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.

Small-molecule modulators of Hedgehog signaling: identification and characterization of Smoothened agonists and antagonists

BACKGROUND

The Hedgehog (Hh) signaling pathway is vital to animal development as it mediates the differentiation of multiple cell types during embryogenesis. In adults, Hh signaling can be activated to facilitate tissue maintenance and repair. Moreover, stimulation of the Hh pathway has shown therapeutic efficacy in models of neuropathy. The underlying mechanisms of Hh signal transduction remain obscure, however: little is known about the communication between the pathway suppressor Patched (Ptc), a multipass transmembrane protein that directly binds Hh, and the pathway activator Smoothened (Smo), a protein that is related to G-protein-coupled receptors and is capable of constitutive activation in the absence of Ptc.

RESULTS

We have identified and characterized a synthetic non-peptidyl small molecule, Hh-Ag, that acts as an agonist of the Hh pathway. This Hh agonist promotes cell-type-specific proliferation and concentration-dependent differentiation in vitro, while in utero it rescues aspects of the Hh-signaling defect in Sonic hedgehog-null, but not Smo-null, mouse embryos. Biochemical studies with Hh-Ag, the Hh-signaling antagonist cyclopamine, and a novel Hh-signaling inhibitor Cur61414, reveal that the action of all these compounds is independent of Hh-protein ligand and of the Hh receptor Ptc, as each binds directly to Smo.

CONCLUSIONS

Smo can have its activity modulated directly by synthetic small molecules. These studies raise the possibility that Hh signaling may be regulated by endogenous small molecules in vivo and provide potent compounds with which to test the therapeutic value of activating the Hh-signaling pathway in the treatment of traumatic and chronic degenerative conditions.

A role for immune complexes in enhanced respiratory syncytial virus disease

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and viral pneumonia in infants and young children. Administration of a formalin inactivated vaccine against RSV to children in the 1960s resulted in increased morbidity and mortality in vaccine recipients who subsequently contracted RSV. This incident precluded development of subunit RSV vaccines for infants for over 30 years, because the mechanism of illness was never clarified. An RSV vaccine for infants is still not available. Here, we demonstrate that enhanced RSV disease is mediated by immune complexes and abrogated in complement component C3 and B cell-deficient mice but not in controls. Further, we show correlation with the enhanced disease observed in children by providing evidence of complement activation in postmortem lung sections from children with enhanced RSV disease.

Immunoglobulin M antibodies present in the acute phase of Kawasaki syndrome lyse cultured vascular endothelial cells stimulated by gamma interferon

Kawasaki syndrome (KS) is characterized by diffuse vasculitis and marked T cell and B cell activation. In this study, sera from 16 patients with acute KS, 15 patients in the convalescent phase of KS, and 19 age-matched controls were assessed for complement dependent cytotoxic activity against 111In-labeled human umbilical vein endothelial (HUVE) cells, Neither sera from patients with KS nor sera from controls had cytotoxic effects on HUVE cells cultivated under standard conditions. Since activated T cells such as those present in acute KS secrete gamma interferon (gamma-IFN), we also examined the effects of sera from acute KS on HUVE cells preincubated with gamma-IFN. We report here that immunoglobulin M (IgM) antibodies in sera from patients with acute KS cause significant (P less than 0.01) killing of gamma-IFN-treated HUVE cells. Pretreatment with interleukin 2, gamma-IFN, or beta-IFN failed to render HUVE susceptible to lysis with acute KS sera. The observed effects were not mediated via immune complexes. The cytotoxic antibodies in acute KS seem to be directed against inducible monomorphic antigenic determinants present on gamma-IFN-treated HUVE cells but not on control or gamma-IFN treated autologous human dermal fibroblasts (HDF). Similarly, acute KS sera also induced lysis of gamma-IFN-treated human saphenous vein endothelial (HSVE) cells but not gamma-IFN treated human saphenous vein smooth muscle (HSVSM) cells. Since gamma-IFN induces the same level of class I and class II major histocompatibility complex (MHC) antigen expression on HDF, HUVE, HSVE, and HSVSM cells, our results suggest that the anti-endothelial cell antibodies in acute KS are directed to gamma-IFN-inducible molecules other than MHC determinants. These observations are further substantiated by the failure of human B cells or monocytes to absorb the anti-endothelial cell activity. Since most vasculitides, including acute KS, are characterized both by marked immune activation and the secretion of lymphokines, antibodies directed to gamma-IFN-inducible endothelial cell antigens may represent a general mechanism for vascular injury.

