Intracellular IL-1 Receptor Antagonist Isoform 1 Released from Keratinocytes upon Cell Death Acts as an Inhibitor for the Alarmin IL-1α.

The inflammatory effects of IL-1α/ß are controlled by IL-1R antagonist (IL-1Ra). One IL-1Ra isoform is secreted, whereas three other isoforms (intracellular IL-1Ra [icIL-1Ra] 1, 2, and 3) are supposed to remain intracellular because of the absence of a signal peptide. In contrast to the well-characterized function of the secreted isoform, the biological role of the intracellular isoforms remains largely unclear. icIL-1Ra1 represents the major isoform in keratinocytes. We created icIL-1Ra1-/- mice and investigated the role of icIL-1Ra1 in Aldara (5% imiquimod)-induced psoriasis-like skin inflammation. Naive icIL-1Ra1-/- mice bred habitually and exhibited a normal phenotype. icIL-1Ra1 deficiency aggravated Aldara-induced skin inflammation, as demonstrated by increased ear thickness and increased mRNA levels of key proinflammatory cytokines. No intracellular effect of icIL-1Ra1 could be detected in isolated keratinocytes using RNA-sequencing analysis; however, Aldara treatment led to caspase 1/11-, caspase 8-, and RIPK3-independent keratinocyte cell death accompanied by the release of both icIL-1Ra1 and IL-1α. Furthermore, blocking IL-1α attenuated the clinical severity of Aldara-induced ear thickening in icIL-1Ra1-/- mice. Our data suggest that upon keratinocyte damage icIL-1Ra1 acts extracellularly as an antagonist of the alarmin IL-1α to immediately counteract its inflammatory effects.

Unopposed IL-18 signaling leads to severe TLR9-induced macrophage activation syndrome in mice.

The term macrophage activation syndrome (MAS) defines a severe, potentially fatal disorder characterized by overwhelming inflammation and multiorgan involvement. Interleukin-18 (IL-18) is a proinflammatory cytokine belonging to the IL-1 family, the activity of which is regulated by its endogenous inhibitor IL-18 binding protein (IL-18BP). Elevated IL-18 levels have been reported in patients with MAS. Herein, we show that on repeated toll-like receptor 9 (TLR9) stimulation with unmethylated cytosine guanine dinucleotide containing single-stranded DNA (CpG), IL-18BP-/- mice display severe MAS manifestations, including increased weight loss, splenomegaly, anemia, thrombocytopenia, hyperferritinemia, and bone marrow hemophagocytosis as compared with wild-type mice. Serum-free IL-18 was detected in CpG-treated IL-18BP-/- mice only. Levels of interferon-γ (IFN-γ) and of IFN-γ signature genes, such as the chemokine Cxcl9 or the transcription factor CIIta, were significantly increased in IL-18BP-/- mice. Blocking IL-18 receptor signaling attenuated the severity of MAS and IFN-γ responses in IL-18BP-/- mice. Blocking IFN-γ had comparable effects to IL-18 inhibition on most MAS manifestations. Our data indicate that endogenous IL-18BP exerts a protective role in CpG-induced MAS and that IL-18, which acts upstream of IFN-γ, is involved in the severity of MAS.

Deficiency in IL-1 Receptor Type 2 Aggravates K/BxN Serum Transfer-Induced Arthritis in Mice but Has No Impact on Systemic Inflammatory Responses.

The biological activity of IL-1 is tightly regulated by the specific receptor antagonist (IL-1Ra) and the decoy receptor IL-1 receptor type 2 (IL-1R2). The role of IL-1Ra has been well demonstrated in IL-1Ra-deficient mice. In contrast, the role of endogenous IL-1R2 remains widely unknown. To define the functional role of endogenous IL-1R2 in the K/BxN serum transfer arthritis model and in IL-1ß- or LPS-induced systemic inflammation in vivo, IL-1R2-/- mice were created and compared with wild type mice. IL-1R2-/- mice bred habitually and exhibited a normal phenotype. IL-1R2 deficiency aggravated arthritis severity and increased mRNA levels for key cytokines and chemokines such as IL-6, IL-1ß, Cxcl-1, and Cxcl-2 significantly in ankles. There was no effect of IL-1R2 deficiency on the cell-autonomous cytokine response to IL-1ß in the tested cell types, i.e., neutrophils, macrophages, and fibroblasts, but IL-1R2 deficiency on neutrophils increased the IL-1-induced response of fibroblasts in trans. Furthermore, IL-1ß induced shedding of IL-1R2 in vivo. Inflammatory responses to IL-1ß and LPS-induced mortality were not different in IL-1R2-/- compared with wild type mice. Our data demonstrate that the decoy receptor IL-1R2 plays an important inhibitory role in local IL-1- and neutrophil-dependent tissue inflammation as shown in the K/BxN serum transfer arthritis model. In contrast to IL-1Ra, IL-1R2 appears to be less crucial for systemic responses to acute administration of IL-1 or LPS.

