Jolien Suurmond

Retraction Note: Mast cells are the main interleukin 17-positive cells in anticitrullinated protein antibody-positive and -negative rheumatoid arthritis and osteoarthritis synovium

IntroductionMast cells have been implicated to play a functional role in arthritis, especially in autoantibody-positive disease. Among the cytokines involved in rheumatoid arthritis (RA), IL-17 is an important inflammatory mediator. Recent data suggest that the synovial mast cell is a main producer of IL-17, although T cells have also been implicated as prominent IL-17 producers as well. We aimed to identify IL-17 expression by mast cells and T cells in synovium of arthritis patients.MethodsSynovial samples of anticitrullinated protein antibody-positive (ACPA+) and ACPA-negative (ACPA-) RA and osteoarthritis (OA) patients were stained for IL-17 in combination with CD117 (mast cells), CD3 (T cells) and CD68 (macrophages). Concentrations of IL-17 in synovial fluid were determined by ELISA.ResultsThe number of IL-17+ cells in synovium was comparable in all groups. Although the vast majority of IL-17+ cells are mast cells, no difference in the percentage of IL-17+ mast cells was observed. Nonetheless, levels of IL-17 in synovial fluid were increased in ACPA+ RA patients compared to ACPA- RA and OA patients.ConclusionsThe synovial mast cell is the main IL-17+ cell in all three arthritis groups analyzed. These data are relevant for studies aimed at blocking IL-17 in the treatment of arthritis.

Chemotherapy-resistant osteosarcoma is highly susceptible to IL-15-activated allogeneic and autologous NK cells.

High-grade osteosarcoma occurs predominantly in adolescents and young adults and has an overall survival rate of about 60%, despite chemotherapy and surgery. Therefore, novel treatment modalities are needed to prevent or treat recurrent disease. Natural killer (NK) cells are lymphocytes with cytotoxic activity toward virus-infected or malignant cells. We explored the feasibility of autologous and allogeneic NK cell–mediated therapies for chemotherapy-resistant and chemotherapy-sensitive high-grade osteosarcoma. The expression by osteosarcoma cells of ligands for activating NK cell receptors was studied in vitro and in vivo, and their contribution to NK cell–mediated cytolysis was studied by specific antibody blockade. Chromium release cytotoxicity assays revealed chemotherapy-sensitive and chemotherapy-resistant osteosarcoma cell lines and osteosarcoma primary cultures to be sensitive to NK cell–mediated cytolysis. Cytolytic activity was strongly enhanced by IL-15 activation and was dependent on DNAM-1 and NKG2D pathways. Autologous and allogeneic activated NK cells lysed osteosarcoma primary cultures equally well. Osteosarcoma patient–derived NK cells were functionally and phenotypically unimpaired. In conclusion, osteosarcoma cells, including chemoresistant variants, are highly susceptible to lysis by IL-15-induced NK cells from both allogeneic and autologous origin. Our data support the exploitation of NK cells or NK cell–activating agents in patients with high-grade osteosarcoma.

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.

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.

Communication between human mast cells and CD4(+) T cells through antigen-dependent interactions.

Mast cells (MCs) are immune cells residing in tissues where pathogens are first encountered. It has been indicated that MCs might also be involved in setting the outcome of T‐cell responses. However, little is known about the capacity of human MCs to express MHC class II and/or to capture and present antigens to CD4+ T cells. To study the T‐cell stimulatory potential of human MCs, CD34+ stem cell derived MCs were generated. These cells expressed HLA‐DR when stimulated with IFN‐γ, and, importantly, presented peptide and protein for activation of antigen‐specific CD4+ T cells. The interplay between MC and T cell led to increased HLA‐DR expression on MCs. MCs were present in close proximity to T cells in tonsil and expressed HLA‐DR and CD80, indicating their ability to present antigens to CD4+ T cells in T‐cell areas of human LNs. Our data show that MCs can present native antigens to human CD4+ T cells and that HLA‐DR expressing MCs are present in tonsil tissue, indicating that human MCs can directly activate T cells and provide a rationale to study the potential of MCs to prime and/or skew human T‐cell responses.

Differential TLR-induced cytokine production by human mast cells is amplified by FcɛRI triggering

Mast cells are mainly present in strategic locations, where they may have a role in defence against parasites and bacteria. These pathogens can be recognized by mast cells via Toll‐like receptors (TLR). Allergic symptoms are often increased in the presence of pathogens at the site of allergen exposure, but it is unknown which cytokines can mediate such an effect.

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 rheumatic disease

Rheumatoid Arthritis is a chronic autoimmune disease with a complex disease pathogenesis leading to inflammation and destruction of synovial tissue in the joint. Several molecules lead to activation of immune pathways, including autoantibodies, Toll-Like Receptor ligands and cytokines. These pathways can cooperate to create the pro-inflammatory environment that results in tissue destruction. Each of these pathways can activate mast cells, inducing the release of a variety of inflammatory mediators, and in combination can markedly enhance mast cell responses. Mast cell-derived cytokines, chemokines, and proteases have the potential to induce recruitment of other leukocytes able to evoke tissue remodeling or destruction. Likewise, mast cells can secrete a plethora of factors that can contribute to tissue remodeling and fibroblast activation. Although the functional role of mast cells in arthritis pathogenesis in mice is not yet elucidated, the increased numbers of mast cells and mast cell-specific mediators in synovial tissue of rheumatoid arthritis patients suggest that mast cell activation in rheumatoid arthritis may contribute to its pathogenesis.

Anti-citrullinated protein antibodies in rheumatoid arthritis: a functional role for mast cells and basophils?

This article has been retracted.