IgA1-containing immune complexes in IgA nephropathy differentially affect proliferation of mesangial cells

BACKGROUND

Sera of patients with IgA nephropathy (IgAN) contain circulating immune complexes (CIC) composed of galactose-deficient IgA1 complexed with antiglycan antibodies. The role of these CIC in the pathogenesis of IgAN is not known.

METHODS

We studied how proliferation of cultured mesangial cells (MC) is affected by CIC prepared from sera of IgAN patients and healthy control subjects using size-exclusion chromatography. CIC-containing fractions were added to serum-starved MC in culture, and cell proliferation was measured using (3)H-thymidine incorporation. The results were confirmed by staining MC using an antibody against proliferating cell nuclear antigen.

RESULTS

The incubation of starved MC with serum fractions with M(r) 800 to 900 kD, rich with galactose-deficient IgA1, stimulated proliferation, while fractions with smaller complexes were inhibitory. Furthermore, CIC-containing larger molecular mass fractions isolated from serum of an IgAN patient collected during an episode of macroscopic hematuria stimulated MC proliferation more than CIC obtained during a subsequent quiescent phase. To examine the role of IgA, we removed IgA1 from serum before fractionation. The resultant IgA1-depleted fractions were devoid of stimulatory IgA-CIC. Sera of IgAN patients were also fractionated after addition of desialylated galactose-deficient polymeric IgA1 to form additional immune complexes. Supplementation with a small quantity of this IgA1 increased cellular proliferation in assays using serum fractions of M(r)>/=800 to 900 kD; uncomplexed IgA1 did not affect MC proliferation significantly. In contrast, supplementation with a larger quantity of this IgA1 inhibited cellular proliferation in assays using serum fractions of M(r) 700 to 800 kD.

CONCLUSION

Overall, these findings suggest that CIC containing aberrantly glycosylated IgA1 affect proliferation of MC in vitro and, thus, likely play a role in the pathogenesis of IgAN.

B cells and antibodies in CNS demyelinating disease

There is much evidence to implicate B cells, plasma cells, and their products in the pathogenesis of MS. Despite unequivocal evidence that the animal model for MS, EAE, is initiated by myelin-specific T cells, there is accumulating evidence of a role for B cells, plasma cells, and their products in EAE pathogenesis. The role(s) played by B cells, plasma cells, and antibodies in CNS inflammatory demyelinating diseases are likely to be multifactorial and complex, involving distinct and perhaps opposing roles for B cells versus antibody.

Nephritogenic autoantibodies in lupus: current concepts and continuing controversies

In summary, we suggest that the following statements regarding lupus nephritis are best supported by the existing data. 1) Lupus nephritis is an immunologically complex disorder. Autoantibodies directed against multiple epitopes on chromatin, including but not limited to dsDNA, may contribute to nephritis. 2) The presence of charged residues within autoantibody heavy chain CDR regions, particularly CDR3, may be essential to the property of nephritogenicity. 3) Chromatin/antichromatin immune complexes (formed either in the circulation or in situ in the GBM) are likely the proximal cause of lupus nephritis. Cross-reactive autoantibodies or antibodies reacting directly to glomerular antigens are less likely to play a major pathogenic role. 4) The induction of lupus nephritis may relate to the propensity of chromatin or its components to bind to the GBM by virtue of the interactions of histones with type IV collagen and heparan-sulfated glycosaminoglycans. Nonetheless, as indicated above, there are numerous issues that remain to be addressed and clarified with respect to lupus nephritis. Insight into these issues is not only of theoretical interest, but may lead to new approaches to diagnostic testing and more specific therapies to replace currently use nonspecific immunosuppressive drugs, which have substantial toxicities.