Severe neutrophil-dominated inflammation and enhanced myelopoiesis in IL-33-overexpressing CMV/IL33 mice.

IL-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous studies emphasized a role for IL-33 in shaping innate and adaptive immune responses. IL-33 was also reported to modulate myelopoiesis and myeloid cell activity. In this article, we describe IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, which display an inflammatory phenotype associated with growth retardation and paw swelling. The phenotype of CMV/IL33 mice is dependent on activation of the ST2 receptor and is characterized by extensive neutrophil infiltration into different organs, including the paws. Local or systemic levels of proinflammatory mediators such as IL-1ß, Cxcl-1, G-CSF, and IL-6 are increased. CMV/IL-33 mice also suffer from anemia, thrombocytosis, and a marked dysregulation of myelopoiesis, leading to an important increase in myeloid cell production or accumulation in bone marrow (BM), spleen, and peripheral blood. Consistently, recombinant IL-33 induced proliferation of myeloid lineage cells in BM-derived granulocyte cultures, whereas IL-33 knockout mice exhibited minor deficiencies in spleen and BM myeloid cell populations. Our observations reveal a neutrophil-dominated inflammatory phenotype in IL-33-overexpressing CMV/IL33 and LysM/IL33 mice, and highlight important regulatory effects of IL-33 on myelopoiesis in vitro and in vivo, where excessive IL-33 signaling can translate into the occurrence of a myeloproliferative disorder.

Interleukin-36 potently stimulates human M2 macrophages, Langerhans cells and keratinocytes to produce pro-inflammatory cytokines.

Interleukin (IL)-36 cytokines belong to the IL-1 family and include three agonists, IL-36 α, ß and γ and one inhibitor, IL-36 receptor antagonist (IL-36Ra). IL-36 and IL-1 (α and ß) activate similar intracellular pathways via their related heterodimeric receptors, IL-36R/IL-1RAcP and IL-1R1/IL-1RAcP, respectively. However, excessive IL-36 versus IL-1 signaling induces different phenotypes in humans, which may be related to differential expression of their respective receptors. We examined the expression of IL-36R, IL-1R1 and IL-1RAcP mRNA in human peripheral blood, tonsil and skin immune cells by RT-qPCR. Monocyte-derived dendritic cells (MDDC), M0, M1 or M2-polarized macrophages, primary keratinocytes, dermal macrophages and Langerhans cells (LC) were stimulated with IL-1ß or IL-36ß. Cytokine production was assessed by RT-qPCR and immunoassays. The highest levels of IL-36R mRNA were found in skin-derived keratinocytes, LC, dermal macrophages and dermal CD1a(+) DC. In the blood and in tonsils, IL-36R mRNA was predominantly found in myeloid cells. By contrast, IL-1R1 mRNA was detected in almost all cell types with higher levels in tonsil and skin compared to peripheral blood immune cells. IL-36ß was as potent as IL-1ß in stimulating M2 macrophages, keratinocytes and LC, less potent than IL-1ß in stimulating M0 macrophages and MDDC, and exerted no effects in M1 and dermal macrophages. Levels of IL-1Ra diminished the ability of M2 macrophages to respond to IL-1. Taken together, these data are consistent with the association of excessive IL-36 signaling with an inflammatory skin phenotype and identify human LC and M2 macrophages as new IL-36 target cells.

The interleukin (IL)-1 cytokine family--Balance between agonists and antagonists in inflammatory diseases.

