Alternative macrophage activation is essential for survival during schistosomiasis and downmodulates T helper 1 responses and immunopathology

Fibrosis is regulated by Th2 and Th17 responses and by dynamic interactions between fibroblasts and macrophages

Dysregulated wound healing leads to fibrosis, whereby fibroblasts synthesize excess extracellular matrix and scarring impairs proper organ function. Although fibrotic diseases arise from diverse causes and display heterogeneous features, fibrosis commonly associates with chronic inflammation. Recent discoveries reinforce the idea that communication between fibroblasts, macrophages, and CD4 T cells integrates the processes of wound healing and host defense. Signals between macrophages and fibroblasts can exacerbate, suppress, or reverse fibrosis. Fibroblasts and macrophages are activated by T cells, but their activation also engages negative feedback loops that reduce fibrosis by restraining the immune response, particularly when the Th2 cytokine IL-13 contributes to pathology. Thus the interactions among fibroblasts, macrophages, and CD4 T cells likely play general and critical roles in initiating, perpetuating, and resolving fibrosis in both experimental and clinical conditions.

Regulation of homeostasis and inflammation in the intestine

The gastrointestinal tract is the largest immune interface with the environment. Exposure to large numbers of dietary and microbial antigens requires complex and highly regulated immune responses by different mucosal cell types, which result in the induction and maintenance of intestinal homeostasis. Defects in this equilibrium can disrupt the homeostatic mechanisms and lead to chronic intestinal inflammation. We review the cell populations and mechanisms involved in the control of intestinal homeostasis and inflammation, focusing on inflammatory bowel diseases. We describe some aspects of gut immunity that could alter the delicate balance between inflammatory and tolerogenic responses and result in chronic gastrointestinal tract inflammation in patients.

Obstacles and opportunities for understanding macrophage polarization

Macrophages are now routinely categorized into phenotypic subtypes based on gene expression induced in response to cytokine and pathogen-derived stimulation. In the broadest division, macrophages are described as being CAMs (M1 macrophages) or AAMs (M2 macrophages) based on their exposure to TLR and IFN signals or Th2 cytokines, respectively. Despite the prolific use of this simple classification scheme, little is known about the precise functions of effector molecules produced by AAMs, especially how representative the CAM and AAM subtypes are of tissue macrophages in homeostasis, infection, or tissue repair and how plasticity in gene expression regulates macrophage function in vivo. Furthermore, correlations between mouse and human tissue macrophages and their representative subtypes are lacking and are a major barrier to understanding human immunity. Here, we briefly summarize current features of macrophage polarization and discuss the roles of various macrophage subpopulations and macrophage-associated genes in health and disease.

Multiple helminth infection of the skin causes lymphocyte hypo-responsiveness mediated by Th2 conditioning of dermal myeloid cells

Infection of the mammalian host by schistosome larvae occurs via the skin, although nothing is known about the development of immune responses to multiple exposures of schistosome larvae, and/or their excretory/secretory (E/S) products. Here, we show that multiple (4x) exposures, prior to the onset of egg laying by adult worms, modulate the skin immune response and induce CD4⁺ cell hypo-responsiveness in the draining lymph node, and even modulate the formation of hepatic egg-induced granulomas. Compared to mice exposed to a single infection (1x), dermal cells from multiply infected mice (4x), were less able to support lymph node cell proliferation. Analysis of dermal cells showed that the most abundant in 4x mice were eosinophils (F4/80⁺MHC-II⁻), but they did not impact the ability of antigen presenting cells (APC) to support lymphocyte proliferation to parasite antigen in vitro. However, two other cell populations from the dermal site of infection appear to have a critical role. The first comprises arginase-1⁺, Ym-1⁺ alternatively activated macrophage-like cells, and the second are functionally compromised MHC-II^hi cells. Through the administration of exogenous IL-12 to multiply infected mice, we show that these suppressive myeloid cell phenotypes form as a consequence of events in the skin, most notably an enrichment of IL-4 and IL-13, likely resulting from an influx of RELMα-expressing eosinophils. We further illustrate that the development of these suppressive dermal cells is dependent upon IL-4Rα signalling. The development of immune hypo-responsiveness to schistosome larvae and their effect on the subsequent response to the immunopathogenic egg is important in appreciating how immune responses to helminth infections are modulated by repeated exposure to the infective early stages of development.

Schistosomiasis is a major helminth disease that infects more than 200 million people in the tropics. Free-swimming aquatic cercariae infect through the skin after contact with contaminated water, and in endemic areas this can occur frequently. However, nothing is known about how multiple exposures affects innate immunity in the skin, and/or whether it impacts the acquired immune response. Consequently, we have developed an infection model in the mouse to examine the immune response to multiple infections prior to the production of eggs. We show that multiple exposures to schistosome larvae cause lymphocyte hypo-responsiveness, partly mediated by macrophages and dendritic cells from the skin which have a ‘down-regulated’ phenotype and are not able to act as efficient antigen presenting cells (APCs). These regulated APCs are conditioned amongst high levels of the cytokines IL-4 and IL-13 which follow an influx of abundant eosinophils. In the absence of the regulatory APCs, and in the absence of the common receptor chain for IL-4 and IL-13 (i.e. IL-4Rα), lymphocyte proliferation is restored. These findings are important in understanding how dermal immune responses are modulated so that we can devise new strategies for vaccine delivery, or the treatment of chronic inflammatory conditions of the skin.

From inflammation to wound healing: using a simple model to understand the functional versatility of murine macrophages

Macrophages are fundamental cells of the innate immune system. Their activation is essential for such distinct immune functions as inflammation (pathogen-killing) and tissue repair (wound healing). An open question has been the functional stability of an individual macrophage cell: whether it can change its functional profile between different immune responses such as between the repair pathway and the inflammatory pathway. We studied this question theoretically by constructing a rate equation model for the key substrate, enzymes and products of the pathways; we then tested the model experimentally. Both our model and experiments show that individual macrophages can switch from the repair pathway to the inflammation pathway but that the reverse switch does not occur.

Interleukin-13 (IL-13)/IL-13 receptor alpha1 (IL-13Ralpha1) signaling regulates intestinal epithelial cystic fibrosis transmembrane conductance regulator channel-dependent Cl- secretion

Interleukin-13 (IL-13) has been linked to the pathogenesis of inflammatory diseases of the gastrointestinal tract. It is postulated that IL-13 drives inflammatory lesions through the modulation of both hematopoietic and nonhematopoietic cell function in the intestine. To delineate the relevant contribution of elevated levels of intestinal IL-13 to intestinal structure and function, we generated an intestinal IL-13 transgenic mouse (iIL-13Tg). We show that constitutive overexpression of IL-13 in the small bowel induces modification of intestinal epithelial architecture (villus blunting, goblet cell hyperplasia, and increased epithelial proliferation) and epithelial function (altered basolateral → apical Cl(-) ion conductance). Pharmacological analyses in vitro and in vivo determined that elevated Cl(-) conductance is mediated by altered cystic fibrosis transmembrane conductance regulator expression and activity. Generation of iIL-13Tg/Il13rα1(-/-), iIL-13Tg/Il13rα2(-/-), and iIL-13Tg/Stat6(-/-) mice revealed that IL-13-mediated dysregulation of epithelial architecture and Cl(-) conductance is dependent on IL-13Rα1 and STAT-6. These observations demonstrate a central role for the IL-13/IL-13Rα1 pathway in the regulation of intestinal epithelial cell Cl(-) secretion via up-regulation of cystic fibrosis transmembrane conductance regulator, suggesting an important role for this pathway in secretory diarrhea.