Physiological and pathological aspects of circulating immune complexes

Complement participates in the elimination of IC in many circumstances. When antigen/antibody IC first form in the circulation, complement inhibits their aggregation because the covalent binding of C3b to the IC modifies their biophysical properties and they remain soluble. Such opsonized (C3b coated) IC attach to cells bearing C3b receptors (CR1) in the circulation, in particular to erythrocytes, since in humans 85 to 90% of CR1 in the blood is located on these cells. This immune adherence binding reaction appears to be a physiological system that allows IC to be transported through the circulation to the fixed macrophages of the MPS where they are safely eliminated. The deposition of circulating complement-fixing IC in various organs such as the kidney may be considered as a failure of this transport system. This is apparent in complement deficient and depleted states, and also for non-complement-fixing IC (IgA IC). The formation of insoluble IC (by definition immune deposits found in human pathology are insoluble) produces complement activation and inflammation at the site of the immune aggregate.

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.

Fc gamma receptor IIB on follicular dendritic cells regulates the B cell recall response

Generation of the B cell recall response appears to involve interaction of Ag, in the form of an immune complex (IC) trapped on follicular dendritic cells (FDCs), with germinal center (GC) B cells. Thus, the expression of receptors on FDC and B cells that interact with ICs could be critical to the induction of an optimal recall response. FDCs in GCs, but not in primary follicles, express high levels of the IgG Fc receptor Fc gamma RIIB. This regulated expression of Fc gamma RIIB on FDC and its relation to recall Ab responses were examined both in vitro and in vivo. Trapping of IC in spleen and lymph nodes of Fc gamma RII-/- mice was significantly reduced compared with that in wild-type controls. Addition of ICs to cultures of Ag-specific T and B cells elicited pronounced Ab responses only in the presence of FDCs. However, FDCs derived from Fc gamma RIIB-/- mice supported only low level Ab production in this situation. Similarly, when Fc gamma RIIB-/- mice were transplanted with wild-type Ag-specific T and B cells and challenged with specific Ag, the recall responses were significantly depressed compared with those of controls with wild-type FDC. These results substantiate the hypothesis that FcgammaRIIB expression on FDCs in GCs is important for FDCs to retain ICs and to mediate the conversion of ICs to a highly immunogenic form and for the generation of strong recall responses.

An in vitro model of immune complex-mediated basement membrane zone separation caused by pemphigoid antibodies, leukocytes, and complement

In this study, an in vitro model of immune complex-mediated basement membrane zone separation caused by periphigoid antibodies, serum complement, and peripheral blood leukocytes is described. When cryostat sections of fresh-frozen normal human skin were treated with either of 4 bullous pemiphigoid sera containing complement-activating anti-basement membrane zone antibodies and subsequently incubated at 37 degrees C with normal human peripheral blood leukocytes and fresh human serum, leukocytes attached to 96% of the basement membrane zone in 100% of sections. Sixty-seven percent of the sections developed focal areas of basement membrane zone separation resembling dermal-epidermal separation described in early pemphigoid lesions. In control sections in which either leukocytes, pemphigoid antibody or fresh human serum were omitted, significantly less leukocyte attachment and basement membrane zone separation occurred. Evidence that leukocytes caused separation was supported by an absolute requirement for viable leukocytes during incubation, a high correlation between leukocyte attachment and separation and experiments showing that leukocytes attached to the basement membrane zone were activated. This study provides the first in vitro evidence directly supporting a functional role for immune-complex mediated inflammation in the pathogenesis of basement membrane zone separation and blisters in bullous pemphigoid.

Mechanisms of immune-deposit formation and the mediation of immune renal injury

The passive trapping of preformed immune complexes is responsible for some forms of glomerulonephritis that are associated with mesangial or subendothelial deposits. The biochemical characteristics of circulating antigens play important roles in determining the biologic activity of immune complexes in these cases. Examples of circulating immune complex diseases include the classic acute and chronic serum sickness models in rabbits, and human lupus nephritis. Immune deposits also form "in situ". In situ immune deposit formation may occur at subepithelial, subendothelial, and mesangial sites. In situ immune-complex formation has been most frequently studied in the Heymann nephritis models of membranous nephropathy with subepithelial immune deposits. While the autoantigenic target in Heymann nephritis has been identified as megalin, the pathogenic antigenic target in human membranous nephropathy had been unknown until the recent identification of neutral endopeptidase as one target. It is likely that there is no universal antigen in human membranous nephropathy. Immune complexes can damage glomerular structures by attracting circulating inflammatory cells or activating resident glomerular cells to release vasoactive substances, cytokines, and activators of coagulation. However, the principal mediator of immune complex-mediated glomerular injury is the complement system, especially C5b-9 membrane attack complex formation. C5b-9 inserts in sublytic quantities into the membranes of glomerular cells, where it produces cell activation, converting normal cells into resident inflammatory effector cells that cause injury. Excessive activation of the complement system is normally prevented by a series of circulating and cell-bound complement regulatory proteins. Genetic deficiencies or mutations of these proteins can lead to the spontaneous development of glomerular disease. The identification of specific antigens in human disease may lead to the development of fundamental therapies. Particularly promising future therapeutic approaches include selective immunosuppression and interference in complement activation and C5b-9-mediated cell injury.

