Resistance of Fc receptor- deficient mice to fatal glomerulonephritis

Fcγ receptor deficiency attenuates diabetic nephropathy

Among patients with diabetes, increased production of immunoglobulins against proteins modified by diabetes is associated with proteinuria and cardiovascular risk, suggesting that immune mechanisms may contribute to the development of diabetes complications, such as nephropathy. We investigated the contribution of IgG Fcγ receptors to diabetic renal injury in hyperglycemic, hypercholesterolemic mice. We used streptozotocin to induce diabetes in apolipoprotein E-deficient mice and in mice deficient in both apolipoprotein E and γ-chain, the common subunit of activating Fcγ receptors. After 15 weeks, the mice lacking Fcγ receptors had significantly less albuminuria and renal hypertrophy, despite similar degrees of hyperglycemia and hypercholesterolemia, immunoglobulin production, and glomerular immune deposits. Moreover, diabetic Fcγ receptor-deficient mice had less mesangial matrix expansion, inflammatory cell infiltration, and collagen and α-smooth muscle actin content in their kidneys. Accordingly, expression of genes involved in leukocyte infiltration, fibrosis, and oxidative stress was significantly reduced in diabetic kidneys and in mesangial cells cultured from Fcγ receptor-deficient mice. In summary, preventing the activation of Fcγ receptors alleviates renal hypertrophy, inflammation, and fibrosis in hypercholesterolemic mice with diabetes, suggesting that modulating Fcγ receptor signaling may be renoprotective in diabetic nephropathy.

Increased survival and reduced renal injury in MRL/lpr mice treated with a human Fcγ receptor II (CD32) peptide

Systemic lupus erythematosus (SLE) is a multisystem chronic inflammatory disease affecting many organs. The deposition in kidney tissue of immune complexes and their interaction with macrophages is thought to trigger the inflammatory response leading to glomerulonephritis. It has been demonstrated that inhibition of this interaction in murine models can alleviate the disease. Six synthetic peptides were derived from the membrane-proximal extracellular domain (EC2) of human Fcγ receptor II (huFcγRII). Of these, one peptide, huRII6, was shown to be a potent competitive inhibitor of IgG binding to recombinant FcγRII in vitro. To examine the possible therapeutic impact of huRII6 in vivo, this peptide, or a control, was given by subcutaneous injection to female MRL/lpr mice from weeks 7 to 36, resulting in an enhanced survival rate compared with control-treated animals and a reduction of proteinuria. Histopathological examination of the kidneys showed a reduction in deposition of immune complexes and preservation of structure. Such a functional peptide should prove useful for examining the role of IgG-FcγR interactions in experimental models of disease and may provide for the development of FcR-targeting drugs to treat autoimmune disorders.

Murine membranous nephropathy: immunization with α3(IV) collagen fragment induces subepithelial immune complexes and FcγR-independent nephrotic syndrome

Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults and a significant cause of end-stage renal disease, yet current therapies are nonspecific, toxic, and often ineffective. The development of novel targeted therapies requires a detailed understanding of the pathogenic mechanisms, but progress is hampered by the lack of a robust mouse model of disease. We report that DBA/1 mice as well as congenic FcγRIII(-/-) and FcRγ(-/-) mice immunized with a fragment of α3(IV) collagen developed massive albuminuria and nephrotic syndrome, because of subepithelial deposits of mouse IgG and C3 with corresponding basement membrane reaction and podocyte foot process effacement. The clinical presentation and histopathologic findings were characteristic of MN. Although immunized mice produced genuine anti-α3NC1 autoantibodies that bound to kidney and lung basement membranes, neither crescentic glomerulonephritis nor alveolitis ensued, likely because of the predominance of mouse IgG1 over IgG2a and IgG2b autoantibodies. The ablation of activating IgG Fc receptors did not ameliorate injury, implicating subepithelial deposition of immune complexes and consequent complement activation as a major effector pathway. We have thus established an active model of murine MN. This model, leveraged by the availability of genetically engineered mice and mouse-specific reagents, will be instrumental in studying the pathogenesis of MN and evaluating the efficacy of novel experimental therapies.

A role for adhesion and degranulation-promoting adapter protein in collagen-induced platelet activation mediated via integrin α(2) β(1)

BACKGROUND

Collagen-induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α(2) β(1) . Adhesion and degranulation-promoting adapter protein (ADAP) regulates α(IIb) β(3) in platelets and α(L) β(2) in T cells, and is phosphorylated in GPVI-deficient platelets activated by collagen.

OBJECTIVES

To determine whether ADAP plays a role in collagen-induced platelet activation and in the regulation and function of α(2) β(1).

METHODS

Using ADAP(-/-) mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation.

RESULTS AND CONCLUSIONS

Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP(-/-) platelets. However, aggregation and signaling induced by collagen-related peptide (CRP), a GPVI-selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α(2) β(1) -selective ligand GFOGER and to a peptide (III-04), which supports adhesion that is dependent on both GPVI and α(2) β(1), was reduced in ADAP(-/-) platelets. An impedance-based label-free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non-fluorescent differential-interference contrast microscopy, which revealed reduced filpodia formation in ADAP(-/-) platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen-binding integrin α(2) β(1). In addition, we found that ADAP(-/-) mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild-type animals. This may reflect increased removal of platelets from the circulation.

Immunoglobulin treatment ameliorates myocardial injury in experimental autoimmune myocarditis associated with suppression of reactive oxygen species

AIMS

We tested the hypothesis that immunoglobulin ameliorated experimental autoimmune myocarditis (EAM) in mice attributing to the suppression of reactive oxygen species (ROS)-mediated myocardial injury.

METHODS

We intraperitoneally administered intact type of human immunoglobulin (Ig) or F(ab')2 fragments of human immunoglobulin, 1g/kg/day daily for 3 weeks, to male BALB/c mice with heart failure due to EAM.

RESULTS

The results showed that intact type of Ig, but not F(ab')2 type, reduced the severity of myocarditis by comparing the heart weight/body weight and lung weight/body weight ratios, pericardial effusion score, macroscopic and microscopic scores. Tissue superoxide production was marked in untreated mice with EAM, which was suppressed by the treatment of immunoglobulins. The cytotoxic activities of lymphocytes in mice with EAM treated with Ig were reduced compared with untreated controls. The shift from Th1 toward Th2 cytokine balance was demonstrated by the treatment of immunoglobulins both in vitro and in vivo.

CONCLUSION

ROS may be involved in the development of myocarditis. Intact Ig ameliorates myocardial damage in mice with myocarditis associated with suppression of ROS and cytotoxic activity of lymphocytes.