Croix CM, Ray A, Ray P. Rapid host defense against Aspergillus fumigatus involves alveolar macrophages with a predominance of alternatively activated phenotype

The ubiquitous fungus Aspergillus fumigatus is associated with chronic diseases such as invasive pulmonary aspergillosis in immunosuppressed patients and allergic bronchopulmonary aspergillosis (ABPA) in patients with cystic fibrosis or severe asthma. Because of constant exposure to this fungus, it is critical for the host to exercise an immediate and decisive immune response to clear fungal spores to ward off disease. In this study, we observed that rapidly after infection by A. fumigatus, alveolar macrophages predominantly express Arginase 1 (Arg1), a key marker of alternatively activated macrophages (AAMs). The macrophages were also found to express Ym1 and CD206 that are also expressed by AAMs but not NOS2, which is expressed by classically activated macrophages. The expression of Arg1 was reduced in the absence of the known signaling axis, IL-4Rα/STAT6, for AAM development. While both Dectin-1 and TLR expressed on the cell surface have been shown to sense A. fumigatus, fungus-induced Arg1 expression in CD11c⁺ alveolar macrophages was not dependent on either Dectin-1 or the adaptor MyD88 that mediates intracellular signaling by most TLRs. Alveolar macrophages from WT mice efficiently phagocytosed fungal conidia, but those from mice deficient in Dectin-1 showed impaired fungal uptake. Depletion of macrophages with clodronate-filled liposomes increased fungal burden in infected mice. Collectively, our studies suggest that alveolar macrophages, which predominantly acquire an AAM phenotype following A. fumigatus infection, have a protective role in defense against this fungus.

BALB/c mice deficient in CD4 T cell IL-4Rα expression control Leishmania mexicana Load although female but not male mice develop a healer phenotype

Immunologically intact BALB/c mice infected with Leishmania mexicana develop non-healing progressively growing lesions associated with a biased Th2 response while similarly infected IL-4Rα-deficient mice fail to develop lesions and develop a robust Th1 response. In order to determine the functional target(s) for IL-4/IL-13 inducing non-healing disease, the course of L. mexicana infection was monitored in mice lacking IL-4Rα expression in specific cellular compartments. A deficiency of IL-4Rα expression on macrophages/neutrophils (in LysM(cre)IL-4Rα(-/lox) animals) had minimal effect on the outcome of L. mexicana infection compared with control (IL-4Rα(-/flox)) mice. In contrast, CD4(+) T cell specific (Lck(cre)IL-4Rα(-/lox)) IL-4Rα(-/-) mice infected with L. mexicana developed small lesions, which subsequently healed in female mice, but persisted in adult male mice. While a strong Th1 response was manifest in both male and female CD4(+) T cell specific IL-4Rα(-/-) mice infected with L. mexicana, induction of IL-4 was manifest in males but not females, independently of CD4(+) T cell IL-4 responsiveness. Similar results were obtained using pan-T cell specific (iLck(cre)IL-4Rα(-/lox)) IL-4Rα(-/-) mice. Collectively these data demonstrate that upon infection with L. mexicana, initial lesion growth in BALB/c mice is dependent on non-T cell population(s) responsive to IL-4/IL-13 while progressive infection is dependent on CD4(+) T cells responsive to IL-4.

IL-4Rα-responsive smooth muscle cells contribute to initiation of TH2 immunity and pulmonary pathology in Nippostrongylus brasiliensis infections

Nippostrongylus brasiliensis infections generate pulmonary pathologies that can be associated with strong T(H)2 polarization of the host's immune response. We present data demonstrating N. brasiliensis-driven airway mucus production to be dependent on smooth muscle cell interleukin 4 receptor-α (IL-4Rα) responsiveness. At days 7 and 10 post infection (PI), significant airway mucus production was found in IL-4Rα(-/lox) control mice, whereas global knockout (IL-4Rα(-/-)) and smooth muscle-specific IL-4Rα-deficient mice (SM-MHC(Cre) IL-4Rα(-/lox)) showed reduced airway mucus responses. Furthermore, interleukin (IL)-13 and IL-5 cytokine production in SM-MHC(Cre) IL-4Rα(-/lox) mice was impaired along with a transient reduction in T-cell numbers in the lung. In vitro treatment of smooth muscle cells with secreted N. brasiliensis excretory-secretory antigen (NES) induced IL-6 production. Decreased protein kinase C (PKC)-dependent smooth muscle cell proliferation associated with cell cycle arrest was found in cells stimulated with NES. Together, these data demonstrate that both IL-4Rα and NES-driven responses by smooth muscle cells make important contributions in initiating T(H)2 responses against N. brasiliensis infections.

gp130 on macrophages/granulocytes modulates inflammation during experimental tuberculosis

gp130 is a common receptor chain for cytokines such as interleukin (IL)-27 and IL-6. During experimental tuberculosis (TB), IL-27 prevents optimal antimycobacterial protection and limits the pathological sequelae of chronic inflammation. The anti-inflammatory properties of IL-27 have been attributed mainly to its suppressive effect on T helper (TH) cells. However, because gp130 cytokines also suppress the inflammatory immune response of macrophages, IL-27 may also regulate inflammation by limiting the secretion of pro-inflammatory cytokines. To specifically address the role of gp130 cytokines on macrophages, the outcome of experimental TB was analysed in macrophage/neutrophil-specific gp130-deficient (LysM(cre) gp130(loxP/loxP)) mice. In these mice, the enhanced induction of inflammatory cytokines and increased expression of the inducible nitric oxide synthase (NOS2) and LRG47 was linked to a greatly augmented TH17 immune response and matrix metalloproteinase (MMP)-9 expression. However, this amplified inflammatory immune response in Mtb-infected LysM(cre) gp130(loxP/loxP) mice was not associated with reduced bacterial loads and/or accelerated pathology. Our study revealed an immunoregulatory function of gp130 cytokines on macrophages/granulocytes, which is, however, not critical for modulating the outcome of TB.

