Immunology

Effector memory T helper 2 (Th2) cells that accumulate in target organs (i.e. skin or bronchial mucosa) have a central role in the pathogenesis of allergic disorders. To date, the factors that selectively trigger local production of Th2‐attracting chemokines remain poorly understood. In mucosa, at the sites of allergen entry, immature dendritic cells (DC) are in close contact with mast cells. Histamine and prostaglandin E₂ (PGE₂) are two mediators released by allergen‐activated mast cells that favour the polarization of maturing DC into Th2‐polarizing cells. We analysed here the effects of histamine and PGE₂ on the prototypic, Th2‐(CCL17, CCL22) versus Th1‐(CXCL10) chemokine production by human DC. We report that histamine and PGE₂ dose‐dependently up‐regulate CCL17 and CCL22 by monocyte‐derived immature DC. These effects were potentiated by tumour necrosis factor‐α, still observed in the presence of the Th1‐cytokine interferon‐γ (IFN‐γ) and abolished by the immunomodulatory cytokine interleukin‐10. In addition, histamine and PGE₂ down‐regulated IFN‐γ‐induced CXCL10 production by monocyte‐derived DC. These properties of histamine and PGE₂ were observed at the transcriptional level and were mediated mainly through H2 receptors for histamine and through EP2 and EP4 receptors for PGE₂. Finally, histamine and PGE₂ also up‐regulated CCL17 and CCL22 and decreased IFN‐γ‐induced CXCL10 production by purified human myeloid DC. In conclusion, these data show that, in addition to polarizing DC into mature cells that promote naïve T‐cell differentiation into Th2 cells, histamine and PGE₂ may act on immature DC to trigger local Th2 cell recruitment through a selective control of Th1/Th2‐attracting chemokine production, thereby contributing to maintain a microenvironment favourable to persistent immunoglobulin E synthesis.

Toll‐like receptors (TLRs) signal through two main pathways: a myeloid differentiation factor (MyD)88‐dependent pathway that acts via nuclear factor κB (NF‐κB) to induce proinflammatory cytokines such as tumour necrosis factor‐α (TNF‐α) and a MyD88‐independent pathway that acts via type I interferons to increase the expression of interferon‐inducible genes. Repeated signalling through TLR4 and a number of other TLRs has been reported to result in a reduction in the subsequent proinflammatory cytokine response, a phenomenon known as TLR tolerance. In this study we have shown that, whilst NF‐κB activation and production of TNF‐α and interleukin‐12 by murine RAW264.7 and J774.2 cells in response to stimulation by TLR4, ‐5, ‐7 or ‐9, was reduced by prior stimulation with TLR4, ‐5, ‐7 or ‐9 ligands, the primary stimulation of TLR3, which does not use the MyD88 pathway, did not reduce the TNF‐α or interleukin‐12 responses to subsequent TLR stimulation. The response to TLR3 stimulation was not diminished by prior TLR ligand exposure. Furthermore, the production of interferon‐β (IFN‐β) following stimulation of TLR3 or ‐4, which is MyD88‐independent, was increased by prior activation of TLR4, ‐5, ‐7 or ‐9. In contrast, TLR9 ligand‐induced IFN‐β production, which is MyD88‐dependent, was tolerized by prior TLR stimulation. These results are consistent with differential regulation of MyD88‐dependent and MyD88‐independent cytokine production following serial activation of TLRs.

Recently air pollutants and irritants have been labelled as possible exogenous risk factors for allergic disorders. Although the underlying causes of allergic disorders such as atopic dermatitis and asthma remain unclear, the T helper type 2 (Th2) cell‐mediated allergic inflammatory cascade may contribute to their pathogenesis. In the last decade, it has been documented that one of the candidates for triggering Th2 commitment is thymic stromal lymphopoietin (TSLP), the expression of which is up‐regulated in the lesions of allergic patients. Here, we describe TSLP function in a fluorescein isothiocyanate (FITC) ‐induced contact hypersensitivity (CHS) model. A cytokine profile indicated that the model was dominantly mediated by the Th2 milieu. Interestingly, TSLP was increased in the skin during the sensitization phase when stimulated by a solvent, dibutyl phthalate (DBP), but not by FITC hapten or another solvent, acetone. Ear swelling in FITC‐induced CHS was totally abrogated by removing DBP from the sensitization or elicitation phase, and was restored by complementary injection of TSLP. Inversely, the ear swelling was suppressed by injection of small interfering RNA against TSLP during the sensitization phase, which was concomitant with decreasing expression of interleukin‐4 at the swollen skin site. Taken together, DBP‐induced TSLP during the sensitization phase plays a role in establishing FITC‐induced CHS and may be one of the causes of Th2 commitment in the model, suggesting that certain environmental toxins, such as DBP, may endow pro‐allergic and atopic predisposition in humans or animals.