The interleukin (IL)-1 family of cytokines comprises 11 members, including 7 pro-inflammatory agonists (IL-1α, IL-1ß, IL-18, IL-33, IL-36α, IL-36ß, IL-36γ) and 4 defined or putative antagonists (IL-1R antagonist (IL-1Ra), IL-36Ra, IL-37, and IL-38) exerting anti-inflammatory activities. Except for IL-1Ra, IL-1 cytokines do not possess a leader sequence and are secreted via an unconventional pathway. In addition, IL-1ß and IL-18 are produced as biologically inert pro-peptides that require cleavage by caspase-1 in their N-terminal region to generate active proteins. N-terminal processing is also required for full activity of IL-36 cytokines. The IL-1 receptor (IL-1R) family comprises 10 members and includes cytokine-specific receptors, co-receptors and inhibitory receptors. The signaling IL-1Rs share a common structure with three extracellular immunoglobulin (Ig) domains and an intracellular Toll-like/IL-1R (TIR) domain. IL-1 cytokines bind to their specific receptor, which leads to the recruitment of a co-receptor and intracellular signaling. IL-1 cytokines induce potent inflammatory responses and their activity is tightly controlled at the level of production, protein processing and maturation, receptor binding and post-receptor signaling by naturally occurring inhibitors. Some of these inhibitors are IL-1 family antagonists, while others are IL-1R family members acting as membrane-bound or soluble decoy receptors. An imbalance between agonist and antagonist levels can lead to exaggerated inflammatory responses. Several genetic modifications or mutations associated with dysregulated IL-1 activity and autoinflammatory disorders were identified in mouse models and in patients. These findings paved the road to the successful use of IL-1 inhibitors in diseases that were previously considered as untreatable.

Secretory lysosomes of mouse mast cells store and exocytose active caspase-3 in a strictly granzyme B dependent manner.

In this study, we report that cytoplasmic granules from in vivo and in vitro derived mouse mast cells (MCs) contain active granzyme B (gzmB) and caspase-3, which is consistent with recent findings. Studying WT and gzmB-deficient mice, we observed that BM-derived MCs (BMMCs) from both strains contain cytosolic pro-caspase-3, but only WT BMMCs expressed active caspase-3 limited to their secretory lysosomes. Confocal microscopy revealed colocalization of active caspase-3 and gzmB in these cytoplasmic granules. The combined data demonstrate that the generation and storage of active caspase-3 is gzmB-dependent. The finding that BMMCs secrete caspase-3 and gzmB after Ag stimulation suggests that both proteases contribute to extracellular MC-mediated proteolytic events. Although the extracellular function of MC-derived caspase-3 remains unclear, we show that BMMC-secreted caspase-3 cleaves IL-33, a cytokine that contributes to the development of asthma and arthritis. We also show that an in vitro propagated cytolytic T-lymphocyte line constitutively expresses gzmB together with active caspase-3, suggesting a novel interaction of these proteases in the execution of multiple innate and adaptive immune responses.

IL-36 signaling amplifies Th1 responses by enhancing proliferation and Th1 polarization of naive CD4+ T cells.

The interleukin-1 (IL-1) superfamily of cytokines comprises a set of pivotal mediators of inflammation. Among them, the action of IL-36 cytokines in immune responses has remained elusive. In a recent study, we demonstrated a direct effect of IL-36 on immune cells. Here we show that, among T cells, the IL-36 receptor is predominantly expressed on naive CD4(+) T cells and that IL-36 cytokines act directly on naive T cells by enhancing both cell proliferation and IL-2 secretion. IL-36ß acts in synergy with IL-12 to promote Th1 polarization and IL-36 signaling is also involved in mediating Th1 immune responses to Bacillus Calmette-Guerin infection in vivo. Our findings point toward a critical function of IL-36 in the priming of Th1 cell responses in vitro, and in adaptive immunity in a model of mycobacterial infection in vivo.

Immune-mediated experimental arthritis in IL-33 deficient mice.