CD28 and IL-4: two heavyweights controlling the balance between immunity and inflammation

The costimulatory receptor CD28 and IL-4Rα-containing cytokine receptors play key roles in controlling the size and quality of pathogen-specific immune responses. Thus, CD28-mediated costimulation is needed for effective primary T-cell expansion and for the generation and activation of regulatory T-cells (Treg cells), which protect from immunopathology. Similarly, IL-4Rα signals are required for alternative activation of macrophages, which counteract inflammation by type 1 responses. Furthermore, immune modulation by CD28 and IL-4 is interconnected through the promotion of IL-4 producing T-helper 2 cells by CD28 signals. Using conditionally IL-4Rα and CD28 deleting mice, as well as monoclonal antibodies, which block or stimulate CD28, or mAb that deplete Treg cells, we have studied the roles of CD28 and IL-4Rα in experimental mouse models of virus (influenza), intracellular bacteria (L. monocytogenes, M. tuberculosis), and parasite infections (T. congolense, L. major). We observed that in some, but not all settings, Treg cells and type 2 immune deviation, including activation of alternative macrophages can be manipulated to protect the host either from infection or from immunopathology with an overall beneficial outcome. Furthermore, we provide direct evidence that secondary CD8 T-cell responses to i.c. bacteria are dependent on CD28-mediated costimulation.

Interleukin-25 fails to activate STAT6 and induce alternatively activated macrophages

Interleukin-25 (IL-25), a T helper type 2 (Th2) -related factor, inhibits the production of inflammatory cytokines by monocytes/macrophages. Since Th2 cytokines antagonize classically activated monocytes/macrophages by inducing alternatively activated macrophages (AAMs), we here assessed the effect of IL-25 on the alternative activation of human monocytes/macrophages. The interleukins IL-25, IL-4 and IL-13 were effective in reducing the expression of inflammatory chemokines in monocytes. This effect was paralleled by induction of AAMs in cultures added with IL-4 or IL-13 but not with IL-25, regardless of whether cells were stimulated with lipopolysaccharide or interferon-γ. Moreover, pre-incubation of cells with IL-25 did not alter the ability of both IL-4 and IL-13 to induce AAMs. Both IL-4 and IL-13 activated signal transducer and activator of transcription 6 (STAT6), and silencing of this transcription factor markedly reduced the IL-4/IL-13-driven induction of AAMs. In contrast, IL-25 failed to trigger STAT6 activation. Among Th2 cytokines, only IL-25 and IL-10 were able to activate p38 mitogen-activated protein kinase. These results collectively indicate that IL-25 fails to induce AAMs and that Th2-type cytokines suppress inflammatory responses in human monocytes by activating different intracellular signalling pathways.

CD11c depletion severely disrupts Th2 induction and development in vivo

Although dendritic cells (DCs) are adept initiators of CD4⁺ T cell responses, their fundamental importance in this regard in Th2 settings remains to be demonstrated. We have used CD11c–diphtheria toxin (DTx) receptor mice to deplete CD11c⁺ cells during the priming stage of the CD4⁺ Th2 response against the parasitic helminth Schistosoma mansoni. DTx treatment significantly depleted CD11c⁺ DCs from all tissues tested, with 70–80% efficacy. Even this incomplete depletion resulted in dramatically impaired CD4⁺ T cell production of Th2 cytokines, altering the balance of the immune response and causing a shift toward IFN-γ production. In contrast, basophil depletion using Mar-1 antibody had no measurable effect on Th2 induction in this system. These data underline the vital role that CD11c⁺ antigen-presenting cells can play in orchestrating Th2 development against helminth infection in vivo, a response that is ordinarily balanced so as to prevent the potentially damaging production of inflammatory cytokines.

Peroxiredoxin: a central player in immune modulation

Peroxiredoxins (Prx) are a family of anti-oxidants that protect cells from metabolically produced reactive oxygen species (ROS). The presence of these enzymes in the secretomes of many parasitic helminths suggests they provide protection against ROS released by host immune effector cells. However, we recently reported that helminth-secreted Prx also contribute to the development of Th2-responses via a mechanism involving the induction of alternatively activated macrophages. In this review, we discuss the role helminth Prx may play in modulating the immune responses of their hosts.

Macrophages: master regulators of inflammation and fibrosis

Macrophages are found in close proximity with collagen-producing myofibroblasts and indisputably play a key role in fibrosis. They produce profibrotic mediators that directly activate fibroblasts, including transforming growth factor-beta1 and platelet-derived growth factor, and control extracellular matrix turnover by regulating the balance of various matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases. Macrophages also regulate fibrogenesis by secreting chemokines that recruit fibroblasts and other inflammatory cells. With their potential to act in both a pro- and antifibrotic capacity, as well as their ability to regulate the activation of resident and recruited myofibroblasts, macrophages and the factors they express are integrated into all stages of the fibrotic process. These various, and sometimes opposing, functions may be performed by distinct macrophage subpopulations, the identification of which is a growing focus of fibrosis research. Although collagen-secreting myofibroblasts once were thought of as the master "producers" of fibrosis, this review will illustrate how macrophages function as the master "regulators" of fibrosis.

Alternatively activated macrophages inhibit T-cell proliferation by Stat6-dependent expression of PD-L2

Alternatively activated macrophages (AAM) accumulate in tissues during Th2-associated immune responses like helminth infections and allergic disorders. These cells differentiate in response to interleukin 4 (IL-4)/IL-13-mediated activation of Stat6 and possess potent inhibitory activity against T cells. The molecular mechanism that leads to T-cell suppression remains unclear and could involve soluble factors or inhibitory ligands. Microarray analysis revealed that the inhibitory ligand, programmed death ligand 2 (PD-L2) was strongly induced by IL-4 in macrophages from wild-type but not Stat6-deficient mice. PD-L2 expression correlated with other established markers for AAM-like Relm-α/Fizz1, arginase1, or Ym1 and thereby serves as useful surface marker to identify and isolate AAM from tissues. Antibodies against PD-L2 blocked the inhibitory activity of AAM and retroviral expression of PD-L2 in macrophages from Stat6(-/-) mice was sufficient to inhibit T-cell proliferation, which demonstrates that PD-L2 mediates potent and nonredundant inhibition of T cells independently of other Stat6-regulated genes. Infection of conditional IL-4/IL-13-deficient mice with the helminth Nippostrongylus brasiliensis further showed that PD-L2 expression was dependent on IL-4/IL-13 from Th2 cells. In vivo blockade of PD-L2 during N brasiliensis infection caused an enhanced Th2 response in the lung, indicating that AAM inhibit Th2 cells by expression of PD-L2.

Expression of IL-4 receptor alpha on smooth muscle cells is not necessary for development of experimental allergic asthma

BACKGROUND

Airflow in the lungs of patients with allergic asthma is impaired by excessive mucus production and airway smooth muscle contractions. Elevated levels of the cytokines IL-4 and IL-13 are associated with this pathology. In vitro studies have suggested that IL-4 receptor alpha (IL-4Ralpha) signaling on smooth muscle cells is critical for airway inflammation and airway hyperresponsiveness.

OBJECTIVE

To define the contribution of IL-4 and IL-13 to the onset of asthmatic pathology, the role of their key receptor IL-4Ralpha in smooth muscle cells was examined in vivo.

