la prevalencia de la bacteria Porphyromonas gingivalis y el desarrollo de artritis reumatoide. Como fundamento de esta relación, se ha observado que esta bacteria secreta una enzima, peptidil-arginina deiminasa, que es capaz de citrulinar proteínas del hospedero y así favorecer una respuesta autoinmune. Sin embargo, debido a la heterogeneidad de diseños experimentales, selección de pacientes y valoración de los desenlaces, los resultados no han mostrado la reproducibilidad deseada. Asimismo, observaciones recientes apuntan a que la actividad enzimática podría ser generada por otras especies bacterianas, lo que hace más compleja su relación. Sin embargo, por otro lado, algunos estudios sugieren que el tratamiento periodontal puede limitar el desarrollo de la artritis reumatoide.

Various studies have suggested a link between severe periodontitis,the prevalence of Porphyromonas gingivalis, and the development ofrheumatoid arthritis. As evidence of this relationship, P. gingivalis hasbeen found to secrete an enzyme, peptidyl arginine deiminase, which isable to citrullinate host proteins and thus help activate an autoimmuneresponse. However, due to the heterogeneity of experimental designs,patient selection, and assessment of clinical outcomes, the results havenot shown the desired reproducibility. Furthermore, recent fi ndingsindicate that the enzymatic activity may be produced by other species ofbacteria, which suggests the relationship is more complex. However, anumber of studies have shown that periodontal treatment could inhibitthe development of rheumatoid arthritis.

Microparticles That Form Immune Complexes as Modulatory Structures in Autoimmune Responses.

Microparticles (MPs) are induced during apoptosis, cell activation, and even "spontaneous" release. Initially MPs were considered to be inert cellular products with no biological function. However, an extensive research and functional characterization have shown that the molecular composition and the effects of MPs depend upon the cellular background and the mechanism inducing them. They possess a wide spectrum of biological effects on intercellular communication by transferring different molecules able to modulate other cells. MPs interact with their target cells through different mechanisms: membrane fusion, macropinocytosis, and receptor-mediated endocytosis. However, when MPs remain in the extracellular milieu, they undergo modifications such as citrullination, glycosylation, and partial proteolysis, among others, becoming a source of neoantigens. In rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), reports indicated elevated levels of MPs with different composition, content, and effects compared with those isolated from healthy individuals. MPs can also form immune complexes amplifying the proinflammatory response and tissue damage. Their early detection and characterization could facilitate an appropriate diagnosis optimizing the pharmacological strategies, in different diseases including cancer, infection, and autoimmunity. This review focuses on the current knowledge about MPs and their involvement in the immunopathogenesis of SLE and RA.

Toll-like receptor 2 controls acute immune complex-driven arthritis in mice by regulating the inhibitory Fcγ receptor IIB.

OBJECTIVE: Previous studies have demonstrated a protective role of Toll-like receptor 2 (TLR-2) and a proinflammatory function of TLR-4 during chronic T cell-driven arthritis. The involvement of TLRs in T cell-independent arthritic processes, however, remains unclear. This study was undertaken to determine the functional significance of TLR-2 and TLR-4 in T cell-independent immune complex-driven arthritis. METHODS: Serum-transfer arthritis was induced in wild-type and TLR-deficient mice by intraperitoneal injections of arthritogenic K/BxN mouse serum. Arthritis was assessed macroscopically and by histologic analysis. The influence of TLR-2 on macrophage cytokine profile, Fcγ receptor (FcγR) expression, and response to immune complexes was determined. RESULTS: While TLR-4, unexpectedly, did not play any significant role, TLR-2 deficiency accelerated the onset and markedly increased the severity of acute immune complex-driven arthritis in mice. TLR-2 deficiency resulted in a substantial increase in joint inflammation, bone erosion, and cartilage pathology, indicating a protective function of TLR-2 in passive FcγR-driven disease. Ex vivo study of the macrophage inflammatory phenotype revealed increased production of tumor necrosis factor α (TNFα) and interleukin-6 (IL-6) despite similar levels of IL-10, along with a significant increase in FcγR-specific response, in TLR-2-/- mouse macrophages early in the disease. Although distinct FcγR messenger RNA expression was not affected, cell surface protein expression of the inhibitory FcγRIIB in TLR-2-/- naive primary macrophages was specifically diminished, resulting in a higher proinflammatory response. Accordingly, TLR-2 stimulation specifically up-regulated FcγRIIB, but not the activating FcγR, on macrophages. CONCLUSION: TLR-2 regulates acute immune complex-driven arthritis by controlling macrophage FcγR response. Our findings indicate that the protective role of TLR-2 is extended beyond its previously described role in promoting Treg cells during T cell-mediated arthritis.

