New insights in the pathogenesis of immunoglobulin A vasculitis (Henoch-Schönlein purpura)

Immunoglobulin A vasculitis (IgAV), also referred to as Henoch-Schönlein purpura, is the most common form of childhood vasculitis. The pathogenesis of IgAV is still largely unknown. The disease is characterized by IgA1-immune deposits, complement factors and neutrophil infiltration, which is accompanied with vascular inflammation. Incidence of IgAV is twice as high during fall and winter, suggesting an environmental trigger associated to climate. Symptoms can resolve without intervention, but some patients develop glomerulonephritis with features similar to IgA nephropathy that include hematuria, proteinuria and IgA deposition in the glomerulus. Ultimately, this can lead to end-stage renal disease. In IgA nephropathy immune complexes containing galactose-deficient (Gd-)IgA1 are found and thought to play a role in pathogenesis. Although Gd-IgA1 complexes are also present in patients with IgAV with nephritis, their role in IgAV is disputed. Alternatively, it has been proposed that in IgAV IgA1 antibodies are generated against endothelial cells. We anticipate that such IgA complexes can activate neutrophils via the IgA Fc receptor FcαRI (CD89), thereby inducing neutrophil migration and activation, which ultimately causes tissue damage in IgAV. In this Review, we discuss the putative role of IgA, IgA receptors, neutrophils and other factors such as infections, genetics and the complement system in the pathogenesis of IgA vasculitis

DCs facilitate B cell responses against microbial DNA via DC-SIGN

<div><p>Microbial DNA is highly immunostimulatory and is sensed by endosomal pattern recognition receptors after release from internalized microbes. It is unclear how extracellular DNA released from dead microbes is delivered to endosomal PRRs to induce immune responses. Here we have investigated the ability of DCs to bind and internalize extracellular <i>E</i>.<i>coli</i> DNA as well as synthetic DNA. DCs internalized <i>E</i>.<i>coli</i> and synthetic DNA, which was dependent on the C-type lectin receptor DC-SIGN. Notably, endosomal uptake of DNA by DCs enhanced TLR9-dependent responses of B cells against DNA. Hence, we have identified DC-SIGN as a cell surface receptor for DNA that facilitates immune responses directed against DNA.</p></div

DC-SIGN binds class A ODN and microbial DNA.

<p>(<b>A</b>,<b>B</b>,<b>D</b>-<b>H</b>) <i>E</i>. <i>coli</i> DNA (<b>A</b>,<b>B</b>,<b>D</b>), human DNA (<b>D</b>) or indicated ODNs (<b>E-H</b>) were coated on high binding plates and recombinant DC-SIGN binding to coated ligands was measured by ELISA. (<b>C</b>) Recombinant DC-SIGN was coated on high binding plates and binding to DNAse-treated or untreated biotin-labeled <i>E</i>. <i>coli</i> DNA or Fucose was measured by ELISA. (<b>I-K</b>) Binding of parental Raji cells or Raji cells stably expressing DC-SIGN or Langerin to FITC-labeled <i>E</i>. <i>coli</i> DNA (<b>I,J</b>) or FITC-labeled ODN-2216 (<b>K</b>) was analyzed by flow cytometry. 10 μg/ml DNA or 5 μM ODN was used in all experiments unless stated otherwise. Data are collated (mean ± s.d.) of four independent experiments (<b>G</b>) or representative of at least four (<b>I</b>), three (<b>E</b>) or two (<b>A-D,F,H</b>,<b>J</b>,<b>K</b>) independent experiments (mean ± s.d. of duplicates in <b>A</b>-<b>F,H</b>). *P<0.05, **P<0.01 (student’s t-test). EC-DNA: <i>E</i>. <i>coli</i> DNA.</p

Dendritic cells interact with both class A ODN and microbial DNA via DC-SIGN.

<p>(<b>A</b>-<b>D</b>). Flow cytometry analysis of monocyte-derived DCs incubated with EC-DNA-FITC (<b>A,B</b>) or ODN-2216-FITC (<b>C,D</b>) for 10 min in the presences or absence of EGTA, IgG1 isotype control or blocking antibodies directed against DC-SIGN. (<b>E</b>,<b>F</b>) Confocal imaging of EC-DNA-FITC (green, <b>E</b>) or ODN-2216-FITC (green, <b>F</b>), early endosome antigen 1 (EEA1, red), DC-SIGN (turquoise) and DNA (Hoechst, blue) in monocyte-derived DCs stimulated with EC-DNA-FITC (<b>E</b>) or ODN-2216-FITC (<b>F</b>). 10 μg/ml DNA or 5 μM ODN was used in all experiments. Data are collated (mean ± s.d.) of three (<b>B</b>,<b>D</b>) independent experiments with different donors or are representative of at least three (<b>A,C</b>) or two (<b>E,F</b>) independent experiments with different donors. *P<0.05, **P<0.01 (student’s t-test).EC-DNA: <i>E</i>.<i>coli</i> DNA, ROI: region of interest.</p

Dendritic cells produce type I IFN or cytokines in response to synthetic and microbial DNA.

