Acute cysticercosis favours rapid and more severe lesions caused by Leishmania major and Leishmania mexicana infection, a role for alternatively activated macrophages

Co-infection: the outcome of Plasmodium infection differs according to the time of pre-existing helminth infection

Although helminth-Plasmodium coinfections are common in tropical regions, the implications of this co-existence for the host immune response are poorly understood. In order to understand the effect of helminth infection at different times of coinfection on the immune response against Plasmodium infection, BALB/c mice were intraperitoneally infected with Taenia crassiceps (Tc). At 2 (Tc2) or 8 (Tc8) weeks post-infection, mice were intravenously infected with 1 × 10³ Plasmodium yoelii (Py) 17XL-parasitized red blood cells. Py 17XL-single-infected mice developed cachexia, splenomegaly, and anemia, and died at 11 days post-infection. Importantly, Tc2 + Py-coinfected mice showed increased survival of 58% on day 11, but developed pathology (cachexia and splenomegaly) and succumbed on day 18 post-coinfection, this latter associated with high levels of IL-1β and IL-12, and reduced IFN-γ in serum compared with Py 17XL-single-infected mice. Interestingly, Tc8 + Py-coinfected mice showed increased survival up to 80% on day 11 and succumbed on day 30 post-coinfection. This increased survival rate conferred by chronic helminth infection was associated with a decreased pathology and mixed inflammatory-type 1/anti-inflammatory-type 2 immune profile as evidenced by the production of high levels of IL-12 and IL-10, and reduced TNF-α from macrophages, high levels of IL-4 and IL-10, and low levels of IFN-γ from spleen cells. Also high serum levels of IL-1β, TNF-α, IL-12, IL-4, and IL-10, but a significant reduction of IFN-γ were observed. Together, these data indicate that polarization of the cell-mediated response modulated by a pre-existing helminth infection differentially impacts on the host immune response to Py 17XL in a time-dependent manner.

Tegumentary leishmaniasis and coinfections other than HIV

BACKGROUND

Tegumentary leishmaniasis (TL) is a disease of skin and/or mucosal tissues caused by Leishmania parasites. TL patients may concurrently carry other pathogens, which may influence the clinical outcome of TL.

METHODOLOGY AND PRINCIPAL FINDINGS

This review focuses on the frequency of TL coinfections in human populations, interactions between Leishmania and other pathogens in animal models and human subjects, and implications of TL coinfections for clinical practice. For the purpose of this review, TL is defined as all forms of cutaneous (localised, disseminated, or diffuse) and mucocutaneous leishmaniasis. Human immunodeficiency virus (HIV) coinfection, superinfection with skin bacteria, and skin manifestations of visceral leishmaniasis are not included. We searched MEDLINE and other databases and included 73 records: 21 experimental studies in animals and 52 studies about human subjects (mainly cross-sectional and case studies). Several reports describe the frequency of Trypanosoma cruzi coinfection in TL patients in Argentina (about 41%) and the frequency of helminthiasis in TL patients in Brazil (15% to 88%). Different hypotheses have been explored about mechanisms of interaction between different microorganisms, but no clear answers emerge. Such interactions may involve innate immunity coupled with regulatory networks that affect quality and quantity of acquired immune responses. Diagnostic problems may occur when concurrent infections cause similar lesions (e.g., TL and leprosy), when different pathogens are present in the same lesions (e.g., Leishmania and Sporothrix schenckii), or when similarities between phylogenetically close pathogens affect accuracy of diagnostic tests (e.g., serology for leishmaniasis and Chagas disease). Some coinfections (e.g., helminthiasis) appear to reduce the effectiveness of antileishmanial treatment, and drug combinations may cause cumulative adverse effects.

CONCLUSIONS AND SIGNIFICANCE

In patients with TL, coinfection is frequent, it can lead to diagnostic errors and delays, and it can influence the effectiveness and safety of treatment. More research is needed to unravel how coinfections interfere with the pathogenesis of TL.

