Quantitative appraisal of murine filariasis confirms host strain differences but reveals that BALB/c females are more susceptible than males to Litomosoides sigmodontis

Host genetic backgrounds: the key to determining parasite-host adaptation

Parasitic diseases pose a significant threat to global public health, particularly in developing countries. Host genetic factors play a crucial role in determining susceptibility and resistance to infection. Recent advances in molecular and biological technologies have enabled significant breakthroughs in understanding the impact of host genes on parasite adaptation. In this comprehensive review, we analyze the host genetic factors that influence parasite adaptation, including hormones, nitric oxide, immune cells, cytokine gene polymorphisms, parasite-specific receptors, and metabolites. We also establish an interactive network to better illustrate the complex relationship between host genetic factors and parasite-host adaptation. Additionally, we discuss future directions and collaborative research priorities in the parasite-host adaptation field, including investigating the impact of host genes on the microbiome, developing more sophisticated models, identifying and characterizing parasite-specific receptors, utilizing patient-derived sera as diagnostic and therapeutic tools, and developing novel treatments and management strategies targeting specific host genetic factors. This review highlights the need for a comprehensive and systematic approach to investigating the underlying mechanisms of parasite-host adaptation, which requires interdisciplinary collaborations among biologists, geneticists, immunologists, and clinicians. By deepening our understanding of the complex interactions between host genetics and parasite adaptation, we can develop more effective and targeted interventions to prevent and treat parasitic diseases. Overall, this review provides a valuable resource for researchers and clinicians working in the parasitology field and offers insights into the future directions of this critical research area.

Filarial infections compromise influenza vaccination efficacy: Lessons from the mouse

Helminth parasites infect more than a quarter of the human population and inflict significant changes to the immunological status of their hosts. Several human studies report impaired responses to vaccinations in helminth-infected individuals. Analysing the impact of helminth infections on the efficacy of influenza vaccinations in the mouse system helps to elucidate the underlying immunological processes. Concurrent infection with the parasitic nematode Litomosoides sigmodontis reduced the quantity and quality of antibody responses to vaccination against seasonal influenza in BALB/c and C57BL/6 mice. This led to impaired vaccination-induced protection against challenge infections with the human pathogenic 2009 pandemic H1N1 influenza A virus in helminth-infected mice. Impaired responses were also observed if vaccinations were performed after immune-driven or drug-induced clearance of a previous helminth infection. Mechanistically, the suppression was associated with a systemic and sustained expansion of IL-10-producing CD4⁺CD49b⁺LAG-3⁺ type 1 regulatory T cells and partially abrogated by in vivo blockade of the IL-10 receptor. In summary, these findings raise the concern that individuals in helminth-endemic areas may not always benefit from vaccinations, even in the absence of an acute and diagnosable helminth infection.

Sex—the most underappreciated variable in research: insights from helminth-infected hosts

The sex of a host affects the intensity, prevalence, and severity of helminth infection. In many cases, one sex has been found to be more susceptible than the other, with the prevalence and intensity of helminth infections being generally higher among male than female hosts; however, many exceptions exist. This observed sex bias in parasitism results primarily from ecological, behavioural, and physiological differences between males and females. Complex interactions between these influences modulate the risk of infection. Indeed, an interplay among sex hormones, sex chromosomes, the microbiome and the immune system significantly contributes to the generation of sex bias among helminth-infected hosts. However, sex hormones not only can modulate the course of infection but also can be exploited by the parasites, and helminths appear to have developed molecules and pathways for this purpose. Furthermore, host sex may influence the efficacy of anti-helminth vaccines; however, although little data exist regarding this sex-dependent efficacy, host sex is known to influence the response to vaccines. Despite its importance, host sex is frequently overlooked in parasitological studies. This review focuses on the key contributors to sex bias in the case of helminth infection. The precise nature of the mechanisms/factors determining these sex-specific differences generally remains largely unknown, and this represents an obstacle in the development of control methods. There is an urgent need to identify any protective elements that could be targeted in future therapies to provide optimal disease management with regard to host sex. Hence, more research is needed into the impact of host sex on immunity and protection.