In recent years, nitric oxide (NO), a gas previously considered to be a potentially toxic chemical, has been established as a diffusible universal messenger that mediates cell–cell communication throughout the body. Constitutive and inducible NO production regulate numerous essential functions of the gastrointestinal mucosa, such as maintenance of adequate perfusion, regulation of microvascular and epithelial permeability, and regulation of the immune response. Up‐regulation of the production of NO via expression of inducible nitric oxide synthase (iNOS) represents part of a prompt intestinal antibacterial response; however, NO has also been associated with the initiation and maintenance of inflammation in human inflammatory bowel disease (IBD). Recent studies on animal models of experimental IBD have shown that constitutive and inducible NO production seems to be beneficial during acute colitis, but sustained up‐regulation of NO is detrimental. This fact is also supported by studies on mice genetically deficient in various NOS isoforms. However, the mechanism by which NO proceeds from being an indispensable homeostatic regulator to a harmful destructor remains unknown. Furthermore, extrapolation of data from animal colitis models to human IBD is questionable. The purpose of this review is to update our knowledge about the role of this universal mediator and the enzymes that generate it in the pathogenesis of IBD.

The colonic epithelium provides an interface between the host and micro‐organisms colonising the gastrointestinal tract. Molecular recognition of bacteria is facilitated through Toll‐like receptors (TLR). The colonic epithelium expresses relatively high levels of mRNA for TLR3 and less for TLR2 and ‐4. Little is known of the expression patterns and mode of induction of expression for these pattern recognition receptors in human colon. The aim of this study was to investigate their localization in the gut and induction of expression in epithelial cell lines by mucosal bacteria. TLR2 and ‐4 were expressed only in crypt epithelial cells, expression was lost as the cells matured and moved towards the gut lumen. In contrast, TLR3 was only produced in mature epithelial cells. HT29 and CACO‐2 had different levels of expression for TLR1–4. Co‐culture of HT29 cells with different mucosal isolates showed that they were highly responsive to bacterial challenge, with up‐regulation of mRNA for TLR1–4. In contrast, CACO‐2 cells were refractive to bacterial challenge, showing little difference in mRNA levels. TLR3 was induced in HT29 only by Gram‐positive commensals with up‐regulation of both mRNA and protein and an enhancement of the antiviral immune response. This pattern of expression allows induction of responsiveness to bacteria only by the crypt epithelium so that tolerance to commensal organisms can be maintained. In contrast, mature columnar epithelium is able to respond to viral pathogens, which are not part of the normal gut commensal microbiota.

Viral infections frequently induce acute and chronic inflammatory diseases, yet the contribution of the innate immune response to a detrimental host response remains poorly understood. In virus‐infected cells, double‐stranded RNA (dsRNA) is generated as an intermediate during viral replication. Cell necrosis (and the release of endogenous dsRNA) is a common event during both sterile and infectious inflammatory processes. The discovery of Toll‐like receptor 3 (TLR3) as an interferon‐inducing dsRNA sensor led to the assumption that TLR3 was the master sentinel against viral infections. This simplistic view has been challenged by the discovery of at least three members of the DExd/H‐box helicase cytosolic sensors of dsRNA that share with TLR3 the Toll–interleukin‐1 receptor (TIR) ‐adapter molecule TIR domain‐containing adaptor protein interferon‐β (TRIF) for downstream type I interferon signalling. Data are conflicting on the role of TLR3 in protective immunity against viruses in the mouse model. Varying susceptibility to infection and disease outcomes have been reported in TLR3‐immunodeficient mice. Surprisingly, the susceptibility to develop herpes simplex virus‐1 encephalitis in humans with inborn defects of the TLR3 pathway varies, and TLR3‐deficient humans do not show increased susceptibility to other viral infections. Therefore, a current challenge is to understand the protective versus pathogenic contribution of TLR3 in viral infections. We review recent advances in the identification of TLR3‐signalling pathways, endogenous and virus‐induced negative regulators of the TLR3 cascade, and discuss the protective versus pathogenic role of TLR3 in viral pathogenesis.

