The gene regulatory basis of bystander activation in CD8+ T cells

CD8+ T cells are classically recognized as adaptive lymphocytes based on their ability to recognize specific foreign antigens and mount memory responses. However, recent studies indicate that some antigen-inexperienced CD8+ T cells can respond to innate cytokines alone in the absence of cognate T cell receptor stimulation, a phenomenon referred to as bystander activation. Here, we demonstrate that neonatal CD8+ T cells undergo a robust and diverse program of bystander activation, which corresponds to enhanced innate-like protection against unrelated pathogens. Using a multi-omics approach, we found that the ability of neonatal CD8+ T cells to respond to innate cytokines derives from their capacity to undergo rapid chromatin remodeling, resulting in the usage of a distinct set of enhancers and transcription factors typically found in innate-like T cells. We observed that the switch between innate and adaptive functions in the CD8+ T cell compartment is mediated by changes in the abundance of distinct subsets of cells. The innate CD8+ T cell subset that predominates in early life was also present in adult mice and humans. Our findings provide support for the layered immune hypothesis and indicate that the CD8+ T cell compartment is more functionally diverse than previously thought.

Basophil responses in susceptible AKR mice upon infection with the intestinal helminth parasite Trichuris muris

Intestinal helminth infection promotes a Type 2 inflammatory response in resistant C57BL/6 mice that is essential for worm clearance. The study of inbred mouse strains has revealed factors that are critical for parasite resistance and delineated the role of Type 1 versus Type 2 immune responses in worm clearance. In C57BL/6 mice, basophils are key innate immune cells that promote Type 2 inflammation and are programmed via the Notch signalling pathway during infection with the helminth Trichuris muris. However, how the host genetic background influences basophil responses and basophil expression of Notch receptors remains unclear. Here we use genetically susceptible inbred AKR/J mice that have a Type 1-skewed immune response during T. muris infection to investigate basophil responses in a susceptible host. Basophil population expansion occurred in AKR/J mice even in the absence of fulminant Type 2 inflammation during T. muris infection. However, basophils in AKR/J mice did not robustly upregulate expression of the Notch2 receptor in response to infection as occurred in C57BL/6 mice. Blockade of the Type 1 cytokine interferon-γ in infected AKR/J mice was not sufficient to elicit infection-induced basophil expression of the Notch2 receptor. These data suggest that the host genetic background, outside of the Type 1 skew, is important in regulating basophil responses during T. muris infection in susceptible AKR/J mice.

Interleukin-6 neutralization and regulatory T cells are additive in chondroprotection from IL-1β-induced inflammation

Anti-inflammatory Regulatory T cells (Tregs) are enriched in the joints of patients with osteoarthritis (OA) compared to healthy joints. Tregs maintain homeostasis through secretion of anti-inflammatory cytokines and cell-to-cell interactions including immune checkpoint signaling. Interleukin-6 (IL-6) is a pleiotropic cytokine secreted by inflamed synoviocytes and chondrocytes that can inhibit or alter Treg function. This study tested the hypothesis that neutralization of IL-6 would enable Treg anti-inflammatory function to resolve inflammation and catabolism elicited by IL-1β in an equine chondrocyte/synoviocyte/Treg tri-culture OA model. Synoviocyte/chondrocyte co-cultures were stimulated with IL-1β, and treated with αIL-6 neutralizing antibody. Activated Tregs secreting IL-10 were added in direct contact with synoviocytes to create a tri-culture. Neutralization of IL-6 partially restored Treg anti-inflammatory functions and, in combination, reduced IL-1β-stimulated synoviocyte MMP13 expression to control levels and restored Acan expression in chondrocytes. IL-6 neutralization alone decreased Il6 expression in chondrocytes and synoviocytes, mitigating IL-6 positive feedback loop. Although Tregs were the primary producers of anti-inflammatory IL-10 and IL-4, they also produced pro-inflammatory IL-17A, as detected by ELIA, which may have been responsible for incomplete rescue of synoviocyte/chondrocyte homeostasis following IL-1β stimulation. Treg secretion of IL-10, IL-4, and IL-17A was not altered by tri-culture conditions or presence of αIL-6, therefore, it was unlikely that Treg phenotype instability occurred. The significant effect of chondrocyte/synoviocyte donor, but not Treg donor, on gene expression and IL-6 concentration in conditioned media, indicated that personalized therapy considering the patient's OA status might be needed for successful implementation of immunotherapy in the context of OA.

T Helper 17-Like Regulatory T Cells in Equine Synovial Fluid Are Associated With Disease Severity of Naturally Occurring Posttraumatic Osteoarthritis

BACKGROUND

Infiltration of cluster of differentiation (CD) 3⁺ (CD3⁺) T cells into the synovium and synovial fluid occurs in most patients with posttraumatic osteoarthritis. During disease progression, proinflammatory T helper 17 cells and anti-inflammatory regulatory T cells infiltrate the joint in response to inflammation. This study aimed to characterize the dynamics of regulatory T and T helper 17 cell populations in synovial fluid from equine clinical patients with posttraumatic osteoarthritis to determine whether phenotype and function are associated with potential immunotherapeutic targets.

HYPOTHESIS

An imbalance of the ratio of regulatory T cells and T helper 17 cells would be associated with disease progression in posttraumatic osteoarthritis, suggesting opportunities for immunomodulatory therapy.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Synovial fluid was aspirated from the joints of equine clinical patients undergoing arthroscopic surgery for posttraumatic osteoarthritis resulting from intra-articular fragmentation. Joints were classified as having mild or moderate posttraumatic osteoarthritis. Synovial fluid was also obtained from nonoperated horses with normal cartilage. Peripheral blood was obtained from horses with normal cartilage and those with mild and moderate posttraumatic osteoarthritis. Synovial fluid and peripheral blood cells were analyzed by flow cytometry, and native synovial fluid was analyzed by enzyme-linked immunosorbent assay.