Plasmacytoid dendritic cells and C1q differentially regulate inflammatory gene induction by lupus immune complexes.

Immune complexes (ICs) play a pivotal role in causing inflammation in systemic lupus erythematosus (SLE). Yet, it remains unclear what the dominant blood cell type(s) and inflammation-related gene programs stimulated by lupus ICs are. To address these questions, we exposed normal human PBMCs or CD14(+) isolated monocytes to SLE ICs in the presence or absence of C1q and performed microarray analysis and other tests for cell activation. By microarray analysis, we identified genes and pathways regulated by SLE ICs that are both type I IFN dependent and independent. We also found that C1q-containing ICs markedly reduced expression of the majority of IFN-response genes and also influenced the expression of multiple other genes induced by SLE ICs. Surprisingly, IC activation of isolated CD14(+) monocytes did not upregulate CD40 and CD86 and only modestly stimulated inflammatory gene expression. However, when monocyte subsets were purified and analyzed separately, the low-abundance CD14(dim) ("patrolling") subpopulation was more responsive to ICs. These observations demonstrate the importance of plasmacytoid dendritic cells, CD14(dim) monocytes, and C1q as key regulators of inflammatory properties of ICs and identify many pathways through which they act.

[The activation of TRIF-dependent signaling pathway in THP-1 cells induced by ß2 GPI/anti-ß2 GPI antibodies complex].

AIM: To observe whether the TIR-domain-containing adaptor inducing interferon-ß (TRIF) is activated in THP-1 cells treated with ß2 GPI/anti-ß2 GPI complex and investigate the roles of TRIF-dependent signaling pathway of Toll-like receptor 4 (TLR4) in antiphospholipid syndrome (APS). METHODS: The total RNA was extracted and the protein lysates were collected from THP-1 cells stimulated with ß2 GPI/anti-ß2 GPI complex. And the level of TRIF mRNA in THP-1 cells was detected by Real-time PCR (RT-PCR), TRIF protein expression was investigated by western blotting, respectively. Furthermore, whether TLR4 inhibitor, TAK-242, could interrupt the expression of TRIF as well as some inflammatory cytokines such as IL-6, IL-8 and TNF-α in THP-1 cells stimulated with ß2 GPI/anti-ß2 GPI complex was also investigated. RESULTS: Both mRNA and protein levels of TRIF could be significantly increased in THP-1 cells with treatment of ß2 GPI/anti-ß2 GPI complex (100 mg/L). The expression of TRIF was shown in a manner of time-dependence, with the maximal levels at 1 hour (mRNA) and 2 hour (protein) stimulation respectively. The ß2 GPI/anti-ß2 GPI complex-induced TRIF and inflammatory cytokines including IL-6, IL-8 and TNF-α expression in THP-1 cells could be inhibited by TAK-242 (5 µmol/L). CONCLUSION: TRIF-dependent signaling pathway of Toll-like receptor 4 is involved in the activation of THP-1 cells induced by ß2 GPI/anti-ß2GPI complex, suggesting that TRIF may play an important role in the pathogenesis of antiphospholipid syndrome.

CD23-dependent transcytosis of IgE and immune complex across the polarized human respiratory epithelial cells.

IgE-mediated allergic inflammation occurs when allergens cross-link IgE on the surface of immune cells, thereby triggering the release of inflammatory mediators as well as enhancing Ag presentations. IgE is frequently present in airway secretions, and its level can be enhanced in human patients with allergic rhinitis and bronchial asthma. However, it remains completely unknown how IgE appears in the airway secretions. In this study, we show that CD23 (FcεRII) is constitutively expressed in established or primary human airway epithelial cells, and its expression is significantly upregulated when airway epithelial cells were subjected to IL-4 stimulation. In a transcytosis assay, human IgE or IgE-derived immune complex (IC) was transported across a polarized Calu-3 monolayer. Exposure of the Calu-3 monolayer to IL-4 stimulation also enhanced the transcytosis of either human IgE or the IC. A CD23-specific Ab or soluble CD23 significantly reduced the efficiency of IgE or IC transcytosis, suggesting a specific receptor-mediated transport by CD23. Transcytosis of both IgE and the IC was further verified in primary human airway epithelial cell monolayers. Furthermore, the transcytosed Ag-IgE complexes were competent in inducing degranulation of the cultured human mast cells. Because airway epithelial cells are the first cell layer to come into contact with inhaled allergens, our study implies CD23-mediated IgE transcytosis in human airway epithelial cells may play a critical role in initiating and contributing to the perpetuation of airway allergic inflammation.