Nitric oxide produced by the inducible enzyme, nitric oxide synthase (iNOS), is implicated in immunological and inflammatory processes. We determined the effects of T‐helper (Th)2‐derived cytokines on the induction of iNOS from an epithelial A549 cell line and human airway epithelial cells stimulated by a mixture of interleukin‐1β (IL‐1β), interferon‐γ (IFN‐γ) and tumour necrosis factor‐α (TNF‐α). Interleukin‐4 (IL‐4) and interleukin‐13 (IL‐13) but not interleukin‐10 (IL‐10) inhibited both iNOS mRNA expression and nitrite release in A549 cells. On human airway epithelial cells, IL‐4 and IL‐13 reduced iNOS mRNA expression. Dexamethasone also inhibited both iNOS expression and nitrite release. Th2 cytokines, IL‐4 and IL‐13, inhibit iNOS upregulation by Th1 cytokines, indicating an important reciprocal role of Th1 and Th2 T‐cell subsets on lung epithelial cells.

The lectin pathway of complement is activated upon binding of mannan‐binding lectin (MBL) or ficolins (FCNs) to their targets. Upon recognition of targets, the MBL‐and FCN‐associated serine proteases (MASPs) are activated, allowing them to generate the C3 convertase C4b2a. Recent findings indicate that the MASPs also activate components of the coagulation system. We have previously shown that MASP‐1 has thrombin‐like activity whereby it cleaves and activates fibrinogen and factor XIII. MASP‐2 has factor Xa‐like activity and activates prothrombin through cleavage to form thrombin. We now report that purified L‐FCN‐MASPs complexes, bound from serum to N‐acetylcysteine‐Sepharose, or MBL‐MASPs complexes, bound to mannan‐agarose, generate clots when incubated with calcified plasma or purified fibrinogen and factor XIII. Plasmin digestion of the clot and analysis using anti‐D‐dimer antibodies revealed that the clot was made up of fibrin and was similar to that generated by thrombin in normal human plasma. Fibrinopeptides A and B (FPA and FPB, respectively) were released after fibrinogen cleavage by L‐FCN‐MASPs complexes captured on N‐acetylcysteine‐Sepharose. Studies of inhibition of fibrinopeptide release indicated that the dominant pathway for clotting catalysed by the MASPs is via MASP‐2 and prothrombin activation, as hirudin, a thrombin inhibitor that does not inhibit MASP‐1 and MASP‐2, substantially inhibits fibrinopeptide release. In the light of their potent chemoattractant effects on neutrophil and fibroblast recruitment, the MASP‐mediated release of FPA and FPB may play a role in early immune activation. Additionally, MASP‐catalysed deposition and polymerization of fibrin on the surface of micro‐organisms may be protective by limiting the dissemination of infection.

Since the first World Health Organization notification on 31 December 2019, coronavirus disease 2019 (COVID‐19), the respiratory disease caused by the coronavirus severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2), has been responsible for over four million confirmed infections and almost 300 000 deaths worldwide. The pandemic has led to over half of the world's population living under lockdown conditions. To allow normal life to resume, public health interventions will be needed to prevent further waves of infections as lockdown measures are lifted. As one of the most effective countermeasures against infectious diseases, an efficacious vaccine is considered crucial to containing the COVID‐19 pandemic. Following the publication of the genome sequence of SARS‐CoV‐2, vaccine development has accelerated at an unprecedented pace across the world. Here we review the different platforms employed to develop vaccines, the standard timelines of development and how they can be condensed in a pandemic situation. We focus on vaccine development in the UK and vaccines that have entered clinical trials around the world.

Amongst the families of intracellular molecules that chaperone and assist with the trafficking of other proteins, notably during conditions of cellular stress, heat shock protein (hsp) 70 is one of the most studied. Although its name suggests that expression is exclusively induced during cellular hyperthermia, members of the hsp70 family of proteins can be constitutively expressed and/or induced by a range of other cellular insults. The ubiquitous presence of hsp70 in eukaryotic and prokaryotic cells, combined with its high degree of sequence homology and intrinsic immunogenicity, have prompted the suggestion that inappropriate immune reactivity to hsp70 might lead to pro‐inflammatory responses and the development of autoimmune disease. Indeed, hsp70 has been shown to be a potent activator of innate immunity and aberrant expression of hsp70 in certain organs promotes immunopathology. However, studies also suggest that hsp70 might have immunotherapeutic potential, as hsp70 purified from malignant and virally infected cells can transfer and deliver antigenic peptides to antigen‐presenting cells to elicit peptide‐specific immunity and, in contrast to its reported pro‐inflammatory effects, the administration of recombinant hsp70 can attenuate experimental autoimmune disease. This review focuses on the immunoregulatory capacity of hsp70 and its potential therapeutic value.