Regulation of immunity by Taeniids: lessons from animal models and in vitro studies

Taeniidae is the largest family of the Cyclophyllidea order of parasites despite being composed of just two genera: Taenia spp and Echinococcus spp. These parasites are flatworms with a terrestrial life cycle, having an immature or larval stage called metacestode, which develops into the mature form within the intestine of the primary host after being consumed in raw or poorly cooked meat. Consumed eggs hatch into oncospheres, penetrate the intestinal walls and are transported via the bloodstream to later develop into metacestodes within the muscles and internal organs of secondary and sometimes primary hosts, thereby initiating the cycle again. Larval stages of both Taenia spp and Echinococcus spp are well known to produce tissue-dwelling, long-lasting infections; in this stage, these parasites can reach centimetres (macroparasites) and both genera may cause life-threatening diseases in humans. Establishing such long-term infections requires an exceptional ability to modulate host immunity for long periods of time. In this review, we analyse the immunoregulatory mechanisms induced by these tapeworms and their products, mainly discussing the importance of taeniid strategies to successfully colonize their hosts, such as antigen-presenting cell phenotype manipulation and the consequent induction of T-cell anergy, among others.

ATaenia crassicepsfactor induces apoptosis of spleen CD4+T cells and TFG-β andFoxp3gene expression in mice

This study was undertaken to determine whether a parasite substance produces structural pathology in the mouse spleen. A low-molecular-weightTaenia crassicepsmetacestode factor (MF) isolated from the peritoneal fluid of female mice infected withT. crassicepsmetacestodes induced pathological and immunological changes in mouse spleen cellsin vivo.Electron microscopy and confocal microscopy revealed severe changes in the spleen histoarchitecture ofT. crassiceps-infected and MF-treated mice. Apoptotic degenerated spleen cells were observed in the white and red pulps and were more conspicuous in the white pulp of the spleen from theT. crassiceps-infected mice than in that of the MF-treated mice. Flow cytometry analysis revealed that the numbers of spleen CD4+T cells were significantly lower in both experimental groups than in control mice. Theex vivoexpression of transforming growth factor (TGF)-β and factor Foxp3 were significantly higher in splenocytes of the experimental mice than the basal expression observed in the control cells. These findings may have potential applications for a better understanding of the host–parasite relationship in human neurocysticercosis.

Identification and biochemical characterization of macrophage migration inhibitory factor-2 (MIF-2) homologue of human lymphatic filarial parasite, Wuchereria bancrofti

Homologues of human macrophage migration inhibitory factor (hMIF) have been reported from vertebrates, invertebrates and prokaryotes, as well as plants. Filarial parasites produce two homologues of hMIF viz., MIF-1 and MIF-2, which play important role in the host immune modulation. Earlier, we have characterized MIF-1 (Wba-mif-1) from Wuchereria bancrofti, the major causal organism of human lymphatic filariasis. Here, we are reporting the molecular and biochemical characterization of MIF-2 from this parasite (Wba-mif-2). The complete Wba-mif-2 gene and its cDNA were amplified, cloned and sequenced. The size of Wba-mif-2 gene and cDNA were found to be 4.275 kb and 363 bp, respectively. The gene annotation revealed the presence of a large intron of 3.912 kb interspersed with two exons of 183 bp and 180 bp. The alignment of derived amino acid sequences of Wba-MIF-2 with Wba-MIF-1 showed 44% homology. The conserved CXXC oxido-reductase catalytic site present in Wba-mif-1 was found absent in Wba-mif-2 coding sequence. The amplified Wba-mif-2 cDNA was cloned into an expression vector pRSET-B and transformed into salt inducible Escherichia coli strain GJ1158. The expressed recombinant Wba-MIF-2 protein showed tautomerase activity against L-dopachrome methyl ester and the specific activity was determined to be 18.57±0.77 μmol/mg/min. Three known inhibitors of hMIF tautomerase activity significantly inhibited the tautomerase activity of recombinant Wba-MIF-2. Although the conserved CXXC oxido-reductase motif is absent in Wba-mif-2, the recombinant protein showed significant oxido-reductase activity in the insulin reduction assay, possibly because of the presence of vicinal cysteine residues.

