Neutrophils clear bacteria associated with parasitic nematodes augmenting the development of an effective Th2-type response

The Roles of Various Immune Cell Populations in Immune Response against Helminths

Helminths are multicellular parasites that are a substantial problem for both human and veterinary medicine. According to estimates, 1.5 billion people suffer from their infection, resulting in decreased life quality and burdens for healthcare systems. On the other hand, these infections may alleviate autoimmune diseases and allergy symptoms. The immune system is programmed to combat infections; nevertheless, its effector mechanisms may result in immunopathologies and exacerbate clinical symptoms. This review summarizes the role of the immune response against worms, with an emphasis on the Th2 response, which is a hallmark of helminth infections. We characterize non-immune cells (enteric tuft cells-ETCs) responsible for detecting parasites, as well as the role of hematopoietic-derived cells (macrophages, basophils, eosinophils, neutrophils, innate lymphoid cells group 2-ILC2s, mast cells, T cells, and B cells) in initiating and sustaining the immune response, as well as the functions they play in granulomas. The aim of this paper is to review the existing knowledge regarding the immune response against helminths, to attempt to decipher the interactions between cells engaged in the response, and to indicate the gaps in the current knowledge.

Effects of helminths on the human immune response and the microbiome

Helminths have evolved sophisticated immune regulating mechanisms to prevent rejection by their mammalian host. Our understanding of how the human immune system responds to these parasites remains poor compared to mouse models of infection and this limits our ability to develop vaccines as well as harness their unique properties as therapeutic strategies against inflammatory disorders. Here, we review how recent studies on human challenge infections, self-infected individuals, travelers, and endemic populations have improved our understanding of human type 2 immunity and its effects on the microbiome. The heterogeneity of responses between individuals and the limited access to tissue samples beyond the peripheral blood are challenges that limit human studies on helminths, but also provide opportunities to transform our understanding of human immunology. Organoids and single-cell sequencing are exciting new tools for immunological analysis that may aid this pursuit. Learning about the genetic and immunological basis of resistance, tolerance, and pathogenesis to helminth infections may thus uncover mechanisms that can be utilized for therapeutic purposes.

Hookworm infections: Reappraising the evidence for a role of Neutrophils in light of NETosis

In Hookworm infection, neutrophils have long had the image of the villain, being recruited to the site of larval migration because of damage but participating themselves in tissue injury. With recent developments in neutrophil biology, there is an increasing body of evidence for the role of neutrophils as effector cells in hookworm immunity. In particular, their ability to release extracellular traps, or neutrophil extracellular traps (NETs), confer neutrophils a larvicidal activity. Here, we review recent evidence in this nascent field and discuss the avenue for future research on NETs/hookworm interactions.

Lessons in type 2 immunity: Neutrophils in Helminth infections

Neutrophils constitute the body's first line of defense against invading pathogens. Equipped with a large array of tools, these immune cells are highly efficient in eliminating bacterial and viral infections, yet their activity can at the same time be detrimental to the host itself - this is the broad consensus on these granulocytes. However, the last decade has proven that neutrophils are a much more sophisticated cell type with unexpected and underappreciated functions in health and disease. In this review, we look at the latest discoveries in neutrophil biology with a focus on their role during the hallmark setting of type 2 immunity - helminth infection. We discuss the involvement of neutrophils in various helminth infection models and summarize the latest findings regarding neutrophil regulation and effector function. We will show that neutrophils have much more to offer than previously thought and while studies of neutrophils in helminth infections are still in its infancy, recent discoveries highlight more than ever that these cells are a key cog of the immune system, even during type 2 responses.

Synergistic Activation of Bovine CD4+ T Cells by Neutrophils and IL-12

CD4+ T cell activation requires inflammatory cytokines to provide a third signal (3SI), such as interleukin-12 (IL-12). We recently reported that bovine neutrophils can enhance the activation of bovine CD4+ T cells. To explore the interactions between neutrophils and third signal cytokines in bovine CD4+ T cell activation, naïve CD4+ T cells were isolated from cattle lymph nodes and stimulated for 3.5 days with anti-bovine CD3 (first signal; 1SI), anti-bovine CD28 (second signal; 2SI), and recombinant human IL-12 (3SI) in the presence or absence of neutrophils harvested from the same animals. Indeed, the strongest activation was achieved in the presence of all three signals, as demonstrated by CD25 upregulation, IFNγ production in CD4+ T cells, and secretion of IFNγ and IL-2 in cell supernatants. More importantly, 1SI plus neutrophils led to enhanced CD25 expression that was further increased by IL-12, suggesting synergistic action by IL-12 and neutrophils. Consistently, neutrophils significantly increased IFNγ production in 1SI plus IL-12-stimulated CD4+ T cells. Our data suggest the synergy of neutrophils and IL-12 as a novel regulator on bovine CD4+ T cell activation in addition to three signals. This knowledge could assist the development of immune interventions for the control of infectious diseases in cattle.