Anti-CD40L immune complexes potently activate platelets in vitro and cause thrombosis in FCGR2A transgenic mice.

Anti-CD40L immunotherapy in systemic lupus erythematosus patients was associated with thromboembolism of unknown cause. We previously showed that monoclonal anti-CD40L immune complexes (ICs) activated platelets in vitro via the IgG receptor (FcgammaRIIa). In this study, we examined the prothrombotic effects of anti-CD40L ICs in vivo. Because mouse platelets lack FcgammaRIIa, we used FCGR2A transgenic mice. FCGR2A mice were injected i.v. with preformed ICs consisting of either anti-human CD40L mAb (M90) plus human CD40L, or a chimerized anti-mouse CD40L mAb (hMR1) plus mouse CD40L. ICs containing an aglycosylated form of hMR1, which does not bind FcgammaRIIa, were also injected. M90 IC caused shock and thrombocytopenia in FCGR2A but not in wild-type mice. Animals injected with hMR1 IC also experienced these effects, whereas those injected with aglycosylated-hMR1 IC did not, demonstrating that anti-CD40L IC-induced platelet activation in vivo is FcgammaRIIa-dependent. Sequential injections of individual IC components caused similar effects, suggesting that ICs were able to assemble in circulation. Analysis of IC-injected mice revealed pulmonary thrombi consisting of platelet aggregates and fibrin. Mice pretreated with a thrombin inhibitor became moderately thrombocytopenic in response to anti-CD40L ICs and had pulmonary platelet-thrombi devoid of fibrin. In conclusion, we have shown for the first time that anti-CD40L IC-induced thrombosis can be replicated in mice transgenic for FcgammaRIIa. This molecular mechanism may be important for understanding thrombosis associated with CD40L immunotherapy. The FCGR2A mouse model may also be useful for assessing the hemostatic safety of other therapeutic Abs.

Complement receptor 1 expression on mouse erythrocytes mediates clearance of Streptococcus pneumoniae by immune adherence.

Complement-containing immune complexes can be presented to phagocytes by human erythrocytes bearing complement receptor 1 (CR1). Although this has long been assumed to be a mechanism by which humans are able to protect themselves from "extracellular" bacteria such as pneumococci, there is little direct evidence. In these studies we have investigated this question by comparing results for erythrocytes from transgenic mice expressing human CR1 on their erythrocytes to the results for wild-type mouse erythrocytes that do not express CR1. We demonstrate that human CR1 expression on murine erythrocytes allows immune adherence to beads opsonized with either mouse or human serum as a source of complement. The role of CR1 in immune adherence was supported by studies showing that it was blocked by the addition of antibody to human CR1. Furthermore, human CR1 expression enhances the immune adherence of opsonized pneumococci to erythrocytes in vitro, and the pneumococci attached to erythrocytes via CR1 can be transferred in vitro to live macrophages. Even more importantly, we observed that if complement-opsonized pneumococci are injected intravenously with CR1(+) mouse erythrocytes into wild-type mice (after a short in vitro incubation), they are cleared faster than opsonized pneumococci similarly injected with wild-type mouse erythrocytes. Finally, we have shown that the intravenous (i.v.) injection of pneumococci into CR1(+) mice also results in more rapid blood clearance than in wild-type mice. These data support that immune adherence via CR1 on erythrocytes likely plays an important role in the clearance of opsonized bacteria from human blood.

Hypersensitivity pneumonitis caused by fungi.

Hypersensitivity pneumonitis (HP) is a complex syndrome caused by an exaggerated immune response to the inhalation of a large variety of organic particles. The most frequent antigens that cause HP worldwide are bird proteins (pigeon breeders' disease) and bacteria (Saccharopolyspora rectivirgula). However, fungi are also implicated in many cases, including occupational and nonoccupational outbreaks. The clinical course of the disease is highly variable and its diagnosis clinically challenging since no specific test or biomarker allows a consistent diagnosis. Therefore, a combination of symptoms, bronchoalveolar lavage findings, chest imaging, lab tests, and often biopsies are needed for an accurate diagnosis. Regardless of the cause or the responsible environment, the histopathology is similar and usually consists of a granulomatous interstitial bronchiolocentric pneumonitis characterized by the presence of poorly formed granulomas and a prominent interstitial infiltrate composed of lymphocytes, plasma cells, and macrophages. However, some patients may show a "nonspecific interstitial pneumonia" pattern, or even a usual interstitial pneumonia-like pattern. Importantly, patients with chronic HP may evolve to interstitial fibrosis or develop emphysematous changes, although the reason(s) for these different pathological responses are presently unclear. This review provides a general overview of HP, emphasizing its fungal etiologies, and also examines the currently used clinical criteria for diagnosis and proposes an alternative classification. Challenges for future research include identification of biomarkers that may predict outcome and progression (primarily of chronic HP), and the need for a better understanding of the underlying molecular and genetic mechanisms of the disease.