Helminth parasites alter protection against Plasmodium infection

More than one-third of the world's population is infected with one or more helminthic parasites. Helminth infections are prevalent throughout tropical and subtropical regions where malaria pathogens are transmitted. Malaria is the most widespread and deadliest parasitic disease. The severity of the disease is strongly related to parasite density and the host's immune responses. Furthermore, coinfections between both parasites occur frequently. However, little is known regarding how concomitant infection with helminths and Plasmodium affects the host's immune response. Helminthic infections are frequently massive, chronic, and strong inductors of a Th2-type response. This implies that infection by such parasites could alter the host's susceptibility to subsequent infections by Plasmodium. There are a number of reports on the interactions between helminths and Plasmodium; in some, the burden of Plasmodium parasites increased, but others reported a reduction in the parasite. This review focuses on explaining many of these discrepancies regarding helminth-Plasmodium coinfections in terms of the effects that helminths have on the immune system. In particular, it focuses on helminth-induced immunosuppression and the effects of cytokines controlling polarization toward the Th1 or Th2 arms of the immune response.

Evolution, molecular epidemiology and perspectives on the research of taeniid parasites with special emphasis on Taenia solium

Human cysticercosis is known since old historical times in Greece and China; however, human infections by tapeworms have accompanied human beings for more that hundred thousand years. The disease is tightly bound to poverty and lack of hygiene, and has been eradicated in developed countries, but continues being a public health problem in developing countries of Latin-American, Sub-Saharan Africa and Asia, and is also remerging in a number of non endemic countries. It is considered a neglected disease. Here we revise a number of key scientific contributions on taeniid biology that open new avenues for more effective approaches to the control of cysticercosis. The evolution of flatworms and class Cestoda is analyzed, with special emphasis on the emergence of taeniid parasites and the colonization of the human species by tapeworms. The complex molecular host-parasite interplay in this relationship as result of co-evolution between two distantly related organisms. The relevant host and parasite's factors, in the prospect of identifying species-specific molecular markers useful in epidemiological studies carried out in endemic countries. The new possibilities arising with the characterization of the genomes for several species of tapeworms, including a deeper understanding of these organisms, as well as improved tools for diagnosis, vaccination and drug treatment. The need to revise the current control and management strategies for this tropical neglected disease.

Alternatively Activated Macrophages Revisited: New Insights into the Regulation of Immunity, Inflammation and Metabolic Function following Parasite Infection

The role of macrophages in homeostatic conditions and the immune system range from clearing debris to recognizing and killing pathogens. While classically activated macrophages (CAMacs) are induced by T helper type 1 (Th1) cytokines and exhibit microbicidal properties, Th2 cytokines promote alternative activation of macrophages (AAMacs). AAMacs contribute to the killing of helminth parasites and mediate additional host-protective processes such as regulating inflammation and wound healing. Yet, other parasites susceptible to Th1 type responses can exploit alternative activation of macrophages to diminish Th1 immune responses and prolong infection. In this review, we will delineate the factors that mediate alternative activation (e.g. Th2 cytokines and chitin) and the resulting downstream signaling events (e.g. STAT6 signaling). Next, the specific AAMac-derived factors (e.g. Arginase1) that contribute to resistance or susceptibility to parasitic infections will be summarized. Finally, we will conclude with the discussion of additional AAMac functions beyond immunity to parasites, including the regulation of inflammation, wound healing and the regulation of metabolic disorders.

Immunoregulation by Taenia crassiceps and its antigens

Taenia crassiceps is a cestode parasite of rodents (in its larval stage) and canids (in its adult stage) that can also parasitize immunocompromised humans. We have studied the immune response elicited by this helminth and its antigens in mice and human cells, and have discovered that they have a strong capacity to induce chronic Th2-type responses that are primarily characterized by high levels of Th2 cytokines, low proliferative responses in lymphocytes, an immature and LPS-tolerogenic profile in dendritic cells, the recruitment of myeloid-derived suppressor cells and, specially, alternatively activated macrophages. We also have utilized the immunoregulatory capabilities of this helminth to successfully modulate autoimmune responses and the outcome of other infectious diseases. In the present paper, we review the work of others and ourselves with regard to the immune response induced by T. crassiceps and its antigens, and we compare the advances in our understanding of this parasitic infection model with the knowledge that has been obtained from other selected models.

