Felice Rivellese

Activation of human basophils by combined toll-like receptor- and FcεRI-triggering can promote Th2 skewing of naive T helper cells.

Basophils are mostly known for their involvement in allergic reactions. Recent studies in mice indicate a role for basophils in the induction of adaptive immunity, especially T helper 2 (Th2) responses. Therefore, it would be highly important to understand how basophils respond to pathogen‐associated molecules, such as ligands for toll‐like receptors (TLRs), and if the basophils could promote Th2 responses via these stimuli. To this end, the activation of basophils via TLRs in combination with activation via IgE was studied, as well as its effect on T helper cell skewing. Using quantitative PCR, we demonstrated the presence of mRNA for TLRs 1–8 in human basophils. Basophils responded to TLR triggering with differential cytokine production, but not with degranulation. Simultaneous triggering of TLRs and IgE led to synergy in production of IL‐4, IL‐8, IL‐13, and RANTES. Furthermore, the synergistic effects on basophils mediated by IgE and TLR‐4 triggering allowed robust Th2 skewing upon activation of naïve human CD4+ T cells.

Resolution of inflammation by interleukin-9-producing type 2 innate lymphoid cells

Inflammatory diseases such as arthritis are chronic conditions that fail to resolve spontaneously. While the cytokine and cellular pathways triggering arthritis are well defined, those responsible for the resolution of inflammation are incompletely characterized. Here we identified interleukin (IL)-9-producing type 2 innate lymphoid cells (ILC2s) as the mediators of a molecular and cellular pathway that orchestrates the resolution of chronic inflammation. In mice, the absence of IL-9 impaired ILC2 proliferation and activation of regulatory T (Treg) cells, and resulted in chronic arthritis with excessive cartilage destruction and bone loss. In contrast, treatment with IL-9 promoted ILC2-dependent Treg activation and effectively induced resolution of inflammation and protection of bone. Patients with rheumatoid arthritis in remission exhibited high numbers of IL-9+ ILC2s in joints and the circulation. Hence, fostering IL-9-mediated ILC2 activation may offer a novel therapeutic approach inducing resolution of inflammation rather than suppression of inflammatory responses.

Toll-like receptor triggering augments activation of human mast cells by anti-citrullinated protein antibodies

Objective Mast cells may play a role in rheumatoid arthritis (RA), but activation of human mast cells in autoimmune settings has been little studied. Toll-like receptors (TLR) and Fcγ receptors (FcγR) are important receptors for cellular activation in the joint, but expression and stimulation of these receptors in human mast cells or the functional interplay between these pathways is poorly understood. Here, we analysed triggering of human mast cells via these receptors in the context of anti-citrullinated protein antibody-positive (ACPA+) RA. Methods RNA and protein expression of TLRs and FcγR was quantified using PCR and flow cytometry, respectively. Mast cells were stimulated with TLR ligands (including HSP70) combined with IgG immune complexes and IgG-ACPA. Results Human mast cells expressed TLRs and produced cytokines in response to TLR ligands. Both cultured and synovial mast cells expressed FcγRIIA, and triggering of this receptor by IgG immune complexes synergised with activation by TLR ligands, leading to two- to fivefold increased cytokine levels. Mast cells produced cytokines in response to ACPA immune complexes in a citrulline-specific manner, which synergised in the presence of HSP70. Conclusions Our data show that synovial mast cells express FcγRIIA and that mast cells can be activated by IgG-ACPA and TLR ligands. Importantly, combined stimulation via TLRs and immune complexes leads to synergy in cytokine production. These findings suggest mast cells are important targets for TLR ligands and immune complexes, and that combined activation of mast cells via these pathways greatly enhances inflammation in synovial tissue of RA patients.