Previous work suggested implication of the interleukin (IL)-1 family cytokine IL-33, signaling through its receptor ST2, in the pathogenesis of human and mouse arthritis. In this study, we directly investigated the role of endogenous IL-33 in antigen-induced arthritis (AIA) and collagen-induced arthritis (CIA) using IL-33 KO mice. AIA was induced by injection of methylated bovine serum albumin (mBSA) into knee joints of previously immunized mice. CIA was induced by immunization with bovine type II collagen. Disease severity was evaluated by clinical and histological scoring and cellular immune responses were assessed in cultured draining lymph node cells. Our results indicate that the development of AIA or CIA, as assessed by clinical or histological evaluation, is not impaired in IL-33 deficient mice. We did not observe any consistent modifications in humoral or cellular immune responses in IL-33 KO mice, although IL-33 deficiency enhanced antigen-specific IFN-γ production, proliferation or IgG2a titers in some experiments, suggesting that endogenous IL-33 may contribute to shaping the adaptive immune response. In conclusion, our data suggest that IL-33 plays a modifying rather than a pivotal role in disease development in two models of immune-mediated arthritis.

IL-36R ligands are potent regulators of dendritic and T cells.

IL-36α (IL-1F6), IL-36ß (IL-1F8), and IL-36γ (IL-1F9) are members of the IL-1 family of cytokines. These cytokines bind to IL-36R (IL-1Rrp2) and IL-1RAcP, activating similar intracellular signals as IL-1, whereas IL-36Ra (IL-1F5) acts as an IL-36R antagonist (IL-36Ra). In this study, we show that both murine bone marrow-derived dendritic cells (BMDCs) and CD4(+) T lymphocytes constitutively express IL-36R and respond to IL-36α, IL-36ß, and IL-36γ. IL-36 induced the production of proinflammatory cytokines, including IL-12, IL-1ß, IL-6, TNF-α, and IL-23 by BMDCs with a more potent stimulatory effect than that of other IL-1 cytokines. In addition, IL-36ß enhanced the expression of CD80, CD86, and MHC class II by BMDCs. IL-36 also induced the production of IFN-γ, IL-4, and IL-17 by CD4(+) T cells and cultured splenocytes. These stimulatory effects were antagonized by IL-36Ra when used in 100- to 1000-fold molar excess. The immunization of mice with IL-36ß significantly and specifically promoted Th1 responses. Our data thus indicate a critical role of IL-36R ligands in the interface between innate and adaptive immunity, leading to the stimulation of T helper responses.

Articular inflammation is controlled by myeloid cell-derived interleukin 1 receptor antagonist during the acute phase of arthritis in mice.

OBJECTIVES: To define the cell type (myeloid vs other cells) specific effect of interleukin 1 (IL-1) receptor antagonist (IL-1Ra) deficiency on the acute inflammatory phase of arthritis. METHODS: Arthritis was induced by K/BxN serum transfer in wild-type (WT), IL-1Ra-deficient (IL-1Ra(-/-)) and conditional knockout mice. In the latter, IL-1Ra production was specifically targeted in myeloid cells (IL-1Ra(ΔM)) or in both hepatocytes and myeloid cells (IL-1Ra(ΔH+M)). Arthritis severity was clinically evaluated and ankle sections were scored for synovial inflammation and cartilage erosion. Quantitative RT-PCR, western blot and immunohistochemical analyses measured expression, localisation and cellular sources of the different IL-1Ra isoforms in arthritic joints. RESULTS: Total and myeloid cell-specific IL-1Ra deficiency was associated with increased arthritis severity, although disease incidence was similar to that of WT mice. Increased clinical scores were associated with exacerbated synovial inflammation. All IL-1Ra isoforms, except for intracellular (ic)IL-1Ra2, were expressed in arthritic joints of WT mice. In contrast, production of secreted (s)IL-1Ra and icIL-1Ra3 isoforms was markedly decreased in arthritic joints of both IL-1Ra(ΔM) and IL-1Ra(ΔH+M) mice. Immunohistochemical and western blot analyses suggested that the icIL-1Ra1 isoform is produced primarily by synovial fibroblasts. CONCLUSION: Myeloid cell-derived IL-1Ra, including both sIL-1Ra and icIL-1Ra3 isoforms, controls articular inflammation during the acute phase of K/BxN serum transfer-induced arthritis.

Granzyme B-induced and caspase 3-dependent cleavage of gelsolin by mouse cytotoxic T cells modifies cytoskeleton dynamics.