Cestode regulation of inflammation and inflammatory diseases

Helminth parasites are masters of immune regulation; a likely prerequisite for long-term survival by circumventing their hosts' attempt to eradicate them. From a translational perspective, knowledge of immune events as a response to infection with a helminth parasite could be used to reduce the intensity of unwanted inflammatory reactions. Substantial data have accumulated showing that inflammatory reactions that promote a variety of auto-inflammatory diseases are dampened as a consequence of infection with helminth parasites, via either the mobilization of an anti-worm spectrum of immune events or by the direct effect of secretory/excretory bioactive immunomodulatory molecules released from the parasite. However, many issues are outstanding in the definition of the mechanism(s) by which infection with helminth parasites can affect the outcome, positively or negatively, of concomitant disease. We focus on a subgroup of this complex group of metazoan parasites, the cestodes, summarizing studies from rodent models that illustrate if, and by what mechanisms, infection with tapeworms ameliorate or exaggerate disease in their host. The ability of infection with cestodes, or other classes of helminth, to worsen a disease course or confer susceptibility to intracellular pathogens should be carefully considered in the context of 'helminth therapy'. In addition, poorly characterised cestode extracts can regulate murine and human immunocyte function, yet the impact of these in the context of autoimmune or allergic diseases is poorly understood. Thus, studies with cestodes, as representative helminths, have helped cement the concept that infection with parasitic helminths can inhibit concomitant disease; however, issues relating to long-term effects, potential side-effects, mixed pathogen infections and purification of immunomodulatory molecules from the parasite remain as challenges that need to be addressed in order to achieve the use of helminths as anti-inflammatory agents for human diseases.

Signal Transducers and Activators of Transcription (STAT) family members in helminth infections

Helminth parasites are a diverse group of multicellular organisms. Despite their heterogeneity, helminths share many common characteristics, such as the modulation of the immune system of their hosts towards a permissive state that favors their development. They induce strong Th2-like responses with high levels of IL-4, IL-5 and IL-13 cytokines, and decreased production of proinflammatory cytokines such as IFN-γ. IL-4, IFN-γ and other cytokines bind with their specific cytokine receptors to trigger an immediate signaling pathway in which different tyrosine kinases (e.g. Janus kinases) are involved. Furthermore, a seven-member family of transcription factors named Signal Transducers and Activators of Transcription (STAT) that initiate the transcriptional activation of different genes are also involved and regulate downstream the JAK/STAT signaling pathway. However, how helminths avoid and modulate immune responses remains unclear; moreover, information concerning STAT-mediated immune regulation during helminth infections is scarce. Here, we review the research on mice deficient in STAT molecules, highlighting the importance of the JAK/STAT signaling pathway in regulating susceptibility and/or resistance in these infections.

From host immunity to pathogen invasion: the effects of helminth coinfection on the dynamics of microparasites

Concurrent infections with multiple parasites are ubiquitous in nature. Coinfecting parasites can interact with one another in a variety of ways, including through the host's immune system via mechanisms such as immune trade-offs and immunosuppression. These within-host immune processes mediating interactions among parasites have been described in detail, but how they scale up to determine disease dynamic patterns at the population level is only beginning to be explored. In this review, we use helminth-microparasite coinfection as a model for examining how within-host immunological effects may influence the ecological outcome of microparasitic diseases, with a specific focus on disease invasion. The current literature on coinfection between helminths and major microparasitic diseases includes many studies documenting the effects of helminths on individual host responses to microparasites. In many cases, the observed host responses map directly onto parameters relevant for quantifying disease dynamics; however, there have been few attempts at integrating data on individual-level effects into theoretical models to extrapolate from the individual to the population level. Moreover, there is considerable variability in the particular combination of disease parameters affected by helminths across different microparasite systems. We develop a conceptual framework identifying some potential sources of such variability: Pathogen persistence and severity, and resource availability to hosts. We also generate testable hypotheses regarding diseases and the environmental contexts when the effects of helminths on microparasite dynamics should be most pronounced. Finally, we use a case study of helminth and mycobacterial coinfection in the African buffalo to illustrate both progress and challenges in understanding the population-level consequences of within-host immunological interactions, and conclude with suggestions for future research that will help improve our understanding of the effects of coinfection on dynamics of infectious diseases.