RESULTS

CD3⁺ T cells represented 81% of lymphocytes in synovial fluid, which increased in animals with moderate posttraumatic osteoarthritis to 88.3% (P = .02). CD14⁺ macrophages were doubled in those with moderate posttraumatic osteoarthritis compared with mild posttraumatic osteoarthritis and controls (P < .001). Less than 5% of CD3⁺ T cells found within the joint were forkhead box P3 protein⁺ (Foxp3⁺) regulatory T cells, but a 4- to 8-times higher percentage of nonoperated and mild posttraumatic osteoarthritis joint regulatory T cells secreted interleukin (IL)-10 than peripheral blood Tregs (P < .005). T regulatory-1 cells that secreted IL-10 but did not express Foxp3 accounted for approximately 5% of CD3⁺ T cells in all joints. T helper 17 cells and Th17-like regulatory T cells were increased in those with moderate posttraumatic osteoarthritis (P < .0001) compared with mild and nonoperated patients. IL-10, IL-17A, IL-6, chemokine (C-C motif) ligand (CCL) 2 (CCL2), and CCL5 concentrations detected by enzyme-linked immunosorbent assay in synovial fluid were not different between groups.

CONCLUSIONS

An imbalance of the ratio of regulatory T cells and T helper 17 cells and an increase in T helper 17 cell-like regulatory T cells in synovial fluid from joints with more severe disease provide novel insights into immunological mechanisms that are associated with posttraumatic osteoarthritis progression and pathogenesis.

CLINICAL RELEVANCE

Early and targeted use of immunotherapeutics in the mitigation of posttraumatic osteoarthritis may improve patient clinical outcomes.

TSLP, IL-33, and IL-25: Not just for allergy and helminth infection

The release of cytokines from epithelial and stromal cells is critical for the initiation and maintenance of tissue immunity. Three such cytokines, thymic stromal lymphopoietin, IL-33, and IL-25, are important regulators of type 2 immune responses triggered by parasitic worms and allergens. In particular, these cytokines activate group 2 innate lymphoid cells, TH2 cells, and myeloid cells, which drive hallmarks of type 2 immunity. However, emerging data indicate that these tissue-associated cytokines are not only involved in canonical type 2 responses but are also important in the context of viral infections, cancer, and even homeostasis. Here, we provide a brief review of the roles of thymic stromal lymphopoietin, IL-33, and IL-25 in diverse immune contexts, while highlighting their relative contributions in tissue-specific responses. We also emphasize a biologically motivated framework for thinking about the integration of multiple immune signals, including the 3 featured in this review.

E-Protein Inhibition in ILC2 Development Shapes the Function of Mature ILC2s during Allergic Airway Inflammation

E-protein transcription factors limit group 2 innate lymphoid cell (ILC2) development while promoting T cell differentiation from common lymphoid progenitors. Inhibitors of DNA binding (ID) proteins block E-protein DNA binding in common lymphoid progenitors to allow ILC2 development. However, whether E-proteins influence ILC2 function upon maturity and activation remains unclear. Mice that overexpress ID1 under control of the thymus-restricted proximal Lck promoter (ID1^tg/WT) have a large pool of primarily thymus-derived ILC2s in the periphery that develop in the absence of E-protein activity. We used these mice to investigate how the absence of E-protein activity affects ILC2 function and the genomic landscape in response to house dust mite (HDM) allergens. ID1^tg/WT mice had increased KLRG1^- ILC2s in the lung compared with wild-type (WT; ID1^WT/WT) mice in response to HDM, but ID1^tg/WT ILC2s had an impaired capacity to produce type 2 cytokines. Analysis of WT ILC2 accessible chromatin suggested that AP-1 and C/EBP transcription factors but not E-proteins were associated with ILC2 inflammatory gene programs. Instead, E-protein binding sites were enriched at functional genes in ILC2s during development that were later dynamically regulated in allergic lung inflammation, including genes that control ILC2 response to cytokines and interactions with T cells. Finally, ILC2s from ID1^tg/WT compared with WT mice had fewer regions of open chromatin near functional genes that were enriched for AP-1 factor binding sites following HDM treatment. These data show that E-proteins shape the chromatin landscape during ILC2 development to dictate the functional capacity of mature ILC2s during allergic inflammation in the lung.

Helminths make themselves at home

Drurey et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20211140) show that excretory/secretory products from the parasitic helminth Heligmosomoides polygyrus suppress the host-protective small intestinal epithelial response. These findings establish that helminths directly modulate the tissue in which they live, shining new light on the host-parasite interaction.

PGD2 and CRTH2 counteract Type 2 cytokine-elicited intestinal epithelial responses during helminth infection

Type 2 inflammation is associated with epithelial cell responses, including goblet cell hyperplasia, that promote worm expulsion during intestinal helminth infection. How these epithelial responses are regulated remains incompletely understood. Here, we show that mice deficient in the prostaglandin D2 (PGD2) receptor CRTH2 and mice with CRTH2 deficiency only in nonhematopoietic cells exhibited enhanced worm clearance and intestinal goblet cell hyperplasia following infection with the helminth Nippostrongylus brasiliensis. Small intestinal stem, goblet, and tuft cells expressed CRTH2. CRTH2-deficient small intestinal organoids showed enhanced budding and terminal differentiation to the goblet cell lineage. During helminth infection or in organoids, PGD2 and CRTH2 down-regulated intestinal epithelial Il13ra1 expression and reversed Type 2 cytokine-mediated suppression of epithelial cell proliferation and promotion of goblet cell accumulation. These data show that the PGD2-CRTH2 pathway negatively regulates the Type 2 cytokine-driven epithelial program, revealing a mechanism that can temper the highly inflammatory effects of the anti-helminth response.

