The fate of the filaria Litomosoides sigmodontis in susceptible and naturally resistant mice

Unbalanced Arginine pathway and altered maturation of pleural macrophages in Th2-deficient mice during Litomosoides sigmodontis filarial infection

Filarial parasites are tissue dwelling worms transmitted by hematophagous vectors. Understanding the mechanisms regulating microfilariae (the parasite offspring) development is a prerequisite for controlling transmission in filarial infections. Th2 immune responses are key for building efficient anti-parasite responses but have been shown to also lead to detrimental tissue damage in the presence of microfilariae. Litomosoides sigmodontis, a rodent filaria residing in the pleural cavity was therefore used to characterize pleuropulmonary pathology and associated immune responses in wild-type and Th2 deficient mice. Wild-type and Th2-deficient mice (Il-4rα^-/-/Il-5^-/-) were infected with L. sigmodontis and parasite outcome was analyzed during the patent phase (when microfilariae are in the general circulation). Pleuropulmonary manifestations were investigated and pleural and bronchoalveolar cells were characterized by RNA analysis, imaging and/or flow cytometry focusing on macrophages. Il-4rα^-/-/Il-5^-/- mice were hypermicrofilaremic and showed an enhanced filarial survival but also displayed a drastic reduction of microfilaria-driven pleural cavity pathologies. In parallel, pleural macrophages from Il-4rα^-/-/Il-5^-/- mice lacked expression of prototypical alternative activation markers RELMα and Chil3 and showed an altered balance of some markers of the arginine metabolic pathway. In addition, monocytes-derived F4/80^intermediate macrophages from infected Il-4rα^-/-/Il-5^-/- mice failed to mature into resident F4/80^high large macrophages. Altogether these data emphasize that the presence of both microfilariae and IL-4R/IL-5 signaling are critical in the development of the pathology and in the phenotype of macrophages. In Il-4rα^-/-/Il-5^-/- mice, the balance is in favor of parasite development while limiting the pathology associated with the host immune response.

Microfilaria-dependent thoracic pathology associated with eosinophilic and fibrotic polyps in filaria-infected rodents

BACKGROUND

Pulmonary manifestations are regularly reported in both human and animal filariasis. In human filariasis, the main known lung manifestations are the tropical pulmonary eosinophilia syndrome. Its duration and severity are correlated with the presence of microfilariae. Litomosoides sigmodontis is a filarial parasite residing in the pleural cavity of rodents. This model is widely used to understand the immune mechanisms that are established during infection and for the screening of therapeutic molecules. Some pulmonary manifestations during the patent phase of infection with L. sigmodontis have been described in different rodent hosts more or less permissive to infection.

METHODS

Here, the permissive Mongolian gerbil (Meriones unguiculatus) was infected with L. sigmodontis. Prevalence and density of microfilariae and adult parasites were evaluated. Lungs were analyzed for pathological signatures using immunohistochemistry and 3D imaging techniques (two-photon and light sheet microscopy).

RESULTS

Microfilaremia in gerbils was correlated with parasite load, as amicrofilaremic individuals had fewer parasites in their pleural cavities. Fibrotic polypoid structures were observed on both pleurae of infected gerbils. Polyps were of variable size and developed from the visceral mesothelium over the entire pleura. The larger polyps were vascularized and strongly infiltrated by immune cells such as eosinophils, macrophages or lymphocytes. The formation of these structures was induced by the presence of adult filariae since small and rare polyps were observed before patency, but they were exacerbated by the presence of gravid females and microfilariae.

CONCLUSIONS

Altogether, these data emphasize the role of host-specific factors in the pathogenesis of filarial infections.

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.

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).

Migratory phase of Litomosoides sigmodontis filarial infective larvae is associated with pathology and transient increase of S100A9 expressing neutrophils in the lung

Filarial infections are tropical diseases caused by nematodes of the Onchocercidae family such as Mansonella perstans. The infective larvae (L3) are transmitted into the skin of vertebrate hosts by blood-feeding vectors. Many filarial species settle in the serous cavities including M. perstans in humans and L. sigmodontis, a well-established model of filariasis in mice. L. sigmodontis L3 migrate to the pleural cavity where they moult into L4 around day 9 and into male and female adult worms around day 30. Little is known of the early phase of the parasite life cycle, after the L3 is inoculated in the dermis by the vector and enters the afferent lymphatic vessels and before the moulting processes in the pleural cavity. Here we reveal a pulmonary phase associated with lung damage characterized by haemorrhages and granulomas suggesting L3 reach the lung via pulmonary capillaries and damage the endothelium and parenchyma by crossing them to enter the pleural cavity. This study also provides evidence for a transient inflammation in the lung characterized by a very early recruitment of neutrophils associated with high expression levels of S100A8 and S100A9 proteins.