Toxoplasma gondii peroxiredoxin promotes altered macrophage function, caspase-1-dependent IL-1β secretion enhances parasite replication

Alternatively activated macrophages (AAM) are a key feature Th2 immunity and have been associated with a variety of roles during helminth infection. The role this cell subset plays in protzoan infection remain relatively unexplored, herein we describe the effects of a redox enzyme (rTgPrx) derived from Toxoplasma gondii on murine macrophage phenotype in vitro. RTgPrx has been previously associated with the maintainence of parasite oxidative balance. Here our experiments show that rTgPrx promotes AAM as indicated by high arginase-1 (arg-1), YM1 and FIZZ expression via both signal transducer and activator of transcription (STAT)6-dependent and -independent mechanisms. Additionally rTgPrx treatment reduced caspase-1 activity and IL-1β secretion, while simultaneously increasing IL-10 release. Furthermore the in vitro replication of T. gondii (RH strain) was enhanced when macrophages were treated with rTgPrx. This is in contrast with the previously described effects of a Plasmodium berghei ANKA 2-cys-peroxiredoxin that promotes pro-inflammatory cytokine production. These results highlight the role of T. gondii derived redox enzymes as important immune modulators and potentially indicate a role for AAM in modulating immunopathology and promoting parasite replication during T. gondii infection.

Qualitative and quantitative immunohistochemical evaluation of iNOS expression in the spleen of dogs naturally infected with Leishmania chagasi

Nitric oxide (NO), the product of the nitric oxide synthase enzymes has been detected in Leishmania-infected animals. Besides its role on the immunity to infection, the role of NO and the inducible nitric oxide synthase (iNOS) in the pathogenesis of canine visceral leishmaniasis (CVL) is not well understood. This study aimed at evaluating immunohistochemically the iNOS expression in the spleen of dogs naturally infected (ID) with Leishmania (L.) chagasi compared with non-infected dogs (NID). The ID was grouped according to the clinical form and the parasite load. Symptomatic dogs (SD) presented higher parasite load in relation to oligosymptomatic (OD) and asymptomatic (AD). The qualitative expression of iNOS was observed only in ID. SD presented strong and prominent labeling of iNOS, followed by OD and AD. Quantitatively, the results showed that the median expression of iNOS was higher in SD and OD compared to NID. Also, dog spleens with high parasitism load showed marked iNOS expression. Taken together, the results suggest that the expression of iNOS in the spleen of infected dogs with CVL was associated with clinical worsening of the disease and with high parasitism.

Taenia crassiceps infection abrogates experimental autoimmune encephalomyelitis

Helminth infections induce strong immunoregulation that can modulate subsequent pathogenic challenges. Taenia crassiceps causes a chronic infection that induces a Th2-biased response and modulates the host cellular immune response, including reduced lymphoproliferation in response to mitogens, impaired antigen presentation and the recruitment of suppressive alternatively activated macrophages (AAMФ). In this study, we aimed to evaluate the ability of T. crassiceps to reduce the severity of experimental autoimmune encephalomyelitis (EAE). Only 50% of T. crassiceps-infected mice displayed EAE symptoms, which were significantly less severe than uninfected mice. This effect was associated with both decreased MOG-specific splenocyte proliferation and IL-17 production and limited leukocyte infiltration into the spinal cord. Infection with T. crassiceps induced an anti-inflammatory cytokine microenvironment, including decreased TNF-α production and high MOG-specific production of IL-4 and IL-10. While the mRNA expression of TNF-α and iNOS was lower in the brain of T. crassiceps-infected mice with EAE, markers for AAMФ were highly expressed. Furthermore, in these mice, there was reduced entry of CD3(+)Foxp3(-) cells into the brain. The T. crassiceps-induced immune regulation decreased EAE severity by dampening T cell activation, proliferation and migration to the CNS.

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.

Taenia crassiceps infection attenuates multiple low-dose streptozotocin-induced diabetes

Taenia crassiceps, like other helminths, can exert regulatory effects on the immune system of its host. This study investigates the effect of chronic T. crassiceps infection on the outcome of Multiple Low Dose Streptozotocin-Induced Diabetes (MLDS). Healthy or previously T. crassiceps-infected mice received MLDS and type 1 diabetes (T1D) symptoms were evaluated for 6 weeks following the induction of MLDS. T. crassiceps-infected mice displayed lower blood glucose levels throughout the study. A significantly lower percentage of T. crassiceps-infected mice (40%) developed T1D compared to the uninfected group (100%). Insulitis was remarkably absent in T. crassiceps-infected mice, which had normal pancreatic insulin content, whereas uninfected mice showed a dramatic reduction in pancreatic insulin. Infected mice that received MLDS did not show an increase in their regulatory T cell population, however, they had a greater number of alternatively activated macrophages, higher levels of the cytokine IL-4, and lower levels of TNF-α. Therefore, infection with T. crassiceps causes an immunomodulation that modifies the incidence and development of MLDS-induced autoimmune diabetes.