Plasmacytoid Dendritic Cells Facilitate Th Cell Cytokine Responses throughout Schistosoma mansoni Infection

Plasmacytoid dendritic cells (pDCs) are potent producers of type I IFN (IFN-I) during viral infection and respond to IFN-I in a positive feedback loop that promotes their function. IFN-I shapes dendritic cell responses during helminth infection, impacting their ability to support Th2 responses. However, the role of pDCs in type 2 inflammation is unclear. Previous studies have shown that pDCs are dispensable for hepatic or splenic Th2 responses during the early stages of murine infection with the trematode Schistosoma mansoni at the onset of parasite egg laying. However, during S. mansoni infection, an ongoing Th2 response against mature parasite eggs is required to protect the liver and intestine from acute damage and how pDCs participate in immune responses to eggs and adult worms in various tissues beyond acute infection remains unclear. We now show that pDCs are required for optimal Th2 cytokine production in response to S. mansoni eggs in the intestinal-draining mesenteric lymph nodes throughout infection and for egg-specific IFN-γ at later time points of infection. Further, pDC depletion at chronic stages of infection led to increased hepatic and splenic pathology as well as abrogated Th2 cell cytokine production and activation in the liver. In vitro, mesenteric lymph node pDCs supported Th2 cell responses from infection-experienced CD4+ T cells, a process dependent on pDC IFN-I responsiveness, yet independent of Ag. Together, these data highlight a previously unappreciated role for pDCs and IFN-I in maintaining and reinforcing type 2 immunity in the lymph nodes and inflamed tissue during helminth infection.

Prostaglandin regulation of type 2 inflammation: From basic biology to therapeutic interventions

Type 2 immunity is critical for the protective and repair responses that mediate resistance to parasitic helminth infection. This immune response also drives aberrant inflammation during atopic diseases. Prostaglandins are a class of critical lipid mediators that are released during type 2 inflammation and are integral in controlling the initiation, activation, maintenance, effector functions, and resolution of Type 2 inflammation. In this review, we explore the roles of the different prostaglandin family members and the receptors they bind to during allergen‐ and helminth‐induced Type 2 inflammation and the mechanism through which prostaglandins promote or suppress Type 2 inflammation. Furthermore, we discuss the potential role of prostaglandins produced by helminth parasites in the regulation of host–pathogen interactions, and how prostaglandins may regulate the inverse relationship between helminth infection and allergy. Finally, we discuss opportunities to capitalize on our understanding of prostaglandin pathways to develop new therapeutic options for humans experiencing Type 2 inflammatory disorders that have a significant prostaglandin‐driven component including allergic rhinitis and asthma.

Lin28b promotes innate functions in CD8+ T cells through rapid chromatin remodeling

Lin28b serves as a master regulator of fetal lymphopoiesis and promotes the development of more innate-like lymphocytes in early life. However, it is still unclear how Lin28b alters the function of CD8+ T cells and protects the host against infection. In this report we examined how Lin28b transcriptionally and epigenetically programs neonatal CD8+ T cells for innate immune defense. We found that murine neonatal CD8+ T cells possess innate-like transcriptomes, are more responsive to innate cytokines in the absence of antigen and produce a broader spectrum of cytokines compared to their adult counterparts. This unique program of bystander activation in early life corresponded with enhanced innate-like protection against irrelevant strains of bacterial, helminth and virus infections. To understand how this diversity of immune function is mediated, we used flow cytometry and scRNAseq and discovered that the neonatal CD8+ T cell pool is composed of many subsets of cells expressing distinct cytokines. ATACseq analysis revealed that neonatal CD8+ T cells are programmed to display innate-like properties and produce a diverse array of cytokines, which may be due to their ability to undergo extensive chromatin remodeling upon cytokine stimulation. These unique properties may be driven by high Lin28b expression in fetal progenitor cells, as adult cells expressing Lin28b initiated a program of bystander activation and immune protection that was analogous to neonates. Collectively, our data suggests that Lin28b enables neonatal CD8+ T cells to undergo a more diverse program of bystander activation, allowing them to deploy a bet-hedging immune strategy and protect the host against a rapidly changing pathogenic environment.

Astrocytes promote a protective immune response to brain Toxoplasma gondii infection via IL-33-ST2 signaling

It is of great interest to understand how invading pathogens are sensed within the brain, a tissue with unique challenges to mounting an immune response. The eukaryotic parasite Toxoplasma gondii colonizes the brain of its hosts, and initiates robust immune cell recruitment, but little is known about pattern recognition of T. gondii within brain tissue. The host damage signal IL-33 is one protein that has been implicated in control of chronic T. gondii infection, but, like many other pattern recognition pathways, IL-33 can signal peripherally, and the specific impact of IL-33 signaling within the brain is unclear. Here, we show that IL-33 is expressed by oligodendrocytes and astrocytes during T. gondii infection, is released locally into the cerebrospinal fluid of T. gondii-infected animals, and is required for control of infection. IL-33 signaling promotes chemokine expression within brain tissue and is required for the recruitment and/or maintenance of blood-derived anti-parasitic immune cells, including proliferating, IFN-γ-expressing T cells and iNOS-expressing monocytes. Importantly, we find that the beneficial effects of IL-33 during chronic infection are not a result of signaling on infiltrating immune cells, but rather on radio-resistant responders, and specifically, astrocytes. Mice with IL-33 receptor-deficient astrocytes fail to mount an adequate adaptive immune response in the CNS to control parasite burden-demonstrating, genetically, that astrocytes can directly respond to IL-33 in vivo. Together, these results indicate a brain-specific mechanism by which IL-33 is released locally, and sensed locally, to engage the peripheral immune system in controlling a pathogen.