Group-2 innate lymphoid cell-dependent regulation of tissue neutrophil migration by alternatively activated macrophage-secreted Ear11

Type-2 immunity is characterised by interleukin (IL)-4, IL-5 and IL-13, eosinophilia, mucus production, IgE, and alternatively activated macrophages (AAM). However, despite the lack of neutrophil chemoattractants such as CXCL1, neutrophils, a feature of type-1 immunity, are observed in type-2 responses. Consequently, alternative mechanisms must exist to ensure that neutrophils can contribute to type-2 immune reactions without escalation of deleterious inflammation. We now demonstrate that type-2 immune-associated neutrophil infiltration is regulated by the mouse RNase A homologue, eosinophil-associated ribonuclease 11 (Ear11), which is secreted by AAM downstream of IL-25-stimulated ILC2. Transgenic overexpression of Ear11 resulted in tissue neutrophilia, whereas Ear11-deficient mice have fewer resting tissue neutrophils, whilst other type-2 immune responses are not impaired. Notably, administration of recombinant mouse Ear11 increases neutrophil motility and recruitment. Thus, Ear11 helps maintain tissue neutrophils at homoeostasis and during type-2 reactions when chemokine-producing classically activated macrophages are infrequently elicited.

Anisakis spp. in toothed and baleen whales from Japanese waters with notes on their potential role as biological tags

In this study, Anisakis nematodes isolated from toothed and baleen whales from localities around Japan were molecularly (PCR-RFLP) identified. In Wakayama, common bottlenose dolphins (Tursiops truncatus) were infected with A. simplex sensu stricto (s.s.), A. typica and A. pegreffii, while A. typica was the only species found in pantropical spotted dolphin (Stenella attenuata) and striped dolphin (S. coeruleoalba). Offshore common minke whales (Balaenoptera acutorostrata) and sei whales (B. borealis) were almost exclusively infected with A. simplex s.s.. However, in common minke whales from two Hokkaido localities, mature worms mostly consisted of A. simplex s.s. in some individuals and of A. pegreffii in others, but immature worms were mainly A. simplex s.s.. Gross and histopathological examination on gastric mucosa attached by anisakids resulted in mild and superficial reactions by the two baleen whale species in contrast to severe inflammatory reaction associated with ulcer formations by common bottlenose dolphin. Host specificity and adaptability of Anisakis spp. in these baleen and toothed whales were discussed from the points of view of adult worm size, worm population and pathological reactions by hosts. Interestingly, most of the common minke whales predominantly harboring mature A. pegreffii adults belonged to the Yellow Sea - East China Sea stock (J stock), which migrates through the Sea of Japan, whereas most of those mainly parasitized by mature A. simplex s.s. adults were from the Okhotsk Sea - West Pacific stock (O stock), mostly inhabiting the Pacific side, suggesting that these sibling species may have utility as biological tags to differentiate whale stocks. These results represent the first definitive host records for A. pegreffi in the Northwestern Pacific Ocean.

Neutrophils Enhance Their Own Influx to Sites of Bacterial Infection via Endosomal TLR-Dependent Cxcl2 Production

