Induction of the specific allergic immune response is independent of proteases from the fungus Alternaria alternata

We have analyzed the importance of proteases for the induction of allergic responses against the mold Alternaria alternata. Responses induced in vivo with untreated or heat treated (protease inactivated) extracts were compared in BALB/c, C57BL/6, TLR4 KO, and MyD88 KO mice. In BALB/c mice, both extracts induced similar lung inflammation, upregulation of inflammatory mediators, Th2 cytokines, and Alternaria‐specific antibodies. However heat inactivation abrogated polyclonal IgE production. Similar results were obtained in C57BL/6 albeit lung expression of some Th2 mediators was decreased in mice stimulated with the heat‐treated extract. Treatment of the extract with protease inhibitors did not affect the induction of the allergic response either, except again for the polyclonal IgE response. Th2 responses and lung inflammation were readily induced in TLR4 knockout mice. In contrast, lung inflammation, Th2 responses, cytokine productions, and antibody synthesis were strongly suppressed in MyD88‐deficient mice. Early lung IL‐33 and IL‐1‐α expression were also suppressed. In conclusion, albeit some heat labile proteases are required for the stimulation of the polyclonal IgE secretion, fungal proteases, and TLR4 signaling are not required while MyD88 is essential for triggering the systemic immune response and for the development of lung allergic inflammation in response to Alternaria extracts.     

IL-23 and the Th17 pathway promote inflammation and impair antifungal immune resistance

Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases. We found here that the IL-23/IL-17 developmental pathway acted as a negative regulator of the Th1-mediated immune resistance to fungi and played an inflammatory role previously attributed to uncontrolled Th1 cell responses. Both inflammation and infection were exacerbated by a heightened Th17 response against Candida albicans and Aspergillus fumigatus, two major human fungal pathogens. IL-23 acted as a molecular connection between uncontrolled fungal growth and inflammation, being produced by dendritic cells in response to a high fungal burden and counter-regulating IL-12p70 production. Both IL-23 and IL-17 subverted the inflammatory program of neutrophils, which resulted in severe tissue inflammatory pathology associated with infection. Our data are the first demonstrating that the IL-23/IL-17 pathway promotes inflammation and susceptibility in an infectious disease model. As IL-23-driven inflammation promotes infection and impairs antifungal resistance, modulation of the inflammatory response represents a potential strategy to stimulate protective immune responses to fungi.     

The immune response to Prevotella bacteria in chronic inflammatory disease

The microbiota plays a central role in human health and disease by shaping immune development, immune responses and metabolism, and by protecting from invading pathogens. Technical advances that allow comprehensive characterization of microbial communities by genetic sequencing have sparked the hunt for disease‐modulating bacteria. Emerging studies in humans have linked the increased abundance of Prevotella species at mucosal sites to localized and systemic disease, including periodontitis, bacterial vaginosis, rheumatoid arthritis, metabolic disorders and low‐grade systemic inflammation. Intriguingly, Prevotella abundance is reduced within the lung microbiota of patients with asthma and chronic obstructive pulmonary disease. Increased Prevotella abundance is associated with augmented T helper type 17 (Th17) ‐mediated mucosal inflammation, which is in line with the marked capacity of Prevotella in driving Th17 immune responses in vitro. Studies indicate that Prevotella predominantly activate Toll‐like receptor 2, leading to production of Th17‐polarizing cytokines by antigen‐presenting cells, including interleukin‐23 (IL‐23) and IL‐1. Furthermore, Prevotella stimulate epithelial cells to produce IL‐8, IL‐6 and CCL20, which can promote mucosal Th17 immune responses and neutrophil recruitment. Prevotella‐mediated mucosal inflammation leads to systemic dissemination of inflammatory mediators, bacteria and bacterial products, which in turn may affect systemic disease outcomes. Studies in mice support a causal role of Prevotella as colonization experiments promote clinical and inflammatory features of human disease. When compared with strict commensal bacteria, Prevotella exhibit increased inflammatory properties, as demonstrated by augmented release of inflammatory mediators from immune cells and various stromal cells. These findings indicate that some Prevotella strains may be clinically important pathobionts that can participate in human disease by promoting chronic inflammation.     