TREM-2, triggering receptor expressed on myeloid cell-2, negatively regulates TLR responses in dendritic cells

DCs play a key role in defense against infections and also in preventing inflammatory and autoimmune diseases. The response of DCs to pathogens is tightly regulated by many mechanisms to allow for appropriate, but not pathogenic, responses. We previously showed that DCs with deficiencies for two ITAM-bearing signaling adapters, DAP12 and FcRγ, produce more inflammatory cytokines upon treatment with Toll-like receptor (TLR) agonists than WT DCs. Here, we investigated whether the TREM-2 receptor pairs with DAP12 to inhibit TLR responses in DCs. TREM-2-deficient BMDCs showed increased inflammatory cytokine and type I IFN production in response to TLR ligation. Additionally, TREM-2-deficient BMDCs had increased TLR-induced maturation and were more efficient at inducing antigen-specific T-cell proliferation upon CpG DNA stimulation compared with WT BMDCs. Finally, we showed that a TREM-2 ligand is expressed on the surface of BMDCs, suggesting that the TREM-2 receptor transduces inhibitory signals due to recognition of an endogenous ligand.

Role of novel rat-specific Fc receptor in macrophage activation associated with crescentic glomerulonephritis

Crescentic glomerulonephritis (Crgn) is a complex disease where the initial insult is often the glomerular deposition of antibodies against intrinsic or deposited antigens in the glomerulus. The role of Fc receptors in the induction and progression of Crgn is increasingly recognized, and our previous studies have shown that copy number variation in Fcgr3 partially explains the genetic susceptibility of the Wistar-Kyoto (WKY) rat to nephrotoxic nephritis, a rat model of Crgn. The Fcgr3-related sequence (Fcgr3-rs) is a novel rat-specific Fc receptor with a cytoplasmic domain 6 amino acids longer than its paralogue, Fcgr3. The Fcgr3-rs gene is deleted from the WKY rat genome, and this deletion is associated with enhanced macrophage activity in this strain. Here, we investigated the mechanism by which the deletion of Fcgr3-rs in the WKY strain leads to increased macrophage activation. By lentivirus-mediated gene delivery, we generated stably transduced U937 cells expressing either Fcgr3-rs or Fcgr3. In these cells, which lack endogenous Fcgr3 receptors, we show that Fcgr3-rs interacts with the common Fc-γ chain but that Fc receptor-mediated phagocytosis and signaling are defective. Furthermore, in primary macrophages, expression of Fcgr3-rs inhibits Fc receptor-mediated functions, because WKY bone marrow-derived macrophages transduced with Fcgr3-rs had significantly reduced phagocytic activity. This inhibitory effect on phagocytosis was mediated by the novel cytoplasmic domain of Fcgr3-rs. These results suggest that Fcgr3-rs may act to inhibit Fcgr3-mediated signaling and phagocytosis and could be considered as a novel mechanism in the modulation of Fc receptor-mediated cell activation in autoimmune diseases.

Spleen tyrosine kinase promotes acute neutrophil-mediated glomerular injury via activation of JNK and p38 MAPK in rat nephrotoxic serum nephritis

Glomerular antibody deposition induces acute neutrophil-mediated glomerular injury via activation of c-Jun amino terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). However, the link between antibody deposition and activation of JNK/p38 MAPK signalling is unclear. This study tested the postulate that spleen tyrosine kinase (Syk), which is activated via Fcγ-receptor ligation, is required for activation of JNK and p38 signalling and acute neutrophil-mediated glomerular injury. We used a Syk inhibitor (SYKi) in rat nephrotoxic serum nephritis (NTN) in which neutrophil-mediated glomerular injury is dependent upon JNK and p38 signalling. SYKi or vehicle treatment of Sprague-Dawley rats began 30 min before administration of anti-GBM serum with rats killed 3 or 24 h later. Immunostaining identified de novo glomerular Syk activation (p-Tyr 525/526) in untreated NTN, being most prominent in neutrophils. Vehicle and untreated NTN exhibited heavy proteinuria and glomerular thrombosis at 24 h with P-selectin and fibrin immunostaining within capillaries, glomerular macrophage and T cell infiltration, activation of JNK and p38 MAPK signalling, and upregulation of glomerular mRNA levels of pro-inflammatory molecules (TNF-α, NOS2, MMP-12 and CCL2). In contrast, SYKi treatment provided complete protection from proteinuria, with a profound reduction in glomerular thrombosis and immunostaining for P-selectin and fibrin, and a substantial reduction in glomerular mRNA levels of pro-inflammatory molecules. SYKi treatment also reduced the acute glomerular neutrophil influx and pro-inflammatory response at 3 h in NTN. These protective effects were associated with a significant reduction in glomerular JNK and p38 MAPK activation. In addition, activation of Syk, JNK and p38 was identified in human biopsy samples of acute crescentic glomerulonephritis. In conclusion, this study demonstrates that Syk signalling is required for JNK and p38 MAPK signalling and acute neutrophil-dependent glomerular injury in rat NTN. These findings identify Syk as a potential therapeutic target in antibody-dependent kidney disease.

Transferred antigen-specific T(H)17 but not T(H)1 cells induce crescentic glomerulonephritis in mice

To explore the role of antigen-specific CD4(+) T cells in glomerulonephritis, we administered ovalbumin 323-339 peptide conjugated to glomerular-binding polyclonal antibody and induced disease in RAG1(-/-) mice with CD4(+) T cells from OT2 × RAG1(-/-) mice. These OT2 × RAG1(-/-) mice have a transgenic T-cell receptor specific for this peptide. When CD4(+) T cells were primed in vivo, crescentic glomerulonephritis developed after 21 days in mice given peptide-conjugated glomerular-binding antibody but not unconjugated antibody control. We then investigated the relative roles of T(H)1 and T(H)17 cells, using Fab(2) fragments of glomerular-binding antibody to exclude a role for antibody in this model. T cells from OT2 × RAG1(-/-) mice were polarized in vitro, and T(H)1 or T(H)17 cell lines were injected into mice that were also given peptide-conjugated Fab(2) or unconjugated Fab(2) control, giving four experimental groups. After 21 days crescentic glomerulonephritis was seen in mice receiving T(H)17 cells and peptide-conjugated Fab(2) but in none of the other three groups. These results suggest that T(H)17 but not T(H)1 cells can induce crescentic glomerulonephritis.

Attenuation of Immune-Mediated Renal Injury by Telmisartan, an Angiotensin Receptor Blocker and a Selective PPAR-γ Activator

BACKGROUND/AIMS

Anti-glomerular basement membrane (GBM) nephritis is characterized by activation of the renin-angiotensin system. This study aimed to determine the question of whether a temporary angiotensin II blockade at the initial stage of anti-GBM nephritis is able to attenuate the disease as well as differences in renoprotection among angiotensin II receptor blockers (ARBs) with distinct peroxisome proliferator-activated receptor (PPAR)-γ-modulating activities.

METHODS

C57BL/6J mice were immunized with rabbit IgG, followed by intravenous injection of rabbit anti-mouse antibodies. Mice were then treated with telmisartan, losartan, and telmisartan + GW9662 (a PPAR-γ antagonist) for 5 days, or hydralazine for 9 days. On days 8 and 13, mice were sacrificed to obtain tissues for histological analysis.

RESULTS

The temporary administration of telmisartan significantly suppressed glomerular damage compared to hydralazine. Losartan showed a similar effect but was less effective. Co-administration of GW9662 attenuated the renoprotective effect of telmisartan, almost to levels observed with losartan. In particular, it limited the decreased infiltration of inflammatory cells and preservation of capillaries in the glomeruli induced by telmisartan.