Mansonella perstans is a widespread human filarial parasite in Africa responsible for pleural and peritoneal cavity filariasis. Compared to other filarial parasites such as Wuchereria bancrofti, Brugia malayi, and Loa loa, the biology of M. perstans is poorly known. The blood-feeding vectors inject infective larvae (L3) into the host skin during a blood meal. Depending on the species, the L3 will then migrate to its specific site. In the murine model of filariasis Litomosoides sigmodontis L3 also reach the pleural cavity where they moult twice then mate and produce microfilariae. Migration patterns from the skin to the pleural cavity are partially known and involve a lymphatic phase. Here we present a sequential analysis of L3 infection from their inoculation to day 8 when they are settled in the pleural cavity, revealing the presence of L3 in the lung. Pulmonary damage including haemorrhages and granulomas is also observed suggesting that L3 could migrate to the pulmonary circulation and capillaries from where they could exit the lung to reach the pleural cavity. This induces a local inflammatory response characterized by neutrophil activation and upregulation.

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.

Development of Trichosomoides nasalis (Nematoda: Trichinelloidea) in the murid host: evidence for larval growth in striated muscle fibres

Trichosomoides nasalis (Trichinelloidea) is a parasite of Arvicanthis niloticus (Muridae) in Senegal. Female worms that harbour dwarf males in their uteri, occur in the epithelium of the nasal mucosa. Young laboratory-bred A. niloticus were either fed females containing larvated eggs or intraperitoneally injected with motile first-stage larvae recovered from female uteri. Both resulted in successful infection. Organs examined during rodent necropsy were blood and lymphatic circulatory systems (heart, large vessels, lymphnodes), lungs, liver, kidneys, thoracic and abdominal cavities, thoracic and abdominal muscular walls, diaphragm, tongue, and nasal mucosa. Development to adult nasal stages took three weeks. Recovery of newly hatched larvae from the peritoneal fluid at four-eight hours after oral infection suggests a direct passage from the stomach or intestinal wall to the musculature. However, dissemination through the blood, as observed with Trichinella spiralis, cannot be excluded even though newly hatched larvae of T. nasalis are twice as thick (15 μm). Developing larvae were found in histological sections of the striated muscle of the abdominal and thoracic walls, and larvae in fourth moult were dissected from these sites. Adult females were found in the deep nasal mucosa where mating occurred prior to worms settling in the nasal epithelium. The present study shows a remarkable similarity between T. nasalis and Trichinella species regarding muscle tropism, but the development of T. nasalis is not arrested at the late first-larval stage and does not induce transformation of infected fibres into nurse cells. T. nasalis seems a potential model to study molecular relations between trichinelloid larvae and infected muscle fibres.

The chemokine CXCL12 is essential for the clearance of the filaria Litomosoides sigmodontis in resistant mice

Litomosoides sigmodontis is a cause of filarial infection in rodents. Once infective larvae overcome the skin barrier, they enter the lymphatic system and then settle in the pleural cavity, causing soft tissue infection. The outcome of infection depends on the parasite's modulatory ability and also on the immune response of the infected host, which is influenced by its genetic background. The goal of this study was to determine whether host factors such as the chemokine axis CXCL12/CXCR4, which notably participates in the control of immune surveillance, can influence the outcome of the infection. We therefore set up comparative analyses of subcutaneous infection by L. sigmodontis in two inbred mouse strains with different outcomes: one susceptible strain (BALB/c) and one resistant strain (C57BL/6). We showed that rapid parasite clearance was associated with a L. sigmodontis-specific CXCL12-dependent cell response in C57BL/6 mice. CXCL12 was produced mainly by pleural mesothelial cells during infection. Conversely, the delayed parasite clearance in BALB/c mice was neither associated with an increase in CXCL12 levels nor with cell influx into the pleural cavity. Remarkably, interfering with the CXCL12/CXCR4 axis in both strains of mice delayed filarial development, as evidenced by the postponement of the fourth molting process. Furthermore, the in vitro growth of stage 4 filariae was favored by the addition of low amounts of CXCL12. The CXCL12/CXCR4 axis thus appears to have a dual effect on the L. sigmodontis life cycle: by acting as a host-cell restriction factor for infection, and as a growth factor for worms.