The influx of neutrophils to infection sites is a fundamental step in host defenses against the frequent human pathogen group B Streptococcus (GBS) and other extracellular bacteria. Using a mouse model of GBS-induced peritonitis, we show in this study that the chemokines Cxcl1 and Cxcl2 play distinctive roles in enhancing the recruitment and the antibacterial activities of neutrophils in a manner that is linked to differences in the cellular sources of these mediators. Cell depletion experiments demonstrated that neutrophils make a significant contribution to the in vivo production of Cxcl2 but not Cxcl1. In vitro, neutrophils responded weakly to LPS but released high levels of Cxcl2 after stimulation with GBS or other bacteria. Neutrophil-derived Cxcl2 acted in an autocrinous manner to increase its own production and to enhance antibacterial activities, including the release of oxygen radicals. In both neutrophils and macrophages, the production of Cxcl1/2 largely required the presence of functional UNC93B1, a chaperone protein involved in signaling by endosomal TLRs. Moreover, the phenotype of UNC93B1-defective phagocytes could be recapitulated by the simultaneous absence of TLR7, 9, and 13 but not by the absence of individual TLRs. Collectively, our data show that neutrophils recognize Gram-positive and Gram-negative bacteria by means of multiple phagosomal TLRs, resulting in de novo synthesis of Cxcl2, amplification of neutrophil recruitment, and potentiation of their antibacterial activities. These data may be useful to devise alternative therapeutic strategies aimed at enhancing the recruitment and the functional activities of polymorphonuclear leukocytes during infections caused by antibiotic-resistant bacteria.

Role of mast cells in the generation of a T-helper type 2 dominated anti-helminthic immune response

Mast cells are long-lived, innate immune cells of the myeloid lineage which are found in peripheral tissues located throughout the body, and positioned at the interface between the host and the environment. Mast cells are found in high concentrations during helminth infection. Using Kitw-sh mast cell deficient mice, a recently published study in Bioscience Reports by Gonzalez et al. (Biosci. Rep., 2018) focused on the role of mast cells in the immune response to infection by the helminth Hymenolepis diminuta The authors showed that mast cells play a role in the modulation of Th2 immune response characterized by a unique IL-4, IL-5 and IL-13 cytokine profile, as well as subsequent robust worm expulsion during H. diminuta infection. Unlike WT mice which expelled H. diminuta at day 10, Kitw-sh deficient mice displayed delayed worm expulsion (day 14 post infection). Further, a possible role for mast cells in the basal expression of cytokines IL-25, IL-33 and thymic stromal lymphopoietin was described. Deletion of neutrophils in Kitw-sh deficient mice enhanced H. diminuta expulsion, which was accompanied by splenomegaly. However, interactions between mast cells and other innate and adaptive immune cells during helminth infections are yet to be fully clarified. We conclude that the elucidation of mechanisms underlying mast cell interactions with cells of the innate and adaptive immune system during infection by helminths can potentially uncover novel therapeutic applications against inflammatory, autoimmune and neoplastic diseases.

Bioluminescent probe for detecting endogenous hypochlorite in living mice

As a kind of biologically important reactive oxygen species (ROS), hypochlorite (ClO^-) plays a crucial role in many physiological processes. As such, endogenous ClO^- is a powerful antibacterial agent during pathogen invasion. Nonetheless, excessive endogenous ClO^- could pose a health threat to mammalian animals including humans. However, the detection of endogenous ClO^- by bioluminescence probes in vivo remains a considerable challenge. Herein, based on a caged strategy, we developed a turn-on bioluminescent probe 1 for the highly selective detection of ClO^-in vitro and imaging endogenous ClO^- in a mouse inflammation model. We anticipate that such a probe could help us understand the role of endogenous ClO^- in a variety of physiological and pathological processes.

Neutrophils Which Migrate to Lymph Nodes Modulate CD4+ T Cell Response by a PD-L1 Dependent Mechanism

It is well known that neutrophils are rapidly recruited to a site of injury or infection and perform a critical role in pathogen clearance and inflammation. However, they are also able to interact with and regulate innate and adaptive immune cells and some stimuli induce the migration of neutrophils to lymph nodes (LNs). Previously, we demonstrated that the immune complex (IC) generated by injecting OVA into the footpad of OVA/CFA immunized mice induced the migration of OVA⁺ neutrophils to draining LNs (dLNs). Here we investigate the effects of these neutrophils which reach dLNs on CD4⁺ T cell response. Our findings here strongly support a dual role for neutrophils in dLNs regarding CD4⁺ T cell response modulation. On the one hand, the CD4⁺ T cell population expands after the influx of OVA⁺ neutrophils to dLNs. These CD4⁺ T cells enlarge their proliferative response, activation markers and IL-17 and IFN-γ cytokine production. On the other hand, these neutrophils also restrict CD4⁺ T cell expansion. The neutrophils in the dLNs upregulate PD-L1 molecules and are capable of suppressing CD4⁺ T cell proliferation. These results indicate that neutrophils migration to dLNs have an important role in the homeostasis of adaptive immunity. This report describes for the first time that the influx of neutrophils to dLNs dependent on IC presence improves CD4⁺ T cell response, at the same time controlling CD4⁺ T cell proliferation through a PD-L1 dependent mechanism.