Impaired phagocytosis directs human monocyte activation in response to fungal derived β‐glucan particles

Recognition of the fungal cell wall carbohydrate β‐glucan by the host receptor Dectin‐1 elicits broad immunomodulatory responses, such as phagocytosis and activation of oxidative burst. These responses are essential for engulfing and killing fungal pathogens. Phagocytic monocytes are key mediators of these early host inflammatory responses to infection. Remarkably, whether phagocytosis of fungal β‐glucan leads to an inflammatory response in human monocytes remains to be established. Here, we show that phagocytosis of heat‐killed Candida albicans is essential to trigger inflammation and cytokine release. By contrast, inhibition of actin‐dependent phagocytosis of particulate (1‐3,1‐6)‐β‐glucan induces a strong inflammatory signature. Sustained monocyte activation, induced by fungal β‐glucan particles upon actin cytoskeleton disruption, relies on Dectin‐1 and results in the classical caspase‐1 inflammasome formation through NLRP3, generation of an oxidative burst, NF‐κB activation, and increased inflammatory cytokine release. PI3K and NADPH oxidase were crucial for both cytokine secretion and ROS generation, whereas Syk signaling mediated only cytokine production. Our results highlight the mechanism by which phagocytosis tightly controls the activation of phagocytes by fungal pathogens and strongly suggest that actin cytoskeleton dynamics are an essential determinant of the host's susceptibility or resistance to invasive fungal infections.     

IL‐33 citrine reporter mice reveal the temporal and spatial expression of IL‐33 during allergic lung inflammation

Interleukin‐33 (IL‐33) is an IL‐1 family cytokine that signals via its receptor T1/ST2, and is a key regulator of inflammation, notably the type‐2 response implicated in allergic asthma. Critical to our understanding of the role of IL‐33 is the identification of the cellular sources of IL‐33. Although progress has been made in this area, the development of a robust live cell reporter of expression would allow the localisation of IL‐33 during ongoing immune responses. We have generated a fluorescent reporter mouse line, Il33^Cit/+, to define the expression profile of IL‐33 in vivo and demonstrate its temporal and spatial expression during experimental allergic asthma responses. We found that type‐2 pneumocytes constitute the major source of IL‐33 upon allergic lung inflammation following exposure to OVA, fungal extract or ragweed pollen. Using Il33^Cit/Cit mice (IL‐33‐deficient), we establish a role for IL‐33 early in the initiation of type‐2 responses and the induction of nuocytes (ILC2). We also demonstrate a potential mechanism of action by which IL‐33 rapidly initiates type‐2 immune responses. Il33^Cit/+ mice have enabled new insights into the initiation of type‐2 responses and will provide an important tool for further dissection of this important inflammatory pathway in vivo.     

Cytosolic DNA sensing via the stimulator of interferon genes adaptor: Yin and Yang of immune responses to DNA

DNA is immunogenic and many cells express cytosolic DNA sensors that activate the stimulator of interferon genes (STING) adaptor to trigger interferon type I (IFN‐β) release, a potent immune activator. DNA sensing to induce IFN‐β triggers host immunity to pathogens but constitutive DNA sensing can induce sustained IFN‐β release that incites autoimmunity. Here, we focus on cytosolic DNA sensing via the STING/IFN‐β pathway that regulates immune responses. Recent studies reveal that cytosolic DNA sensing via the STING/IFN‐β pathway induces indoleamine 2,3 dioxygenase (IDO), which catabolizes tryptophan to suppress effector and helper T‐cell responses and activate Foxp3‐lineage CD4⁺ regulatory T (Treg) cells. During homeostasis, and in some inflammatory settings, specialized innate immune cells in the spleen and lymph nodes may ingest and sense cytosolic DNA to reinforce tolerance that prevents autoimmunity. However, malignancies and pathogens may exploit DNA‐induced regulatory responses to suppress natural and vaccine‐induced immunity to malignant and infected cells. In this review, we discuss the biologic significance of regulatory responses to DNA and novel approaches to exploit DNA‐induced immune responses for therapeutic benefit. The ability of DNA to drive tolerogenic or immunogenic responses highlights the need to evaluate immune responses to DNA in physiologic settings relevant to disease progression or therapy.     

SP‐D and regulation of the pulmonary innate immune system in allergic airway changes