CONCLUSION

Temporary angiotensin II blockade at the initial stage of anti-GBM disease dramatically inhibited its progression. In addition to a class effect of ARBs, telmisartan modified inflammation and endothelial damage in the kidney through its PPAR-γ-agonistic action.

Novel targets for immunotherapy in glomerulonephritis

Glomerulonephritis is a common cause of chronic kidney disease and end stage renal failure. Current therapy relies on variably effective, nonspecific and toxic immunosuppression. Recent insights into underlying biology and disease pathogenesis in human glomerulonephritis combined with advances in the fields of inflammation and autoimmunity promise a cadre of novel targeted interventions. This review highlights the therapeutic potential of two antigens, alpha3 (IV)NC1 collagen and podocyte neutral endopeptidase, and two cell signaling and effector molecules, IgG Fc receptors and complement, judged to be particularly amenable to therapeutic manipulation in man. It is anticipated that continued dissection of pathogenesis in the diverse disorders that comprise the glomerulonephritides will provide the basis for individualized disease-specific therapy.

Generation of a unique small molecule peptidomimetic that neutralizes lupus autoantibody activity

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the presence of pathogenic autoantibodies, many of which are directed against nuclear antigens, in particular double-stranded (ds) DNA. Both clinical studies and animal models have shown that anti-dsDNA antibodies contribute to kidney disease, which is present in 50% of lupus patients and is a major cause of mortality. We previously demonstrated that a subset of nephrotoxic anti-dsDNA antibodies also recognizes the pentapeptide consensus sequence D/E W D/E Y S/G (DWEYS) present in the NR2A and NR2B subunits of the N-methyl-d-aspartate receptor (NMDAR). Autoantibodies with this specificity are present in ≈40% of lupus patient sera and are both nephrotoxic and neurotoxic. Elevated titers are present in cerebrospinal fluid of patients with central nervous system manifestations of SLE. Administration of the nonnaturally occurring D form of the DWEYS pentapeptide prevents these antibodies from depositing in glomeruli and from mediating neuronal excitotoxicity. To craft a more useful therapeutic, we used the structural features of the DWEYS peptide to design a unique, selective, and potent small molecule peptidomimetic, FISLE-412, which neutralizes anti-dsDNA/NMDAR lupus autoantibodies and prevents their pathogenic interaction with tissue antigens. This compound, or others derived from it, may provide a unique strategy for the development of lupus therapeutics.

Anti-T cell immunoglobulin and mucin domain-2 monoclonal antibody exacerbates collagen-induced arthritis by stimulating B cells

Introduction

T cell immunoglobulin and mucin domain-2 (TIM-2) has been shown to regulate CD4 T cell activation. However, the role of TIM-2 in the autoimmune disease models has not been clarified yet. In this study, we investigated the effects of anti-TIM-2 monoclonal antibodies (mAbs) in collagen-induced arthritis (CIA) to determine whether TIM-2 contributes to the development of T helper (Th) 1 or Th17 cells and joint inflammation.

Methods

DBA/1 mice were treated with anti-TIM-2 mAbs during the early or late phase of CIA. Type II collagen (CII)-specific CD4 T-cell proliferative response and cytokine production were assessed from lymph node cell culture. The serum levels of CII-specific antibody were measured by ELISA. The expression of TIM-2 on CD4 T cells or B cells was determined by flow cytometric analysis.

Results

Administration of anti-TIM-2 mAbs in early phase, but not late phase, significantly exacerbated the development of CIA. Although anti-TIM-2 mAbs treatment did not affect the development of Th1 or Th17 cells in the draining lymph node, the serum levels of anti-CII antibodies were significantly increased in the anti-TIM-2-treated mice. TIM-2 expression was found on splenic B cells and further up-regulated by anti-immunoglobulin (Ig)M, anti-CD40, and interleukin(IL)-4 stimulation. In contrast, CD4 T cells did not express TIM-2 even when stimulated with both anti-CD3 and anti-CD28 mAbs. Interestingly, anti-TIM-2 mAbs enhanced proliferation and antibody production of activated B cells in vitro.

Conclusions

TIM-2 signaling influences both proliferation and antibody production of B cells during the early phase of CIA, but not induction of Th1 or Th17 cells.

[Fcγ receptor and systemic autoimmune disease]

The systemic autoimmune disease such as systemic lupus erythematosus (SLE) is characterized by the deposition of immune complexes in multiple organs. Fcγ receptors (FcγR) recognize the Fc portion of IgG and are important in determining the response of leukocytes to deposited immune complexes. FcγR also provide positive and negative regulation of immune cell responses. The activatory FcγR including the FcR common γ chain take balance with Fcγ RIIB, the only inhibitory FcγR. Development of lupus-like autoimmune disease as well as monocytosis in BXSB mice is dependent on the activatory and inhibitory FcγR. In human SLE, dysregulated expression of FcγRIIB on memory B cells is reported and numbers of associations with genetic polymorphism are also reported. The cell-specific modulation of these activatory or inhibitory FcγRs are expected for the new therapeutic strategy in autoimmune diseases.

Macrophage signaling pathways: a novel target in renal disease

Monocytes/macrophages are a heterogeneous cell population that play a critical role in host defense and tissue homeostasis. However, macrophage activation during acute and chronic inflammation can result in macrophage-mediated renal injury in a variety of settings, including proliferative glomerulonephritis. Macrophages can be activated via a number of intracellular signaling pathways (eg, c-Jun amino terminal kinase, p38 mitogen-activated protein kinase, FcR/Syk, Janus kinase/signal transducer and activator of transcription) that induce production of mediators of renal injury. Thus, targeting selected macrophage signaling pathways is a potential therapeutic strategy to suppress macrophage-mediated renal injury while leaving intact the desirable macrophage functions of host defense and tissue repair.

Leukotriene B4 augments and restores Fc gammaRs-dependent phagocytosis in macrophages

Phagocytosis by macrophages is essential for host defense, i.e. preventing invasion of pathogens and foreign materials. Macrophages engulf immunoglobulin G (IgG)-opsonized particles through the action of the receptors for the Fc of IgG (FcγRs). Leukotriene B(4) (LTB(4)) is a classical lipid chemoattractant derived from arachidonic acid. Leukotriene B(4) receptor 1 (BLT1), a high affinity LTB(4) receptor, is expressed in a variety of immune cells such as neutrophils, macrophages, and dendritic cells. Although LTB(4) has been shown to enhance macrophage phagocytosis, few studies have investigated the intracellular mechanisms involved in this in detail. Furthermore, there have been no reports of the direct cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling. Here, we show that FcγRs-dependent phagocytosis was attenuated in BLT1-deficient macrophages as compared with wild-type (WT) cells. Moreover, cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling was identified at the level of phosphatidylinositol 3-OH kinase (PI3K) and Rac, downstream of Syk. In addition, the trimeric G(i) protein (G(i)) was found to be essential for BLT1-dependent phagocytosis. Surprisingly, we found that LTB(4)-BLT1 signaling restores phagocytosis in the absence of FcγRs signaling. These data indicate that LTB(4)-BLT1 signaling plays a pivotal role in macrophage phagocytosis and innate immunity.