The human parasite Loa loa in cytokine and cytokine receptor gene knock out BALB/c mice: survival, development and localization

Background

Immunological mechanisms involved in the survival and development of human filarial species in the vertebrate host are poorly known due to the lack of suitable experimental models. In order to understand the role of cytokines in the survival and development of filarial larvae in the vertebrate host, we infected different strains of BALB/c mice deficient in a number of cytokine or cytokine receptor genes with Loa loa. The survival and development of larvae were monitored.

Methods

BALB/c mice genetically deficient in IL-4R, IFN-γ, IFN-γ/IL-5, IL-5, and IL-4R/IL-5 cytokine or cytokine receptor genes were infected with a human strain of L. loa and necropsies were performed at different time intervals up to 70 days post infection to monitor the survival and development of L. loa larvae. The larvae were teased out of the skin, muscles, peritoneal and pleural cavities, heart and lung tissues. The length and width of the recovered larvae were measured to assess their growth.

Results

In mice deficient for IL-4R, IFN-γ, IFN-γ/IL-5, IL-5 and IL-4R/IL-5, the larvae survived up to 5, 20, 40, 50 and 70 days respectively. Worms recovered 70 days post infection in IL-4R/IL-5 DKO mice were young adults and measured 10.12 mm in length and 0.1 mm in width. Overall, 47% of larvae were recovered from subcutaneous tissues, 40% from muscles, 6% from the peritoneal cavity and 4% from the pleural cavity, lungs and heart.

Conclusion

L. loa exhibits a differential survival and development in different strains of cytokine or cytokine receptor gene knockout mice with IL-4R and IL-5 playing critical roles in the host resistance to L. loa infection. The knock out BALB/c mouse therefore represents a useful tool to explore the key effectors of adaptive immunity involved in the killing of the L. loa parasite in a mammal host.

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.

Th2 responses to helminth parasites can be therapeutically enhanced by, but are not dependent upon, GITR-GITR ligand costimulation in vivo

The immune suppression that characterizes human helminth infections can hinder the development of protective immunity or help to reduce pathogenic inflammation. Signaling through the T cell costimulator glucocorticoid-induced TNFR-related protein (GITR) counteracts immune downregulation by augmenting effector T cell responses and abrogating suppression by Foxp3(+) regulatory T cells. Thus, superphysiological Ab-mediated GITR costimulation represents a novel therapy for promoting protective immunity toward parasitic helminths, whereas blocking physiological GITR-GITR ligand (GITRL) interactions may provide a mechanism for dampening pathogenic Th2 inflammation. We investigated the superphysiological and physiological roles of the GITR-GITRL pathway in the development of protective and pathogenic Th2 responses in murine infection models of filariasis (Litomosoides sigmodontis) and schistosomiasis (Schistosoma mansoni). Providing superphysiological GITR costimulation using an agonistic anti-GITR mAb over the first 12 d of L. sigmodontis infection initially increased the quantity of Th2 cells, as well as their ability to produce Th2 cytokines. However, as infection progressed, the Th2 responses reverted to normal infection levels, and parasite killing remained unaffected. Despite the Th2-promoting role of superphysiological GITR costimulation, Ab-mediated blockade of the GITR-GITRL pathway did not affect Th2 cell priming or maintenance during L. sigmodontis infection. Blockade of GITR-GITRL interactions during the acute egg phase of S. mansoni infection resulted in reduced Th2 responses, but this effect was confined to the spleen and did not lead to changes in liver pathology. Thus, although superphysiological GITR costimulation can therapeutically enhance Th2 responses, physiological GITR-GITRL interactions are not required for the development of Th2-mediated resistance or pathology in murine models of filariasis and schistosomiasis.