The Influence of Genetic and Environmental Factors and Their Interactions on Immune Response to Helminth Infections

Helminth infection currently affect over 2 billion people worldwide, with those with the most pathologies and morbidities, living in regions with unequal and disproportionate access to effective healthcare solutions. Host genetics and environmental factors play critical roles in modulating and regulating immune responses following exposure to various pathogens and insults. However, the interplay of environment and genetic factors in influencing who gets infected and the establishment, persistence, and clearance of helminth parasites remains unclear. Inbred strains of mice have long been used to investigate the role of host genetic factors on pathogenesis and resistance to helminth infection in a laboratory setting. This review will discuss the use of ecological and environmental mouse models to study helminth infections and how this could be used in combination with host genetic variation to explore the relative contribution of these factors in influencing immune response to helminth infections. Improved understanding of interactions between genetics and the environment to helminth immune responses would be important for efforts to identify and develop new prophylactic and therapeutic options for the management of helminth infections and their pathogenesis.

Pre-existing helminth infection impairs the efficacy of adjuvanted influenza vaccination in mice

The world health organization estimates that more than a quarter of the human population is infected with parasitic worms that are called helminths. Many helminths suppress the immune system of their hosts to prolong their survival. This helminth-induced immunosuppression “spills over” to unrelated antigens and can suppress the immune response to vaccination against other pathogens. Indeed, several human studies have reported a negative correlation between helminth infections and responses to vaccinations. Using mice that are infected with the parasitic nematode Litomosoides sigmodontis as a model for chronic human filarial infections, we reported previously that concurrent helminth infection impaired the vaccination-induced protection against the human pathogenic 2009 pandemic H1N1 influenza A virus (2009 pH1N1). Vaccinated, helminth-infected mice produced less neutralizing, influenza-specific antibodies than vaccinated naïve control mice. Consequently helminth-infected and vaccinated mice were not protected against a challenge infection with influenza virus but displayed high virus burden in the lung and a transient weight loss. In the current study we tried to improve the vaccination efficacy using vaccines that are licensed for humans. We either introduced a prime-boost vaccination regimen using the non-adjuvanted anti-influenza vaccine Begripal or employed the adjuvanted influenza vaccine Fluad. Although both strategies elevated the production of influenza-specific antibodies and protected mice from the transient weight loss that is caused by an influenza challenge infection, sterile immunity was not achieved. Helminth-infected vaccinated mice still had high virus burden in the lung while non-helminth-infected vaccinated mice rapidly cleared the virus. In summary we demonstrate that basic improvements of influenza vaccination regimen are not sufficient to confer sterile immunity on the background of helminth-induced immunosuppression, despite amelioration of pathology i.e. weight loss. Our findings highlight the risk of failed vaccinations in helminth-endemic areas, especially in light of the ongoing vaccination campaign to control the COVID-19 pandemic.

A Combination of Deworming and Prime-Boost Vaccination Regimen Restores Efficacy of Vaccination Against Influenza in Helminth-Infected Mice

Helminths still infect a quarter of the human population. They manage to establish chronic infections by downmodulating the immune system of their hosts. Consequently, the immune response of helminth-infected individuals to vaccinations may be impaired as well. Here we study the impact of helminth-induced immunomodulation on vaccination efficacy in the mouse system. We have previously shown that an underlying Litomosoides sigmodontis infection reduced the antibody (Ab) response to anti-influenza vaccination in the context of a systemic expansion of type 1 regulatory T cells (Tr1). Most important, vaccine-induced protection from a challenge infection with the 2009 pandemic H1N1 influenza A virus (2009 pH1N1) was impaired in vaccinated, L. sigmodontis-infected mice. Here, we aim at the restoration of vaccination efficacy by drug-induced deworming. Treatment of mice with Flubendazole (FBZ) resulted in elimination of viable L. sigmodontis parasites in the thoracic cavity after two weeks. Simultaneous FBZ-treatment and vaccination did not restore Ab responses or protection in L. sigmodontis-infected mice. Likewise, FBZ-treatment two weeks prior to vaccination did not significantly elevate the influenza-specific Ig response and did not protect mice from a challenge infection with 2009 pH1N1. Analysis of the regulatory T cell compartment revealed that L. sigmodontis-infected and FBZ-treated mice still displayed expanded Tr1 cell populations that may contribute to the sustained suppression of vaccination responses in successfully dewormed mice. To outcompete this sustained immunomodulation in formerly helminth-infected mice, we finally combined the drug-induced deworming with an improved vaccination regimen. Two injections with the non-adjuvanted anti-influenza vaccine Begripal conferred 60% protection while MF59-adjuvanted Fluad conferred 100% protection from a 2009 pH1N1 infection in FBZ-treated, formerly L. sigmodontis-infected mice. Of note, applying this improved prime-boost regimen did not restore protection in untreated L. sigmodontis-infected mice. In summary our findings highlight the risk of failed vaccinations due to helminth infection.