The role of immune complexes in atherogenesis.

Atherosclerosis is now recognized as a chronic inflammatory disease and is characterized by features of inflammation at all stages of its development. It also appears to display elements of autoimmunity, and several autoantibodies including those directed against oxidized low-density lipoprotein (ox-LDL) and heat shock proteins (Hsps) have been identified in atherosclerosis. Immune complexes (ICs) may form between these antigens and autoantibodies and via Fc receptor signaling and complement activation may modulate the inflammation in atherosclerosis. Antibody isotype may direct the role that ICs play in atherogenesis, immunoglobulin G (IgG) being potentially pro-atherogenic and immunoglobulin M (IgM) playing a protective role. Therapeutic options targeting complement activation and those which are potentially Fc-receptor mediated have been investigated in animal models, though targeting Fc receptor signaling is an area that needs further investigation.

Antigen-induced oligomerization of the B cell receptor is an early target of Fc gamma RIIB inhibition.

The FcgammaRIIB is a potent inhibitory coreceptor that blocks BCR signaling in response to immune complexes and, as such, plays a decisive role in regulating Ab responses. The recent application of high-resolution live cell imaging to B cell studies is providing new molecular details of the earliest events in the initiation BCR signaling that follow within seconds of Ag binding. In this study, we report that when colligated to the BCR through immune complexes, the FcgammaRIIB colocalizes with the BCR in microscopic clusters and blocks the earliest events that initiate BCR signaling, including the oligomerization of the BCR within these clusters, the active recruitment of BCRs to these clusters, and the resulting spreading and contraction response. Fluorescence resonance energy transfer analyses indicate that blocking these early events may not require molecular proximity of the cytoplasmic domains of the BCR and FcgammaRIIB, but relies on the rapid and sustained association of FcgammaRIIB with raft lipids in the membrane. These results may provide novel early targets for therapies aimed at regulating the FcgammaRIIB to control Ab responses in autoimmune disease.

Antigenic structure of the human muscle nicotinic acetylcholine receptor main immunogenic region.

The main immunogenic region on the alpha1 subunits of muscle nicotinic acetylcholine receptors provokes half or more of the autoantibodies in myasthenia gravis and its animal model. Many of these autoantibodies depend on the native conformation of the receptor for their ability to bind with high affinity. We mapped this region and explained the conformation dependence of its epitopes by making chimeras in which sequences of human muscle alpha1 subunits were replaced in human neuronal alpha7 subunits or Aplysia acetylcholine binding protein. These chimeras also revealed that the main immunogenic region can play a major role in promoting conformational maturation and, consequently, assembly of receptor subunits.

Immune complex/Ig negatively regulate TLR4-triggered inflammatory response in macrophages through Fc gamma RIIb-dependent PGE2 production.

Excessive activation of TLR may induce endotoxin shock and inflammatory diseases, so the negative regulation of TLR-triggered inflammatory response attracts much attention. Nonpathogenic immune complex (IC) and Ig (IC/Ig) have been shown to play important roles in the regulation of immune responses and to be therapeutic in some kinds of autoimmune diseases. However, the role of IC/Ig in the regulation of TLR-triggered inflammatory responses and the underlying mechanisms remain to be fully understood. In this study we demonstrate that IC/Ig can significantly inhibit LPS-induced secretion of TNF-alpha and IL-6 from macrophages by preferentially inducing PGE(2). Pretreatment of mice with IC can protect wild-type mice, but not Fc gammaRIIb(-/-) mice, from lethal endotoxin shock, and significantly reduce the levels of serum TNF-alpha and IL-6 in wild-type mice but not in Fc gammaR IIb(-/-) mice. Furthermore, blockade of PGE(2) by celecoxib restores LPS-induced production of TNF-alpha and IL-6 in the presence of IC both in vitro and in vivo. Accordingly, blockade of PGE(2) production in vivo results in the increased sensitivity of IC-pretreated mice to lethal endotoxin shock. Therefore, IC/Ig can negatively regulate TLR4-triggered inflammatory response in macrophages through Fc gammaRIIb-dependent PGE(2). In addition, our results suggest that down-regulation of NF-kappaB activation and TLR4 expression but activation of protein kinase A pathway in macrophages by IC/Ig contribute to the negative regulatory process. Thus we provide new manner for the immune regulation and mechanistic explanation for nonpathogenic IC/Ig in the treatment of inflammatory or autoimmune diseases.