Toll-Like Receptor 4, but Not Neutrophil Extracellular Traps, Promote IFN Type I Expression to Enhance Th2 Responses to Nippostrongylus brasiliensis

The induction of Th2 responses is thought to be multifactorial, and emerge from specific pathways distinct from those associated with antagonistic antibacterial or antiviral Th1 responses. Here, we show that the recognition of non-viable Nippostrongylus brasiliensis (Nb) in the skin induces a strong recruitment of monocytes and neutrophils and the release of neutrophil extracellular traps (NETs). Nb also activates toll-like receptor 4 (TLR4) signaling with expression of Ifnb transcripts in the skin and the development of an IFN type I signature on helminth antigen-bearing dendritic cells in draining lymph nodes. Co-injection of Nb together with about 10,000 Gram-negative bacteria amplified this TLR4-dependent but NET-independent IFN type I response and enhanced the development of Th2 responses. Thus, a limited activation of antibacterial signaling pathways is able to boost antihelminthic responses, suggesting a role for bacterial sensing in the optimal induction of Th2 immunity.

Th2/1 Hybrid Cells Occurring in Murine and Human Strongyloidiasis Share Effector Functions of Th1 Cells

Infections by the soil-transmitted threadworm Strongyloides stercoralis affect 30–100 million people worldwide, predominantly in tropic and sub-tropic regions. Here we assessed the T helper cell phenotypes in threadworm-infected patients and experimental murine infections with focus on CD4⁺ T cells co-expressing markers of Th2 and Th1 differentiation. We show that mice infected with the close relative S. ratti generate strong Th2 responses characterized by the expansion of CD4⁺ GATA-3⁺ cells expressing IL-4/-5/-13 in blood, spleen, gut-draining lymph nodes, lung and gut tissue. In addition to conventional Th2 cells, significantly increased frequencies of GATA-3⁺T-bet⁺ Th2/1-hybrid cells were detected in all organs and co-expressed Th2- and Th1-cytokines at intermediate levels. Assessing the phenotype of blood-derived CD4⁺ T cells from South Indian patients infected with S. stercoralis and local uninfected control donors we found that GATA-3 expressing Th2 cells were significantly increased in the patient cohort, coinciding with elevated eosinophil and IgE/IgG4 levels. A fraction of IL-4⁺CD4⁺ T cells simultaneously expressed IFN-γ hence displaying a Th2/1 hybrid phenotype. In accordance with murine Th2/1 cells, human Th2/1 cells expressed intermediate levels of Th2 cytokines. Contrasting their murine counterparts, human Th2/1 hybrids were marked by high levels of IFN-γ and rather low GATA-3 expression. Assessing the effector function of murine Th2/1 cells in vitro we found that Th2/1 cells were qualified for driving the classical activation of macrophages. Furthermore, Th2/1 cells shared innate, cytokine-driven effector functions with Th1 cells. Hence, the key findings of our study are that T helper cells with combined characteristics of Th2 and Th1 cells are integral to immune responses of helminth-infected mice, but also occur in helminth-infected humans and we suggest that Th2/1 cells are poised for the instruction of balanced immune responses during nematode infections.

The role of ILC2 in hookworm infection

Hookworm is a major public health concern, yet still relatively little is known about the immunological responses involved in human infection. Animal studies are mainly confined to using the natural rodent helminth Nippostrongylus brasiliensis as this has been proposed as the most accurate model of hookworm infection in the mouse, with both its life cycle and the immune responses it invokes having been extremely well characterized. In this review, we examine the roles that type 2 innate lymphoid cells (ILC2s) play in immunity and host tolerance to hookworm infection, particularly N. brasiliensis. This includes their role in the initiation and regulation of immune responses, as well as in the resolution and limitation of tissue damage required after an infection with a large organism, such as a helminth.