<p>(<b>A</b>,<b>B</b>,<b>D</b>,<b>E</b>) mRNA analysis of monocyte-derived DCs stimulated with EC-DNA (<b>A</b>,<b>B</b>,<b>D</b>), human DNA (<b>D</b>), ODN-2216 or control ODN (<b>E</b>) for indicated time points was measured by real-time PCR, normalized to GAPDH and set as 1 in samples with the highest expression. (<b>C</b>) Similar as in (<b>A</b>), but EC-DNA was treated with DNAse before stimulation. Cells were stimulated with 10 μg/ml DNA or 5μM ODN in all experiments unless stated otherwise. Data are collated (mean ± s.d.) of four (<b>C</b>), three (<b>A</b>,<b>B</b>) or two (<b>D</b>,<b>E</b>) independent experiments with different donors *P<0.05, **P<0.01 (student’s t-test). EC-DNA: <i>E</i>. <i>coli</i> DNA.</p

Peptide mimetics of immunoglobulin A (IgA) and FcαRI block IgA-induced human neutrophil activation and migration

The cross-linking of the IgA Fc receptor (FcαRI) by IgA induces release of the chemoattractant LTB4, thereby recruiting neutrophils in a positive feedback loop. IgA autoantibodies of patients with autoimmune blistering skin diseases therefore induce massive recruitment of neutrophils, resulting in severe tissue damage. To interfere with neutrophil mobilization and reduce disease morbidity, we developed a panel of specific peptides mimicking either IgA or FcαRI sequences. CLIPS technology was used to stabilize three-dimensional structures and to increase peptides’ half-life. IgA and FcαRI peptides reduced phagocytosis of IgA-coated beads, as well as IgA-induced ROS production and neutrophil migration in in vitro and ex vivo (human skin) experiments. Since topical application would be the preferential route of administration, Cetomacrogol cream containing an IgA CLIPS peptide was developed. In the presence of a skin permeation enhancer, peptides in this cream were shown to penetrate the skin, while not diffusing systemically. Finally, epitope mapping was used to discover sequences important for binding between IgA and FcαRI. In conclusion, a cream containing IgA or FcαRI peptide mimetics, which block IgA-induced neutrophil activation and migration in the skin may have therapeutic potential for patients with IgA-mediated blistering skin diseases

Augmented antibody-based anticancer therapeutics boost neutrophil cytotoxicity

Most clinically used anticancer mAbs are of the IgG isotype, which can eliminate tumor cells through NK cell-mediated antibody-dependent cellular cytotoxicity and macrophage-mediated antibody-dependent phagocytosis. IgG, however, ineffectively recruits neutrophils as effector cells. IgA mAbs induce migration and activation of neutrophils through the IgA Fc receptor (FcαRI) but are unable to activate NK cells and have poorer half-life. Here, we combined the agonistic activity of IgG mAbs and FcαRI targeting in a therapeutic bispecific antibody format. The resulting TrisomAb molecules recruited NK cells, macrophages, and neutrophils as effector cells for eradication of tumor cells in vitro and in vivo. Moreover, TrisomAb had long in vivo half-life and strongly decreased B16F10gp75 tumor outgrowth in mice. Importantly, neutrophils of colorectal cancer patients effectively eliminated tumor cells in the presence of anti-EGFR TrisomAb but were less efficient in mediating killing in the presence of IgG anti-EGFR mAb (cetuximab). The clinical application of TrisomAb may provide potential alternatives for cancer patients who do not benefit from current IgG mAb therapy

Augmented antibody-based anticancer therapeutics boost neutrophil cytotoxicity

Most clinically used anticancer mAbs are of the IgG isotype, which can eliminate tumor cells through NK cell-mediated antibody-dependent cellular cytotoxicity and macrophage-mediated antibody-dependent phagocytosis. IgG, however, ineffectively recruits neutrophils as effector cells. IgA mAbs induce migration and activation of neutrophils through the IgA Fc receptor (FcαRI) but are unable to activate NK cells and have poorer half-life. Here, we combined the agonistic activity of IgG mAbs and FcαRI targeting in a therapeutic bispecific antibody format. The resulting TrisomAb molecules recruited NK cells, macrophages, and neutrophils as effector cells for eradication of tumor cells in vitro and in vivo. Moreover, TrisomAb had long in vivo half-life and strongly decreased B16F10gp75 tumor outgrowth in mice. Importantly, neutrophils of colorectal cancer patients effectively eliminated tumor cells in the presence of anti-EGFR TrisomAb but were less efficient in mediating killing in the presence of IgG anti-EGFR mAb (cetuximab). The clinical application of TrisomAb may provide potential alternatives for cancer patients who do not benefit from current IgG mAb therapy

Fc gamma receptor IIa suppresses type I and III interferon production by human myeloid immune cells

Type I and type III interferons (IFNs) are fundamental for antiviral immunity, but prolonged expression is also detrimental to the host. Therefore, upon viral infection high levels of type I and III IFNs are followed by a strong and rapid decline. However, the mechanisms responsible for this suppression are still largely unknown. Here, we show that IgG opsonization of model viruses influenza and respiratory syncytial virus (RSV) strongly and selectively suppressed type I and III IFN production by various human antigen-presenting cells. This suppression was induced by selective inhibition of TLR, RIG-I-like receptor, and STING-dependent type I and III IFN gene transcription. Surprisingly, type I and III IFN suppression was mediated by Syk and PI3K independent inhibitory signaling via FcγRIIa, thereby identifying a novel non-canonical FcγRIIa pathway in myeloid cells. Together, these results indicate that IgG opsonization of viruses functions as a novel negative feedback mechanism in humans, which may play a role in the selective suppression of type I and III IFN responses during the late-phase of viral infections. In addition, activation of this pathway may be used as a tool to limit type I IFN-associated pathology