The airway mucosal surfaces are constantly exposed to inhaled particles that can be potentially toxic, infectious or allergenic and should elicit inflammatory changes. The proximal and distal air spaces, however, are normally infection and inflammation free due to a specialized interplay between cellular and molecular components of the pulmonary innate immune system. Surfactant protein D (SP‐D) is an epithelial‐cell‐derived immune modulator that belongs to the small family of structurally related Ca²⁺‐dependent C‐type collagen‐like lectins. While collectins can be detected in mucosal surfaces of various organs, SP‐A and SP‐D (the ‘lung collectins’) are constitutively expressed in the lung at high concentrations. Both proteins are considered important players of the pulmonary immune responses. Under normal conditions however, SP‐A‐/‐ mice display no pathological features in the lung. SP‐D‐/‐ mice, on the other hand, show chronic inflammatory alterations indicating a special importance of this molecule in regulating immune homeostasis and the function of the innate immune cells. Recent studies in our laboratory and others implied significant associations between changes in SP‐D levels and the presence of airway inflammation both in animal models and patients raising a potential usefulness of this molecule as a disease biomarker. Research on wild‐type and mutant recombinant molecules in vivo and in vitro showed that SP‐D binds carbohydrates, lipids and nucleic acids with a broad spectrum specificity and initiates phagocytosis of inhaled pathogens as well as apoptotic cells. Investigations on gene‐deficient and conditional over expressor mice in addition, provided evidence that SP‐D directly modulates macrophage and dendritic cell function as well as T cell‐dependent inflammatory events. Thus, SP‐D has a unique, dual functional capacity to induce pathogen elimination on the one hand and control of pro‐inflammatory mechanisms on the other, suggesting a potential suitability for therapeutic prevention and treatment of chronic airway inflammation without compromising the host defence function of the airways. This paper will review recent findings on the mechanisms of immune‐protective function of SP‐D in the lung. Cite this as: L. R. Forbes and A. Haczku, Clinical & Experimental Allergy, 2010 (40) 547–562.     

TLR2‐independent induction and regulation of chronic intestinal inflammation

Interactions between the intestinal microflora and host innate immune receptors play a critical role in intestinal homeostasis. Several studies have shown that TLR2 can modulate inflammatory responses in the gut. TLR2 signals enhance tight junction formation and fortify the epithelial barrier, and may play a crucial role in driving acute inflammatory responses towards intestinal bacterial pathogens. In addition, TLR2 agonists can have direct effects on both Th1 cells and Treg. To define the role of TLR2 in the induction and regulation of chronic intestinal inflammation we examined the effects of TLR2 deletion on several complementary models of inflammatory bowel disease. Our results show that TLR2 signals are not required for the induction of chronic intestinal inflammation by either innate or adaptive immune responses. We further show that TLR2^?/? mice harbor normal numbers of Foxp3⁺ Treg that are able to suppress intestinal inflammation as effectively as their WT counterparts. We also did not find any intrinsic role for TLR2 for pathogenic effector T‐cell responses in the gut. Thus, in contrast to their role in acute intestinal inflammation and repair, TLR2 signals may have a limited impact on the induction and regulation of chronic intestinal inflammation.     

Regulatory T cells in human geohelminth infection suppress immune responses to BCG and Plasmodium falciparum

Chronic helminth infections induce T‐cell hyporesponsiveness, which may affect immune responses to other pathogens or to vaccines. This study investigates the influence of Treg activity on proliferation and cytokine responses to BCG and Plasmodium falciparum‐parasitized RBC in Indonesian schoolchildren. Geohelminth‐infected children's in vitro T‐cell proliferation to either BCG or pRBC was reduced compared to that of uninfected children. Although the frequency of CD4⁺CD25^hiFOXP3⁺ T cells was similar regardless of infection status, the suppressive activity differed between geohelminth‐infected and geohelminth‐uninfected groups: Ag‐specific proliferative responses increased upon CD4⁺CD25^hi T‐cell depletion in geohelminth‐infected subjects only. In addition, IFN‐γ production in response to both BCG and parasitized RBC was increased after removal of CD4⁺CD25^hi T cells. These data demonstrate that geohelminth‐associated Treg influence immune responses to bystander Ag of mycobacteria and plasmodia. Geohelminth‐induced immune modulation may have important consequences for co‐endemic infections and vaccine trials.     

Inhibition of murine γδ lymphocyte expansion and effector function by regulatory αβ T cells is cell‐contact‐dependent and sensitive to GITR modulation

γδ T cells are highly cytolytic lymphocytes that produce large amounts of pro‐inflammatory cytokines during immune responses to multiple pathogens. Furthermore, their ability to kill tumor cells has fueled the development of γδ‐T‐cell‐based cancer therapies. Thus, the regulation of γδ‐T‐cell activity is of great biological and clinical relevance. Here, we show that murine CD4⁺CD25⁺ αβ T cells, the vast majority of which express the Treg marker, Foxp3, abolish key effector functions of γδ T cells, namely the production of the pro‐inflammatory cytokines, IFN‐γ and IL‐17, cytotoxicity, and lymphocyte proliferation in vitro and in vivo. We further show that suppression is dependent on cellular contact between Treg and γδ T cells, results in the induction of an anergic state in γδ lymphocytes, and can be partially reversed by manipulating glucocorticoid‐induced TNF receptor‐related protein (GITR) signals. Our data collectively dissect a novel mechanism by which the expansion and pro‐inflammatory functions of γδ T cells are regulated.     