Key players in the immune response to biomaterial scaffolds for regenerative medicine

The compatibility of biomaterials is critical to their structural and biological function in medical applications. The immune system is the first responder to tissue trauma and to a biomaterial implant. The innate immune effector cells, most notably macrophages, play a significant role in the defense against foreign bodies and the formation of a fibrous capsule around synthetic implants. Alternatively, macrophages participate in the pro-regenerative capacity of tissue-derived biological scaffolds. Research is now elucidating the role of the adaptive immune system, and T cells in particular, in directing macrophage response to synthetic and biological materials. Here, we review basic immune cell types and discuss recent research on the role of the immune system in tissue repair and its potential relevance to scaffold design. We will also discuss new emerging immune cell types relevant to biomaterial responses and tissue repair. Finally, prospects for specifically targeting and modulating the immune response to biomaterial scaffolds for enhancing tissue repair and regeneration will be presented.

An overview of the role of neutrophils in innate immunity, inflammation and host-biomaterial integration

Despite considerable recent progress in defining neutrophil functions and behaviors in tissue repair, much remains to be determined with regards to its overall role in the tissue integration of biomaterials. This article provides an overview of the neutrophil’s numerous, important roles in both inflammation and resolution, and subsequently, their role in biomaterial integration. Neutrophils function in three primary capacities: generation of oxidative bursts, release of granules and formation of neutrophil extracellular traps (NETs); these combined functions enable neutrophil involvement in inflammation, macrophage recruitment, M2 macrophage differentiation, resolution of inflammation, angiogenesis, tumor formation and immune system activation. Neutrophils exhibit great flexibility to adjust to the prevalent microenvironmental conditions in the tissue; thus, the biomaterial composition and fabrication will potentially influence neutrophil behavior following confrontation. This review serves to highlight the neutrophil’s plasticity, reiterating that neutrophils are not just simple suicidal killers, but the true maestros of resolution and regeneration.

Neutrophil recruitment to lymph nodes limits local humoral response to Staphylococcus aureus

Neutrophils form the first line of host defense against bacterial pathogens. They are rapidly mobilized to sites of infection where they help marshal host defenses and remove bacteria by phagocytosis. While splenic neutrophils promote marginal zone B cell antibody production in response to administered T cell independent antigens, whether neutrophils shape humoral immunity in other lymphoid organs is controversial. Here we investigate the neutrophil influx following the local injection of Staphylococcus aureus adjacent to the inguinal lymph node and determine neutrophil impact on the lymph node humoral response. Using intravital microscopy we show that local immunization or infection recruits neutrophils from the blood to lymph nodes in waves. The second wave occurs temporally with neutrophils mobilized from the bone marrow. Within lymph nodes neutrophils infiltrate the medulla and interfollicular areas, but avoid crossing follicle borders. In vivo neutrophils form transient and long-lived interactions with B cells and plasma cells, and their depletion augments production of antigen-specific IgG and IgM in the lymph node. In vitro activated neutrophils establish synapse- and nanotube-like interactions with B cells and reduce B cell IgM production in a TGF- β1 dependent manner. Our data reveal that neutrophils mobilized from the bone marrow in response to a local bacterial challenge dampen the early humoral response in the lymph node.

Highly antibiotic resistant Staphylococcus aureus (S. aureus) are an important human pathogen and major cause of hospital acquired infections. An early host defense mechanism against bacterial infection is neutrophil recruitment, which helps eliminate the bacteria at the site of invasion. However, unless quickly neutralized, pathogens such as S. aureus can gain access to nearby lymph nodes via draining lymphatics. Lymph nodes protect the host by mobilizing additional resources that limit further pathogen dissemination. These include recruitment of neutrophils to the lymph node to directly target pathogens and the initiation of adaptive immune mechanisms, such as the humoral immune response, which transforms B lymphocytes capable of making pathogen specific antibodies into antibody producing plasma cells. Using a mouse model that allows direct visualization of lymphocytes, neutrophils, and fluorescently-labeled S. aureus in lymph nodes, we document the rapid appearance of bacteria in the lymph node following local S. aureus infection. We characterize the dynamic influx of neutrophils that occurs as a consequence and reveal direct B cell-neutrophil interactions within the lymph node parenchyma. We find that while lymph node neutrophils rapidly engage bacteria, they limit the subsequent humoral immune response likely by producing Transforming Growth Factor-β1, a factor known to limit B cell responses. These finding have important implication for our understanding of B cell responses against potent pathogens such as S. aureus and for the design of effective vaccines.