Helicobacter pylori Hp(2–20) Promotes Migration and Proliferation of Gastric Epithelial Cells by Interacting with Formyl Peptide Receptors In Vitro and Accelerates Gastric Mucosal Healing In Vivo

Helicobacter pylori-derived peptide RpL1 aa 2–20 (Hp(2–20)) in addition to its antimicrobial action exerts several immunomodulatory effects in eukaryotic cells by interacting with formyl peptide receptors (FPRs). It has recently been shown that activation of FPRs facilitates intestinal epithelial cell restitution. We investigated whether Hp(2–20) induces healing of injured gastric mucosa and assessed the mechanisms underlying any such effect. We investigated the expression of FPRs in two gastric epithelial cell lines (MKN-28 and AGS) at mRNA and protein level. To determine whether FPRs were functional we performed chemotaxis experiments and proliferation assays and studied the Hp(2–20)-activated downstream signaling pathway. The effect of Hp(2–20) on mucosal healing was evaluated in rats after indomethacin-induced injury. Here we show that: (1) FPRs were expressed in both cell lines; (2) Hp(2–20) stimulated migration and proliferation of gastric epithelial cells; (3) this effect was specifically mediated by formyl peptide receptor-like 1 (FPRL1) and FPRL2 and was associated with activation of FPR-related downstream signaling pathways; (4) Hp(2–20) up-regulated the expression and secretion of vascular endothelial growth factor; and (5) Hp(2–20) accelerated healing of rat gastric mucosa after injury brought about by indomethacin at both the macroscopic and microscopic levels. In conclusion, by interacting with FRPL1 and FPRL2, H. pylori-derived Hp(2–20) induces cell migration and proliferation, as well as the expression of vascular endothelial growth factor, thereby promoting gastric mucosal healing. This study provides further evidence of the complexity of the relationship between H. pylori and human gastric mucosa, and it suggests that a bacterial product may be used to heal gastric mucosal injury.

IgE and IL-33-mediated triggering of human basophils inhibits TLR4-induced monocyte activation.

Basophils are circulating granulocytes, best known as effector cells in allergic reactions. Recent studies in mice suggest that they might also participate in the suppression of chronic inflammation. The aim of this study was to assess the ability of purified human basophils to modulate monocyte responses upon IL‐33 and IgE triggering. Activation of human basophils with IL‐33 induced the production of IL‐4 and the release of histamine, and enhanced their IgE‐mediated activation. In addition, basophils triggered with IL‐33 and anti‐IgE significantly suppressed the LPS‐induced production of the proinflammatory cytokine TNF‐α and the upregulation of the costimulatory molecule CD80 by monocytes. These effects were mainly explained by the release of histamine, as they could be inhibited by the histamine receptor 2 antagonist ranitidine, with a smaller contribution of IL‐4. In contrast, basophil‐derived IL‐4 and histamine had opposing effects on the expression of the inhibitory Fc γ receptor IIb and the production of IL‐10 by monocytes. Our data show that basophils can influence monocyte activation and suggest a previously unrecognized role for human basophils in the modulation of monocyte‐mediated immune responses, through the balanced secretion of histamine and IL‐4.

Ability of Interleukin-33– and Immune Complex–Triggered Activation of Human Mast Cells to Down-Regulate Monocyte-Mediated Immune Responses

Mast cells have been implicated in the pathogenesis of rheumatoid arthritis (RA). In particular, their activation by interleukin‐33 (IL‐33) has been linked to the development of arthritis in animal models. The aim of this study was to evaluate the functional responses of human mast cells to IL‐33 in the context of RA.

Mast cells in rheumatoid arthritis: friends or foes?

Mast cells are tissue-resident cells of the innate immunity, implicated in the pathogenesis of many autoimmune diseases, including rheumatoid arthritis (RA). They are present in synovia and their activation has been linked to the potentiation of inflammation in the course of RA. However, recent investigations questioned the role of mast cells in arthritis. In particular, animal models generated conflicting results, so that many of their pro-inflammatory, i.e. pro-arthritogenic functions, even though supported by robust experimental evidence, have been labelled as redundant. At the same time, a growing body of evidence suggests that mast cells can act as tunable immunomodulatory cells. These characteristics, not yet fully understood in the context of RA, could partially explain the inconsistent results obtained with experimental models, which do not account for the pro- and anti-inflammatory functions exerted in more chronic heterogeneous conditions such as RA. Here we present an overview of the current knowledge on mast cell involvement in RA, including the intriguing hypothesis of mast cells acting as subtle immunomodulatory cells and the emerging concept of synovial mast cells as potential biomarkers for patient stratification.