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.

Mucosal immunology of geohelminth infections in humans

There is limited data on the human mucosal immune response to geohelminths, but extensive data from experimental animals. Geohelminth infections may modulate mucosal immunity with effects on parasite expulsion or persistence and mucosal inflammation. Geohelminths are considered to have important effects on immunity to mucosal vaccines, infectious disease susceptibility, and anti-inflammatory effects in inflammatory bowel disease and asthma. This review will discuss the findings of studies of human immunity to geohelminths and their potential effects on non-parasite mucosal immune responses. Such effects are likely to be of public health importance in middle- and low-income countries where these parasites are endemic. There is a need for human studies on the effects of geohelminth infections on mucosal immunity and the potential for anthelmintic treatment to modify these effects. Such studies are likely to provide important insights into the regulation of mucosal immunity and inflammation, and the development of more effective mucosal vaccines.

Alternative activation of macrophages: an immunologic functional perspective

Macrophages are innate immune cells with well-established roles in the primary response to pathogens, but also in tissue homeostasis, coordination of the adaptive immune response, inflammation, resolution, and repair. These cells recognize danger signals through receptors capable of inducing specialized activation programs. The classically known macrophage activation is induced by IFN-gamma, which triggers a harsh proinflammatory response that is required to kill intracellular pathogens. Macrophages also undergo alternative activation by IL-4 and IL-13, which trigger a different phenotype that is important for the immune response to parasites. Here we review the cellular sources of these cytokines, receptor signaling pathways, and induced markers and gene signatures. We draw attention to discrepancies found between mouse and human models of alternative activation. The evidence for in vivo alternative activation of macrophages is also analyzed, with nematode infection as prototypic disease. Finally, we revisit the concept of macrophage activation in the context of the immune response.

Differential response of antigen presenting cells from susceptible and resistant strains of mice to Taenia crassiceps infection

Antigen presenting cells (APCs) are critically involved in the interaction between pathogens and the host immune system. Here, we examined two different populations of APCs in mice that are susceptible (BALB/c) or resistant (C57BL/6) to Taenia crassiceps cysticercosis. Bone marrow-derived dendritic cells (BMDCs) from both strains of mice were exposed to T. crassiceps excreted/secreted antigens (TcES) and, at the same time, to the Toll-like receptor (TLR) ligand LPS. BMDCs from BALB/c mice underwent a partial maturation when incubated with TcES and displayed decreased responses to TLR-dependent stimuli associated with low CD80, CD86, CD40 and CCR7 expression and impaired IL-15 production. These BMDCs-induced impaired allogenic responses. In contrast, BMDCs from C57BL/6 mice displayed normal maturation and induced strong allogenic responses. Moreover, the exposure to TcES resulted in a lower production of IL-12 and TNF-alpha by LPS-activated DCs from BALB/c mice compared to C57BL/6 DCs. Three parameters of macrophage activation were assessed during Taenia infection: LPS+IFN-gamma-induced production of IL-12, TNF-alpha and nitric oxide (NO) in vitro; infection-induced markers for alternatively activated macrophages (Arginase-1, RELM-alpha, Ym-1 and TREM-2 expression) and suppressive activity. The maximum response to LPS+IFN-gamma-induced TNF-alpha, IL-12 and NO production by macrophages from both strains of mice occurred 2 wk post-infection. However, as infection progressed, the production of these molecules by BALB/c macrophages declined. While the BALB/c macrophages displayed impaired pro-inflammatory responses, these macrophages showed strong Arginase-1, Ym-1, RELM-alpha and TREM-2 expression. By contrast, C57BL/6 macrophages maintained a pro-inflammatory profile and low transcripts for alternative activation markers. Macrophages from T. crassiceps-infected BALB/c mice showed stronger suppressive activity than those from C57BL/6 mice. These findings suggest that APC activation at both early and late time points during T. crassiceps infection is a possible mechanism that underlies the differential susceptibility to T. crassiceps infection displayed by these mouse strains.