CCL17 controls mast cells for the defense against filarial larval entry

Filarial parasites have to trespass many barriers to successfully settle within their mammalian host, which is equipped with mechanical borders and complex weaponry of an evolved immune system. However, little is known about mechanisms of early local events in filarial infections. In this study, bone marrow-derived dendritic cells not only upregulated activation markers CD40 and CD80 upon in vitro stimulation with filarial extracts, but also secreted CCL17, a chemokine known to be produced upon microbial challenge. Mice deficient for CCL17 had an up to 4-fold higher worm burden compared with controls by day 10 of infection with the murine filaria Litomosoides sigmodontis. Also, numbers of mast cells (MCs) invading the skin and degranulation were significantly increased, which was associated with enhanced vascular permeability and larval establishment. This phenotype was reverted by inhibition of MC degranulation with disodium cromoglycate or by blockade of histamine. In addition, we showed that CCL17-mediated vascular permeability was dependent on the presence of Wolbachia endosymbionts and TLR2. Our findings reveal that CCL17 controls filarial larval entry by limiting MC-dependent vascular permeability.

Filarial parasites develop faster and reproduce earlier in response to host immune effectors that determine filarial life expectancy

During larval development, filarial nematodes adjust their lifelong reproductive strategy to the presence of anti-parasitic immune cells that determine host resistance and experimental vaccine efficacy.

Humans and other mammals mount vigorous immune assaults against helminth parasites, yet there are intriguing reports that the immune response can enhance rather than impair parasite development. It has been hypothesized that helminths, like many free-living organisms, should optimize their development and reproduction in response to cues predicting future life expectancy. However, immune-dependant development by helminth parasites has so far eluded such evolutionary explanation. By manipulating various arms of the immune response of experimental hosts, we show that filarial nematodes, the parasites responsible for debilitating diseases in humans like river blindness and elephantiasis, accelerate their development in response to the IL-5 driven eosinophilia they encounter when infecting a host. Consequently they produce microfilariae, their transmission stages, earlier and in greater numbers. Eosinophilia is a primary host determinant of filarial life expectancy, operating both at larval and at late adult stages in anatomically and temporally separate locations, and is implicated in vaccine-mediated protection. Filarial nematodes are therefore able to adjust their reproductive schedules in response to an environmental predictor of their probability of survival, as proposed by evolutionary theory, thereby mitigating the effects of the immune attack to which helminths are most susceptible. Enhancing protective immunity against filarial nematodes, for example through vaccination, may be less effective at reducing transmission than would be expected and may, at worst, lead to increased transmission and, hence, pathology.

Many organisms are able to adapt their development to the severity of their environment based on specific cues, and we have identified such a phenomenon, termed phenotypic plasticity, in the filarial parasite Litomosoides sigmodontis. Filarial nematodes infect about 200 million people worldwide, and much effort is going into finding a vaccine that would complement current drug treatments. Although anti-filarial immunity can be achieved, we show, in accord with evolutionary theory, that when these parasites infect a new host, they are able to adjust their development and reproduction to the presence of immune cells specialized in anti-helminth attack. These developmental schedules are determined within hours and impact their lifelong reproductive strategy; when immune attack is strong, and thus mortality is likely to be high, they produce offspring earlier and in greater numbers. Because current experimental vaccines rely on the very immune elements to which these nematodes adjust their development, their phenotypic plasticity could mitigate the expected reduction of disease burden in vaccinated populations.

Litomosoides sigmodontis: a simple method to infect mice with L3 larvae obtained from the pleural space of recently infected jirds (Meriones unguiculatus)

Litomosoides sigmodontis is a filarial nematode that is used as a mouse model for human filarial infections. The life cycle of L. sigmodontis comprises rodents as definitive hosts and tropical rat mites as alternate hosts. Here, we describe a method of infecting mice with third stage larvae (L3) extracted from the pleural space of recently infected jirds (Meriones unguiculatus). This method enables infection of mice with a known number of L3 larvae without the time-consuming dissection of L3 larvae from mites and results in higher worm recovery and patency rates than conventional methods. Additionally, this method allows for geographical separation of the facility maintaining the L. sigmodontis life cycle from the institution at which mice are infected.