METHODS

By using transgenic smooth muscle myosin heavy chain(cre)IL-4Ralpha(-/lox) mice deficient in IL-4Ralpha in smooth muscle cells, in vivo effects of impaired IL-4Ralpha signaling in smooth muscle cells on the outcome of asthmatic disease were investigated for the first time. Allergic asthma was introduced in mice by repeated sensitization with ovalbumin/aluminum hydroxide on days 0, 7, and 14, followed by intranasal allergen challenge on days 21 to 23. Mice were investigated for the presence of airway hyperresponsiveness, airway inflammation, allergen-specific antibody production, T(h)2-type cytokine responses, and lung pathology.

RESULTS

Airway hyperresponsiveness, airway inflammation, mucus production, T(h)2 cytokine production, and specific antibody responses were unaffected in smooth muscle myosin heavy chain(cre)IL-4Ralpha(-/lox) mice compared with control animals.

CONCLUSION

The impairment of IL-4Ralpha on smooth muscle cells had no effect on major etiologic markers of allergic asthma. These findings suggest that IL-4Ralpha responsiveness in airway smooth muscle cells during the early phase of allergic asthma is not, as suggested, necessary for the outcome of the disease.

Protection of cattle against a natural infection of Fasciola hepatica by vaccination with recombinant cathepsin L1 (rFhCL1)

The liver fluke, Fasciola hepatica causes liver fluke disease, or fasciolosis, in ruminants such as cattle and sheep. An effective vaccine against the helminth parasite is essential to reduce our reliance on anthelmintics, particularly in light of frequent reports of resistance to some frontline drugs. In our study, Friesian cattle (13 per group) were vaccinated with recombinant F. hepatica cathepsin L1 protease (rFhCL1) formulated in mineral-oil based adjuvants, Montanide ISA 70VG and ISA 206VG. Following vaccination the animals were exposed to fluke-contaminated pastures for 13 weeks. At slaughter, there was a significant reduction in fluke burden of 48.2% in the cattle in both vaccinated groups, relative to the control non-vaccinated group, at p<or=0.05. All vaccinated animals showed a sharp rise in total IgG levels to rFhCL1 post-vaccination which was maintained over the course of the 13-week challenge infection and was significantly higher than levels reached in the control group. Arginase levels in the macrophages of vaccinated cattle were significantly lower than those of the control cattle, indicating that the parasite-induced alternative-activation of the macrophages was altered by vaccination. The data demonstrate the potential for recombinant FhCL1 vaccine in controlling fasciolosis in cattle under field conditions.

IL-4R{alpha}-responsive smooth muscle cells increase intestinal hypercontractility and contribute to resistance during acute Schistosomiasis

Interleukin-(IL)-4 and IL-13 signal through heterodimeric receptors containing a common IL-4 receptor-alpha (IL-4Ralpha) subunit, which is important for protection against helminth infections, including schistosomiasis. Previous studies demonstrated important roles for IL-4Ralpha-responsive hematopoietic cells, including T cells and macrophages in schistosomiasis. In this study, we examined the role of IL-4Ralpha responsiveness by nonhematopoietic smooth muscle cells during experimental acute murine schistosomiasis. Comparative Schistosoma mansoni infection studies with smooth muscle cell-specific IL-4Ralpha-deficient (SM-MHC(cre)IL-4Ralpha(-/flox)) mice, heterozygous control (IL-4Ralpha(-/flox)) mice, and global IL-4Ralpha-deficient (IL-4Ralpha(-/-)) mice were conducted. S. mansoni-infected SM-MHC(cre)IL-4Ralpha(-/flox) mice showed increased weight loss and earlier mortalities compared with IL-4Ralpha(-/flox) mice, despite comparable T(H)2/type 2 immune responses. In contrast to highly susceptible IL-4Ralpha-deficient mice, increased susceptibility in SM-MHC(cre)IL-4Ralpha(-/flox) mice was not accompanied by intestinal tissue damage and subsequent sepsis. However, both susceptible mutant mouse strains failed to efficiently expel eggs, demonstrated by egg reduction in the feces compared with control mice. Reduced egg expulsion was accompanied by impaired IL-4/IL-13-mediated hypercontractile intestinal responses, which was present in the more resistant control mice. Together, we conclude that IL-4Ralpha responsiveness by smooth muscle cells and subsequent IL-4- and IL-13-mediated hypercontractility are required for host protection during acute schistosomiasis to efficiently expel S. mansoni eggs and to prevent premature mortality.

Alternative activation of macrophages: mechanism and functions

The concept of an alternative pathway of macrophage activation has stimulated interest in its definition, mechanism, and functional significance in homeostasis and disease. We assess recent research in this field, argue for a restricted definition, and explore pathways by which the T helper 2 (Th2) cell cytokines interleukin-4 (IL-4) and IL-13 mediate their effects on macrophage cell biology, their biosynthesis, and responses to a normal and pathological microenvironment. The stage is now set to gain deeper insights into the role of alternatively activated macrophages in immunobiology.

IL-4Ralpha-independent expression of mannose receptor and Ym1 by macrophages depends on their IL-10 responsiveness

IL-4Ralpha-dependent responses are essential for granuloma formation and host survival during acute schistosomiasis. Previously, we demonstrated that mice deficient for macrophage-specific IL-4Ralpha (LysM(cre)Il4ra(-/lox)) developed increased hepatotoxicity and gut inflammation; whereas inflammation was restricted to the liver of mice lacking T cell-specific IL-4Ralpha expression (iLck(cre)Il4ra(-/lox)). In the study presented here we further investigated their role in liver granulomatous inflammation. Frequencies and numbers of macrophage, lymphocyte or granulocyte populations, as well as Th1/Th2 cytokine responses were similar in Schistosoma mansoni-infected LysM(cre)Il4ra(-/lox) liver granulomas, when compared to Il4ra(-/lox) control mice. In contrast, a shift to Th1 responses with high IFN-gamma and low IL-4, IL-10 and IL-13 was observed in the severely disrupted granulomas of iLck(cre)Il4ra(-/lox) and Il4ra(-/-) mice. As expected, alternative macrophage activation was reduced in both LysM(cre)Il4ra(-/lox) and iLck(cre)Il4ra(-/lox) granulomas with low arginase 1 and heightened nitric oxide synthase RNA expression in granuloma macrophages of both mouse strains. Interestingly, a discrete subpopulation of SSC(high)CD11b+I-A/I-E(high)CD204+ macrophages retained expression of mannose receptor (MMR) and Ym1 in LysM(cre)Il4ra(-/lox) but not in iLck(cre)Il4ra(-/lox) granulomas. While aaMphi were in close proximity to the parasite eggs in Il4ra(-/lox) control mice, MMR+Ym1+ macrophages in LysM(cre)Il4ra(-/lox) mice were restricted to the periphery of the granuloma, indicating that they might have different functions. In vivo IL-10 neutralisation resulted in the disappearance of MMR+Ym1+ macrophages in LysM(cre)Il4ra(-/lox) mice. Together, these results show that IL-4Ralpha-responsive T cells are essential to drive alternative macrophage activation and to control granulomatous inflammation in the liver. The data further suggest that in the absence of macrophage-specific IL-4Ralpha signalling, IL-10 is able to drive mannose receptor- and Ym1-positive macrophages, associated with control of hepatic granulomatous inflammation.