Exercising Immunity: Interleukin-13 Flexes Muscle

Endurance exercise drives physiological changes in the muscle to optimize performance. In a recent study in Science, Knudsen et al. report a role for the type 2 cytokine interleukin-13 in orchestrating metabolic reprogramming that drives adaptation to endurance exercise.

The Prostaglandin D2 Receptor CRTH2 Promotes IL-33-Induced ILC2 Accumulation in the Lung

Group 2 innate lymphoid cells (ILC2s) are rare innate immune cells that accumulate in tissues during allergy and helminth infection, performing critical effector functions that drive type 2 inflammation. ILC2s express ST2, the receptor for the cytokine IL-33, and chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2), a receptor for the bioactive lipid prostaglandin D2 (PGD2). The IL-33-ST2 and the PGD2-CRTH2 pathways have both been implicated in promoting ILC2 accumulation during type 2 inflammation. However, whether these two pathways coordinate to regulate ILC2 population size in the tissue in vivo remains undefined. In this study, we show that ILC2 accumulation in the murine lung in response to systemic IL-33 treatment was partially dependent on CRTH2. This effect was not a result of reduced ILC2 proliferation, increased apoptosis or cell death, or differences in expression of the ST2 receptor in the absence of CRTH2. Rather, data from adoptive transfer studies suggested that defective accumulation of CRTH2-deficient ILC2s in response to IL-33 was due to altered ILC2 migration patterns. Whereas donor wild-type ILC2s preferentially accumulated in the lungs compared with CRTH2-deficient ILC2s following transfer into IL-33-treated recipients, wild-type and CRTH2-deficient ILC2s accumulated equally in the recipient mediastinal lymph node. These data suggest that CRTH2-dependent effects lie downstream of IL-33, directly affecting the migration of ILC2s into inflamed lung tissues. A better understanding of the complex interactions between the IL-33 and PGD2-CRTH2 pathways that regulate ILC2 population size will be useful in understanding how these pathways could be targeted to treat diseases associated with type 2 inflammation.

Elevated circulating Th2 but not group 2 innate lymphoid cell responses characterize canine atopic dermatitis

Atopic dermatitis (AD) is an allergic skin disease that causes significant morbidity and affects multiple species. AD is highly prevalent in companion dogs, and the clinical management of the disease remains challenging. An improved understanding of the immunologic and genetic pathways that lead to disease could inform the development of novel treatments. In allergic humans and mouse models of AD, the disease is associated with Th2 and group 2 innate lymphoid cell (ILC2) activation that drives type 2 inflammation. Type 2 inflammation also appears to be associated with AD in dogs, but gaps remain in our understanding of how key type 2-associated cell types such as canine Th2 cells and ILC2s contribute to the pathogenesis of canine AD. Here, we describe previously uncharacterized canine ILC2-like cells and Th2 cells ex vivo that produced type 2 cytokines and expressed the transcription factor Gata3. Increased circulating Th2 cells were associated with chronic canine AD. Single-cell RNA sequencing revealed a unique gene expression signature in T cells in dogs with AD. These findings underline the importance of pro-allergic Th2 cells in orchestrating AD and provide new methods and pathways that can inform the development of improved therapies.

Lung Innate Lymphoid Cell Composition Is Altered in Primary Graft Dysfunction

Rationale: Primary graft dysfunction (PGD) is the leading cause of early morbidity and mortality after lung transplantation, but the immunologic mechanisms are poorly understood. Innate lymphoid cells (ILC) are a heterogeneous family of immune cells regulating pathologic inflammation and beneficial tissue repair. However, whether changes in donor-derived lung ILC populations are associated with PGD development has never been examined.Objectives: To determine whether PGD in chronic obstructive pulmonary disease or interstitial lung disease transplant recipients is associated with alterations in ILC subset composition within the allograft.Methods: We performed a single-center cohort study of lung transplantation patients with surgical biopsies of donor tissue taken before, and immediately after, allograft reperfusion. Donor immune cells from 18 patients were characterized phenotypically by flow cytometry for single-cell resolution of distinct ILC subsets. Changes in the percentage of ILC subsets with reperfusion or PGD (grade 3 within 72 h) were assessed.Measurements and Main Results: Allograft reperfusion resulted in significantly decreased frequencies of natural killer cells and a trend toward reduced ILC populations, regardless of diagnosis (interstitial lung disease or chronic obstructive pulmonary disease). Seven patients developed PGD (38.9%), and PGD development was associated with selective reduction of the ILC2 subset after reperfusion. Conversely, patients without PGD exhibited significantly higher ILC1 frequencies before reperfusion, accompanied by elevated ILC2 frequencies after allograft reperfusion.Conclusions: The composition of donor ILC subsets is altered after allograft reperfusion and is associated with PGD development, suggesting that ILCs may be involved in regulating lung injury in lung transplant recipients.

Impact of Interleukin-27p28 on T and B Cell Responses during Toxoplasmosis

Interleukin-27 (IL-27) is a heterodimeric cytokine composed of the subunits IL-27p28 and EBi3, and while the IL-27 heterodimer influences T cell activities, there is evidence that IL-27p28 can have EBi3-independent activities; however, their relevance to infection is unclear. Therefore, the studies presented here compared how IL-27p28 transgenics and IL-27p28-/- mice responded to the intracellular parasite Toxoplasma gondii While the loss of IL-27p28 and its overexpression both result in increased susceptibility to T. gondii, the basis for this phenotype reveals distinct roles for IL-27p28. As a component of IL-27, IL-27p28 is critical to limit infection-induced T cell-mediated pathology, whereas the ectopic expression of IL-27p28 reduced the effector T cell population and had a major inhibitory effect on parasite-specific antibody titers and a failure to control parasite replication in the central nervous system. Indeed, transfer of immune serum to infected IL-27p28 transgenics resulted in reduced parasite burden and pathology. Thus, IL-27p28, independent of its role as a component of IL-27, can act as a negative regulator of humoral and cellular responses during toxoplasmosis.