Delay of LPS-induced acute lung injury resolution by soluble immune complexes is neutrophil dependent.

The pathophysiological role of soluble immune complexes (SICXs) and its relationship with neutrophils were investigated in LPS-induced acute lung injury (ALI) animal model (Sprague-Dawley rat) and through the in vitro studies. Results showed that LPS-induced SICX was timely related to changes of tumor necrosis factor alpha and macrophage inflammatory protein 2 (inflammatory cytokines) in bronchoalveolar lavage fluid. In vitro study showed that SICX can bind to Fc gammaR (CD64 and CD32 or CD16) to prevent the apoptosis of neutrophils. The SICX-mediated apoptosis inhibition was extracellular signal-regulated kinase (ERK) or phosphoinositide 3 kinase dependent and was interrupted by PD98059 and LY294002. In vivo, additional amount of SICX exacerbated the lung injury caused by LPS. LPS-induced lung injury and macrophage inflammatory protein 2 release, however, were prevented by CD64 and CD32 blockers (decoy antibodies). In conclusion, excessive amount of SICX in lung can act through Fc gammaRs to protect bronchoalveolar lavage fluid neutrophils from apoptosis that eventually lead to delayed resolution of ALI caused by LPS. Blockade of SICX engagement of CD32 and CD64 (with decoy antibodies) could interrupt SICX-mediated protection of neutrophils and protect lung from LPS-induced injury. The decoy antibodies may therefore have therapeutic utility in ALI.

[Causes and mechanisms of systemic vasculitides]. / Causes et mécanismes à l'origine des vascularites.

Multiple mechanisms contribute to the pathogenesis of systemic vasculitides: 1. vasculitides resulting from the deposition of circulating immune complexes, comprising polyarteritis nodosa associated with hepatitis B virus infection, cryoglobulinemia associated systemic with vasculitides, mainly the consequence of hepatitis C virus infection, and Schonlein Henoch purpura, which results from the deposition in the mesangium and vessels of IgA forming complexes; 2. vasculitides associated with anti-neutrophil cytoplasm antibodies (ANCA), comprising Wegener's granulomatosis associated with anti-proteinase 3 ANCA, and microscopic polyangiitis and Churg-Strauss syndrome, associated with anti-myeloperoxydase ANCA. The pathogenic role of ANCA has been demonstrated in vitro and in vivo in the case of anti-myéloperoxydase antibodies, whereas it has only been demonstrated in vitro in the case of anti-proteinase 3 antibodies; 3. polyarteritis nodosa unrelated to viral infection results from rheologic phenomenon that explain the localisation of vasculitis lesions at the bifurcation of arteries and the presence of microaneurysm.

IgEb immune complexes activate macrophages through FcgammaRIV binding.

Because functional analysis of Fc receptors (FcRs) relies heavily on mouse models, the identification of another Fcgamma receptor is particularly noteworthy. We demonstrate that FcgammaRIV, identified here as the mouse ortholog of primate FcgammaRIII, required association of the FcR gamma-chain for optimal expression and function on myeloid cells; its signaling potential was also enhanced by a cytoplasmic 'YEEP' motif that was able to recruit the adaptor molecule Crk-L and phosphatidylinositol-3-OH kinase. Unexpectedly, FcgammaRIV 'preferentially' bound immunoglobulin E antibodies of the 'b' allotype (IgE(b)) as well as IgG2a and IgG2b antibodies. Ligation of FcgammaRIV by antigen-IgE(b) immune complexes promoted macrophage-mediated phagocytosis, presentation of antigen to T cells, production of proinflammatory cytokines and the late phase of cutaneous allergic reactions. IgE(b) antibody-mediated modification of macrophage responses may therefore influence mouse asthma models and strain-dependent differences in parasite susceptibility.

Murine dendritic cell type I IFN production induced by human IgG-RNA immune complexes is IFN regulatory factor (IRF)5 and IRF7 dependent and is required for IL-6 production.