Lack of collagen XVIII/endostatin exacerbates immune-mediated glomerulonephritis

Collagen XVIII is a component of the highly specialized extracellular matrix associated with basement membranes of epithelia and endothelia. In the normal kidney, collagen XVIII is distributed throughout glomerular and tubular basement membranes, mesangial matrix, and Bowman's capsule. Proteolytic cleavage within its C-terminal domain releases the fragment endostatin, which has antiangiogenic properties. Because damage to the glomerular basement membrane (GBM) accompanies immune-mediated renal injury, we investigated the role of collagen XVIII/endostatin in this disorder. We induced anti-GBM glomerulonephritis in collagen XVIII alpha1-null and wild-type mice and compared the resulting matrix accumulation, inflammation, and capillary rarefaction. Anti-GBM disease upregulated collagen XVIII/endostatin expression within the GBM and Bowman's capsule of wild-type mice. Collagen XVIII/endostatin-deficient mice developed more severe glomerular and tubulointerstitial injury than wild-type mice. Collagen XVIII/endostatin deficiency altered matrix remodeling, enhanced the inflammatory response, and promoted capillary rarefaction and vascular endothelial cell damage, but did not affect endothelial proliferation. Supplementing collagen XVIII-deficient mice with exogenous endostatin did not affect the progression of anti-GBM disease. Taken together, these results suggest that collagen XVIII/endostatin preserves the integrity of the extracellular matrix and capillaries in the kidney, protecting against progressive glomerulonephritis.

Pattern recognition and renal defence in crescentic glomerulonephritis

Chavele et al. studied the role of the mannose receptor (MR) in crescentic nephritis. The accelerated nephrotoxic model of glomerulonephritis was induced on both wild-type (WT) and MR-deficient mice. The mice lacking the MR showed a markedly altered phenotype. They were largely protected against the development of glomerulonephritis with less affected glomeruli, less proteinuria and much better renal function when compared to the WT mice. Whilst more infiltrating macrophages were present in the WT animals, there was no difference in the deposition of sheep and mouse immunoglobulins. To elucidate the mechanism of MR-mediated damage, the authors additionally performed a series of in vitro experiments. They show that the Fab portion of sheep immunoglobulins binds to MR binding domains. Interestingly, the MR seems to interact with FcR-mediated cellular reactions. When MR-deficient macrophages and mesangial cells were treated with immune complexes, the macrophages showed a significantly poorer oxygen burst when compared with WT cells. In line with this possible interaction between Fc-receptors and MR, co-localization of Fcã-receptors and MR was demonstrated on macrophages. Additionally, the authors observed that MR-deficient mesangial cells in culture proliferated more abundantly and showed a markedly increased rate of spontaneous apoptosis compared with WT cells. These findings led the authors to test whether this increased apoptosis could play a role in the suppression of glomerular inflammation. Indeed, TNF-á production by LPS-stimulated macrophages was markedly reduced in the presence or apoptotic cells. The anti-inflammatory effect of apoptotic mesangial cells was increased when MR-deficient macrophages were used instead of WT cells (Figure 1). Fig. 1 Proposed role for mannose receptor in crescentic glomerulonephritis. (A) IgG may bind to the Fc receptor with the Fc domain and to the mannose receptor with the Fab portion leading to an increased oxygen burst (left). If the mannose receptor is absent, the oxygen burst is decreased (right). (B) Absence of the mannose receptor leads to an increased proliferation and apoptosis of mesangial cells. These apoptotic cells induce a non-inflammatory macrophage phenotype with IL-10 and TGF-beta production (right). If the mannose receptor is present, mesangial cells proliferate less and are less apoptotic, and a pro-inflammatory macrophage phenotype with TNF-alpha production prevails.

Critical role of Src-Syk-PLC{gamma}2 signaling in megakaryocyte migration and thrombopoiesis

Migration of megakaryocytes (MKs) from the proliferative osteoblastic niche to the capillary-rich vascular niche is essential for proplatelet formation and platelet release. In this study, we explore the role of surface glycoprotein receptors and signaling proteins in regulating MK migration and platelet recovery after immune-induced thrombocytopenia. We show that spreading and migration of mouse primary bone marrow-derived MKs on a fibronectin matrix are abolished by the Src family kinases inhibitor PP1, the Syk kinase inhibitor R406 and the integrin alphaIIbbeta3 antagonist lotrafiban. We also demonstrate that these responses are inhibited in primary phospholipase C gamma2 (PLCgamma2)-deficient MKs. Conversely, MK spreading and migration were unaltered in the absence of the collagen receptor, the glycoprotein VI-FcRgamma-chain complex. We previously reported a correlation between a defect in MK migration and platelet recovery in the absence of platelet endothelial cell adhesion molecule-1 and the tyrosine phosphatase CD148. This correlation also holds for mice deficient in PLCgamma2. This study identifies a model in which integrin signaling via Src family kinases and Syk kinase to PLCgamma2 is required for MK spreading, migration, and platelet formation.

IL-6 increases B-cell IgG production in a feed-forward proinflammatory mechanism to skew hematopoiesis and elevate myeloid production

Src homology 2 domain-containing inositol 5-phosphatase (SHIP(-/-)) animals display an age-related increase in interleukin-6 (IL-6), a decrease in B lymphopoiesis, and an elevation in myelopoiesis. We investigated the origin of the IL-6 production and show that it is largely produced by peritoneal and splenic macrophages. IL-6 production by these macrophages is not a direct result of the loss of SHIP: IL-6 production is not spontaneous, is absent from bone marrow-derived macrophages, declines with prolonged culture of macrophages, and requires a stimulus present in vivo. The IL-6-rich peritoneal cavity of SHIP(-/-) mice shows more than 700-fold more immunoglobulin G (IgG) than wild-type, approximately 20% of which is aggregated or in an immune complex and contains B220(+) cells that secrete IgG. The SHIP-deficient peritoneal macrophages show evidence of IgG receptor stimulation. Animals lacking both the signal-transducing gamma-chain of IgG receptors and SHIP or Ig and SHIP produce less IL-6. The data indicate a feed-forward process in which peripheral macrophages, responding through IgG receptors to secreted IgG, produce IL-6, to support further B-cell production of IgG. Because of the proinflammatory phenotype of SHIP(-/-) animals, these findings emphasize the importance of IL-6-neutralizing strategies in autoimmune and proinflammatory diseases.

Decreased NK Cell FcRgamma in HIV-1 infected individuals receiving combination antiretroviral therapy: a cross sectional study

Background

FcRγ is an immunoreceptor tyrosine-based activation motif (ITAM)-signalling protein essential for immunoreceptor signaling and monocyte, macrophage and NK cell function. Previous study from our laboratory showed that FcRγ is down-regulated in HIV-infected macrophages in vitro. FcRγ expression in immune cells present in HIV-infected individuals is unknown.