Embracing nature's complexity: Immunoparasitology in the wild

A wealth of research is dedicated to understanding how resistance against parasites is conferred and how parasite-driven pathology is regulated. This research is in part driven by the hope to better treatments for parasitic diseases of humans and livestock, and in part by immunologists who use parasitic infections as biomedical tools to evoke physiological immune responses. Much of the current mechanistic knowledge has been discovered in laboratory studies using model organisms, especially the laboratory mouse. However, wildlife are also hosts to a range of parasites. Through the study of host-parasite interactions in these non-laboratory systems we can gain a deeper understanding of parasite immunology in a more natural, complex environment. With a focus on helminth parasites, we here explore the insights gained into parasite-induced immune responses through (for immunologists) non-conventional experimental systems, and how current core findings from laboratory studies are reflected in these more natural conditions. The quality of the immune response is undoubtedly a central player in susceptibility versus resistance, as many laboratory studies have shown. Yet, in the wild, parasite infections tend to be chronic diseases. Whilst reading our review, we encourage the reader to consider the following questions which may (only) be answered by studying naturally occurring parasites in the wild: a) what type of immune responses are mounted against parasites in different hosts in the wild, and how do they vary within an individual over time, between individuals of the same species and between species? b) can we use wild or semi-wild study systems to understand the evolutionary drivers for tolerance versus resistance towards a parasite? c) what determines the ability of the host to cope with an infection and is there a link with the type of immune response mounted? d) can we modulate environmental factors to manipulate a wild animal's immune response to parasitic infections, with translation potential for humans, wildlife, and livestock? and e) in context of this special issue, what lessons for Type 2 immunity can we glean from studying animals in their natural environments? Further, we aim to integrate some of the knowledge gained in semi-wild and wild settings with knowledge gained from traditional laboratory-based research, and to raise awareness for the opportunities (and challenges) that come with integrating a multitude of naturally-occurring variables into immunoparasitological research.

Helminth Infections Suppress the Efficacy of Vaccination against Seasonal Influenza

Helminth parasites infect more than a quarter of the human population and inflict significant changes to the immunological status of their hosts. Here, we analyze the impact of helminth infections on the efficacy of vaccinations using Litomosoides sigmodontis-infected mice. Concurrent helminth infection reduces the quantity and quality of antibody responses to vaccination against seasonal influenza. Vaccination-induced protection against challenge infections with the human pathogenic 2009 pandemic H1N1 influenza A virus is drastically impaired in helminth-infected mice. Impaired responses are also observed if vaccinations are performed after clearance of a previous helminth infection, suggesting that individuals in helminth-endemic areas may not always benefit from vaccinations, even in the absence of an acute and diagnosable helminth infection. Mechanistically, the suppression is associated with a systemic and sustained expansion of interleukin (IL)-10-producing CD4⁺CD49⁺LAG-3⁺ type 1 regulatory T cells and partially abrogated by in vivo blockade of the IL-10 receptor.

The immune response of inbred laboratory mice to Litomosoides sigmodontis: A route to discovery in myeloid cell biology

Litomosoides sigmodontis is the only filarial nematode where the full life cycle, from larval delivery to the skin through to circulating microfilaria, can be completed in immunocompetent laboratory mice. It is thus an invaluable tool for the study of filariasis. It has been used for the study of novel anti‐helminthic therapeutics, the development of vaccines against filariasis, the development of immunomodulatory drugs for the treatment of inflammatory disease and the study of basic immune responses to filarial nematodes. This review will focus on the latter and aims to summarize how the L sigmodontis model has advanced our basic understanding of immune responses to helminths, led to major discoveries in macrophage biology and provided new insights into the immunological functions of the pleural cavity. Finally, and most importantly L sigmodontis represents a suitable platform to study how host genotype affects immune responses, with the potential for further discovery in myeloid cell biology and beyond.