Granule-associated perforin and granzymes (gzms) are key effector molecules of cytotoxic T lymphocytes (Tc cells) and natural killer cells and play a critical role in the control of intracellular pathogens and cancer. Based on the notion that many gzms, including A, B, C, K, H, and M exhibit cytotoxic activity in vitro, all gzms are believed to serve a similar function in vivo. However, more recent evidence supports the concept that gzms are not unidimensional but, rather, possess non-cytotoxic potential, including stimulation of pro-inflammatory cytokines and anti-viral activities. The present study shows that isolated mouse gzmB cleaves the actin-severing mouse protein, cytoplasmic gelsolin (c-gelsolin) in vitro. However, when delivered to intact target cells by ex vivo immune Tc cells, gzmB mediates c-gelsolin proteolysis via activation of caspases 3/7. The NH(2)-terminal c-gelsolin fragment generated by either gzmB or caspase 3 in vitro constitutively severs actin filaments without destroying the target cells. The observation that gzmB secreted by Tc cells initiates a caspase cascade that disintegrates the actin cytoskeleton in target cells suggests that this intracellular process may contribute to anti-viral host defense.

IL-1 Family Antagonists in Mouse and Human Skin Inflammation.

Interleukin (IL)-1 family cytokines initiate inflammatory responses, and shape innate and adaptive immunity. They play important roles in host defense, but excessive immune activation can also lead to the development of chronic inflammatory diseases. Dysregulated IL-1 family signaling is observed in a variety of skin disorders. In particular, IL-1 family cytokines have been linked to the pathogenesis of psoriasis and atopic dermatitis. The biological activity of pro-inflammatory IL-1 family agonists is controlled by the natural receptor antagonists IL-1Ra and IL-36Ra, as well as by the regulatory cytokines IL-37 and IL-38. These four anti-inflammatory IL-1 family members are constitutively and highly expressed at steady state in the epidermis, where keratinocytes are a major producing cell type. In this review, we provide an overview of the current knowledge concerning their regulatory roles in skin biology and inflammation and their therapeutic potential in human inflammatory skin diseases. We further highlight some common misunderstandings and less well-known observations, which persist in the field despite recent extensive interest for these cytokines.

ß Cell-Specific Deletion of the IL-1 Receptor Antagonist Impairs ß Cell Proliferation and Insulin Secretion.

Interleukin-1 receptor antagonist (IL-1Ra) is elevated in the circulation during obesity and type 2 diabetes (T2D) but is decreased in islets from patients with T2D. The protective role of local IL-1Ra was investigated in pancreatic islet ß cell (ßIL-1Ra)-specific versus myeloid-cell (myeloIL-1Ra)-specific IL-1Ra knockout (KO) mice. Deletion of IL-1Ra in ß cells, but not in myeloid cells, resulted in diminished islet IL-1Ra expression. Myeloid cells were not the main source of circulating IL-1Ra in obesity. ßIL-1Ra KO mice had impaired insulin secretion, reduced ß cell proliferation, and decreased expression of islet proliferation genes, along with impaired glucose tolerance. The key cell-cycle regulator E2F1 partly reversed IL-1ß-mediated inhibition of potassium channel Kir6.2 expression and rescued impaired insulin secretion in IL-1Ra knockout islets. Our findings provide evidence for the importance of ß cell-derived IL-1Ra for the local defense of ß cells to maintain normal function and proliferation.

Granzyme A Contributes to Inflammatory Arthritis in Mice Through Stimulation of Osteoclastogenesis.