Alternatively activated macrophages elicited by helminth infection can be reprogrammed to enable microbial killing

The prime function of classically activated macrophages (activated by Th1-type signals, such as IFN-gamma) is microbial destruction. Alternatively activated macrophages (activated by Th2 cytokines, such as IL-4 and IL-13) play important roles in allergy and responses to helminth infection. We utilize a murine model of filarial infection, in which adult nematodes are surgically implanted into the peritoneal cavity of mice, as an in vivo source of alternatively activated macrophages. At 3 wk postinfection, the peritoneal exudate cell population is dominated by macrophages, termed nematode-elicited macrophages (NeMphi), that display IL-4-dependent features such as the expression of arginase 1, RELM-alpha (resistin-like molecule alpha), and Ym1. Since increasing evidence suggests that macrophages show functional adaptivity, the response of NeMphi to proinflammatory Th1-activating signals was investigated to determine whether a switch between alternative and classical activation could occur in macrophages differentiated in an in vivo infection setting. Despite the long-term exposure to Th2 cytokines and antiinflammatory signals in vivo, we found that NeMphi were not terminally differentiated but could develop a more classically activated phenotype in response to LPS and IFN-gamma. This was reflected by a switch in the enzymatic pathway for arginine metabolism from arginase to inducible NO synthase and the reduced expression of RELM-alpha and Ym1. Furthermore, this enabled NeMphi to become antimicrobial, as LPS/IFN-gamma-treated NeMphi produced NO that mediated killing of Leishmania mexicana. However, the adaptation to antimicrobial function did not extend to key regulatory pathways, such as IL-12 production, which remained unaltered.

Orthologs of macrophage migration inhibitory factor from parasitic nematodes

Chronic helminth infections are associated with modulation of host cellular immune responses, presumably to prolong parasite survival within the mammalian host. This phenomenon is attributed, at least in part, to the elaboration of parasite molecules, including orthologs of host cytokines and receptors, at the host-parasite interface. This review describes recent progress in the characterization of macrophage migration inhibitory factor (MIF) orthologs from parasitic nematodes. The roles of these molecules in parasite developmental biology and pathogenesis are discussed. Further knowledge of the species-specific activities and three-dimensional structures of human and parasitic nematode MIF molecules should make them ideal targets for drug- and/or vaccine-based strategies aimed at nematode disease control.

The divergent roles of alternatively activated macrophages in helminthic infections

Macrophages play crucial roles in the immune response, as they can initiate, modulate and also be final effector cells during immune responses to infections. Macrophages are derived from myeloid precursor cells in bone marrow and are widely distributed in every tissue of the body. Over the past 10 years, the concepts about macrophage activation have clearly changed; macrophages are not called activated or inactivated as they used to be. These changes in the concept of macrophage response is the result of many in vitro and in vivo studies, but the major support for the current concept of alternatively activated macrophages (AAMphi) comes from parasitic helminth infections. Parasitic helminths have developed complex mechanisms to evade and modulate host immunity. Infections with these parasites induce strong polarized Th2-type immune responses frequently associated with impaired T-cell proliferative responses to parasitic or unrelated antigens. Given the recent advances in understanding the immunoregulatory capabilities of helminthic infections, it has been suggested that macrophages can be a target for immunomodulation. Furthermore, they become altered when a host experiences chronic exposure to helminth parasites or their by-products, which favour the induction of AAMphi. How AAMphi participate in modulating host immunity during helminth infections and what their roles are in clearing or favouring parasite survival remains elusive. Here we review the most recent advances in the literature on AAMphi at the host-parasite interface, including three classes of helminths: nematodes (Brugia, Nippostrongylus, Litomosoides, Heligmosomoides), trematodes (Schistosoma, Fasciola) and cestodes (Taenia, Echinococcus, Hymenolepis).