IL-17 and neutrophils: unexpected players in the type 2 immune response

The study of immunity to helminth infection has been central to understanding the function of type 2 cytokines and their targets. Although type 2 cytokines are considered anti-inflammatory and promote tissue repair, they also contribute to allergy and fibrosis. Here, we utilise data from helminth infection models, to illustrate that IL-17 and neutrophils, typically associated with pro-inflammatory responses, are intimately linked with type 2 immunity. Neutrophils work with IL-4Rα-activated macrophages to control incoming larvae but this comes at a cost of enhanced tissue damage. Chitinase like proteins (CLPs) bridge these diverse outcomes, inducing both protective IL-17 and reparative Th2 responses. Dysregulation of CLPs, IL-17 and neutrophils likely contribute to disease severity and pathology associated with type 2 immunity.

Perinatal exposure to a low dose of bisphenol A impaired systemic cellular immune response and predisposes young rats to intestinal parasitic infection

Perinatal exposure to the food contaminant bisphenol A (BPA) in rats induces long lasting adverse effects on intestinal immune homeostasis. This study was aimed at examining the immune response to dietary antigens and the clearance of parasites in young rats at the end of perinatal exposure to a low dose of BPA. Female rats were fed with BPA [5 µg/kg of body weight/day] or vehicle from gestational day 15 to pup weaning. Juvenile female offspring (day (D)25) were used to analyze immune cell populations, humoral and cellular responses after oral tolerance or immunization protocol to ovalbumin (OVA), and susceptibility to infection by the intestinal nematode Nippostrongylus brasiliensis (N. brasiliensis). Anti-OVA IgG titers following either oral tolerance or immunization were not affected after BPA perinatal exposure, while a sharp decrease in OVA-induced IFNγ secretion occurred in spleen and mesenteric lymph nodes (MLN) of OVA-immunized rats. These results are consistent with a decreased number of helper T cells, regulatory T cells and dendritic cells in spleen and MLN of BPA-exposed rats. The lack of cellular response to antigens questioned the ability of BPA-exposed rats to clear intestinal infections. A 1.5-fold increase in N. brasiliensis living larvae was observed in the intestine of BPA-exposed rats compared to controls due to an inappropriate Th1/Th2 cytokine production in infected jejunal tissues. These results show that perinatal BPA exposure impairs cellular response to food antigens, and increases susceptibility to intestinal parasitic infection in the juveniles. This emphasized the maturing immune system during perinatal period highly sensitive to low dose exposure to BPA, altering innate and adaptative immune response capacities in early life.

Neutrophils prime a long-lived effector macrophage phenotype that mediates accelerated helminth expulsion

We examined the role of innate cells in acquired resistance to the natural murine parasitic nematode, Nippostrongylus brasiliensis. Macrophages obtained from lungs as late as 45 d after N. brasiliensis inoculation were able to transfer accelerated parasite clearance to naive recipients. Primed macrophages adhered to larvae in vitro and triggered increased mortality of parasites. Neutrophil depletion in primed mice abrogated the protective effects of transferred macrophages and inhibited their in vitro binding to larvae. Neutrophils in parasite-infected mice showed a distinct transcriptional profile and promoted alternatively activated M2 macrophage polarization through secretory factors including IL-13. Differentially activated neutrophils in the context of a type 2 immune response therefore prime a long-lived effector macrophage phenotype that directly mediates rapid nematode damage and clearance.

Opportunities to exploit non-neutralizing HIV-specific antibody activity

Antibodies act as a nexus between innate and adaptive immunity: they provide a means to engage a spectrum of innate immune effector cells in order to clear viral particles and infected cells and prime antigen presentation. This functional landscape is remarkably complex, and depends on antibody isotype, subclass, and glycosylation; the expression levels and patterns of a suite of Fc receptors with both complementary and opposing activities; and a host of innate immune cells capable of differential responses to opsonized particles and present at different sites. In vivo, even neutralizing antibodies rely on their ability to act as molecular beacons and recruit innate immune effector cells in order to provide protection, and results from both human and macaque studies have implicated these effector functions in vaccinemediated protection. Thus, while enhancing effector function is a tractable handle for potentiating antibody-mediated protection from HIV infection, success will depend critically on leveraging understanding of the means by which antibodies with specific functional profiles could be elicited, which effector functions could provide optimal protection, and perhaps most critically, how to efficiently recruit the innate effector cells present at sites of infection.