Arginase I suppresses IL-12/IL-23p40-driven intestinal inflammation during acute schistosomiasis

Alternatively activated macrophages prevent lethal intestinal pathology caused by worm ova in mice infected with the human parasite Schistosoma mansoni through mechanisms that are currently unclear. This study demonstrates that arginase I (Arg I), a major product of IL-4- and IL-13-induced alternatively activated macrophages, prevents cachexia, neutrophilia, and endotoxemia during acute schistosomiasis. Specifically, Arg I-positive macrophages promote TGF-beta production and Foxp3 expression, suppress Ag-specific T cell proliferation, and limit Th17 differentiation. S. mansoni-infected Arg I-deficient bone marrow chimeras develop a marked accumulation of worm ova within the ileum but impaired fecal egg excretion compared with infected wild-type bone marrow chimeras. Worm ova accumulation in the intestines of Arg I-deficient bone marrow chimeras was associated with intestinal hemorrhage and production of molecules associated with classical macrophage activation (increased production of IL-6, NO, and IL-12/IL-23p40), but whereas inhibition of NO synthase-2 has marginal effects, IL-12/IL-23p40 neutralization abrogates both cachexia and intestinal inflammation and reduces the number of ova within the gut. Thus, macrophage-derived Arg I protects hosts against excessive tissue injury caused by worm eggs during acute schistosomiasis by suppressing IL-12/IL-23p40 production and maintaining the Treg/Th17 balance within the intestinal mucosa.

Regulation of learning and memory by meningeal immunity: a key role for IL-4

Proinflammatory cytokines have been shown to impair cognition; consequently, immune activity in the central nervous system was considered detrimental to cognitive function. Unexpectedly, however, T cells were recently shown to support learning and memory, though the underlying mechanism was unclear. We show that one of the steps in the cascade of T cell-based support of learning and memory takes place in the meningeal spaces. Performance of cognitive tasks led to accumulation of IL-4-producing T cells in the meninges. Depletion of T cells from meningeal spaces skewed meningeal myeloid cells toward a proinflammatory phenotype. T cell-derived IL-4 was critical, as IL-4(-/-) mice exhibited a skewed proinflammatory meningeal myeloid cell phenotype and cognitive deficits. Transplantation of IL-4(-/-) bone marrow into irradiated wild-type recipients also resulted in cognitive impairment and proinflammatory skew. Moreover, adoptive transfer of T cells from wild-type into IL-4(-/-) mice reversed cognitive impairment and attenuated the proinflammatory character of meningeal myeloid cells. Our results point to a critical role for T cell-derived IL-4 in the regulation of cognitive function through meningeal myeloid cell phenotype and brain-derived neurotrophic factor expression. These findings might lead to the development of new immune-based therapies for cognitive impairment associated with immune decline.

Macrophages and immunologic inflammation of the kidney

Monocyte-derived tissue effector cells, macrophages, are present in large numbers in all forms of kidney disease with inflammation. Their roles in inflammation and the molecular effectors of macrophage function have been difficult to decipher. With the advent of modern genetic tools and mouse models of human disease, great insight into monocyte/macrophage biology has been forthcoming. This review places macrophage study in its historical context, defines immunologic diseases of the kidney, broadens its definition to encompass current thinking of the immune response to kidney injury, highlights key advances of the study of monocyte/macrophages in kidney diseases, and identifies new therapeutic pathways and targets that hinge around macrophage function. This article advances the case that targeting macrophage activation and phenotype is leading to new therapies in the treatment of many acute and chronic kidney diseases.

Blood fluke exploitation of non-cognate CD4+ T cell help to facilitate parasite development

Schistosoma blood flukes, which infect over 200 million people globally, co-opt CD4⁺ T cell-dependent mechanisms to facilitate parasite development and egg excretion. The latter requires Th2 responses, while the mechanism underpinning the former has remained obscure. Using mice that are either defective in T cell receptor (TCR) signaling or that lack TCRs that can respond to schistosomes, we show that naïve CD4⁺ T cells facilitate schistosome development in the absence of T cell receptor signaling. Concurrently, the presence of naïve CD4⁺ T cells correlates with both steady-state changes in the expression of genes that are critical for the development of monocytes and macrophages and with significant changes in the composition of peripheral mononuclear phagocyte populations. Finally, we show that direct stimulation of the mononuclear phagocyte system restores blood fluke development in the absence of CD4⁺ T cells. Thus we conclude that schistosomes co-opt innate immune signals to facilitate their development and that the role of CD4⁺ T cells in this process may be limited to the provision of non-cognate help for mononuclear phagocyte function. Our findings have significance for understanding interactions between schistosomiasis and other co-infections, such as bacterial infections and human immunodeficiency virus infection, which potently stimulate innate responses or interfere with T cell help, respectively. An understanding of immunological factors that either promote or inhibit schistosome development may be valuable in guiding the development of efficacious new therapies and vaccines for schistosomiasis.

Schistosomes, or blood flukes, cause a debilitating illness in millions of people worldwide, which manifests when inflammation develops in response to parasite eggs that become trapped in the liver and other organs. Paradoxically, schistosomes require signals from the host's immune system in order to develop fully into egg-producing adults. Previously, we showed that CD4⁺ T cells facilitate schistosome development. Here, we show that the mere presence of CD4⁺ T cells is sufficient for schistosome development to proceed. There is no requirement for these cells to respond to the parasite, or to exhibit any typical “effector” response. Two pieces of data suggest this effect on parasite development is mediated by antigen-presenting cells of the innate immune system such as monocytes and macrophages, which interact with CD4⁺ T cells by expressing MHC class II molecules. First, the presence of naïve CD4⁺ T cells correlates with baseline changes in the development of monocyte/macrophage populations. Second, direct stimulation of the monocyte-macrophage system restores parasite development, bypassing the requirement for CD4⁺ T cells in schistosome development. Understanding the mechanisms that promote or inhibit blood fluke infection may facilitate the development of new treatments and vaccines for schistosomiasis.

