Early IL-6 signalling promotes IL-27 dependent maturation of regulatory T cells in the lungs and resolution of viral immunopathology.

Interleukin-6 is a pleiotropic, pro-inflammatory cytokine that can promote both innate and adaptive immune responses. In humans with respiratory virus infections, such as Respiratory Syncytial Virus (RSV), elevated concentrations of IL-6 are associated with more severe disease. In contrast the polymorphisms in the Il6 promoter which favour lower IL-6 production are associated with increased risk of both RSV and Rhinovirus infections. To determine the precise contribution of IL-6 to protection and pathology we used murine models of respiratory virus infection. RSV infection resulted in increased IL-6 production both in the airways and systemically which remained heightened for at least 2 weeks. IL-6 depletion early, but not late, during RSV or Influenza A virus infection resulted in significantly increased disease associated with an influx of virus specific TH1 and cytotoxic CD8+ T cells, whilst not affecting viral clearance. IL-6 acted by driving production of the immunoregulatory cytokine IL-27 by macrophages and monocytes, which in turn promoted the local maturation of regulatory T cells. Concordantly IL-27 was necessary to regulate TH1 responses in the lungs, and sufficient to limit RSV induced disease. Overall we found that during respiratory virus infection the prototypic inflammatory cytokine IL-6 is a critical anti-inflammatory regulator of viral induced immunopathology in the respiratory tract through its induction of IL-27.

Targeting the ICOS/ICOS-L pathway in a mouse model of established allergic asthma disrupts T follicular helper cell responses and ameliorates disease.

BACKGROUND: Allergic asthma is characterized by chronic inflammation and remodelling of the airways, associated with dysregulated type 2 immune responses and allergen-specific IgE. T follicular helper cells (TFH ) are crucial in T-dependent B-cell responses and have been implicated in allergic airway disease (AAD). TFH , unlike other CD4+ T cells, are uniquely reliant on continuous ICOS signalling to maintain their phenotype after T-cell priming; therefore, disrupting this signal can impair TFH responses. However, the contribution of TFH to disease during chronic aero-allergen exposure and the therapeutic potential of targeting these cells have not been evaluated. METHODS: To establish AAD, female BALB/c mice were repeatedly exposed to house dust mite or Alternaria alternata three times a week for up to 5 weeks. To examine the impact of TFH on AAD, mice were allergen exposed for 5 weeks and co-administered anti-ICOS Ligand-targeted antibodies, three times a week for the last 2 weeks. RESULTS: TFH were first observed in the lung-draining lymph nodes and with further exposure were also found locally within the lungs. TFH accumulated with sustained allergen exposure, alongside germinal centre (GC) B cells. Blockade of ICOS signalling after AAD establishment successfully depleted TFH but did not affect the differentiation of other CD4+ T-cell subsets. This reduced GC responses, allergen-specific IgE, inflammation, pulmonary IL-13 and airway hyper-responsiveness. CONCLUSIONS: TFH are crucial in the regulation of AAD and the ICOS/ICOS-L pathway could represent a novel therapeutic target in allergic asthma.

Neutrophils drive alveolar macrophage IL-1ß release during respiratory viral infection.

BACKGROUND: Alveolar macrophages are sentinels of the airways that must exhibit immune restraint to innocuous antigens but elicit a robust inflammatory response to pathogenic threats. How distinction between these dichotomous functions is controlled is poorly defined.Neutrophils are the first responders to infection, and we hypothesised that they may free alveolar macrophages from their hyporesponsive state, promoting their activation. Activation of the inflammasome and interleukin (IL)-1ß release is a key early inflammatory event that must be tightly regulated. Thus, the role of neutrophils in defining inflammasome activation in the alveolar macrophage was assessed. METHODS: Mice were infected with the X31 strain of influenza virus and the role of neutrophils in alveolar macrophage activation established through administration of a neutrophil-depleting (1A8) antibody. RESULTS: Influenza elicited a robust IL-1ß release that correlated (r=0.6849; p<0.001) with neutrophil infiltrate and was ablated by neutrophil depletion. Alveolar macrophages were shown to be the prominent source of IL-1ß during influenza infection, and virus triggered the expression of Nod-like receptor protein 3 (NLRP3) inflammasome and pro-IL-1ß in these cells. However, subsequent activation of the inflammasome complex and release of mature IL-1ß from alveolar macrophages were critically dependent on the provision of a secondary signal, in the form of antimicrobial peptide mCRAMP, from infiltrating neutrophils. CONCLUSIONS: Neutrophils are critical for the activation of the NLRP3 inflammasome in alveolar macrophages during respiratory viral infection. Accordingly, we rationalise that neutrophils are recruited to the lung to confront a viable pathogenic threat and subsequently commit alveolar macrophages to a pro-inflammatory phenotype to combat infection.