Methodology/Principal Findings

We compared FcRγ expression in peripheral blood mononuclear cells isolated from HIV-1-infected individuals receiving combination antiretroviral therapy and healthy, HIV-1-uninfected individuals. FcRγ mRNA and protein levels were measured using quantitative real-time PCR and immunoblotting, respectively. CD56⁺ CD94⁺ lymphocytes isolated from blood of HIV-1 infected individuals had reduced FcRγ protein expression compared to HIV-uninfected individuals (decrease = 76.8%, n = 18 and n = 12 respectively, p = 0.0036). In a second group of patients, highly purified NK cells had reduced FcRγ protein expression compared to uninfected controls (decrease = 50.2%, n = 9 and n = 8 respectively, p = 0.021). Decreased FcRγ expression in CD56+CD94+ lymphocytes was associated with reduced mRNA (51.7%, p = 0.021) but this was not observed for the smaller group of patients analysed for NK cell expression (p = 0.36).

Conclusion/Significance

These data suggest biochemical defects in ITAM-dependent signalling within NK cells in HIV-infected individuals which is present in the context of treatment with combination antiretroviral therapy.

The other side of immunoglobulin G: suppressor of inflammation

Immunoglobulin G (IgG) molecules can have two completely opposite functions. On one hand, they induce proinflammatory responses and recruit innate immune effector cells during infection with pathogenic microorganisms or autoimmune disease. On the other hand, intravenous infusion of high doses of pooled IgG molecules from thousands of donors [intravenous IG (IVIG) therapy] represents an efficient anti-inflammatory treatment for many autoimmune diseases. Whereas our understanding of the mechanism of the proinflammatory activity of IgG is quite advanced, we are only at the very beginning to comprehend how the anti-inflammatory activity comes about and what cellular and molecular players are involved in this activity. This review will summarize our current knowledge and focus upon the two major models of either IVIG-mediated competition for IgG-triggered effector functions or IVIG-mediated adjustment of cellular activation thresholds used to explain the mechanism of the anti-inflammatory activity.

Direct recognition of the mycobacterial glycolipid, trehalose dimycolate, by C-type lectin Mincle

Tuberculosis remains a fatal disease caused by Mycobacterium tuberculosis, which contains various unique components that affect the host immune system. Trehalose-6,6′-dimycolate (TDM; also called cord factor) is a mycobacterial cell wall glycolipid that is the most studied immunostimulatory component of M. tuberculosis. Despite five decades of research on TDM, its host receptor has not been clearly identified. Here, we demonstrate that macrophage inducible C-type lectin (Mincle) is an essential receptor for TDM. Heat-killed mycobacteria activated Mincle-expressing cells, but the activity was lost upon delipidation of the bacteria; analysis of the lipid extracts identified TDM as a Mincle ligand. TDM activated macrophages to produce inflammatory cytokines and nitric oxide, which are completely suppressed in Mincle-deficient macrophages. In vivo TDM administration induced a robust elevation of inflammatory cytokines in sera and characteristic lung inflammation, such as granuloma formation. However, no TDM-induced lung granuloma was formed in Mincle-deficient mice. Whole mycobacteria were able to activate macrophages even in MyD88-deficient background, but the activation was significantly diminished in Mincle/MyD88 double-deficient macrophages. These results demonstrate that Mincle is an essential receptor for the mycobacterial glycolipid, TDM.

Fcgamma receptor-dependent expansion of a hyperactive monocyte subset in lupus-prone mice

OBJECTIVE

Lupus-prone BXSB mice develop monocytosis characterized by selective accumulation of the Gr-1- monocyte subset. The aim of this study was to explore the possible role of activating IgG Fc receptors (FcgammaR) in the development of monocytosis and to characterize the functional phenotype of the Gr-1- subset that accumulates in lupus-prone mice bearing the NZB-type defective Fcgr2b allele for the inhibitory FcgammaRIIB.

METHODS

The development of monocytosis was analyzed in BXSB and anti-IgG2a rheumatoid factor-transgenic C57BL/6 mice deficient in activating FcgammaR. Moreover, we assessed the expression levels of activating FcgammaR and inhibitory FcgammaRIIB on Gr-1+ and Gr-1- monocyte subsets in C57BL/6 mice bearing the C57BL/6-type or the NZB-type Fcgr2b allele.

RESULTS

We observed monocytosis with expansion of the Gr-1- subset in anti-IgG2a-transgenic C57BL/6 mice expressing IgG2a, but not in those lacking IgG2a. Moreover, monocytosis barely developed in BXSB and anti-IgG2a-transgenic C57BL/6 mice deficient in activating FcgammaR. The Gr-1- subset that accumulated in lupus-prone mice displayed a unique hyperactive phenotype. It expressed very low levels of inhibitory FcgammaRIIB, due to the presence of the NZB-type Fcgr2b allele, but high levels of activating FcgammaRIV. This was in contrast to high levels of FcgammaRIIB expression and no FcgammaRIV expression on the Gr-1+ subset.

CONCLUSION

Our results demonstrated a critical role of activating FcgammaR in the development of monocytosis and in the expansion of a Gr-1-FcgammaRIIB(low)FcgammaRIV+ hyperactive monocyte subset in lupus-prone mice. Our findings further highlight the importance of the NZB-type Fcgr2b susceptibility allele in murine lupus, the presence of which induces increased production of hyperactive monocytes as well as dysregulated activation of autoreactive B cells.

Both complement and IgG fc receptors are required for development of attenuated antiglomerular basement membrane nephritis in mice

To elucidate the mechanisms of glomerulonephritis, including Goodpasture's syndrome, mouse models are used that use heterologous Abs against the glomerular basement membrane (GBM) with or without preimmunization with foreign IgG from the same species. These studies have revealed the requirement of either FcgammaR or complement, depending on the experimental model used. In this study, we provide evidence that both FcgammaR and complement are obligatory for a full-blown inflammation in a novel attenuated passive model of anti-GBM disease. We demonstrate that administration of subnephritogenic doses of rabbit anti-GBM Abs followed by a fixed dose of mouse mAbs to rabbit IgG, allowing timing and dosing for the induction of glomerulonephritis, resulted in reproducible complement activation via the classical pathway of complement and albuminuria in wild-type mice. Because albuminuria was absent in FcR-gamma-chain(-/-) mice and reduced in C3(-/-) mice, a role for both FcgammaR and complement is postulated. Because C1q(-/-) and C4(-/-) mice lacking a functional classical and lectin pathway did develop albuminuria, we suggest involvement of the alternative pathway of complement. Anti-GBM glomerulonephritis occurs acutely following the administration of mouse anti-rabbit IgG, and proceeds in a chronic fashion dependent on both FcgammaR and complement. This novel attenuated model allows elucidating the relative contribution of different mediator systems of the immune system to the development of renal injury, and also provides a platform for the assessment of different treatment protocols and evaluation of drugs that ultimately may be beneficial for the treatment of anti-GBM mediated glomerulonephritides.