Notch Signaling Orchestrates Helminth-Induced Type 2 Inflammation

Infection with helminth parasites poses a significant challenge to the mammalian immune system. The type 2 immune response to helminth infection is critical in limiting worm-induced tissue damage and expelling parasites. Conversely, aberrant type 2 inflammation can cause debilitating allergic disease. Recent studies have revealed that key type 2 inflammation-associated immune and epithelial cell types respond to Notch signaling, broadly regulating gene expression programs in cell development and function. Here, we discuss new advances demonstrating that Notch is active in the development, recruitment, localization, and cytokine production of immune and epithelial effector cells during type 2 inflammation. Understanding how Notch signaling controls type 2 inflammatory processes could inform the development of Notch pathway modulators to treat helminth infections and allergies.

The Immunobiology of the Interleukin-12 Family: Room for Discovery

The discovery of interleukin (IL)-6 and its receptor subunits provided a foundation to understand the biology of a group of related cytokines: IL-12, IL-23, and IL-27. These family members utilize shared receptors and cytokine subunits and influence the outcome of cancer, infection, and inflammatory diseases. Consequently, many facets of their biology are being therapeutically targeted. Here, we review the landmark discoveries in this field, the combinatorial biology inherent to this family, and how patient datasets have underscored the critical role of these pathways in human disease. We present significant knowledge gaps, including how similar signals from these cytokines can mediate distinct outcomes, and discuss how a better understanding of the biology of the IL-12 family provides new therapeutic opportunities.

The Notch signaling pathway promotes basophil responses during helminth-induced type 2 inflammation

Basophils promote type 2 inflammation that mediates worm clearance during murine infection with the gastrointestinal helminth parasite Trichuris muris. Webb et al. show for the first time that basophil–intrinsic Notch signaling is required for basophil gene expression and a functional program that support helminth expulsion.

Type 2 inflammation drives the clearance of gastrointestinal helminth parasites, which infect over two billion people worldwide. Basophils are innate immune cells that support host-protective type 2 inflammation during murine infection with the helminth Trichuris muris. However, the mechanisms required for basophil function and gene expression regulation in this context remain unclear. We show that during T. muris infection, basophils localized to the intestine and up-regulated Notch receptor expression, rendering them sensitive to Notch signals that rapidly regulate gene expression programs. In vitro, Notch inhibition limited basophil cytokine production in response to cytokine stimulation. Basophil-intrinsic Notch signaling was required for T. muris–elicited changes in genome-wide basophil transcriptional programs. Mice lacking basophil-intrinsic functional Notch signaling had impaired worm clearance, decreased intestinal type 2 inflammation, altered basophil localization in the intestine, and decreased CD4⁺ T helper 2 cell responses following infection. These findings demonstrate that Notch is required for basophil gene expression and effector function associated with helminth expulsion during type 2 inflammation.

Isolation and Identification of Innate Lymphoid Cells (ILCs) for Immunotoxicity Testing

Innate lymphoid cells (ILCs) comprise a family of innate immune cells that orchestrate mucosal immune responses: initiating, sustaining, and even curbing immune responses. ILCs are relatively rare (≤1% of lymphocytes in mucosal tissues), lack classical cell-surface markers, and can be divided into three subsets (type 1-3 ILCs) based on differences in cytokine production, phenotype, and developmental pathway. Because ILCs can only be identified by combinations of cell-surface markers and cytokine production, multicolor flow cytometry is the most reliable method to purify, characterize, and assess the functionality of ILCs. Here, we describe the methods for cell preparation, flow cytometric analysis, and purification of murine ILCs from the lung.

Cytokines and beyond: Regulation of innate immune responses during helminth infection

Parasitic helminth infection elicits a type 2 cytokine-mediated inflammatory response. During type 2 inflammation, damaged or stimulated epithelial cells exposed to helminths and their products produce alarmins and cytokines including IL-25, IL-33, and thymic stromal lymphopoietin. These factors promote innate immune cell activation that supports the polarization of CD4⁺ T helper type 2 (Th2) cells. Activated innate and Th2 cells produce the cytokines IL-4, -5, -9, and -13 that perpetuate immune activation and act back on the epithelium to cause goblet cell hyperplasia and increased epithelial cell turnover. Together, these events facilitate worm expulsion and wound healing processes. While the role of Th2 cells in this context has been heavily studied, recent work has revealed that epithelial cell-derived cytokines are drivers of key innate immune responses that are critical for type 2 anti-helminth responses. Cutting-edge studies have begun to fully assess how other factors and pathways, including lipid mediators, chemokines, Fc receptor signaling, danger-associated molecular pattern molecules, and direct cell-cell interactions, also participate in shaping innate cell-mediated type 2 inflammation. In this review, we discuss how these pathways intersect and synergize with pathways controlled by epithelial cell-derived cytokines to coordinate innate immune responses that drive helminth-induced type 2 inflammation.