Development and validation of an Onchocerca ochengi adult male worm gerbil model for macrofilaricidal drug screening

BACKGROUND

Onchocerciasis currently afflicts an estimated 15 million people and is the second leading infectious cause of blindness world-wide. The development of a macrofilaricide to cure the disease has been hindered by the lack of appropriate small laboratory animal models. This study therefore, was aimed at developing and validating the Mongolian gerbil, as an Onchocerca ochengi (the closest in phylogeny to O. volvulus) adult male worm model.

METHODOLOGY/PRINCIPAL FINDINGS

Mongolian gerbils (Meriones unguiculatus) were each implanted with 20 O. ochengi male worms (collected from infected cattle), in the peritoneum. Following drug or placebo treatments, the implanted worms were recovered from the animals and analyzed for burden, motility and viability. Worm recovery in control gerbils was on average 35%, with 89% of the worms being 100% motile. Treatment of the gerbils implanted with male worms with flubendazole (FBZ) resulted in a significant reduction (p = 0.0021) in worm burden (6.0% versus 27.8% in the control animals); all recovered worms from the treated group had 0% worm motility versus 91.1% motility in control animals. FBZ treatment had similar results even after four different experiments. Using this model, we tested a related drug, oxfendazole (OFZ), and found it to also significantly (p = 0.0097) affect worm motility (22.7% versus 95.0% in the control group).

CONCLUSIONS/SIGNIFICANCE

We have developed and validated a novel gerbil O. ochengi adult male worm model for testing new macrofilaricidal drugs in vivo. It was also used to determine the efficacy of oxfendazole in vivo.

Myeloid cell recruitment versus local proliferation differentiates susceptibility from resistance to filarial infection

Both T_H2-dependent helminth killing and suppression of the T_H2 effector response have been attributed to macrophages (MΦ) activated by IL-4 (M(IL-4)). To investigate how M(IL-4) contribute to diverse infection outcomes, the MΦ compartment of susceptible BALB/c mice and more resistant C57BL/6 mice was profiled during infection of the pleural cavity with the filarial nematode, Litomosoides sigmodontis. C57BL/6 mice exhibited a profoundly expanded resident MΦ (resMΦ) population, which was gradually replenished from the bone marrow in an age-dependent manner. Infection status did not alter the bone-marrow derived contribution to the resMΦ population, confirming local proliferation as the driver of resMΦ expansion. Significantly less resMΦ expansion was observed in the susceptible BALB/c strain, which instead exhibited an influx of monocytes that assumed an immunosuppressive PD-L2⁺ phenotype. Inhibition of monocyte recruitment enhanced nematode killing. Thus, the balance of monocytic vs. resident M(IL-4) numbers varies between inbred mouse strains and impacts infection outcome.

Use of the Litomosoides sigmodontis Infection Model of Filariasis to Study Type 2 Immunity

Helminth parasites infect over 2 billion people worldwide resulting in huge global health and economic burden. Helminths typically stimulate Type 2 immune responses and excel at manipulating or suppressing host-immune responses resulting in chronic infections that can last for years to decades. Alongside the importance for the development of helminth treatments and vaccines, studying helminth immunity has unraveled many fundamental aspects of Type 2 immunity and immune regulation with implications for the treatment of autoimmunity and Type 2-mediated diseases, such as allergies. Here we describe the maintenance and use of Litomosoides sigmodontis, a murine model for studying host-parasite interactions, Type 2 immunity, and vaccines to tissue-dwelling filarial nematodes, which in humans cause lymphatic filariasis (e.g., Brugia malayi) and onchocerciasis (Onchocerca volvulus).