Possible role for Toll-like receptors in interaction of Fasciola hepatica excretory/secretory products with bovine macrophages

Alternative activation of macrophages (Mphi) during helminth infection is a characteristic feature of the host immune response. Alternatively activated macrophages (AAMphi) are distinguished from others by high arginase 1 (Arg-1) activity, low nitric oxide (NO), and high interleukin 10 (IL-10) production. In murine models, these cells have been shown to possess anti-inflammatory properties. They have also been implicated in exacerbating a subsequent infection with a secondary pathogen. In this study we used cattle experimentally infected with Fasciola hepatica to monitor the kinetics of IL-4 and IL-10 over the course of infection. Using naïve Mphi in vitro, we examined the effects of exposure to F. hepatica excretory/secretory products (FhepES) alone or in combination with IL-4. Our results suggest that FhepES may work in combination with IL-4 to produce AAMphi. The effects of FhepES on the subsequent responses to lipopolysaccharide (LPS) and purified protein derivative from Mycobacterium bovis (PPD-B), which are bovine Toll-like receptor 4 (TLR4) and TLR2 antagonists, respectively, were also examined. We found that Mphi stimulated with FhepES together with LPS or PPD-B have reduced NO or gamma interferon production, respectively. The ability of FhepES to produce AAMphi was found to be heat labile and partially dependent on glycan residues. A possible role for TLR recognition is discussed.

Infection with parasitic nematodes confounds vaccination efficacy

T helper (Th) cells produce signature cytokine patterns, induced largely by intracellular versus extracellular pathogens that provide the cellular and molecular basis for counter regulatory expression of protective immunity during concurrent infections. The production of IL-12 and IFN-gamma, for example, resulting from exposure to many bacterial, viral, and protozoan pathogens is responsible for Th1-derived protective responses that also can inhibit development of Th2-cells expressing IL-4-dependent immunity to extracellular helminth parasites and vice versa. In a similar manner, concurrent helminth infection alters optimal vaccine-induced responses in humans and livestock; however, the consequences of this condition have not been adequately studied especially in the context of a challenge infection following vaccination. Demands for new and effective vaccines to control chronic and emerging diseases, and the need for rapid deployment of vaccines for bio security concerns requires a systematic evaluation of confounding factors that limit vaccine efficacy. One common albeit overlooked confounder is the presence of gastrointestinal nematode parasites in populations of humans and livestock targeted for vaccination. This is particularly important in areas of the world were helminth infections are prevalent, but the interplay between parasites and emerging diseases that can be transmitted worldwide make this a global issue. In addition, it is not clear if the epidemic in allergic disease in industrialized countries substitutes for geohelminth infection to interfere with effective vaccination regimens. This presentation will focus on recent vaccination studies in mice experimentally infected with Heligmosomoides polygyrus to model the condition of gastrointestinal parasite infestation in mammalian populations targeted for vaccination. In addition, a large animal vaccination and challenge model against Mycoplasma hyopneumonia in swine exposed to Ascaris suum will provide a specific example of the need for further work in this area, and for controlled field studies to assess the impact of other similar scenarios.

Alternatively activated macrophages in helminth infections

Helminthic parasites can trigger highly polarized immune responses typically associated with increased numbers of CD4(+) Th2 cells, eosinophils, mast cells, and basophils. These cell populations are thought to coordinate an effective response ultimately leading to parasite expulsion, but they also play a role in the regulation of associated pathologic inflammation. Recent studies suggest that macrophages, conventionally associated with IFN-gamma-dominant Th1-type responses to many bacteria and viruses, also play an essential role in the Th2-type inflammatory response. These macrophages are referred to as alternatively activated macrophages (AAMPhis) as they express a characteristic pattern of cell surface and secreted molecules distinct from that of classically activated macrophages (CAMPhis) associated with microbe infections. In this review, we will discuss recent findings regarding the role of AAMPhis in the development of disease and host protection following helminth infection.

Alternatively activated macrophages in protozoan infections

A type 1 cytokine-dependent pro-inflammatory response inducing classically activated macrophages is crucial for parasite control during protozoan infections but can also contribute to the development of immunopathological disease symptoms. Accumulating evidence indicates that interleukins 4, 13 and 10, transforming growth factor-beta, immune complexes and apoptotic cells elicited during these infections induce alternative activation states of macrophages, affecting disease outcome by, on the one hand, promoting parasite survival and proliferation and, on the other hand, limiting collateral tissue damage because of excessive type 1 inflammation. Thus, modulation of macrophage activation may be instrumental in allowing parasite persistence and long-term host survival.