The prostaglandin D2 receptor CRTH2 suppresses epithelial cell responses during intestinal helminth infection

Type 2 inflammation is required for expulsion of intestinal helminth parasites and is characterized by immune cell activation, type 2 cytokine production, and increased mucin production by epithelial goblet cells. Previous studies show that the prostaglandin D2 (PGD2) receptor chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) promotes type 2 inflammation in the lung via effects on immune cells, but how CRTH2 influences helminth-induced type 2 inflammation in the intestine was unclear. Here we show that CRTH2-deficient mice cleared infection with the mouse-adapted helminth parasite Nippostrongylus brasiliensis more efficiently and had increased numbers of mucin-producing goblet cells in the small intestine post-infection compared to wild type mice, despite similar levels of worm establishment. These data suggest that CRTH2 restrains goblet cell responses during intestinal helminth infection, in contrast to its pro-inflammatory role in the lung. Consistent with this idea, bone marrow-chimeric mice in which only non-hematopoietic cells lacked CRTH2 also cleared parasites more efficiently than chimeric wild type mice and had increased numbers of small intestinal goblet cells. In addition, murine small intestinal organoids that were stimulated with type 2 cytokines downregulated expression of goblet cell-associated genes following culture with PGD2. Taken together, these data suggest that the PGD2-CRTH2 pathway suppresses epithelial goblet cell responses following helminth-induced type 2 inflammation in the intestine. These findings may inform the development and use of drugs that inhibit this pathway during intestinal type 2 inflammatory disease, such as food allergy.

Emerging concepts and future challenges in innate lymphoid cell biology

Innate lymphoid cells (ILCs) are innate immune cells that are ubiquitously distributed in lymphoid and nonlymphoid tissues and enriched at mucosal and barrier surfaces. Three major ILC subsets are recognized in mice and humans. Each of these subsets interacts with innate and adaptive immune cells and integrates cues from the epithelium, the microbiota, and pathogens to regulate inflammation, immunity, tissue repair, and metabolic homeostasis. Although intense study has elucidated many aspects of ILC development, phenotype, and function, numerous challenges remain in the field of ILC biology. In particular, recent work has highlighted key new questions regarding how these cells communicate with their environment and other cell types during health and disease. This review summarizes new findings in this rapidly developing field that showcase the critical role ILCs play in directing immune responses through their ability to interact with a variety of hematopoietic and nonhematopoietic cells. In addition, we define remaining challenges and emerging questions facing the field. Finally, this review discusses the potential application of basic studies of ILC biology to the development of new treatments for human patients with inflammatory and infectious diseases in which ILCs play a role.

Label-Free Imaging of Eosinophilic Esophagitis Mouse Models Using Optical Coherence Tomography

Eosinophilic esophagitis (EoE) is an immune-mediated disorder characterized by esophageal inflammation and related structural changes causing symptoms such as feeding difficulties and food impaction. The pathophysiological mechanisms underlying EoE remain poorly understood. Preclinical studies using mouse models have been critical in comprehending human disease mechanisms and associated pathways. In this chapter, we describe an experimental method using a noninvasive label-free optical imaging technique, optical coherence tomography, to characterize the pathophysiological changes in the esophagus of mice with EoE-like disease ex vivo.

Flt3 Ligand Is Essential for Survival and Protective Immune Responses during Toxoplasmosis

Dendritic cells (DCs) are critical for resistance to Toxoplasma gondii, and infection with this pathogen leads to increased numbers of DCs at local sites of parasite replication and in secondary lymphoid organs, but the factors that regulate this expansion are poorly understood. The cytokine Flt3 ligand (Flt3L) is critical for the generation and maintenance of DCs, and Flt3L(-/-) mice were found to be highly susceptible to acute toxoplasmosis. This phenotype correlated with decreased production of IL-12 and IFN-γ, as well as impaired NK cell responses. Surprisingly, despite low basal numbers of DCs, Flt3L(-/-) mice infected with T. gondii displayed an expansion of CD8α(+) and CD11b(lo)CD8α(-) DCs. Infection also induced an expansion of parasite-specific CD4(+) and CD8(+) T cells in Flt3L(-/-) mice; however, these cells were reduced in number and displayed impaired ability to produce IFN-γ relative to wild-type controls. Exogenous IL-12 treatment partially restored NK and T cell responses in Flt3L(-/-) mice, as well as acute resistance; however, these mice eventually succumbed to toxoplasmic encephalitis, despite the presence of large numbers of DCs and T cells in the brain. These results highlight the importance of Flt3L for resistance to toxoplasmosis and demonstrate the existence of Flt3L-independent pathways that can mediate infection-induced expansion of DCs and T cell priming.

Basophils promote innate lymphoid cell responses in inflamed skin

Type 2 inflammation underlies allergic diseases such as atopic dermatitis, which is characterized by the accumulation of basophils and group 2 innate lymphoid cells (ILC2s) in inflamed skin lesions. Although murine studies have demonstrated that cutaneous basophil and ILC2 responses are dependent on thymic stromal lymphopoietin, whether these cell populations interact to regulate the development of cutaneous type 2 inflammation is poorly defined. In this study, we identify that basophils and ILC2s significantly accumulate in inflamed human and murine skin and form clusters not observed in control skin. We demonstrate that murine basophil responses precede ILC2 responses and that basophils are the dominant IL-4-enhanced GFP-expressing cell type in inflamed skin. Furthermore, basophils and IL-4 were necessary for the optimal accumulation of ILC2s and induction of atopic dermatitis-like disease. We show that ILC2s express IL-4Rα and proliferate in an IL-4-dependent manner. Additionally, basophil-derived IL-4 was required for cutaneous ILC2 responses in vivo and directly regulated ILC2 proliferation ex vivo. Collectively, these data reveal a previously unrecognized role for basophil-derived IL-4 in promoting ILC2 responses during cutaneous inflammation.