Dendritic cell (DC) activation by nucleic acid-containing IgG complexes is implicated in systemic lupus erythematosus (SLE) pathogenesis. However, it has been difficult to definitively examine the receptors and signaling pathways by which this activation is mediated. Because mouse FcgammaRs recognize human IgG, we hypothesized that IgG from lupus patients might stimulate mouse DCs, thereby facilitating this analysis. In this study, we show that sera and purified IgG from lupus patients activate mouse DCs to produce IFN-alpha, IFN-beta, and IL-6 and up-regulate costimulatory molecules in a FcgammaR-dependent manner. This activation is only seen in sera with reactivity against ribonucleoproteins and is completely dependent on TLR7 and the presence of RNA. As anticipated, IFN regulatory factor (IRF)7 is required for IFN-alpha and IFN-beta production. Unexpectedly, however, IRF5 plays a critical role in IFN-alpha and IFN-beta production induced not only by RNA-containing immune complexes but also by conventional TLR7 and TLR9 ligands. Moreover, DC production of IL-6 induced by these stimuli is dependent on a functional type I IFNR, indicating the need for a type I IFN-dependent feedback loop in the production of inflammatory cytokines. This system may also prove useful for the study of receptors and signaling pathways used by immune complexes in other human diseases.

Soluble TNF-like cytokine (TL1A) production by immune complexes stimulated monocytes in rheumatoid arthritis.

TNF-like cytokine (TL1A) is a newly identified member of the TNF superfamily of ligands that is important for T cell costimulation and Th1 polarization. However, despite increasing information about its functions, very little is known about expression of TL1A in normal or pathological states. In this study, we report that mononuclear phagocytes appear to be a major source of TL1A in rheumatoid arthritis (RA), as revealed by their strong TL1A expression in either synovial fluids or synovial tissue of rheumatoid factor (RF)-seropositive RA patients, but not RF-/RA patients. Accordingly, in vitro experiments revealed that human monocytes express and release significant amounts of soluble TL1A when stimulated with insoluble immune complexes (IC), polyethylene glycol precipitates from the serum of RF+/RA patients, or with insoluble ICs purified from RA synovial fluids. Monocyte-derived soluble TL1A was biologically active as determined by its capacity to induce apoptosis of the human erythroleukemic cell line TF-1, as well as to cooperate with IL-12 and IL-18 in inducing the production of IFN-gamma by CD4(+) T cells. Because RA is a chronic inflammatory disease with autoimmune etiology, in which ICs, autoantibodies (including RF), and various cytokines contribute to its pathology, our data suggest that TL1A could be involved in its pathogenesis and contribute to the severity of RA disease that is typical of RF+/RA patients.

R406, an orally available spleen tyrosine kinase inhibitor blocks fc receptor signaling and reduces immune complex-mediated inflammation.

Recent compelling evidence has lead to renewed interest in the role of antibodies and immune complexes in the pathogenesis of several autoimmune disorders, such as rheumatoid arthritis. These immune complexes, consisting of autoantibodies to self-antigens, can mediate inflammatory responses largely through binding and activating the immunoglobulin Fc receptors (FcRs). Using cell-based structure activity relationships with cultured human mast cells, we have identified the small molecule R406 [N4-(2,2-dimethyl-3-oxo-4H-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine] as a potent inhibitor of immunoglobulin E (IgE)- and IgG-mediated activation of Fc receptor signaling (EC(50) for degranulation = 56-64 nM). Here we show that the primary target for R406 is the spleen tyrosine kinase (Syk), which plays a key role in the signaling of activating Fc receptors and the B-cell receptor (BCR). R406 inhibited phosphorylation of Syk substrate linker for activation of T cells in mast cells and B-cell linker protein/SLP65 in B cells. R406 bound to the ATP binding pocket of Syk and inhibited its kinase activity as an ATP-competitive inhibitor (K(i) = 30 nM). Furthermore, R406 blocked Syk-dependent FcR-mediated activation of monocytes/macrophages and neutrophils and BCR-mediated activation of B lymphocytes. R406 was selective as assessed using a large panel of Syk-independent cell-based assays representing both specific and general signaling pathways. Consistent with Syk inhibition, oral administration of R406 to mice reduced immune complex-mediated inflammation in a reverse-passive Arthus reaction and two antibody-induced arthritis models. Finally, we report a first-inhuman study showing that R406 is orally bioavailable, achieving exposures capable of inhibiting Syk-dependent IgE-mediated basophil activation. Collectively, the results show R406 potential for modulating Syk activity in human disease.