Patency of Litomosoides sigmodontis infection depends on Toll-like receptor 4 whereas Toll-like receptor 2 signalling influences filarial-specific CD4(+) T-cell responses

BALB/c mice develop a patent state [release of microfilariae (Mf), the transmission life-stage, into the periphery] when exposed to the rodent filariae Litomosoides sigmodontis. Interestingly, only a portion of the infected mice become patent, which reflects the situation in human individuals infected with Wuchereria bancrofti. Since those individuals had differing filarial-specific profiles, this study compared differences in immune responses between Mf(+) and Mf(-) infected BALB/c mice. We demonstrate that cultures of total spleen or mediastinal lymph node cells from Mf(+) mice produce significantly more interleukin-5 (IL-5) to filarial antigens but equal levels of IL-10 when compared with Mf(-) mice. However, isolated CD4(+) T cells from Mf(+) mice produced significantly higher amounts of all measured cytokines, including IL-10, when compared with CD4(+) T-cell responses from Mf(-) mice. Since adaptive immune responses are influenced by triggering the innate immune system we further studied the immune profiles and parasitology in infected Toll-like receptor-2-deficient (TLR2(-/-)) and TLR4(-/-) BALB/c mice. Ninety-three per cent of L. sigmodontis-exposed TLR4(-/-) BALB/c mice became patent (Mf(+)) although worm numbers remained comparable to those in Mf(+) wild-type controls. Lack of TLR2 had no influence on patency outcome or worm burden but infected Mf(+) mice had significantly lower numbers of Foxp3(+) regulatory T cells and dampened peripheral immune responses. Interestingly, in vitro culturing of CD4(+) T cells from infected wild-type mice with granulocyte-macrophage colony-stimulating factor-derived TLR2(-/-) dendritic cells resulted in an overall diminished cytokine profile to filarial antigens. Hence, triggering TLR4 or TLR2 during chronic filarial infection has a significant impact on patency and efficient CD4(+) T-cell responses, respectively.

Litomosoides sigmodontis: a jird urine metabolome study

The neglected tropical disease onchocerciasis affects more than 35 million people worldwide with over 95% in Africa. Disease infection initiates from the filarial parasitic nematode Onchocerca volvulus, which is transmitted by the blackfly vector Simulium sp. carrying infectious L3 larvae. New treatments and diagnostics are required to eradicate this parasitic disease. Herein, we describe that a previously discovered biomarker for onchocerciasis, N-acetyltyramine-O-glucuronide (NATOG) is also present in urine samples of jirds infected with the onchocerciasis model nematode Litomosoides sigmodontis. Increased NATOG values paralleled a progressing infection and demonstrated that quantification of NATOG in this rodent model can be utilized to track its infectivity. Moreover, our findings suggest how NATOG monitoring may be used for evaluating potential drug candidates.

Macrophage proliferation, provenance, and plasticity in macroparasite infection

Macrophages have long been center stage in the host response to microbial infection, but only in the past 10–15 years has there been a growing appreciation for their role in helminth infection and the associated type 2 response. Through the actions of the IL-4 receptor α (IL-4Rα), type 2 cytokines result in the accumulation of macrophages with a distinctive activation phenotype. Although our knowledge of IL-4Rα-induced genes is growing rapidly, the specific functions of these macrophages have yet to be established in most disease settings. Understanding the interplay between IL-4Rα-activated macrophages and the other cellular players is confounded by the enormous transcriptional heterogeneity within the macrophage population and by their highly plastic nature. Another level of complexity is added by the new knowledge that tissue macrophages can be derived either from a resident prenatal population or from blood monocyte recruitment and that IL-4 can increase macrophage numbers through proliferative expansion. Here, we review current knowledge on the contribution of macrophages to helminth killing and wound repair, with specific attention paid to distinct cellular origins and plasticity potential.

Nematode-induced interference with vaccination efficacy targets follicular T helper cell induction and is preserved after termination of infection