OBJECTIVE: Granzyme A (GzmA) levels are elevated in the plasma and synovium of patients with rheumatoid arthritis (RA), suggesting involvement of this protease in the pathogenesis of the disease. GzmA contributes to sepsis by regulating the production of proinflammatory cytokines. The purpose of this study was to evaluate the contribution of GzmA to the pathogenesis of RA in vivo and to examine the possibility that GzmA acting via tumor necrosis factor (TNF) stimulates osteoclastogenesis. METHODS: Inflammatory arthritis induced by type II collagen was evaluated in wild-type, GzmA-deficient, and perforin-deficient mice. The osteoclastogenic potential of GzmA was examined in vitro using bone marrow cells and colony-forming unit-granulocyte-macrophage (CFU-GM) cells and in vivo using GzmA-deficient mice. RESULTS: Gene deletion of GzmA attenuated collagen-induced arthritis, including serum levels of proinflammatory cytokines, joint damage, and bone erosion in affected mice, suggesting that osteoclast activity is reduced in the absence of GzmA. Accordingly, GzmA-treated bone marrow cells produced multinucleated cells that fulfilled the criteria for mature osteoclasts: tartrate-resistant acid phosphatase (TRAP) activity, ß integrin expression, calcitonin receptor expression, and resorptive activity on dentin slices. GzmA appeared to act without accessory cells, and its activity was not affected by osteoprotegerin, suggesting a minor contribution of RANKL. It also induced the expression and secretion of TNF. Neutralization of TNF or stimulation of CFU-GM cells from TNF-/- mice prevented GzmA-induced osteoclastogenesis. GzmA-deficient mice had reduced osteoclastogenesis in vivo (fewer calcitonin receptor-positive multinucleated cells and fewer transcripts for cathepsin K, matrix metalloproteinase 9, and TRAP in joints) and reduced serum levels of C-terminal telopeptide of type I collagen. CONCLUSION: GzmA contributes to the joint destruction of RA partly by promoting osteoclast differentiation.

Atherosclerosis severity is not affected by a deficiency in IL-33/ST2 signaling.

Interleukin (IL)-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous work demonstrated that the systemic administration of recombinant IL-33 reduces the development of atherosclerosis in apolipoprotein E-deficient (ApoE(-/-)) mice by inducing a Th1-to-Th2 shift. The objective of our study was to examine the role of endogenous IL-33 and ST2 in atherosclerosis. ApoE(-/-), IL-33(-/-)ApoE(-/-), and ST2(-/-)ApoE(-/-) mice were fed with a cholesterol-rich diet for 10 weeks. Additionally, a group of ApoE(-/-) mice was injected with a neutralizing anti-ST2 or an isotype control antibody during the period of the cholesterol-rich diet. Atherosclerotic lesion development was measured by Oil Red O staining in the thoracic-abdominal aorta and the aortic sinus. There were no significant differences in the lipid-staining area of IL-33(-/-)ApoE(-/-), ST2(-/-)ApoE(-/-), or anti-ST2 antibody-treated ApoE(-/-) mice, compared to ApoE(-/-) controls. The absence of IL-33 signaling had no major and consistent impact on the Th1/Th2 cytokine responses in the supernatant of in vitro-stimulated lymph node cells. In summary, deficiency of the endogenously produced IL-33 and its receptor ST2 does not impact the development of atherosclerosis in ApoE-deficient mice.

The severity of experimental arthritis is independent of IL-36 receptor signaling.

INTRODUCTION: Interleukin (IL)-36 refers to three related IL-1 family cytokines, IL-36α, IL-36ß, and IL-36γ, that bind to the IL-36 receptor (IL-36R). IL-36 exerts proinflammatory effects in skin and lung and stimulates T cell responses. In the present study, we examined the expression and function of IL-36R and its ligands in experimental arthritis. METHODS: Collagen-induced arthritis (CIA), antigen-induced arthritis (AIA), and K/BxN serum transfer-induced arthritis were induced according to standard protocols. Messenger RNA levels for IL-36R and its ligands in the joints of mice with CIA were determined by RT-qPCR. Mice with CIA were injected with a blocking monoclonal anti-IL-36R, a blocking anti-IL-1RI, or their isotype-matched control antibodies at the time of arthritis onset. Anti-IL-36R or control antibodies were also injected at the time of AIA induction. Finally, IL-36R-deficient mice were examined in AIA and serum transfer-induced arthritis. The development and severity of arthritis were assessed by clinical and histological scoring. RESULTS: IL-36R, IL-36Ra and IL-36γ mRNA were detected in the joints of mice with CIA, but their levels did not correlate with arthritis severity. As opposed to anti-IL-1RI antibody treatment, the injection of an anti-IL-36R antibody was devoid of effect on the development and severity of CIA. The severity of joint inflammation and structural damage in AIA was also unaltered by anti-IL-36R antibody treatment. Finally, the severity of AIA and K/BxN serum transfer-induced arthritis was similar in IL-36R-deficient and wild-type mice. CONCLUSIONS: The development and severity of experimental arthritis are independent of IL-36R signaling.