Staging the Initiation of Autoantibody-Induced Arthritis: A Critical Role for Immune Complexes

In the K/BxN mouse model of arthritis, autoantibodies against glucose-6-phosphate isomerase cause joint-specific inflammation and destruction. We have shown using micro-positron emission tomography that these glucose-6-phosphate isomerase-specific autoantibodies rapidly localize to distal joints of mice. In this study we used micro-positron emission tomography to delineate the stages involved in the development of arthritis. Localization of Abs to the joints depended upon mast cells, neutrophils, and FcRs, but not on C5. Surprisingly, anti-type II collagen Abs alone did not accumulate in the distal joints, but could be induced to do so by coinjection of irrelevant preformed immune complexes. Control Abs localized to the joint in a similar manner. Thus, immune complexes are essential initiators of arthritis by sequential activation of neutrophils and mast cells to allow Abs access to the joints, where they must bind a target Ag to initiate inflammation. Our findings support a four-stage model for the development of arthritis and identify checkpoints where the disease is reversible.

Rheumatoid factor (RF) production during anamnestic immune responses in the mouse. III. Activation of RF precursor cells is induced by their interaction with immune complexes and carrier-specific helper T cells

IgG1 immune complexes were identified as the humoral stimuli responsible for the synthesis of IgG1-specific IgM rheumatoid factor (RF), which occurs in the mouse during the early stages of secondary immune responses to protein antigens. The specificity of this phenomenon was illustrated by the fact that complexes made with IgG1 F(ab')2 fragments or with antibodies of a different isotype failed to induce significant anti-IgG1 RF synthesis. The importance of immune complexes in the induction of RF was further underscored by the substantial increase in the titers of isotype-specific RF observed in the serum of mice immunized with IgG1- or IgG2a-complexed antigen rather than with antigen alone. The RF-inducing capacity of the complexes varied with the antigen/antibody ratio: it was maximal in antibody excess or at equivalence, but dramatically reduced in large antigen excess. The importance of T cell priming in RF precursor cell activation by immune complexes was demonstrated by the failure of T cell-deprived spleen cells to reconstitute the capability of irradiated mice to produce RF, and by the optimal RF responses observed after reconstitution of irradiated recipients with primed T cells and naive B cells. The involvement of T cells in this process could not be explained by the release of nonspecific B cell activators, because antigenic stimulation of primed T cells failed to enhance the activation of RF precursor cells by immune complexes of unrelated antigen.

Molecular recognition by induced fit: how fit is the concept?

Induced fit explains why biomolecules can bind together even if they are not optimized for binding. However, induced fit can lead to a kinetic bottleneck and does not describe every interaction in the absence of prior complementarity. Preselection of a fitting conformer is an alternative to induced fit.

Elasticity and adhesion force mapping reveals real-time clustering of growth factor receptors and associated changes in local cellular rheological properties

Cell surface macromolecules such as receptors and ion channels serve as the interface link between the cytoplasm and the extracellular region. Their density, distribution, and clustering are key spatial features influencing effective and proper physical and biochemical cellular responses to many regulatory signals. In this study, the effect of plasma-membrane receptor clustering on local cell mechanics was obtained from maps of interaction forces between antibody-conjugated atomic force microscope tips and a specific receptor, a vascular endothelial growth factor (VEGF) receptor. The technique allows simultaneous measurement of the real-time motion of specific macromolecules and their effect on local rheological properties like elasticity. The clustering was stimulated by online additions of VEGF, or antibody against VEGF receptors. VEGF receptors are found to concentrate toward the cell boundaries and cluster rapidly after the online additions commence. Elasticity of regions under the clusters is found to change remarkably, with order-of-magnitude stiffness reductions and fluidity increases. The local stiffness reductions are nearly proportional to receptor density and, being concentrated near the cell edges, provide a mechanism for cell growth and angiogenesis.