Differential polarization of alveolar macrophages and bone marrow-derived monocytes following chemically and pathogen-induced chronic lung inflammation

Alveolar macrophages and BDMCs undergo sequential biochemical changes during the chronic inflammatory response to chemically induced lung carcinogenesis in mice. Herein, we examine two chronic lung inflammation models-repeated exposure to BHT and infection with Mycobacterium tuberculosis-to establish whether similar macrophage phenotype changes occur in non-neoplastic pulmonary disease. Exposure to BHT or M. tuberculosis results in pulmonary inflammation characterized by an influx of macrophages, followed by systemic effects on the BM and other organs. In both models, pulmonary IFN-gamma and IL-4 production coincided with altered polarization of alveolar macrophages. Soon after BHT administration or M. tuberculosis infection, IFN-gamma content in BALF increased, and BAL macrophages became classically (M1) polarized, as characterized by increased expression of iNOS. As inflammation progressed in both models, the amount of BALF IFN-gamma content and BAL macrophage iNOS expression decreased, and BALF IL-4 content and macrophage arginase I expression rose, indicating alternative/M2 polarization. Macrophages present in M. tuberculosis-induced granulomas remained M1-polarized, implying that these two pulmonary macrophage populations, alveolar and granuloma-associated, are exposed to different activating cytokines. BDMCs from BHT-treated mice displayed polarization profiles similar to alveolar macrophages, but BDMCs in M. tuberculosis-infected mice did not become polarized. Thus, only alveolar macrophages in these two models of chronic lung disease exhibit a similar progression of polarization changes; polarization of BDMCs was specific to BHT-induced pulmonary inflammation, and polarization of granuloma macrophages was specific to the M. tuberculosis infection.

Epithelial interleukin-4 receptor expression promotes colon tumor growth

Inflammatory mediators are of considerable interest as potential therapeutic targets in various cancers. Here we investigate whether interleukin (IL)-4 receptor alpha (IL4Ralpha), a component of the receptor complex for the T helper 2 cytokines IL4 and IL13, plays a role in colonic tumorigenesis. IL4Ralpha protein expression was seen in tumor cells of 28/48 human colon adenocarcinomas on a tissue microarray. In human and murine colon tumor cell lines analyzed in vitro, all of which expressed IL4Ralpha, treatment with exogenous ligand resulted in dose-dependent increases in proliferation. IL4 decreased apoptosis only in HCT116 cells. An orthotopic allograft model was used to determine in vivo effects of tumor cell-specific IL4Ra ablation. MC38 murine tumor cells with the IL4Ra gene knocked down showed reduced proliferation but no difference in apoptosis compared with controls after implantation in ceca of syngeneic mice. Mice null for IL4Ra and wild-type controls were treated with azoxymethane and dextran sulfate sodium to induce tumor formation. Mice with global deletion of IL4Ra had significantly fewer and smaller tumors. Reduced tumorigenicity correlated with decreased proliferation and increased apoptosis. Systemic blockade of IL4Ralpha-IL4 interactions with a chimeric soluble receptor protein gave similar results in the cecal implant model. Thus, IL4Ralpha, a component of the IL4R and IL13R, contributes to tumor formation in a mouse model of colitis-associated cancer. Proliferation appears to be directly mediated via IL4Ralpha on the epithelial tumor cells. Survival may be an indirect response mediated via other host cells. Our results support therapeutic targeting of IL4Ralpha in colon cancer.

Coordinating innate and adaptive immunity in Fasciola hepatica infection: implications for control

The helminth parasite Fasciola hepatica is responsible for major economic losses in agriculture throughout temperate regions of the globe. Control measures are heavily reliant on chemotherapy resulting in the emergence of drug resistant parasite populations. Novel control strategies based on vaccination ultimately require a deeper knowledge of host-parasite interactions. Herein we discuss recent advances in the understanding of the immune response to F. hepatica placing them in context with previous knowledge and developments from other model systems. Advances in RNAi and proteomics in the context of helminth research should make target identification and characterisation more rapid. In parallel, integration of these technologies with better immunological understanding will be crucial for future research into F. hepatica control measures.

Parasitic helminths: new weapons against immunological disorders

The prevalence of allergic and autoimmune diseases is increasing in developed countries, possibly due to reduced exposure to microorganisms in childhood (hygiene hypothesis). Epidemiological and experimental evidence in support of this hypothesis is accumulating. In this context, parasitic helminths are now important candidates for antiallergic/anti-inflammatory agents. Here we summarize antiallergic/anti-inflammatory effects of helminths together along with our own study of the effects of Schistosoma mansoni on Th17-dependent experimental arthritis. We also discuss possible mechanisms of helminth-induced suppression according to the recent advances of immunology.

Peroxisome proliferator-activated receptor (PPAR): balance for survival in parasitic infections

Parasitic infections induce a magnitude of host responses. At the opposite ends of the spectrum are those that ensure the host's needs to eliminate the invaders and to minimize damage to its own tissues. This review analyzes how parasites would manipulate immunity by activating the immunosuppressive nuclear factor, peroxisome proliferator-activated receptors (PPARs) with type 2 cytokines and free fatty acids from arachidonic acid metabolism. PPARs limit the action of type 1 immunity, in which classically activated macrophages act through the production of proinflammatory signals, to spare the parasites. They also favor the development of alternately activated macrophages which control inflammation so the host would not be destroyed. Possibly, the nuclear factors hold a pivotal role in the establishment of chronic infection by delicately balancing the pro- and anti-inflammatory signaling mechanisms and their ligands may be used as combination therapeutics to limit host pathology.

Induction of type 2 responses by schistosome worms during prepatent infection

During natural schistosome infection, the induction of T helper type 2 (Th2) responses has been ascribed to parasite eggs, because exposure of the host to this life-cycle stage elicits a polarized Th2 response to egg antigens. In the present study, we show that schistosome worms also elicit systemic, antigen-specific type 2 responses during prepatent infection, before egg deposition begins. CD4(+) T cells producing interleukin (IL)-4 were induced by both male and female worms during single-sex infections, demonstrating that this response is independent of exposure to eggs. The Th2 response was accompanied by production of immunoglobulin E and the sensitization of circulating basophils to produce additional IL-4 in response to schistosome antigens. Together, our data show that schistosome worms establish an immunologic milieu where CD4(+) T cells and basophils are both primed to produce IL-4 before eggs are laid, suggesting that worms play a role in establishment of the Th2 response that is critical for host survival and parasite transmission.

Similarity and diversity in macrophage activation by nematodes, trematodes, and cestodes

This review summarizes current knowledge of macrophages in helminth infections, with a focus not only on delineating the striking similarities in macrophage phenotype between diverse infections but also on highlighting the differences. Findings from many different labs illustrate that macrophages in helminth infection can act as anti-parasite effectors but can also act as powerful immune suppressors. The specific role for their alternative (Th2-mediated) activation in helminth killing or expulsion versus immune regulation remains to be determined. Meanwhile, the rapid growth in knowledge of alternatively activated macrophages will require an even more expansive view of their potential functions to include repair of host tissue and regulation of host metabolism.

