Nandini Krishnamoorthy

Activation of c-Kit in dendritic cells regulates T helper cell differentiation and allergic asthma.

Dendritic cells (DCs) are integral to the differentiation of T helper cells into T helper type 1 TH1, TH2 and TH17 subsets. Interleukin-6 (IL-6) plays an important part in regulating these three arms of the immune response by limiting the TH1 response and promoting the TH2 and TH17 responses. In this study, we investigated pathways in DCs that promote IL-6 production. We show that the allergen house dust mite (HDM) or the mucosal adjuvant cholera toxin promotes cell surface expression of c-Kit and its ligand, stem cell factor (SCF), on DCs. This dual upregulation of c-Kit and SCF results in sustained signaling downstream of c-Kit, promoting IL-6 secretion. Intranasal administration of antigen into c-Kit–mutant mice or neutralization of IL-6 in cultures established from the lung-draining lymph nodes of immunized wild-type mice blunted the TH2 and TH17 responses. DCs lacking functional c-Kit or those unable to express membrane-bound SCF secreted lower amounts of IL-6 in response to HDM or cholera toxin. DCs expressing nonfunctional c-Kit were unable to induce a robust TH2 or TH17 response and elicited diminished allergic airway inflammation when adoptively transferred into mice. Expression of the Notch ligand Jagged-2, which has been associated with TH2 differentiation, was blunted in DCs from c-Kit–mutant mice. c-Kit upregulation was specifically induced by TH2- and TH17-skewing stimuli, as the TH1-inducing adjuvant, CpG oligodeoxynucleotide, did not promote either c-Kit or Jagged-2 expression. DCs generated from mice expressing a catalytically inactive form of the p110δ subunit of phosphatidylinositol-3 (PI3) kinase (p110D910A) secreted lower amounts of IL-6 upon stimulation with cholera toxin. Collectively, these results highlight the importance of the c-Kit–PI3 kinase–IL-6 signaling axis in DCs in regulating T cell responses.

Early infection with respiratory syncytial virus impairs regulatory T cell function and increases susceptibility to allergic asthma.

Immune tolerance is instituted early in life, during which time regulatory T (Treg) cells have an important role. Recurrent infections with respiratory syncytial virus (RSV) in early life increase the risk for asthma in adult life. Repeated infection of infant mice tolerized to ovalbumin (OVA) through their mothers milk with RSV induced allergic airway disease in response to OVA sensitization and challenge, including airway inflammation, hyper-reactivity and higher OVA-specific IgE, as compared to uninfected tolerized control mice. Virus infection induced GATA-3 expression and T helper type 2 (TH2) cytokine production in forkhead box P3 (FOXP3)+ Treg cells and compromised the suppressive function of pulmonary Treg cells in a manner that was dependent on interleukin-4 receptor α (IL-4Rα) expression in the host. Thus, by promoting a TH2-type inflammatory response in the lung, RSV induced a TH2-like effector phenotype in Treg cells and attenuated tolerance to an unrelated antigen (allergen). Our findings highlight a mechanism by which viral infection targets a host-protective mechanism in early life and increases susceptibility to allergic disease.

Regulatory T Cells in Many Flavors Control Asthma

That regulatory T cells (Tregs) have a crucial role in controlling allergic diseases such as asthma is now undisputed. The cytokines most commonly implicated in Treg-mediated suppression of allergic asthma are transforming growth factor-β (TGF-β) and interleukin (IL)-10). In addition to naturally occurring Tregs, adaptive Tregs, induced in response to foreign antigens, have been shown in recent studies. The concept of inducible/adaptive Tregs (iTregs) has considerable significance in preventing asthma if generated early enough in life. This is because cytokines such as IL-4 and IL-6 inhibit Foxp3 induction in naive CD4+ T cells and therefore de novo generation of Tregs can be expected to be less efficient when it is concomitant with effector cell development in response to an allergen. However, if iTregs can be induced, the process of infectious tolerance would facilitate expansion of the iTreg pool as suggested in the recent literature. It is tempting to speculate that there is a window of opportunity in early life in the context of a relatively immature immune system that is permissive for the generation of iTregs specific to a spectrum of allergens that would regulate asthma for lifelong. The focus of this review is the relevance of nTregs and iTregs in controlling asthma from early life into adulthood, the mechanisms underlying Treg function, and the prospects for using our current concepts to harness the full potential of Tregs to limit disease development and progression.