Fc gamma RIIa, not Fc gamma RIIb, is constitutively and functionally expressed on skin-derived human mast cells

The expression of FcgammaR by human skin-derived mast cells of the MC(TC) type was determined in the current study. Expression of mRNA was analyzed with microarray gene chips and RT-PCR; protein by Western blotting and flow cytometry; function by release of beta-hexosaminidase, PGD(2), leukotriene C(4) (LTC(4)), IL-5, IL-6, IL-13, GM-CSF, and TNF-alpha. FcgammaRIIa was consistently detected along with FcepsilonRI at the mRNA and protein levels; FcgammaRIIc was sometimes detected only by RT-PCR; but FcgammaRIIb, FcgammaRI, and FcgammaRIII mRNA and protein were not detected. FcgammaRIIa-specific mAb caused skin MC(TC) cells to degranulate and secrete PGD(2), LTC(4), GM-CSF, IL-5, IL-6, IL-13, and TNF-alpha in a dose-dependent fashion. FcepsilonRI-specific mAb caused similar amounts of each mediator to be released with the exception of LTC(4), which was not released by this agonist. Simultaneous but independent cross-linking of FcepsilonRI and FcgammaRIIa did not substantially alter mediator release above or below levels observed with each agent alone. Skin MC(TC) cells sensitized with dust-mite-specific IgE and IgG, when coaggregated by Der p2, exhibited enhanced degranulation compared with sensitization with either IgE or IgG alone. These results extend the known capabilities of human skin mast cells to respond to IgG as well as IgE-mediated signals.

Immune complexes can trigger specific, T cell-dependent, autoanti-IgG antibody production in mice

Immunization of mice with a combination of passively administered syngeneic IgG (anti-p-azophenylarsonate [anti-Ars]) antibody and a soluble, multivalent form of the antibody's corresponding antigen (Limulus polyphemus hemocyanin conjugated with Ars [Lph-Ars]) resulted in specific autoanti-IgG Fc (rheumatoid factor) production. The response was rapid and only anti-IgG of the IgM isotype is found. Because immunization with either the IgG antibody or the antigen alone did not result in rheumatoid antibody production, immune complexes appear to be the active form of the immunogens. Antibody/antigen ratios that resulted in maximal anti-IgG antibody responses were the same as those required for peak in vitro immunoprecipitation, i.e., equivalence. Previous exposure of the mice to the exogenously supplied antigen was not required for the response. The response to immune complexes is specific because mice immunized with IgG2a-containing complexes produced autoanti-IgG2a, while mice immunized with IgG1-containing complexes produced anti-IgG1 with little reactivity to other IgG isotypes. IgG2a blocked in its complement-fixing capacity was more effective in eliciting the anti-IgG2a response than native IgG2a, suggesting a possible role for the complement system in modulating the anti-IgG2a response. Induction of rheumatoid factor production by immune complexes could be induced in xid mice but not in nu/nu mice, indicating T lymphocyte dependence of the response. In contrast, the B lymphocyte activator lipopolysaccharide was able to elicit vigorous rheumatoid factor production in both nu/nu and normal mice, demonstrating that nu/nu mice contain B cells capable of making the response. Rheumatoid antibody produced in the immune complex- or LPS-induced responses is Fc specific and has relatively low affinity for IgG that is not bound to antigen.

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.

Requirement for DNA CpG content in TLR9-dependent dendritic cell activation induced by DNA-containing immune complexes