Enhanced IL-2 in early life limits the development of TFH and protective antiviral immunity.

T follicular helper cell (TFH)-dependent antibody responses are critical for long-term immunity. Antibody responses are diminished in early life, limiting long-term protective immunity and allowing prolonged or recurrent infection, which may be important for viral lung infections that are highly prevalent in infancy. In a murine model using respiratory syncytial virus (RSV), we show that TFH and the high-affinity antibody production they promote are vital for preventing disease on RSV reinfection. Following a secondary RSV infection, TFH-deficient mice had significantly exacerbated disease characterized by delayed viral clearance, increased weight loss, and immunopathology. TFH generation in early life was compromised by heightened IL-2 and STAT5 signaling in differentiating naive T cells. Neutralization of IL-2 during early-life RSV infection resulted in a TFH-dependent increase in antibody-mediated immunity and was sufficient to limit disease severity upon reinfection. These data demonstrate the importance of TFH in protection against recurrent RSV infection and highlight a mechanism by which this is suppressed in early life.

Abnormal pro-gly-pro pathway and airway neutrophilia in pediatric cystic fibrosis.

BACKGROUND: Proline-glycine-proline (PGP) is a bioactive fragment of collagen generated by the action of matrix metalloproteinase-9 (MMP-9) and prolylendopeptidase (PE), and capable of eliciting neutrophil chemotaxis and epithelial remodelling. PGP is normally then degraded by leukotriene A4 hydrolase (LTA4H) to limit inflammation and remodelling. This study hypothesized that early and persistent airway neutrophilia in Cystic Fibrosis (CF) may relate to abnormalities in the PGP pathway and sought to understand underlying mechanisms. METHODS: Broncho-alveolar lavage (BAL) fluid was obtained from 38 CF (9 newborns and 29 older children) and 24 non-CF children. BAL cell differentials and levels of PGP, MMP-9, PE and LTA4H were assessed. RESULTS: Whilst PGP was present in all but one of the older CF children tested, it was absent in non-CF controls and the vast majority of CF newborns. BAL levels of MMP-9 and PE were elevated in older children with CF relative to CF newborns and non-CF controls, correlating with airway neutrophilia and supportive of PGP generation. Furthermore, despite extracellular LTA4H commonly being greatly elevated concomitantly with inflammation to promote PGP degradation, this was not the case in CF children, potentially owing to degradation by neutrophil elastase. CONCLUSIONS: A striking imbalance between PGP-generating and -degrading enzymes enables PGP accumulation in CF children from early life and potentially supports airway neutrophilia.

Neutrophils restrain allergic airway inflammation by limiting ILC2 function and monocyte-dendritic cell antigen presentation.

Neutrophil mobilization, recruitment, and clearance must be tightly regulated as overexuberant neutrophilic inflammation is implicated in the pathology of chronic diseases, including asthma. Efforts to target neutrophils therapeutically have failed to consider their pleiotropic functions and the implications of disrupting fundamental regulatory pathways that govern their turnover during homeostasis and inflammation. Using the house dust mite (HDM) model of allergic airway disease, we demonstrate that neutrophil depletion unexpectedly resulted in exacerbated T helper 2 (TH2) inflammation, epithelial remodeling, and airway resistance. Mechanistically, this was attributable to a marked increase in systemic granulocyte colony-stimulating factor (G-CSF) concentrations, which are ordinarily negatively regulated in the periphery by transmigrated lung neutrophils. Intriguingly, we found that increased G-CSF augmented allergic sensitization in HDM-exposed animals by directly acting on airway type 2 innate lymphoid cells (ILC2s) to elicit cytokine production. Moreover, increased systemic G-CSF promoted expansion of bone marrow monocyte progenitor populations, which resulted in enhanced antigen presentation by an augmented peripheral monocyte-derived dendritic cell pool. By modeling the effects of neutrophil depletion, our studies have uncovered previously unappreciated roles for G-CSF in modulating ILC2 function and antigen presentation. More broadly, they highlight an unexpected regulatory role for neutrophils in limiting TH2 allergic airway inflammation.