Thymic stromal lymphopoietin-mediated extramedullary hematopoiesis promotes allergic inflammation

Extramedullary hematopoiesis (EMH) refers to the differentiation of hematopoietic stem cells (HSCs) into effector cells that occurs in compartments outside of the bone marrow. Previous studies linked pattern-recognition receptor (PRR)-expressing HSCs, EMH, and immune responses to microbial stimuli. However, whether EMH operates in broader immune contexts remains unknown. Here, we demonstrate a previously unrecognized role for thymic stromal lymphopoietin (TSLP) in promoting the population expansion of progenitor cells in the periphery and identify that TSLP-elicited progenitors differentiated into effector cells including macrophages, dendritic cells, and granulocytes and that these cells contributed to type 2 cytokine responses. The frequency of circulating progenitor cells was also increased in allergic patients with a gain-of-function polymorphism in TSLP, suggesting the TSLP-EMH pathway might operate in human disease. These data identify that TSLP-induced EMH contributes to the development of allergic inflammation and indicate that EMH is a conserved mechanism of innate immunity.

Characterization of eosinophilic esophagitis murine models using optical coherence tomography

Pre-clinical studies using murine models are critical for understanding the pathophysiological mechanisms underlying immune-mediated disorders such as Eosinophilic esophagitis (EoE). In this study, an optical coherence tomography (OCT) system capable of providing three-dimensional images with axial and transverse resolutions of 5 µm and 10 µm, respectively, was utilized to obtain esophageal images from a murine model of EoE-like disease ex vivo. Structural changes in the esophagus of wild-type (Tslpr(+/+) ) and mutant (Tslpr(-/-) ) mice with EoE-like disease were quantitatively evaluated and food impaction sites in the esophagus of diseased mice were monitored using OCT. Here, the capability of OCT as a label-free imaging tool devoid of tissue-processing artifacts to effectively characterize murine EoE-like disease models has been demonstrated.

Innate lymphoid cells and allergic inflammation

Group 2 innate lymphoid cells (ILC2s) play critical roles in anti-helminth immunity and airway epithelial repair. Recently, these cells have also emerged as key players in the development of allergic inflammation at multiple barrier surfaces. ILC2s arise from common lymphoid progenitors in the bone marrow, are dependent on the transcription factors RORα, GATA3, and TCF-1 and produce the type 2 cytokines IL-4, IL-5, IL-9, and/or IL-13. The epithelial cell-derived cytokines IL-25, IL-33, and TSLP regulate the activation and effector functions of ILC2s, and recent studies suggest that their responsiveness to these cytokines and other factors may depend on their tissue environment. In this review, we focus on recent advances in our understanding of how ILC2s are differentially regulated in the context of allergic inflammation and discuss the therapeutic potential of targeting ILC2s in the treatment of allergic diseases.

Thymic stromal lymphopoietin-elicited basophil responses promote eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is a food allergy-associated inflammatory disease characterized by esophageal eosinophilia. Current management strategies for EoE are nonspecific, and thus there is a need to identify specific immunological pathways that could be targeted to treat this disease. EoE is associated with polymorphisms in the gene that encodes thymic stromal lymphopoietin (TSLP), a cytokine that promotes allergic inflammation, but how TSLP might contribute to EoE disease pathogenesis has been unclear. Here, we describe a new mouse model of EoE-like disease that developed independently of IgE, but was dependent on TSLP and basophils, as targeting TSLP or basophils during the sensitization phase limited disease. Notably, therapeutic TSLP neutralization or basophil depletion also ameliorated established EoE-like disease. In human subjects with EoE, we observed elevated TSLP expression and exaggerated basophil responses in esophageal biopsies, and a gain-of-function TSLP polymorphism was associated with increased basophil responses in patients with EoE. Together, these data suggest that the TSLP-basophil axis contributes to the pathogenesis of EoE and could be therapeutically targeted to treat this disease.

Innate lymphoid cells: balancing immunity, inflammation, and tissue repair in the intestine

Innate lymphoid cells (ILCs) are a recently described group of innate immune cells that can regulate immunity, inflammation, and tissue repair in multiple anatomical compartments, particularly the barrier surfaces of the skin, airways, and intestine. Broad categories of ILCs have been defined based on transcription factor expression and the ability to produce distinct patterns of effector molecules. Recent studies have revealed that ILC populations can regulate commensal bacterial communities, contribute to resistance to helminth and bacterial pathogens, promote inflammation, and orchestrate tissue repair and wound healing. This review will examine the phenotype and function of murine and human ILCs and discuss the critical roles these innate immune cells play in health and disease.

Functional heterogeneity in the basophil cell lineage

CD4(+) T-helper type 2 (Th2) cells, characterized by their expression of interleukin (IL)-4, IL-5, IL-9, and IL-13, are required for immunity to helminth parasites and promote the pathological inflammation associated with asthma and allergic diseases. Recent reports from a number of laboratories have indicated that basophils can influence the induction and/or effector stages of Th2 cytokine-mediated inflammation. However, the impact of basophils appears to depend on the anatomical location and nature of the infectious or inflammatory stimulus. This review highlights the factors that regulate basophil development and activation and describes known basophil effector functions. Further, we discuss the recent identification of phenotypic and functional heterogeneity within murine and human basophil populations and discuss how these findings may explain the context-dependent influence of basophils on either the propagation, regulation, or effector phases of Th2 cytokine-associated inflammation.

The cytokines interleukin 27 and interferon-γ promote distinct Treg cell populations required to limit infection-induced pathology

Interferon-γ (IFN-γ) promotes a population of T-bet(+) CXCR3(+) regulatory T (Treg) cells that limit T helper 1 (Th1) cell-mediated pathology. Our studies demonstrate that interleukin-27 (IL-27) also promoted expression of T-bet and CXCR3 in Treg cells. During infection with Toxoplasma gondii, a similar population emerged that limited T cell responses and was dependent on IFN-γ in the periphery but on IL-27 at mucosal sites. Transfer of Treg cells ameliorated the infection-induced pathology observed in Il27(-/-) mice, and this was dependent on their ability to produce IL-10. Microarray analysis revealed that Treg cells exposed to either IFN-γ or IL-27 have distinct transcriptional profiles. Thus, IFN-γ and IL-27 have different roles in Treg cell biology and IL-27 is a key cytokine that promotes the development of Treg cells specialized to control Th1 cell-mediated immunity at local sites of inflammation.