Increased ILC3s associated with higher levels of IL‐1β aggravates inflammatory arthritis in mice lacking phagocytic NADPH oxidase

The role of redox regulation in immune‐mediated arthritis has been previously described. However, the relationship between innate immune cells, including innate lymphoid cells (ILCs) and phagocyte‐derived ROS, in this process remains unclear. Here, we characterize ILCs and measure the IL‐1 family cytokines along with other cytokines relevant to ILC functions and development in serum‐induced arthritic joints in wild type and phagocytic NADPH oxidase (NOX2)‐deficient Ncf1^?/? mice. We found more severe serum‐induced joint inflammation and increased NCR⁺ ILC3s in inflamed joints of Ncf1^?/? mice. Furthermore, in vitro stimulation with IL‐1β on Tbet⁺ ILC1s from joints facilitated their differentiation into ROR‐γt⁺ ILC3s. Moreover, treatment with IL‐1 antagonists effectively lowered the proportions of NCR⁺ ILC3s and IL‐17A producing ILC3s in Ncf1^?/? arthritic mice and ameliorated the joint inflammation. These results suggest that NOX2 is an essential regulator of ILC transdifferentiation and may mediate this process in a redox‐dependent manner through IL‐1β production in the inflammatory joint. Our findings shed important light on the role of ILCs in the initiation and progression in tissue inflammation and delineate a novel innate immune cell‐mediated pathogenic mechanism through which redox regulation may determine the direction of immune responses in joints.     

Gut hormones: emerging role in immune activation and inflammation

Gut inflammation is characterized by mucosal recruitment of activated cells from both the innate and adaptive immune systems. In addition to immune cells, inflammation in the gut is associated with an alteration in enteric endocrine cells and various biologically active compounds produced by these cells. Although the change in enteric endocrine cells or their products is considered to be important in regulating gut physiology (motility and secretion), it is not clear whether the change plays any role in immune activation and in the regulation of gut inflammation. Due to the strategic location of enteric endocrine cells in gut mucosa, these gut hormones may play an important role in immune activation and promotion of inflammation in the gut. This review addresses the research on the interface between immune and endocrine systems in gastrointestinal (GI) pathophysiology, specifically in the context of two major products of enteric endocrine systems, namely serotonin (5‐hydroxytryptamine: 5‐HT) and chromogranins (Cgs), in relation to immune activation and generation of inflammation. The studies reviewed in this paper demonstrate that 5‐HT activates the immune cells to produce proinflammatory mediators and by manipulating the 5‐HT system it is possible to modulate gut inflammation. In the case of Cgs the scenario is more complex, as this hormone has been shown to play both proinflammatory and anti‐inflammatory functions. It is also possible that interaction between 5‐HT and Cgs may play a role in the modulation of immune and inflammatory responses. In addition to enhancing our understanding of immunoendocrine interaction in the gut, the data generated from the these studies may have implications in understanding the role of gut hormone in the pathogenesis of both GI and non‐GI inflammatory diseases which may lead ultimately to improved therapeutic strategies in inflammatory disorders.     

Epstein–Barr virus‐induced gene 3 suppresses T helper type 1, type 17 and type 2 immune responses after Trypanosoma cruzi infection and inhibits parasite replication by interfering with alternative macrophage activation

The Epstein–Barr virus‐induced gene 3 (EBI3) is a member of the interleukin‐12 (IL)‐12) family structurally related to the subunit p40 of IL‐12 and forms a heterodimer either with the p28 subunit to build IL‐27 or with p35 to form IL‐35. Interleukin‐27 is secreted by antigen‐presenting cells whereas IL‐35 appears to be produced mainly by regulatory T cells and regulatory B cells but both cytokines negatively regulate inflammatory immune responses. We here analysed the function of EBI3 during infection with the intracellular parasite Trypanosoma cruzi. Compared with C57BL/6 wild‐type mice, EBI3‐deficient (EBI3^?/?) mice showed a higher parasitaemia associated with an increased mortality rate. The EBI3^?/? mice displayed an elevated inflammatory immune response with an increased production of T helper type 1 (Th1‐), Th2‐ and Th17‐derived cytokines. The increased Th2 immune response appears to have over‐ridden the otherwise protective Th1 and Th17 immune responses by the induction of arginase‐1‐expressing alternatively activated macrophages in these mice. Hence, neutralization of IL‐4 and arginase‐1 activity partially restored protective immune responses in EBI3^?/? mice. So far, our results demonstrate that EBI3 is an essential general regulator of inflammatory immune responses in experimental Chagas disease and is required for control of T. cruzi infection by inhibiting Th2‐dependent alternative macrophage activation. Further studies are needed to dissect the underlying mechanisms and clarify whether EBI3 association with IL‐27 or/and IL‐35 accounts for its anti‐inflammatory character in parasitic disease.