One-third of the human population is infected with parasitic worms. To avoid being eliminated, these parasites actively dampen the immune response of their hosts. This immune modulation also suppresses immune responses to third-party antigens such as vaccines. Here, we used Litomosoides sigmodontis-infected BALB/c mice to analyse nematode-induced interference with vaccination. Chronic nematode infection led to complete suppression of the humoral response to thymus-dependent vaccination. Thereby the numbers of antigen-specific B cells as well as the serum immunoglobulin (Ig) G titres were reduced. T_H2-associated IgG1 and T_H1-associated IgG2 responses were both suppressed. Thus, nematode infection did not bias responses towards a T_H2 response, but interfered with Ig responses in general. We provide evidence that this suppression indirectly targeted B cells via accessory T cells as number and frequency of vaccine-induced follicular B helper T cells were reduced. Moreover, vaccination using model antigens that stimulate Ig response independently of T helper cells was functional in nematode-infected mice. Using depletion experiments, we show that CD4⁺Foxp3⁺ regulatory T cells did not mediate the suppression of Ig response during chronic nematode infection. Suppression was induced by fourth stage larvae, immature adults and mature adults, and increased with the duration of the infection. By contrast, isolated microfilariae increased IgG2a responses to vaccination. This pro-inflammatory effect of microfilariae was overruled by the simultaneous presence of adults. Strikingly, a reduced humoral response was still observed if vaccination was performed more than 16 weeks after termination of L. sigmodontis infection. In summary, our results suggest that vaccination may not only fail in helminth-infected individuals, but also in individuals with a history of previous helminth infections.

Parasitic worms, called helminths, infect one-third of the world population. Despite exposure to their host's immune system many helminths establish chronic infections and survive several years within their host. They avoid elimination by dampening the immune response of their hosts. This immune suppression also affects immune responses to third-party antigens such as vaccines. Indeed, accumulating evidence suggests that helminth-infected humans display impaired responses to vaccination. Thus, anthelminthic treatment before vaccination is discussed. Here, we use helminth-infected mice to analyse kinetics and mechanism of helminth-induced interference with vaccination efficacy more precisely. We show that chronic helminth infection completely suppressed antibody responses to a model vaccine. Thereby helminths suppressed the antibody-producing B cells indirectly via suppression of accessory T helper cells. The suppression was more pronounced at later time points of infection and still observed in mice that had terminated the helminth infection for more than 16 weeks. In summary, our results suggest that vaccination may not only fail in helminth-infected individuals, but also in individuals with a history of previous helminth infections. Thus, our report highlights the importance to develop vaccination strategies that are functional despite concurrent helminth infection rather than deworming humans before vaccination.

Alternatively activated macrophages derived from monocytes and tissue macrophages are phenotypically and functionally distinct

Macrophages adopt an alternatively activated phenotype (AAMs) when activated by the interleukin-4receptor(R)α. AAMs can be derived either from proliferation of tissue resident macrophages or recruited inflammatory monocytes, but it is not known whether these different sources generate AAMs that are phenotypically and functionally distinct. By transcriptional profiling analysis, we show here that, although both monocyte and tissue-derived AAMs expressed high levels of Arg1, Chi3l3, and Retnla, only monocyte-derived AAMs up-regulated Raldh2 and PD-L2. Monocyte-derived AAMs were also CX3CR1-green fluorescent protein (GFP)(high) and expressed CD206, whereas tissue-derived AAMs were CX3CR1-GFP and CD206 negative. Monocyte-derived AAMs had high levels of aldehyde dehydrogenase activity and promoted the differentiation of FoxP3(+) cells from naïve CD4(+) cells via production of retinoic acid. In contrast, tissue-derived AAMs expressed high levels of uncoupling protein 1. Hence monocyte-derived AAM have properties associated with immune regulation, and the different physiological properties associated with AAM function may depend on the distinct lineage of these cells.

A comprehensive, model-based review of vaccine and repeat infection trials for filariasis

SUMMARY Filarial worms cause highly morbid diseases such as elephantiasis and river blindness. Since the 1940s, researchers have conducted vaccine trials in 27 different animal models of filariasis. Although no vaccine trial in a permissive model of filariasis has provided sterilizing immunity, great strides have been made toward developing vaccines that could block transmission, decrease pathological sequelae, or decrease susceptibility to infection. In this review, we have organized, to the best of our ability, all published filaria vaccine trials and reviewed them in the context of the animal models used. Additionally, we provide information on the life cycle, disease phenotype, concomitant immunity, and natural immunity during primary and secondary infections for 24 different filaria models.