IL-4(-/-) mice with lethal Mesocestoides corti infections--reduced Th2 cytokines and alternatively activated macrophages

Protection against Mesocestoides corti, a cestode that invades vital organs, is dependent on the production of IL-4, as IL-4(-/-) mice were found to have higher parasite burdens when compared with wild-type mice. The goal of this study was to investigate the role of IL-4 in immunity to M. corti, focusing on the immunological profile and on potential mediators of pathology. IL-4(-/-) mice infected with M. corti showed 100% mortality by 32 days, whereas wild-type mice survived for approximately 1 year. Parasite burdens were significantly increased in the liver, peritoneal, and thoracic cavities of IL-4(-/-) mice, associated with impaired recruitment of inflammatory cells and a reduction in monocytes and macrophages. IL-5 production by splenocytes and expression in liver tissue was decreased in infected IL-4(-/-) mice compared with wild-type mice. In contrast, IL-4(-/-) mice produced increased amounts of IFNgamma and TNFalpha. Alternatively activated macrophages were a major feature of liver granulomas in wild-type mice evidenced by Arginase I expression, while livers from infected IL-4(-/-) mice showed impaired alternative macrophage activation without increased classical macrophage activation. Thus, lethality during M. corti infection of IL-4(-/-) mice is associated with decreased Th2 cytokines, increased Th1 cytokines and impairment of alternatively activated macrophages.

Early removal of alternatively activated macrophages leads to Taenia crassiceps cysticercosis clearance in vivo

To determine the role of alternatively activated macrophages in modulating the outcome of experimental cysticercosis caused by Taenia crassiceps, we investigated the effect of removal of alternatively activated macrophage by injecting clodronate-loaded liposomes into susceptible BALB/c mice. Following T. crassiceps infection, mice receiving PBS-loaded liposomes developed a dominant Th2-type response associated with the presence of alternatively activated macrophages together with antigen-specific hyporesponsiveness and high parasite burden. In contrast, similarly infected mice treated with clodronate-loaded liposomes mounted a mixed Th1/Th2-type response, reversed antigen-specific hyporesponsiveness and did not carry notable alternatively activated macrophage populations. These factors were associated with increased resistance to T. crassiceps cysticercosis. Interestingly, early AAM phi depletion was enough to limit parasite growth. However, if macrophages were depleted late in the infection, no effect on parasite burden was observed. These findings demonstrate that alternatively activated macrophages play a critical role in mediating susceptibility to experimental cysticercosis in which their early recruitment may favor parasite survival.

IL-33 induces IL-13-dependent cutaneous fibrosis

IL-33 is constitutively expressed in epithelial barrier tissues, such as skin. Although increased expression of IL-33/IL-33R has been correlated with fibrotic disorders, such as scleroderma and progressive systemic sclerosis, the direct consequences of IL-33 release in skin has not been reported. To determine the effects of dysregulated IL-33 signaling in skin, we administered IL-33 s.c. and monitored its effects at the injection site. Administration of IL-33 resulted in IL-33R-dependent accumulation of eosinophils, CD3(+) lymphocytes, F4/80(+) mononuclear cells, increased expression of IL-13 mRNA, and the development of cutaneous fibrosis. Consistent with extensive cutaneous tissue remodeling, IL-33 resulted in significant modulation of a number of extracellular matrix-associated genes, including collagen VI, collagen III, and tissue inhibitor of metalloproteases-1. We establish that IL-33-induced fibrosis requires IL-13 using IL-13 knockout mice and eosinophils using Delta dblGATA mice. We show that bone marrow-derived eosinophils secrete IL-13 in response to IL-33 stimulation, suggesting that eosinophil-derived IL-13 may promote IL-33-induced cutaneous fibrosis. Collectively, our results identify IL-33 as a previously unrecognized profibrotic mediator in skin and highlight the cellular and molecular pathways by which this pathology develops.

Arginase in parasitic infections: macrophage activation, immunosuppression, and intracellular signals

A type 1 cytokine-dependent proinflammatory response inducing classically activated macrophages (CaMϕs) is crucial for parasite control during protozoan infections but can also contribute to the development of immunopathological disease symptoms. Type 2 cytokines such as IL-4 and IL-13 antagonize CaMϕs inducing alternatively activated macrophages (AaMϕs) that upregulate arginase-1 expression. During several infections, induction of arginase-1-macrophages was showed to have a detrimental role by limiting CaMϕ-dependent parasite clearance and promoting parasite proliferation. Additionally, the role of arginase-1 in T cell suppression has been explored recently. Arginase-1 can also be induced by IL-10 and transforming growth factor-β (TGF-β) or even directly by parasites or parasite components. Therefore, generation of alternative activation states of macrophages could limit collateral tissue damage because of excessive type 1 inflammation. However, they affect disease outcome by promoting parasite survival and proliferation. Thus, modulation of macrophage activation may be instrumental in allowing parasite persistence and long-term host survival.

Intestinal epithelial cell secretion of RELM-beta protects against gastrointestinal worm infection

Th2 cells drive protective immunity against most parasitic helminths, but few mechanisms have been demonstrated that facilitate pathogen clearance. We show that IL-4 and IL-13 protect against intestinal lumen-dwelling worms primarily by inducing intestinal epithelial cells (IECs) to differentiate into goblet cells that secrete resistin-like molecule (RELM) β. RELM-β is essential for normal spontaneous expulsion and IL-4–induced expulsion of Nippostrongylus brasiliensis and Heligmosomoides polygyrus, which both live in the intestinal lumen, but it does not contribute to immunity against Trichinella spiralis, which lives within IEC. RELM-β is nontoxic for H. polygyrus in vitro but directly inhibits the ability of worms to feed on host tissues during infection. This decreases H. polygyrus adenosine triphosphate content and fecundity. Importantly, RELM-β–driven immunity does not require T or B cells, alternative macrophage activation, or increased gut permeability. Thus, we demonstrate a novel mechanism for host protection at the mucosal interface that explains how stimulation of epithelial cells by IL-4 and IL-13 contributes to protection against parasitic helminthes that dwell in the intestinal lumen.

IL-33 amplifies the polarization of alternatively activated macrophages that contribute to airway inflammation

Alternatively activated macrophages (AAM) play a crucial role in type 2 immunity. Mice deficient in ST2, a receptor for the latest member of the IL-1 family, IL-33, have impaired type 2 immune responses. We therefore reasoned that IL-33/ST2 signaling may be involved in the differentiation and activation of AAM during airway inflammation. We report here that IL-33 changed the quiescent phenotype of alveolar macrophages toward an AAM phenotype that expressed mannose receptor, IL-4Ralpha, and produced high levels of CCL24 and CCL17 in an IL-13-dependent manner during IL-33-induced airway inflammation. Neutralization of AAM-derived CCL24 led to an amelioration of IL-33-induced eosinophilia in the lungs. Moreover, depletion of alveolar macrophages reduced IL-33-induced airway inflammation. Additionally, the attenuated OVA-induced airway inflammation in ST2(-/-) mice was associated with a decrease in AAM differentiation. In vitro, IL-33 amplified IL-13-induced polarization of alveolar- and bone marrow-derived macrophage toward an AAM phenotype by increasing the expression of arginase I, Ym1, as well as the production of CCL24 and CCL17. IL-13/IL-4Ralpha signaling was crucial for IL-33-driven AAM amplification by inducing the expression of ST2L. Finally, we showed that IL-33 was more abundantly expressed in the lung epithelial cells of asthma patients than those from healthy controls, suggesting that IL-33 may be involved in lung macrophage activation in clinical asthma. Taken together, we demonstrate here that IL-33/ST2 plays a significant role in the amplification of AAM polarization and chemokine production which contribute to innate and Ag-induced airway inflammation.