TLR4/MyD88-induced CD11b + Gr-1 int F4/80 + non-migratory myeloid cells suppress Th2 effector function in the lung

In humans, environmental exposure to a high dose of lipopolysaccharide (LPS) protects from allergic asthma, the immunological underpinnings of which are not well understood. In mice, exposure to a high LPS dose blunted house dust mite-induced airway eosinophilia and T-helper 2 (Th2) cytokine production. Although adoptively transferred Th2 cells induced allergic airway inflammation in control mice, they were unable to do so in LPS-exposed mice. LPS promoted the development of a CD11b+Gr1intF4/80+ lung-resident cell resembling myeloid-derived suppressor cells in a Toll-like receptor 4 and myeloid differentiation factor 88 (MyD88)-dependent manner that suppressed lung dendritic cell (DC)-mediated reactivation of primed Th2 cells. LPS effects switched from suppressive to stimulatory in MyD88−/− mice. Suppression of Th2 effector function was reversed by anti-interleukin-10 (IL-10) or inhibition of arginase 1. Lineageneg bone marrow progenitor cells could be induced by LPS to develop into CD11b+Gr1intF4/80+cells both in vivo and in vitro that when adoptively transferred suppressed allergen-induced airway inflammation in recipient mice. These data suggest that CD11b+Gr1intF4/80+ cells contribute to the protective effects of LPS in allergic asthma by tempering Th2 effector function in the tissue.

Cutting Edge: Maresin-1 Engages Regulatory T Cells To Limit Type 2 Innate Lymphoid Cell Activation and Promote Resolution of Lung Inflammation

Asthma is a chronic inflammatory disease that fails to resolve. Recently, a key role for type 2 innate lymphoid cells (ILC2s) was linked to asthma pathogenesis; however, mechanisms for ILC2 regulation remain to be determined. In this study, metabololipidomics of murine lungs identified temporal changes in endogenous maresin 1 (MaR1) during self-limited allergic inflammation. Exogenous MaR1 reduced lung inflammation and ILC2 expression of IL-5 and IL-13 and increased amphiregulin. MaR1 augmented de novo generation of regulatory T cells (Tregs), which interacted with ILC2s to markedly suppress cytokine production in a TGF-β–dependent manner. Ab-mediated depletion of Tregs interrupted MaR1 control of ILC2 expression of IL-13 in vivo. Together, the findings uncover Tregs as potent regulators of ILC2 activation; MaR1 targets Tregs and ILC2s to restrain allergic lung inflammation, suggesting MaR1 as the basis for a new proresolving therapeutic approach to asthma and other chronic inflammatory diseases.

Cutting Edge: Inhaled Antigen Upregulates Retinaldehyde Dehydrogenase in Lung CD103 + but Not Plasmacytoid Dendritic Cells To Induce Foxp3 De Novo in CD4 + T Cells and Promote Airway Tolerance

Dendritic cell (DC)–T cell interactions that underlie inducible/adaptive regulatory T cell generation and airway tolerance are not well understood. In this study, we show that mice lacking CD11chi lung DCs, but containing plasmacytoid DCs (pDCs), fail tolerization with inhaled Ag and cannot support Foxp3 induction in vivo in naive CD4+ T cells. CD103+ DCs from tolerized mice efficiently induced Foxp3 in cocultured naive CD4+ T cells but pDCs and lung macrophages failed to do so. CD103+ DCs, but not pDCs or lung macrophages, upregulated the expression of retinaldehyde dehydrogenase 2 (aldh1a2), which is key for the production of retinoic acid, a cofactor for TGF-β for Foxp3 induction. Batf3−/− mice, selectively lacking CD103+ DCs, failed tolerization by inhaled Ag. Collectively, our data show that pulmonary tolerance is dependent on CD103+ DCs, correlating with their ability to upregulate aldh1a2, which can promote Foxp3 expression in T cells.

Maresin 1 biosynthesis during platelet–neutrophil interactions is organ-protective

Significance Neutrophil accumulation is fundamental to acute inflammation. In response to tissue injury, circulating neutrophil–platelet aggregates (N-PAs) form for secondary capture. Counterregulation of acute inflammatory processes by specialized proresolving mediators is essential to mitigate collateral injury to healthy bystander tissue. Here, we identified a biosynthetic route in human platelets for the proresolving mediator maresin 1 (MaR1) that is amplified during platelet–neutrophil interactions. In a self-resolving murine model of acute lung injury, N-PAs rapidly formed and a MaR1 counterregulatory circuit was engaged to restrain N-PAs and acute inflammation and restore homeostasis of the injured lung. Unregulated acute inflammation can lead to collateral tissue injury in vital organs, such as the lung during the acute respiratory distress syndrome. In response to tissue injury, circulating platelet–neutrophil aggregates form to augment neutrophil tissue entry. These early cellular events in acute inflammation are pivotal to timely resolution by mechanisms that remain to be elucidated. Here, we identified a previously undescribed biosynthetic route during human platelet–neutrophil interactions for the proresolving mediator maresin 1 (MaR1; 7R,14S-dihydroxy-docosa-4Z,8E,10E,12Z,16Z,19Z-hexaenoic acid). Docosahexaenoic acid was converted by platelet 12-lipoxygenase to 13S,14S-epoxy-maresin, which was further transformed by neutrophils to MaR1. In a murine model of acute respiratory distress syndrome, lipid mediator metabololipidomics uncovered MaR1 generation in vivo in a temporally regulated manner. Early MaR1 production was dependent on platelet–neutrophil interactions, and intravascular MaR1 was organ-protective, leading to decreased lung neutrophils, edema, tissue hypoxia, and prophlogistic mediators. Together, these findings identify a transcellular route for intravascular maresin 1 biosynthesis via platelet–neutrophil interactions that regulates the extent of lung inflammation.