T-cell-derived, but not B-cell-derived, IL-10 suppresses antigen-specific T-cell responses in Litomosoides sigmodontis-infected mice

IL-10, a cytokine with pleiotropic functions is produced by many different cells. Although IL-10 may be crucial for initiating protective Th2 responses to helminth infection, it may also function as a suppressive cytokine preventing immune pathology or even contributing to helminth-induced immune evasion. Here, we show that B cells and T cells produce IL-10 during murine Litomosoides sigmodontis infection. IL-10-deficient mice produced increased amounts of L. sigmodontis-specific IFN-γ and IL-13 suggesting a suppressive role for IL-10 in the initiation of the T-cell response to infection. Using cell type-specific IL-10-deficient mice, we dissected different functions of T-cell- and B-cell-derived IL-10. Litomosoides sigmodontis-specific IFN-γ, IL-5, and IL-13 production increased in the absence of T-cell-derived IL-10 at early and late time points of infection. In contrast, B-cell-specific IL-10 deficiency did not lead to significant changes in L. sigmodontis-specific cytokine production compared to WT mice. Our results suggest that the initiation of Ag-specific cellular responses during L. sigmodontis infection is suppressed by T-cell-derived IL-10 and not by B-cell-derived IL-10.

Future prospects and challenges of vaccines against filariasis

Filarial infections remain a major public health and socio-economic problem across the tropics, despite considerable effort to reduce disease burden or regionally eliminate the infection with mass drug administration programmes. The sustainability of these programmes is now open to question owing to a range of issues, not least of which is emerging evidence for drug resistance. Vaccination, if developed appropriately, remains the most cost-effective means of long-term disease control. The rationale for the feasibility of vaccination against filarial parasites including onchocerciasis (river blindness, Onchocerca volvulus) and lymphatic filariasis (Wuchereria bancrofti or Brugia malayi) is founded on evidence from both humans and animal models for the development of protective immunity. Nonetheless, enormous challenges need to be faced in terms of overcoming parasite-induced suppression without inducing pathology as well as the need to both recognize and tackle evolutionary and ecological obstacles to successful vaccine development. Nonetheless, new technological advances in addition to systems biology approaches offer hope that optimal immune responses can be induced that will prevent infection, disease and/or transmission.

Pathogenic nematodes suppress humoral responses to third-party antigens in vivo by IL-10-mediated interference with Th cell function

One third of the human population is infected with helminth parasites. To promote their longevity and to limit pathology, helminths have developed several strategies to suppress the immune response of their host. As this immune suppression also acts on unrelated third-party Ags, a preexisting helminth infection may interfere with vaccination efficacy. In this study, we show that natural infection with Litomosoides sigmodontis suppressed the humoral response to thymus-dependent but not to thymus-independent model Ags in C57BL/6 mice. Thereby, we provide evidence that reduced humoral responses were mediated by interference with Th cell function rather than by direct suppression of B cells in L. sigmodontis-infected mice. We directly demonstrate suppression of Ag-specific proliferation in OVA-specific Th cells after adoptive transfer into L. sigmodontis-infected mice that led to equally reduced production of OVA-specific IgG. Transferred Th cells displayed increased frequencies of Foxp3(+) after in vivo stimulation within infected but not within naive mice. Helminth-mediated suppression was induced by established L. sigmodontis infections but was completely independent of the individual worm burden. Using DEREG mice, we rule out a central role for host-derived regulatory T cells in the suppression of transferred Th cell proliferation. In contrast, we show that L. sigmodontis-induced, host-derived IL-10 mediated Foxp3 induction in transferred Th cells and significantly contributed to the observed Th cell hypoproliferation within infected mice.

Early recruitment of natural CD4+ Foxp3+ Treg cells by infective larvae determines the outcome of filarial infection