Dectin-2 is a Syk-coupled pattern recognition receptor crucial for Th17 responses to fungal infection

Innate immune cells detect pathogens via pattern recognition receptors (PRRs), which signal for initiation of immune responses to infection. Studies with Dectin-1, a PRR for fungi, have defined a novel innate signaling pathway involving Syk kinase and the adaptor CARD9, which is critical for inducing Th17 responses to fungal infection. We show that another C-type lectin, Dectin-2, also signals via Syk and CARD9, and contributes to dendritic cell (DC) activation by fungal particles. Unlike Dectin-1, Dectin-2 couples to Syk indirectly, through association with the FcRγ chain. In a model of Candida albicans infection, blockade of Dectin-2 did not affect innate immune resistance but abrogated Candida-specific T cell production of IL-17 and, in combination with the absence of Dectin-1, decreased Th1 responses to the organism. Thus, Dectin-2 constitutes a major fungal PRR that can couple to the Syk–CARD9 innate signaling pathway to activate DCs and regulate adaptive immune responses to fungal infection.

Poly(ADP-ribose) polymerase-1 regulates the progression of autoimmune nephritis in males by inducing necrotic cell death and modulating inflammation

Necrotic lesions and necrotic cell death characterize severe autoimmune nephritides, and contribute to local inflammation and to progression of the disease. Poly(ADP-ribose) polymerase-1 (PARP-1), a DNA repair enzyme, is involved in the induction of necrosis and is a key player in the acute and chronic inflammation. Therefore, we hypothesized that PARP-1 controls the severity of nephritis by mediating the induction of necrosis in the kidney. We used lupus and anti-glomerular basement membrane models of nephritis to determine the effects of PARP-1 on the inflammatory response in the kidney. We show in this study that PARP-1 is indeed activated during the course of glomerulonephritis. We also show that the absence of PARP-1 or its pharmacological inhibition results in milder nephritis, with lower blood urea nitrogen levels, reduced necrotic lesions, and higher survival rates. The relevance of PARP-1 showed a strong male sex specificity, and treatment of male mice with 17beta-estradiol prolonged their survival during the course of nephritis. PARP-1 also regulated TNF-alpha expression and up-regulation of adhesion molecules, further supporting a role of PARP-1 in the inflammatory process within the kidney. Our results demonstrate that PARP-1 activation and consequent necrotic cell death play an important role in the pathogenesis of male nephritis, and suggest that PARP-1 can be a novel therapeutic target in glomerulonephritis.

The tyrosine phosphatase CD148 is an essential positive regulator of platelet activation and thrombosis

Platelets play a fundamental role in hemostasis and thrombosis. They are also involved in pathologic conditions resulting from blocked blood vessels, including myocardial infarction and ischemic stroke. Platelet adhesion, activation, and aggregation at sites of vascular injury are regulated by a diverse repertoire of tyrosine kinase–linked and G protein–coupled receptors. Src family kinases (SFKs) play a central role in initiating and propagating signaling from several platelet surface receptors; however, the underlying mechanism of how SFK activity is regulated in platelets remains unclear. CD148 is the only receptor-like protein tyrosine phosphatase identified in platelets to date. In the present study, we show that mutant mice lacking CD148 exhibited a bleeding tendency and defective arterial thrombosis. Basal SFK activity was found to be markedly reduced in CD148-deficient platelets, resulting in a global hyporesponsiveness to agonists that signal through SFKs, including collagen and fibrinogen. G protein–coupled receptor responses to thrombin and other agonists were also marginally reduced. These results highlight CD148 as a global regulator of platelet activation and a novel antithrombotic drug target.

Fc gamma RIII and Fc gamma RIV are indispensable for acute glomerular inflammation induced by switch variant monoclonal antibodies

The relative ability of IgG subclasses to cause acute inflammation and the roles of specific effector mechanisms in this process are not clear. We explored this in an in vivo model of glomerular inflammation in the mouse. Trinitrophenol was planted on the glomerular basement membrane after conjugation to nephrotoxic Ab. The relative nephritogenicity of anti-trinitrophenol switch variant mAbs was then explored and shown to be IgG2a > IgG2b, with no disease caused by IgG1. Using knockout mice, we showed that FcgammaRIII was necessary for both neutrophil influx and glomerular damage induced by IgG2a and IgG2b. Surprisingly, IgG1 did not cause disease although it binds to FcgammaRIII. Using blocking Abs, we showed that this was explained by an additional requirement for FcgammaRIV, which does not bind to IgG1. IgG2a- or IgG2b-induced neutrophil influx was not affected by deficiency of either FcgammaRI or C3. Bone marrow chimeras were constructed to test the effect of combined deficiency of FcgammaRI and C3, and there was no effect on IgG2a- or IgG2b-mediated neutrophil influx. However, IgG2b-induced albuminuria and thrombosis were reduced in C3-deficient mice, showing an additional role for complement in IgG2b-mediated glomerular damage. The results show that IgG2a and IgG2b are the pathogenic subclasses in acute neutrophil-mediated glomerular inflammation, with an indispensable role for both FcgammaRIII and FcgammaRIV. Additionally, complement contributes to IgG2b-induced glomerular injury.

Increased expression of FcgammaRI/CD64 on circulating monocytes parallels ongoing inflammation and nephritis in lupus

Introduction

The high-affinity receptor for IgG Fcγ/CD64 is critical for the development of lupus nephritis (LN). Cross-linking Fc receptor on recruited monocytes by IgG-containing immune complexes is a key step in immune-complex-mediated nephritis in systemic lupus erythematosus (SLE). The goal of this study was to determine whether expression of Fc receptor (FcγR) I on circulating monocytes is associated with systemic inflammation and renal disease in SLE patients.

Methods

We studied 205 SLE patients (132 with LN and 73 without LN) along with 74 healthy control individuals. Surface expression of CD14 (monocytes), FcγRI/CD64, FcγRII/CD32, and FcγRIII/CD16 was evaluated by flow cytometry. Monocyte function was assessed by determining the migratory capacity and the ability to produce CCL2 (monocyte chemotractic protein 1). High-sensitivity C-reactive protein, C3 and C4 were measured by nephelometry.

Results

There was little difference in the expression of FcγRIII/CD16 or FcγRIII/CD32 on circulating monocytes between patients with SLE and control individuals. In contrast, FcγRI/CD64 expression was significantly higher in SLE patients and even higher in patients with LN. FcγRI/CD64 expression was positively associated with serum creatinine and indicators of systemic inflammation. Monocytes from patients with high FcγRI/CD64 expression also exhibited increased chemotaxis and capacity to produce monocyte chemotractic protein 1.

Conclusions

Increased FcγRI/CD64 expression on circulating monocytes parallels systemic inflammation and renal disease in SLE patients. We propose that circulating monocytes activated by immune complexes and/or proinflammatory mediators upregulate surface expression of FcγRI/CD64 in SLE. The enhanced chemotactic and inflammatory potential of the activated monocytes may participate in a vicious cycle of immune cell recruitment and renal injury in SLE.