Fluorescent imaging of antigen released by a skin-invading helminth reveals differential uptake and activation profiles by antigen presenting cells

Infection of the mammalian host by the parasitic helminth Schistosoma mansoni is accompanied by the release of excretory/secretory molecules (ES) from cercariae which aid penetration of the skin. These ES molecules are potent stimulants of innate immune cells leading to activation of acquired immunity. At present however, it is not known which cells take up parasite antigen, nor its intracellular fate. Here, we develop a technique to label live infectious cercariae which permits the imaging of released antigens into macrophages (MΦ) and dendritic cells (DCs) both in vitro and in vivo. The amine reactive tracer CFDA-SE was used to efficiently label the acetabular gland contents of cercariae which are released upon skin penetration. These ES products, termed ‘0-3hRP’, were phagocytosed by MHC-II⁺ cells in a Ca⁺ and actin-dependent manner. Imaging of a labelled cercaria as it penetrates the host skin over 2 hours reveals the progressive release of ES material. Recovery of cells from the skin shows that CFDA-SE labelled ES was initially (3 hrs) taken up by Gr1⁺MHC-II⁻ neutrophils, followed (24 hrs) by skin-derived F4/80⁺MHC-II^lo MΦ and CD11c⁺ MHC-II^hi DC. Subsequently (48 hrs), MΦ and DC positive for CFDA-SE were detected in the skin-draining lymph nodes reflecting the time taken for antigen-laden cells to reach sites of immune priming. Comparison of in vitro-derived MΦ and DC revealed that MΦ were slower to process 0-3hRP, released higher quantities of IL-10, and expressed a greater quantity of arginase-1 transcript. Combined, our observations on differential uptake of cercarial ES by MΦ and DC suggest the development of a dynamic but ultimately balanced response that can be potentially pushed towards immune priming (via DC) or immune regulation (via MΦ).

Schistosomiasis is caused by the parasitic worm Schistosoma with over 200 million people infected across 76 countries. The parasitic larvae (called cercariae) infect mammalian hosts via the skin, but the exact mechanisms by which dermal cells interact with molecules released by invading larvae are unclear. A better understanding of the infection process and stimulation of the early immune response would thus enable a targeted approach towards the development of drugs and vaccines. Here, we have used the fluorescent tracer CFDA-SE to label infectious cercariae and, together with confocal microscopy, have for the first time tracked in real time the parasite infecting via the epidermis and depositing excretory/secretory material in its wake. Phagocytic macrophages and dendritic cells in the skin internalised excretory/secretory molecules released by the larvae, and both cell types were subsequently located in the draining lymph nodes where priming of the acquired immune response occurs. In vitro studies determined that macrophages were slower to process released parasite material than dendritic cells; they also secreted lower levels of pro-inflammatory cytokines but greater quantities of regulatory IL-10. The relative abundance of macrophages versus dendritic cells in the skin infection site and their differential rates of antigen processing may be crucial in determining the success of adaptive immune priming in response to infection.

The role of basophils in helminth infection

Protective immunity against gastrointestinal and tissue dwelling helminths is coordinated by interaction of different effector cells of the innate and adaptive immune system. Helminths induce a strong type 2 immune response which is characterized by high levels of IgE and increased numbers of Th2 cells, eosinophils, mast cells and basophils. Basophils are rapidly mobilized after helminth infection and can be efficiently recruited into lymphoid and peripheral tissues where they execute their effector functions. Recent work demonstrated that basophils contribute to initiation and execution of type 2 immunity. This review discusses the potential role of basophils for protective immunity against helminths.

Toll-like receptor 9 activation is a key mechanism for the maintenance of chronic lung inflammation

RATIONALE

Accumulating evidence supports the hypothesis that the continuous host response to a persistent challenge can polarize the cytokine environment toward a Th2 cytokine phenotype, but the mechanisms responsible for this skewing are not clear.

OBJECTIVES

We investigated the role of Toll-like receptor 9 (TLR9) in a Th2-driven pulmonary granulomatous response initiated via the embolization of Schistosoma mansoni eggs to the lungs of mice.

METHODS

Mice were intravenously injected with S. mansoni eggs. Histological and flow cytometric analysis, cytokine measurement, adoptive transfer of bone marrow (BM)-derived dendritic cells (DCs), and in vitro T-cell treatments with antigen-presenting cells were examined.

MEASUREMENTS AND MAIN RESULTS

In comparison to wild-type mice, TLR9(-/-) mice showed increased pulmonary granuloma size, augmented collagen deposition, increased Th2 cytokine phenotype, and impaired accumulation of DCs. BM-derived DCs, but not macrophages, recovered from animals with developed Th2-type lung granulomas promoted the production of type 2 cytokines from CD4(+) T cells. BM-derived DCs from TLR9(-/-) mice induced impaired Th1 cytokine and enhanced Th2 cytokine production by T cells, compared with DCs from WT mice. Macrophages from TLR9(-/-) mice expressed a significantly higher alternatively activated (M2) phenotype characterized by increased "found in inflammatory zone-1" (FIZZ1) and arginase-1 expression. The adoptive transfer of BM-derived DCs from syngeneic WT mice into TLR9(-/-) mice restored the granuloma phenotype seen in WT mice.

CONCLUSIONS

These studies suggest that TLR9 plays an important mechanistic role in the maintenance of the pulmonary granulomatous response.

T-bet protects against exacerbation of schistosome egg-induced immunopathology by regulating Th17-mediated inflammation

C57BL/6 mice infected with Schistosoma mansoni naturally develop mild CD4(+) T-cell-mediated immunopathology characterized by small hepatic granulomas around parasite eggs. However, immunization with soluble egg Ag in CFA markedly exacerbates the lesions by inducing a potent proinflammatory environment with high levels of IFN-gamma and IL-17, which are signature cytokines of distinct Th1- versus Th17-cell lineages. To determine the relative role of these subsets in disease exacerbation, we examined mice deficient in T-bet (T-bet(-/-)), which is required for Th1 differentiation and IFN-gamma production. We now report that immunization with soluble egg Ag in CFA caused a significantly greater enhancement of egg-induced hepatic immunopathology in T-bet(-/-) mice compared with WT controls, and analysis of their granulomas disclosed a higher proportion of activated DC and CD4(+) T cells, as well as a marked influx of neutrophils. The absence of IFN-gamma in the T-bet(-/-) mice correlated with a marked increase in IL-23p19, IL-17 and TNF-alpha in granulomas and MLN. In contrast, T-bet(-/-) mice had lower levels of IL-4, IL-5 and IL-10 and a reduction in FIZZ1 and FoxP3 expression, suggesting diminished regulatory activity, respectively, by alternatively activated macrophages and Treg. These findings demonstrate that T-bet-dependent signaling negatively regulates Th17-mediated immunopathology in severe schistosomiasis.