Helicobacter Pylori HP(2–20) Induces Eosinophil Activation and Accumulation in Superficial Gastric Mucosa and Stimulates VEGF-α and TGF-β Release by Interacting with Formyl-Peptide Receptors:

Eosinophils participate in the immune response against Helicobacter pylori, but little is known about their role in the gastritis associated to the infection. We recently demonstrated that the Hp(2–20) peptide derived from H. pylori accelerates wound healing of gastric mucosa by interacting with N-formyl peptide receptors (FPRs) expressed on gastric epithelial cells. The aim of the present study was to investigate whether eosinophils play a role in the repair of gastric mucosa tissue during H. pylori infection. Immunohistochemistry and transmission electron microscopy were used to detect eosinophils in gastric mucosal biopsies. Eosinophil re-distribution occurred in the gastric mucosa of H. pylori-infected patients: their density did not change in the deep mucosal layer, whereas it increased in the superficial lamina propria just below the foveolar epithelium; eosinophils entered the epithelium itself as well as the lumen of foveolae located close to the area harboring bacteria, which in turn were also engulfed by eosinophils. The H. pylori-derived peptide Hp(2–20) stimulated eosinophil migration through the engagement of FPR2 and FPR3, and also induced production of VEGF-A and TGF-β, two key mediators of tissue remodeling. We also demonstrate that Hp(2–20) in vivo induced eosinophil infiltration in rat gastric mucosa after injury brought about by indomethacin. This study suggests that eosinophil infiltrate could modulate the capacity of gastric mucosa to maintain or recover its integrity thereby shedding light on the role of eosinophils in H. pylori infection.

LLT1 and CD161 Expression in Human Germinal Centers Promotes B Cell Activation and CXCR4 Downregulation

Germinal centers (GCs) are microanatomical structures critical for the development of high-affinity Abs and B cell memory. They are organized into two zones, light and dark, with coordinated roles, controlled by local signaling. The innate lectin-like transcript 1 (LLT1) is known to be expressed on B cells, but its functional role in the GC reaction has not been explored. In this study, we report high expression of LLT1 on GC-associated B cells, early plasmablasts, and GC-derived lymphomas. LLT1 expression was readily induced via BCR, CD40, and CpG stimulation on B cells. Unexpectedly, we found high expression of the LLT1 ligand, CD161, on follicular dendritic cells. Triggering of LLT1 supported B cell activation, CD83 upregulation, and CXCR4 downregulation. Overall, these data suggest that LLT1–CD161 interactions play a novel and important role in B cell maturation within the GC in humans.

Human mast cells and basophils-How are they similar how are they different?

Mast cells and basophils are key contributors to allergies and other inflammatory diseases since they are the most prominent source of histamine as well as numerous additional inflammatory mediators which drive inflammatory responses. However, a closer understanding of their precise roles in allergies and other pathological conditions has been marred by the considerable heterogeneity that these cells display, not only between mast cells and basophils themselves but also across different tissue locations and species. While both cell types share the ability to rapidly degranulate and release histamine following high‐affinity IgE receptor cross‐linking, they differ markedly in their ability to either react to other stimuli, generate inflammatory eicosanoids or release immunomodulating cytokines and chemokines. Furthermore, these cells display considerable pharmacological heterogeneity which has stifled attempts to develop more effective anti‐allergic therapies. Mast cell‐ and basophil‐specific transcriptional profiling, at rest and after activation by innate and adaptive stimuli, may help to unravel the degree to which these cells differ and facilitate a clearer understanding of their biological functions and how these could be targeted by new therapies.