Fc receptor gamma-chain, a constitutive component of the IL-3 receptor, is required for IL-3-induced IL-4 production in basophils

The Fc receptor common gamma-chain (FcRgamma) is a widely expressed adaptor bearing an immunoreceptor tyrosine-based activation motif (ITAM) that transduces activation signals from various immunoreceptors. We show here that basophils lacking FcRgamma developed normally and proliferated efficiently in response to interleukin 3 (IL-3) but were very impaired in IL-3-induced production of IL-4 and in supporting T helper type 2 differentiation. Through its transmembrane portion, FcRgamma associated constitutively with the common beta-chain of the IL-3 receptor and signaled by recruiting the kinase Syk. Retrovirus-mediated complementation demonstrated the essential function of the ITAM of FcRgamma in IL-3 signal transduction. Our results identify a previously unknown mechanism whereby FcRgamma functions to 'route' selective cytokine-triggered signals into the ITAM-mediated IL-4 production pathway.

CD18-dependent activation of the neutrophil NADPH oxidase during phagocytosis of Escherichia coli or Staphylococcus aureus is regulated by class III but not class I or II PI3Ks

Phagocytosis and activation of the NADPH oxidase are important mechanisms by which neutrophils and macrophages engulf and kill microbial pathogens. We investigated the role of PI3K signaling pathways in the regulation of the oxidase during phagocytosis of Staphylococcus aureus and Escherichia coli by mouse and human neutrophils, a mouse macrophage-like cell line and a human myeloid-like cell line. Phagocytosis of these bacteria was promoted by serum, independent of serum-derived antibodies, and effectively abolished in mouse neutrophils lacking the β2-integrin common chain, CD18. A combination of PI3K isoform-selective inhibitors, mouse knock-outs, and RNA-interference indicated CD18-dependent activation of the oxidase was independent of class I and II PI3Ks, but substantially dependent on the single class III isoform (Vps34). Class III PI3K was responsible for the synthesis of PtdIns(3)P on phagosomes containing either bacteria. The use of mouse neutrophils carrying an appropriate knock-in mutation indicated that PtdIns(3)P binding to the PX domain of their p40phox oxidase subunit is important for oxidase activation in response to both S aureus and E coli. This interaction does not, however, account for all the PI3K sensitivity of these responses, particularly the oxidase response to E coli, suggesting that additional mechanisms for PtdIns(3)P-regulation of the oxidase must exist.

Mincle is an ITAM-coupled activating receptor that senses damaged cells

Macrophage-inducible C-type lectin (Mincle) is expressed mainly in macrophages and is induced after exposure to various stimuli and stresses. Here we show that Mincle selectively associated with the Fc receptor common gamma-chain and activated macrophages to produce inflammatory cytokines and chemokines. Mincle-expressing cells were activated in the presence of dead cells, and we identified SAP130, a component of small nuclear ribonucloprotein, as a Mincle ligand that is released from dead cells. To investigate whether Mincle is required for normal responses to cell death in vivo, we induced thymocyte death by irradiating mice and found that transient infiltration of neutrophils into the thymus could be blocked by injection of Mincle-specific antibody. Our results suggest that Mincle is a receptor that senses nonhomeostatic cell death and thereby induces the production of inflammatory cytokines to drive the infiltration of neutrophils into damaged tissue.

Mast cell regulation of epithelial TSLP expression plays an important role in the development of allergic rhinitis

Epithelial cell-derived thymic stromal lymphopoietin (TSLP) is a master switch for asthma or atopic dermatitis by inducing a dendritic cell-mediated Th2-type allergic inflammation. Allergic rhinitis is also pathologically characterized by Th2-type allergic inflammation. This study demonstrates that mast cells regulate the epithelial TSLP expression in allergic rhinitis. TSLP expression was found to be up-regulated predominantly in the nasal epithelium in the ovalbumin (OVA)-sensitized and -nasally challenged mouse model of allergic rhinitis, which was abolished in mast cell-deficient WBB6F1-W/W(v) in comparison with control WBB6F1-+/+ mice. Similarly, the epithelial TSLP expression was reduced in Fc receptor gamma chain (FcgammaR)-deficient mice, where the high-affinity IgE receptor (FcepsilonRI) is not expressed on mast cells, in comparison with control C57BL/6 mice. Furthermore, the administration of neutralizing TSLP antibody during the challenge phase of OVA inhibited the development of allergic rhinitis. These results suggest that the direct stimulation of epithelial cells by antigens alone may not be sufficient to induce TSLP expression in the nasal epithelium, and that mast cell regulation of epithelial TSLP expression, possibly via FcepsilonRI, plays an important role in the development of allergic rhinitis.

Human neutrophil Fcgamma receptors initiate and play specialized nonredundant roles in antibody-mediated inflammatory diseases

Inflammation mediated by antibody-antigen complexes contributes to autoimmune diseases. Mice deficient in the common Fcgamma-chain are protected from IgG-mediated glomerulonephritis and the reverse passive Arthus (RPA) reaction and FcR-bearing macrophages, and mast cells have been assigned primary roles in these processes. Here we demonstrate that neutrophil-selective transgenic expression of the two uniquely human neutrophil Fc gamma receptors (FcgammaRs), FcgammaRIIA and FcgammaRIIIB, in Fcgamma-chain-deficient mice restored susceptibility to progressive glomerulonephritis and the cutaneous RPA reaction. FcgammaRIIIB and FcgammaRIIA mediated neutrophil accumulation, whereas FcgammaRIIA alone promoted organ injury. In a model of soluble immune complexes deposited within the vasculature, FcgammaRIIIB was responsible for neutrophil slow rolling and adhesion whereas in the cremaster RPA, induced by both vascular and tissue soluble immune complexes, FcgammaRIIA predominated. Thus, human FcgammaRs on neutrophils serve as molecular links between antibody and immunological disease, with FcgammaRIIA promoting tissue injury and FcgammaRIIIB and FcgammaRIIA displaying specialized context-dependent functions in neutrophil recruitment.

Identification of a major GpVI-binding locus in human type III collagen

We have analyzed the adhesion of human and murine platelets, and of recombinant human and murine GpVI ectodomains, to synthetic triple-helical collagen-like peptides. These included 57 peptides derived from the sequence of human type III collagen and 9 peptides derived from the cyanogen bromide fragment of bovine type III collagen, alpha1(III)CB4. We have identified several peptides that interact with GpVI, in particular a peptide designated III-30 with the sequence GAOGLRGGAGPOGPEGGKGAAGPOGPO. Both human and murine platelets bound to peptide III-30 in a GpVI-dependent manner. III-30 also supported binding of recombinant GpVI ectodomains. Cross-linked III-30 induced aggregation of human and murine platelets, although with a lower potency than collagen-related peptide. Modifications of the peptide sequence indicated that the hydroxyproline residues play a significant role in supporting its GpVI reactivity. However, many peptides containing OGP/GPO motifs did not support adhesion to GpVI. These data indicate that the ability of a triple-helical peptide to bind GpVI is not solely determined by the presence or spatial arrangement of these OGP/GPO motifs within the peptides.