Generalized Lévy walks and the role of chemokines in migration of effector CD8+ T cells

Chemokines play a central role in regulating processes essential to the immune function of T cells^1-3, such as their migration within lymphoid tissues and targeting of pathogens in sites of inflammation. Here we track T cells using multi-photon microscopy to demonstrate that the chemokine CXCL10 enhances the ability of CD8⁺ T cells to control the pathogen T. gondii in the brains of chronically infected mice. This chemokine boosts T cell function in two different ways: it maintains the effector T cell population in the brain and speeds up the average migration speed without changing the nature of the walk statistics. Remarkably, these statistics are not Brownian; rather, CD8⁺ T cell motility in the brain is well described by a generalized Lévy walk. According to our model, this surprising feature enables T cells to find rare targets with more than an order of magnitude more efficiency than Brownian random walkers. Thus, CD8⁺ T cell behavior is similar to Lévy strategies reported in organisms ranging from mussels to marine predators and monkeys^4-10, and CXCL10 aids T cells in shortening the average time to find rare targets.

New directions in the basic and translational biology of interleukin-27

Interleukin (IL)-27 is a member of the IL-6 and IL-12 family composed of the IL-27p28 and Epstein-Barr virus-induced gene 3 (EBI3) subunits. Although IL-27 was originally identified as a proinflammatory factor, subsequent studies have revealed the pleiotropic nature of this cytokine. This review discusses recent work that has explored the effect of IL-27 on CD4(+) T cell subsets, including T regulatory type 1 (Tr-1) cells, T follicular helper cells (Tfhs), and forkhead box P3 (Foxp3)(+) T regulatory cells (Tregs). Additionally, we highlight studies that have identified a role for the IL-27p28 subunit as a cytokine receptor antagonist. Much of the recent work on IL-27 has been relevant to human disease states characterized by inappropriate or excessive inflammation, and this review discusses potential opportunities to use IL-27 as a therapeutic agent.

Analysis of behavior and trafficking of dendritic cells within the brain during toxoplasmic encephalitis

Under normal conditions the immune system has limited access to the brain; however, during toxoplasmic encephalitis (TE), large numbers of T cells and APCs accumulate within this site. A combination of real time imaging, transgenic reporter mice, and recombinant parasites allowed a comprehensive analysis of CD11c⁺ cells during TE. These studies reveal that the CNS CD11c⁺ cells consist of a mixture of microglia and dendritic cells (DCs) with distinct behavior associated with their ability to interact with parasites or effector T cells. The CNS DCs upregulated several chemokine receptors during TE, but none of these individual receptors tested was required for migration of DCs into the brain. However, this process was pertussis toxin sensitive and dependent on the integrin LFA-1, suggesting that the synergistic effect of signaling through multiple chemokine receptors, possibly leading to changes in the affinity of LFA-1, is involved in the recruitment/retention of DCs to the CNS and thus provides new insights into how the immune system accesses this unique site.

Toxoplasmic encephalitis (TE), caused by the protozoan parasite Toxoplasma gondii, can be potentially life threatening especially in immuno-compromised individuals. Immune cells including dendritic cells have been shown to accumulate in the brain during chronic toxoplasmosis; however, little is known about their function, their behavior in vivo, and the mechanisms by which they migrate into the brain. In the present studies, we utilize a combination of real time imaging, transgenic reporter mice, and recombinant parasites to reveal the distinct behavior and morphologies of dendritic cells within the brain and their ability to interact with parasites and effector T cells during TE. The CNS DCs were also found to exhibit a unique chemokine receptor expression pattern during infection, and the migration of DCs into the brain was mediated through a pertussis toxin (which blocks signaling downstream of several chemokine receptors) sensitive process and dependent on the integrin LFA-1. There is currently a poor understanding of the events that lead to DC recruitment to the CNS during inflammation in general, and our studies provide new insights into the mechanisms by which antigen-presenting cells gain access to the brain during infection.

Toxoplasma polymorphic effectors determine macrophage polarization and intestinal inflammation

European and North American strains of the parasite Toxoplasma gondii belong to three distinct clonal lineages, type I, type II, and type III, which differ in virulence. Understanding the basis of Toxoplasma strain differences and how secreted effectors work to achieve chronic infection is a major goal of current research. Here we show that type I and III infected macrophages, a cell type required for host immunity to Toxoplasma, are alternatively activated, while type II infected macrophages are classically activated. The Toxoplasma rhoptry kinase ROP16, which activates STAT6, is responsible for alternative activation. The Toxoplasma dense granule protein GRA15, which activates NF-κB, promotes classical activation by type II parasites. These effectors antagonistically regulate many of the same genes, and mice infected with type II parasites expressing type I ROP16 are protected against Toxoplasma-induced ileitis. Thus, polymorphisms in determinants that modulate macrophage activation influence the ability of Toxoplasma to establish a chronic infection.

A role for IL-27 in limiting T regulatory cell populations

IL-27 is a cytokine that regulates Th function during autoimmune and pathogen-induced immune responses. Although previous studies have shown that regulatory T cells (Tregs) express the IL-27R, and that IL-27 inhibits forkhead box P3 upregulation in vitro, little is known about how IL-27 influences Tregs in vivo. The studies presented in this article show that mice that overexpress IL-27 had decreased Treg frequencies and developed spontaneous inflammation. Although IL-27 did not cause mature Tregs to downregulate forkhead box P3, transgenic overexpression in vivo limited the size of a differentiating Treg population in a bone marrow chimera model, which correlated with reduced production of IL-2, a vital cytokine for Treg maintenance. These data identify an indirect role for IL-27 in shaping the Treg pool.