Emerging functions of c-kit and its ligand stem cell factor in dendritic cells : Regulators of T cell differentiation

The receptor tyrosine kinase, c-kit, and its ligand, stem cell factor (SCF), function in a diverse range of biological functions. The role of c-kit in the maintenance and survival of hematopoietic stem cells and of mast cells is well recognized. c-kit also plays an important role in melanogenesis, erythropoiesis and spermatogenesis. Recent work from our laboratory highlights an important role of c-kit in the regulation of expression of two molecules in dendritic cells (DCs), interleukin-6 (IL-6) and Jagged-2 (a ligand of Notch), which are known to regulate T helper cell differentiation. Our study shows that induction of c-kit expression and its signaling in DCs promotes Th2 and Th17 responses but not Th1 response. c-kit inhibition by imatinib mesylate (Gleevec) in DCs was previously shown to promote natural killer cell activation which may be due to dampening of IL-6 production by the DCs. Since dysregulation of c-kit function has been associated with various disease states including cancer, in this perspective we have focused on known and novel functions of c-kit to include molecules such as IL-6 and Notch that were not previously recognized to be within the purview of c-kit biology. We have also reviewed the differential expression pattern of SCF and c-kit on various cell types and its avriation during development or pathology. The recognition of previously unappreciated roles for c-kit will provide better insights into its function within and beyond the immune system and pave the way for developing better therapeutic strategies.

Distinct Responses of Lung and Spleen Dendritic Cells to the TLR9 Agonist CpG Oligodeoxynucleotide

Dendritic cells (DCs) sense various components of invading pathogens via pattern recognition receptors such as TLRs. CpG oligodeoxynucleotides (ODNs), which mimic bacterial DNA, inhibit allergic airways disease and promote responses in the spleen to bacterial components. Because many TLR agonists are currently being tested for potential therapeutic effects, it is important to characterize the expression and function of TLRs in different tissues. We show that both myeloid and plasmacytoid DCs in the spleen express TLR9, the receptor for CpG ODNs, but lung DCs show no detectable expression in either subset. TLR4 expression in contrast was detected on both lung and spleen DCs. LPS was superior to CpG ODN in increasing the allostimulatory potential of lung DCs and their expression of CD40. However, both agonists efficiently stimulated spleen DCs. CpG ODNs administered to mice efficiently inhibited Th2 cytokine production both in the lung draining lymph node and in the spleen. Surprisingly, inhibition of Th2 cytokine production was evident despite high levels of expression of GATA-3 and additional transcription factors that regulate Th2 responses. Although in the spleen CpG ODNs induced IL-6, a key cytokine induced via TLR9-MyD88 signaling, no IL-6 was detectable in lung LN cells. These studies show for the first time that lung DCs lack TLR9 expression, but, despite this deficiency, CpG ODNs induce potent inhibitory effects on Th2 cytokine production in the lung without inducing expression of the proinflammatory cytokine, IL-6, which has been linked to chronic diseases in the lung and the gut.

Signaling of c-kit in dendritic cells influences adaptive immunity

The binding of the receptor tyrosine kinase, c‐kit, to its ligand, stem cell factor (SCF), mediates numerous biological functions. Important roles for c‐kit in hematopoiesis, melanogenesis, erythropoiesis, spermatogenesis, and carcinogenesis are well documented. Similarly, activation of granulocytes, mast cells, and of eosinophils in particular, by c‐kit ligation has long been known to result in degranulation with concomitant release of pro‐inflammatory mediators, including cytokines. However, recent work from a number of laboratories, including our own, highlights previously unappreciated functions for c‐kit in immunologic processes. These novel findings strongly suggest that signaling through the c‐kit–SCF axis could have a significant impact on the pathogenesis of diseases associated with an immunologic component. In our own studies, c‐kit upregulation on dendritic cells via T helper (Th)2‐ and Th17‐inducing stimuli led to c‐kit activation and immune skewing toward these T helper subsets and away from Th1 responses. Others have shown that dendritic cell treatment with inhibitors of c‐kit activation, such as imatinib mesylate (Gleevec), favored breaking of T‐cell tolerance, skewing of responses toward production of Th1 cytokines, and activation of natural killer cells. These data all indicate that deeper understanding of, and ability to control, the c‐kit–SCF axis could lead to improved treatment modalities aimed at redirecting unwanted and/or deleterious immune responses in a wide variety of conditions.