Disease severity in K/BxN serum transfer-induced arthritis is not affected by IL-33 deficiency.

INTRODUCTION: Interleukin (IL)-33 is a cytokine of the IL-1 family, which signals through the ST2 receptor. Previous work suggested implication of the IL-33/ST2 axis in the pathogenesis of human and mouse arthritis. Here, we directly investigated the role of endogenous IL-33 in K/BxN serum transfer-induced arthritis by using IL-33 knockout (KO) mice. METHODS: Arthritis was induced by injection of complete K/BxN serum or purified IgG. Disease severity was monitored by clinical and histological scoring. RESULTS: K/BxN serum transfer induced pronounced arthritis with similar incidence and severity in IL-33 KO and wild-type (WT) mice. In contrast, disease development was significantly reduced in ST2 KO mice. IL-33 expression in synovial tissue was comparable in arthritic WT and ST2 KO mice, and absent in IL-33 KO mice. Transfer of purified arthritogenic IgG instead of complete K/BxN serum also resulted in similar arthritis severity in IL-33 KO and WT mice, excluding a contribution of IL-33 contained in the serum of donor mice to explain this result. We investigated additional potential confounding factors, including purity of genetic background, but the mechanisms underlying reduced arthritis in ST2 KO mice remained unclear. CONCLUSIONS: The data obtained with IL-33 KO mice indicate that endogenous IL-33 is not required for the development of joint inflammation in K/BxN serum transfer-induced arthritis. On the contrary, arthritis severity was reduced in ST2 KO mice. This observation might relate to IL-33 independent effects of ST2, and/or reveal the existence of confounding variables affecting the severity of joint inflammation in these KO strains.

Mouse neutrophils express the decoy type 2 interleukin-1 receptor (IL-1R2) constitutively and in acute inflammatory conditions.

The proinflammatory activities of IL-1 are tightly controlled at different levels. IL-1R2 acts as a decoy receptor and has been shown to regulate the biological effects of IL-1 in vitro and in vivo. However, little is known about its natural expression in the mouse in physiologic and pathologic conditions. In this study, we examined IL-1R2 mRNA and protein expression in isolated cells and tissues in response to different stimulatory conditions. Data obtained using ex vivo CD11b(+)Ly6G(+) peripheral blood cells and in vitro-differentiated CD11b(+)Ly6G(+) BMG indicated that neutrophils are the major source of constitutively expressed IL-1R2 in the mouse. The expression of IL-1R2 on BMG and ex vivo Ly6G(+) peripheral blood cells was highly up-regulated by HC. IL-1R2 pull-down experiments showed that mouse rIL-1ß binds to BMG IL-1R2, whereas binding of IL-1Ra could not be detected. Furthermore, LPS treatment induced shedding of IL-1R2 from the neutrophil membrane in vitro and in vivo, executed mainly by ADAM17. Finally, in in vivo models of inflammation, including thioglycolate-induced acute peritonitis and acute lung injury, infiltrating Ly6G(+) neutrophils, expressed IL-1R2. Our data show that in the mouse, neutrophils mainly express the decoy receptor IL-1R2 under naïve and inflammatory conditions. These data suggest that neutrophils may contribute to the resolution of acute inflammation.

The biology of cytotoxic cell granule exocytosis pathway: granzymes have evolved to induce cell death and inflammation.

The granule exocytosis pathway of cytotoxic lymphocytes (Tc and NK cells) is critical for control of tumor development and viral infections. Granule-associated perforin and granzymes are key components in Tc cell-mediated function(s). On the basis of studies that showed granzymes A, B, C, K and M, to induce apoptosis in vitro, all granzymes were thought to also induce cell death in vivo. This review summarizes our present understanding of the biological processes elicited by purified granzyme A and granzyme as well as the processes induced by the more physiologically relevant cytotoxic cells secreting these proteases. The combined evidence supports the concept that the granule secretion pathway is not mono-specific but rather poly-functional including induction of pro-inflammatory cytokines, besides their widely appreciated apoptotic properties.