Human helminth infections are synonymous with impaired immune responsiveness indicating suppression of host immunity. Using a permissive murine model of filariasis, Litomosoides sigmodontis infection of inbred mice, we demonstrate rapid recruitment and increased in vivo proliferation of CD4(+)Foxp3(+) Treg cells upon exposure to infective L3 larvae. Within 7 days post-infection this resulted in an increased percentage of CD4(+)T cells at the infection site expressing Foxp3. Antibody-mediated depletion of CD25(+) cells prior to infection to remove pre-existing 'natural' CD4(+)CD25(+)Foxp3(+) Treg cells, while not affecting initial larval establishment, significantly reduced the number of adult parasites recovered 60 days post-infection. Anti-CD25 pre-treatment also impaired the fecundity of the surviving female parasites, which had reduced numbers of healthy eggs and microfilaria within their uteri, translating to a reduced level of blood microfilaraemia. Enhanced parasite killing was associated with augmented in vitro production of antigen-specific IL-4, IL-5, IL-13 and IL-10. Thus, upon infection filarial larvae rapidly provoke a CD4(+)Foxp3(+) Treg-cell response, biasing the initial CD4(+) T-cell response towards a regulatory phenotype. These CD4(+)Foxp3(+) Treg cells are predominantly recruited from the 'natural' regulatory pool and act to inhibit protective immunity over the full course of infection.

Of mice, cattle, and humans: the immunology and treatment of river blindness

River blindness is a seriously debilitating disease caused by the filarial parasite Onchocerca volvulus, which infects millions in Africa as well as in South and Central America. Research has been hampered by a lack of good animal models, as the parasite can only develop fully in humans and some primates. This review highlights the development of two animal model systems that have allowed significant advances in recent years and hold promise for the future. Experimental findings with Litomosoides sigmodontis in mice and Onchocerca ochengi in cattle are placed in the context of how these models can advance our ability to control the human disease.

Repeated respiratory Mycoplasma pneumoniae infections in mice: effect of host genetic background

Respiratory Mycoplasma pneumoniae (Mp) infection is involved in several acute and chronic lung diseases including community-acquired pneumonia, asthma and chronic obstructive pulmonary disease. In the chronic disease process, recurrent respiratory bacterial infections could occur, which may result in varying degrees of symptoms and lung inflammation among patients. However, the lung immunologic differences of host responses to repeated bacterial (i.e., Mp) infections remain to be determined. In the present study, we examined cellular and humoral responses to multiple (up to 3) Mp infections in two genetically different strains of mice (BALB/c and C57BL/6). Mice were intranasally inoculated with one Mp infection, two or three Mp infections (4 weeks apart), and sacrificed on days 3, 7 and 14 after the last Mp infection. Overall, compared to C57BL/6 mice, BALB/c mice demonstrated a significantly higher degree of lung tissue inflammatory cell infiltrate, BAL cellularity, and release of pro-inflammatory cytokines (TNF-alpha, keratinocyte-derived chemokine (KC, a mouse homolog of human chemokine Gro-alpha [CXCL1], and IFN-gamma). In addition, BALB/c mice presented higher levels of serum Mp-specific IgG and IgM, but not IgA. Consistently with lung and serum data, Mp load in BAL and lung specimens was significantly higher in BALB/c mice than C57BL/6 mice. Moreover, repeated Mp infections in BALB/c, but not C57BL/6 mice, produced a greater inflammatory response than did a single Mp infection. Our results suggest that hosts with different genetic background may have different susceptibility to repeated respiratory Mp infections along with inflammatory responses.

Co-infected C57BL/6 mice mount appropriately polarized and compartmentalized cytokine responses to Litomosoides sigmodontis and Leishmania major but disease progression is altered

This study examines the capacity of the mammalian host to fully compartmentalize the response to infection with type 1 vs. type 2 inducing organisms that infect different sites in the body. For this purpose, C57BL/6 mice were infected with the rodent filarial nematode Litomosoides sigmodontis followed by footpad infection with the protozoan parasite Leishmania major. In this host, nematode infection is established in the thoracic cavity but no microfilariae circulate in the bloodstream. We utilized quantitative ELISPOT analysis of IL-4 and IFN-gamma producing cells to assess cytokine bias and response magnitude in the lymph nodes draining the sites of infection as well as more systemic responses in the spleen and serum. Contrary to other systems where co-infection has a major impact on bias, cytokine ratios were unaltered in either local lymph node. The most notable effect of co-infection was an unexpected increase in the magnitude of the IFN-gamma response to L. major in mice previously infected with L. sigmodontis. Further, lesion development was significantly delayed in these mice. Thus, despite the ability of the immune system to appropriately compartmentalize the immune response, interactions between responses at distinct infection sites can alter disease progression.