Immune suppression by neutrophils and granulocytic myeloid-derived suppressor cells: similarities and differences

Neutrophils are essential effector cells in the host defense against invading pathogens. Recently, novel neutrophil functions have emerged in addition to their classical anti-microbial role. One of these functions is the suppression of T cell responses. In this respect, neutrophils share similarities with granulocytic myeloid-derived suppressor cells (G-MDSCs). In this review, we will discuss the similarities and differences between neutrophils and G-MDSCs. Various types of G-MDSCs have been described, ranging from immature to mature cells shaping the immune response by different immune suppressive mechanisms. However, all types of G-MDSCs share distinct features of neutrophils, such as surface markers and morphology. We propose that G-MDSCs are heterogeneous and represent novel phenotypes of neutrophils, capable of suppressing the immune response. In this review, we will attempt to clarify the differences and similarities between neutrophils and G-MDSCs and attempt to facilitate further research.

An essential role for TH2-type responses in limiting acute tissue damage during experimental helminth infection

Helminths induce potent Th2-type immune responses that can mediate worm expulsion but the importance of this response in controlling acute tissue damage caused by migrating multi-cellular parasites through vital tissues remains uncertain. We used a helminth infection model where parasitic nematode larvae migrate transiently through the lung causing damage resulting in hemorrhage and inflammation. Our findings showed initial elevations in IL-17 contributed to inflammation and lung damage while subsequent IL-4R signaling controlled IL-17 elevations, enhanced expression of insulin-like growth factor 1 and IL-10 and stimulated development of M2 cells, each of which contributed to rapid resolution of tissue damage. These studies indicate an essential role for the Th2-type immune response in mediating acute wound healing during helminth infection.

Network model of immune responses reveals key effectors to single and co-infection dynamics by a respiratory bacterium and a gastrointestinal helminth

Co-infections alter the host immune response but how the systemic and local processes at the site of infection interact is still unclear. The majority of studies on co-infections concentrate on one of the infecting species, an immune function or group of cells and often focus on the initial phase of the infection. Here, we used a combination of experiments and mathematical modelling to investigate the network of immune responses against single and co-infections with the respiratory bacterium Bordetella bronchiseptica and the gastrointestinal helminth Trichostrongylus retortaeformis. Our goal was to identify representative mediators and functions that could capture the essence of the host immune response as a whole, and to assess how their relative contribution dynamically changed over time and between single and co-infected individuals. Network-based discrete dynamic models of single infections were built using current knowledge of bacterial and helminth immunology; the two single infection models were combined into a co-infection model that was then verified by our empirical findings. Simulations showed that a T helper cell mediated antibody and neutrophil response led to phagocytosis and clearance of B. bronchiseptica from the lungs. This was consistent in single and co-infection with no significant delay induced by the helminth. In contrast, T. retortaeformis intensity decreased faster when co-infected with the bacterium. Simulations suggested that the robust recruitment of neutrophils in the co-infection, added to the activation of IgG and eosinophil driven reduction of larvae, which also played an important role in single infection, contributed to this fast clearance. Perturbation analysis of the models, through the knockout of individual nodes (immune cells), identified the cells critical to parasite persistence and clearance both in single and co-infections. Our integrated approach captured the within-host immuno-dynamics of bacteria-helminth infection and identified key components that can be crucial for explaining individual variability between single and co-infections in natural populations.

Infections with different infecting agents can alter the immune response against any one parasite and the relative abundance and persistence of the infections within the host. This is because the immune system is not compartmentalized but acts as a whole to allow the host to maintain control of the infections as well as repair damaged tissues and avoid immuno-pathology. There is no comprehensive understanding of the immune responses during co-infections and of how systemic and local mechanisms interact. Here we integrated experimental data with mathematical modelling to describe the network of immune responses of single and co-infection by a respiratory bacterium and a gastrointestinal helminth. We were able to identify key cells and functions responsible for clearing or reducing both parasites and showed that some mechanisms differed between type of infection as a result of different signal outputs and cells contributing to the immune processes. This study highlights the importance of understanding the immuno-dynamics of co-infection as a host response, how immune mechanisms differ from single infections and how they may alter parasite persistence, impact and abundance.