Inhibitory ITAM signaling by Fc alpha RI-FcR gamma chain controls multiple activating responses and prevents renal inflammation

Inhibitory signaling is an emerging function of ITAM-bearing immunoreceptors in the maintenance of homeostasis. Monovalent targeting of the IgA Fc receptor (FcalphaRI or CD89) by anti-FcalphaRI Fab triggers potent inhibitory ITAM (ITAM(i)) signaling through the associated FcRgamma chain (FcalphaRI-FcRgamma ITAM(i)) that prevents IgG phagocytosis and IgE-mediated asthma. It is not known whether FcalphaRI-FcRgamma ITAM(i) signaling controls receptors that do not function through an ITAM and whether this inhibition requires Src homology protein 1 phosphatase. We show in this study that FcalphaRI-Fcgamma ITAM(i) signals depend on Src homology protein 1 phosphatase to target multiple non-ITAM-bearing receptors such as chemotactic receptors, cytokine receptors, and TLRs. We found that anti-FcalphaRI Fab treatment in vivo reduced kidney inflammation in models of immune-mediated glomerulonephritis and nonimmune obstructive nephropathy by a mechanism that involved decreased inflammatory cell infiltration and fibrosis development. This treatment also prevented ex vivo LPS activation of monocytes from patients with lupus nephritis or vasculitis, as well as receptor activation through serum IgA complexes from IgA nephropathy patients. These findings point to a crucial role of FcalphaRI-FcRgamma ITAM(i) signaling in the control of multiple heterologous or autologous inflammatory responses. They also identify anti-FcalphaRI Fab as a new potential therapeutic tool for preventing progression of renal inflammatory diseases.

Anti-inflammatory actions of intravenous immunoglobulin

The remarkable success story of the therapeutic application of pooled immunoglobulin G (IgG) preparations from thousands of donors, the so-called intravenous IgG (IVIG) therapy, to patients with a variety of hematological and immunological disorders began more than half a century ago. Since then, the use of this primary blood product has increased constantly, resulting in the serious danger of shortages in supply. Despite its widespread use and therapeutic success, the mechanisms of action, especially of the anti-inflammatory activity, are only beginning to be understood. In this review, we summarize the clinical use of IVIG for different diseases and discuss recent data on the molecular mechanisms that might explain how this potent drug mediates its activity in vivo.

Differential contribution of three activating IgG Fc receptors (FcgammaRI, FcgammaRIII, and FcgammaRIV) to IgG2a- and IgG2b-induced autoimmune hemolytic anemia in mice

Murine phagocytes express three different activating IgG FcgammaR: FcgammaRI is specific for IgG2a; FcgammaRIII for IgG1, IgG2a, and IgG2b; and FcgammaRIV for IgG2a and IgG2b. Although the role of FcgammaRIII in IgG1 and IgG2a anti-RBC-induced autoimmune hemolytic anemia (AIHA) is well documented, the contribution of FcgammaRI and FcgammaRIV to the development of IgG2a- and IgG2b-induced anemia has not yet been defined. In the present study, using mice deficient in FcgammaRI, FcgammaRIII, and C3, in combination with an FcgammaRIV-blocking mAb, we assessed the respective roles of these three FcgammaR in the development of mild and severe AIHA induced by two different doses (50 and 200 microg) of the IgG2a and IgG2b subclasses of the 34-3C anti-RBC monoclonal autoantibody. We observed that the development of mild anemia induced by a low dose of 34-3C IgG2a autoantibody was highly dependent on FcgammaRIII, while FcgammaRI and FcgammaRIV additionally contributed to the development of severe anemia induced by a high dose of this subclass. In contrast, the development of both mild and severe anemia induced by 34-3C IgG2b was dependent on FcgammaRIII and FcgammaRIV. Our results indicate differential roles of the three activating FcgammaR in IgG2a- and IgG2b-mediated AIHA.

Critical but overlapping role of FcgammaRIII and FcgammaRIV in activation of murine neutrophils by immobilized immune complexes

Immune complex-induced activation of neutrophils through cell surface FcRs plays a central role in the pathogenesis of autoimmune inflammatory diseases. These diseases are often modeled using genetically modified mice. However, in contrast to the number of studies on human cells, the identity of FcRs involved in immune complex activation of murine neutrophils is at present unknown. Furthermore, little is known about the cellular functions mediated by the recently identified murine FcgammaRIV. In this study, we tested the identity of FcRs involved in the activation of neutrophils by plate-bound immune complexes, using various knockout mouse strains, function-blocking mAbs, or the combination of both approaches. Activation of murine neutrophils by immobilized IgG immune complexes was abrogated in FcR gamma-chain-deficient cells, but not by the single or combined deficiency of the gamma-chain-associated FcgammaRI and FcgammaRIII, or by blocking Abs against either FcgammaRIII or FcgammaRIV alone. However, treatment of FcgammaRIII-deficient neutrophils with FcgammaRIV-blocking Abs or simultaneous blocking of FcgammaRIII and FcgammaRIV in wild-type cells completely inhibited the immune complex-induced cellular responses. In parallel studies, activation of human neutrophils by immobilized immune complexes was abrogated by blocking Abs against either FcgammaRIIA or FcgammaRIIIB alone. Taken together, neutrophil activation by immobilized immune complexes requires the murine FcgammaRIII/FcgammaRIV or the human FcgammaRIIA/FcgammaRIIIB molecules. Although both of the two human receptors are required for this response, the two murine receptors play overlapping, redundant roles. These results promote our understanding of autoimmune diseases and identify an IgG-dependent cellular function of FcgammaRIV.

Mechanisms of ANCA-mediated leukocyte-endothelial cell interactions in vivo

Anti-myeloperoxidase (anti-MPO) antibodies have been implicated in the pathogenesis of small-vessel vasculitis, but the molecular mechanisms by which these antibodies contribute to disease are unknown. For determination of how anti-MPO antibodies affect inflammatory cell recruitment in small-vessel vasculitis, intravital microscopy was used to monitor leukocyte behavior in the accessible cremasteric microvessels under various experimental conditions. After local pretreatment of the cremaster muscle with cytokines (TNF-alpha, IL-1beta, or keratinocyte-derived chemokine), administration of anti-MPO IgG to wild-type mice reduced leukocyte rolling in favor of augmented adhesion to and transmigration across the endothelium. This led to a decrease in the number of systemic circulating leukocytes and, similar to the early events in the development of vasculitic lesions, an increase in leukocyte recruitment to renal and pulmonary tissue. TNF-alpha led to the greatest recruitment of inflammatory cells, and IL-1beta led to the least. When anti-CD18 was co-administered, anti-MPO IgG did not affect leukocyte rolling, adhesion, or transmigration; similarly, anti-MPO IgG did not produce these effects in Fc receptor gamma chain-/- mice. This study provides direct in vivo evidence of enhanced leukocyte-endothelial cell interactions in the presence of anti-MPO IgG and highlights the critical roles of Fcgamma receptors and beta2 integrins in mediating these interactions. In addition, it suggests that neutrophils primed by cytokines in the presence of anti-MPO IgG can have systemic effects and target specific vascular beds.

Fcgamma receptors as regulators of immune responses

In addition to their role in binding antigen, antibodies can regulate immune responses through interacting with Fc receptors (FcRs). In recent years, significant progress has been made in understanding the mechanisms that regulate the activity of IgG antibodies in vivo. In this Review, we discuss recent studies addressing the multifaceted roles of FcRs for IgG (FcgammaRs) in the immune system and how this knowledge could be translated into novel therapeutic strategies to treat human autoimmune, infectious or malignant diseases.