Although TLR9 was originally thought to specifically recognize microbial DNA, it is now evident that mammalian DNA can be an effective TLR9 ligand. However, the DNA sequence required for TLR9 activation is controversial, as studies have shown conflicting results depending on the nature of the DNA backbone, the route of DNA uptake, and the cell type being studied. In systemic lupus erythematosus, a major route whereby DNA gains access to intracellular TLR9, and thereby activates dendritic cells (DCs), is through uptake as a DNA-containing immune complex. In this report, we used defined dsDNA fragments with a natural (phosphodiester) backbone and show that unmethylated CpG dinucleotides within dsDNA are required for murine DC TLR9 activation induced by a DNA-containing immune complex. The strongest activation is seen with dsDNA fragments containing optimal CpG motifs (purine-purine-CpG-pyrimidine-pyrimidine) that are common in microbial DNA but rare in mammalian DNA. Importantly, however, activation can also be induced by CpG-rich DNA fragments that lack these optimal CpG motifs and that we show are plentiful in CpG islands within mammalian DNA. No activation is induced by DNA fragments lacking CpG dinucleotides, although this CpG-free DNA can induce DC activation if internalized by liposomal transfection instead of as an immune complex. Overall, the data suggest that the release of CpG-rich DNA from mammalian DNA may contribute to the pathogenesis of autoimmune diseases such as systemic lupus erythematosus and psoriasis in which activation of TLR9 in DCs by self DNA has been implicated in disease pathogenesis.

Pathogenic complement activation in collagen antibody-induced arthritis in mice requires amplification by the alternative pathway

Immune complex-induced inflammation can be mediated by the classical pathway of complement. However, using mice genetically deficient in factor B or C4, we have shown that the collagen Ab-induced model of arthritis requires the alternative pathway of complement and is not dependent on the classical pathway. We now demonstrate that collagen Ab-induced arthritis is not altered in mice genetically deficient in either C1q or mannose-binding lectins A and C, or in both C1q and mannose-binding lectins. These in vivo results prove the ability of the alternative pathway to carry out pathologic complement activation in the combined absence of intact classical and lectin pathways. C3 activation was also examined in vitro by adherent collagen-anti-collagen immune complexes using sera from normal or complement-deficient mice. These results confirm the ability of the alternative pathway to mediate immune complex-induced C3 activation when C4 or C1q, or both C1q and mannose-binding lectins, are absent. However, when all three activation pathways of complement are intact, initiation by immune complexes occurs primarily by the classical pathway. These results indicate that the alternative pathway amplification loop, with its ability to greatly enhance C3 activation, is necessary to mediate inflammatory arthritis induced by adherent immune complexes.

IgGs are made for walking on bacterial and viral surfaces

Binding of antibodies to their cognate antigens is fundamental for adaptive immunity. Molecular engineering of antibodies for therapeutic and diagnostic purposes emerges to be one of the major technologies in combating many human diseases. Despite its importance, a detailed description of the nanomechanical process of antibody-antigen binding and dissociation on the molecular level is lacking. Here we utilize high-speed atomic force microscopy to examine the dynamics of antibody recognition and uncover a principle; antibodies do not remain stationary on surfaces of regularly spaced epitopes; they rather exhibit 'bipedal' stochastic walking. As monovalent Fab fragments do not move, steric strain is identified as the origin of short-lived bivalent binding. Walking antibodies gather in transient clusters that might serve as docking sites for the complement system and/or phagocytes. Our findings could inspire the rational design of antibodies and multivalent receptors to exploit/inhibit steric strain-induced dynamic effects.

Fc gamma receptor IIB on dendritic cells enforces peripheral tolerance by inhibiting effector T cell responses

The uptake of immune complexes by FcRs on APCs augments humoral and cellular responses to exogenous Ag. In this study, CD11c+ dendritic cells are shown to be responsible in vivo for immune complex-triggered priming of T cells. We examine the consequence of Ab-mediated uptake of self Ag by dendritic cells in the rat insulin promoter-membrane OVA model and identify a role for the inhibitory FcgammaRIIB in the maintenance of peripheral CD8 T cell tolerance. Effector differentiation of diabetogenic OT-I CD8+ T cells is enhanced in rat insulin promoter-membrane OVA mice lacking FcgammaRIIB, resulting in a high incidence of diabetes. FcgammaRIIB-mediated inhibition of CD8 T cell priming results from suppression of both DC activation and cross-presentation through activating FcgammaRs. Further FcgammaRIIB on DCs inhibited the induction of OVA-specific Th1 effectors, limiting Th1-type differentiation and memory T cell accumulation. In these MHC II-restricted responses, the presence of FcgammaRIIB only modestly affected initial CD4 T cell proliferative responses, suggesting that FcgammaRIIB limited effector cell differentiation primarily by inhibiting DC activation. Thus, FcgammaRIIB can contribute to peripheral tolerance maintenance by inhibiting DC activation alone or by also limiting processing of exogenously acquired Ag.