Soluble extract from the nematode Strongyloides stercoralis induces CXCR2 dependent/IL-17 independent neutrophil recruitment

Neutrophil recruitment via CXCR2 is required for innate and adaptive protective immunity to the larvae of Strongyloides stercoralis in mice. The goal of the present study was to determine the mechanism of CXCR2-mediated neutrophil recruitment to S. stercoralis. Mice deficient in the receptor for IL-17A and IL-17F, upstream mediators of CXCR2 ligand production, were infected with S. stercoralis larvae; there was no difference in larval survival, neutrophil recruitment, or production of CXCR2 ligands compared with wild type mice. In vivo and in vitro stimulation of neutrophils with S. stercoralis soluble extract resulted in significant neutrophil recruitment. In vitro assays demonstrated that the recruitment functioned through both chemokinesis and chemotaxis, was specific for CXCR2, and was a G protein-coupled response involving tyrosine kinase and PI3K. Finally, neutrophil stimulation with S. stercoralis soluble extract induced release of the CXCR2 ligands MIP-2 and KC from neutrophils, thereby potentially enhancing neutrophil recruitment.

Macroparasites, innate immunity and immunoregulation: developing natural models

Innate immune receptors carry out surveillance for infection threats and are a proximal controller of the threshold and intensity at which inflammatory responses occur. As such, they are a natural focus for understanding how inflammatory immune reactivity is regulated. This review highlights how little data there are relating to the effect of macroparasites on systemic innate receptor responses. The idea is developed that studies on innate immune function in wild animals exposed to a natural profile of infections, including macroparasites, might be a valuable model in which to test hypotheses about the ultimate cause of aberrant inflammation in modern human populations.

Granulocytes: effector cells or immunomodulators in the immune response to helminth infection?

Granulocytes are effector cells in defence against helminth infections. We review the current evidence for the role of granulocytes in protective immunity against different helminth infections and note that for each parasite species the role of granulocytes as effector cells can vary. Emerging evidence also points to granulocytes as immunomodulatory cells able to produce many cytokines, chemokines and modulatory factors which can bias the immune response in a particular direction. Thus, the role of granulocytes in an immunomodulatory context is discussed including the most recent data that points to an important role for basophils under this guise.

Osteopontin promotes fibrosis in dystrophic mouse muscle by modulating immune cell subsets and intramuscular TGF-beta

Duchenne muscular dystrophy (DMD) is an X-linked, degenerative muscle disease that is exacerbated by secondary inflammation. Here, we characterized the immunological milieu of dystrophic muscle in mdx mice, a model of DMD, to identify potential therapeutic targets. We identified a specific subpopulation of cells expressing the Vbeta8.1/8.2 TCR that is predominant among TCR-beta+ T cells. These cells expressed high levels of osteopontin (OPN), a cytokine that promotes immune cell migration and survival. Elevated OPN levels correlated with the dystrophic process, since OPN was substantially elevated in the serum of mdx mice and muscle biopsies after disease onset. Muscle biopsies from individuals with DMD also had elevated OPN levels. To test the role of OPN in mdx muscle, mice lacking both OPN and dystrophin were generated and termed double-mutant mice (DMM mice). Reduced infiltration of NKT-like cells and neutrophils was observed in the muscle of DMM mice, supporting an immunomodulatory role for OPN in mdx muscle. Concomitantly, an increase in CD4+ and FoxP3+ Tregs was also observed in DMM muscle, which also showed reduced levels of TGF-beta, a known fibrosis mediator. These inflammatory changes correlated with increased strength and reduced diaphragm and cardiac fibrosis. These studies suggest that OPN may be a promising therapeutic target for reducing inflammation and fibrosis in individuals with DMD.

Dynamics of neutrophil migration in lymph nodes during infection

Although the signals that control neutrophil migration from the blood to sites of infection have been well characterized, little is known about their migration patterns within lymph nodes or the strategies that neutrophils use to find their local sites of action. To address these questions, we used two-photon scanning-laser microscopy to examine neutrophil migration in intact lymph nodes during infection with an intracellular parasite, Toxoplasma gondii. We found that neutrophils formed both small, transient and large, persistent swarms via a coordinated migration pattern. We provided evidence that cooperative action of neutrophils and parasite egress from host cells could trigger swarm formation. Neutrophil swarm formation coincided in space and time with the removal of macrophages that line the subcapsular sinus of the lymph node. Our data provide insights into the cellular mechanisms underlying neutrophil swarming and suggest new roles for neutrophils in shaping immune responses.