Effect of chemotherapeutic treatment on cytokine (IFN-gamma, IL-2, IL-4, IL-5, IL-10) gene transcription in response to specific antigens in Brugia malayi-infected Mastomys coucha

Cytokine (interferon (IFN)-gamma, interleukin (IL)-2, IL-4, IL-5, IL-10) gene transcription in response to filarial antigens was determined in peripheral blood mononuclear cells of Brugia malayi-infected Mastomys coucha in the course of untreated and chemotherapeutically abbreviated infections. Transcript levels in infected untreated animals suggest particular time courses for the various cytokines with ongoing parasite development and differing efficacies of female, male, microfilarial, and L3 antigens in inducing cytokine gene transcription. Gene transcription of both of Th1- and Th2-associated cytokines were initiated in the course of the infection in a manner that does not fit in a simple Th1-Th2 paradigm. IFN-gamma and IL-4 gene transcripts prevailed during prepatency. In case of the other cytokine genes considered in the study, transcription in general peaked around beginning of patency. During the phase of increasing microfilaremia (approximately 120-180 days p. i.) cytokine gene transcription was generally decreased. Later on, when the parasitemia had leveled off, except IFN-gamma, transcript levels often tended to increase. In chemotherapeutically treated animals, the outcome varied with the different efficacies of the drugs employed. The highly microfilaricidal cyclodepsipeptide BAY 44-4400 eliminated circulating microfilariae and partially sterilized adult worms without killing them. This kind of treatment hardly affected cytokine responses. In contrast, the therapy with Flubendazole, a selectively macrofilaricidal benzimidazole, and particularly the application of CGP 20376, a highly efficient microfilaricidal and macrofilaricidal benzthiazole, resulted in enhanced transcription of the Th1-associated IFN-gamma and IL-2 genes as well as of the Th2-associated IL-5 gene 2-3 months after treatment. IL-10 gene transcription seemed transiently increased after 1 month. There was no effect of any treatment on the IL-4 gene transcription.

Description of adult and fourth-stage larva of Litomosoides navonae n. sp. (Nematoda: Onchocercidae), a parasite of five species of sigmodontine rodents from northeastern Argentina

This study describes a new species of Litomosoides Chandler, 1931, parasitic in five different sigmodontine rodents from Misiones, Chaco and Formosa provinces of Argentina. The fourth-stage (L4) larva (male and female) is also described. L. navonae n. sp. exhibits: a bottle-shaped buccal cavity; a buccal capsule with irregularly crenate external walls; four externo-labial papillae and one ventral cephalic papilla; a well differentiated oesophagus; and sigmodontis-type spicules. The microfilaria is fusiform, with a large sheath. The L4 has a buccal capsule which is relatively longer than that of the adults, with narrower walls and a bottle-shaped lumen. It was observed in this species that the oesophagus/body-length ratio increases from larva to adult (female ratio 26.2-28.3 in larva; 88.4 in adults), and the vulva appears to move further posterior to the oesophago-intestinal junction (200-300 microm in larvae vs a mean of 600 microm in adults). L. navonae was found parasitising: Nectomys squamipes from the Reserve UNLP Valle del Arroyo Cuñá Pirú, Misiones; Oligoryzomys chacoensis, Holochilus chacarius and Akodon azarae bibianae from the marshes of Arroyo Bellaco, El Colorado, Formosa; and O. fornesi and H. chacarius from Selvas del Río de Oro (Chaco). Both N. squamipes and H. chacarius harbour other filarioids species, i.e. L. kohnae Bain, Petit & Diagne, 1989 and L. patersoni (Mazza, 1928), respectively, throughout their range, but these filarioids are readily differentiated from L. navonae. These well-differentiated filarial species found in Nectomys and Holochilus could indicate how isolated the populations of rodents are and could be interpreted either as: (a) an early point in the speciation processes that could be taking place in these hosts; or (b) extra support for the capture phenomenon theory of the evolution of Litomosoides. New regions, such as southern Brazil and northern Argentina, need to be studied in order to clarify these alternatives.

Blood-feeding in the young adult filarial worms Litomosoides sigmodontis

In this study with the filarial model Litomosoides sigmodontis, we demonstrate that the worms ingest host red blood cells at a precise moment of their life-cycle, immediately after the fourth moult. The red blood cells (RBC) were identified microscopically in live worms immobilized in PBS at 4 degrees C, and their density assessed. Two hosts were used: Mongolian gerbils, where microfilaraemia is high, and susceptible BALB/c mice with lower microfilaraemia. Gerbils were studied at 12 time-points, between day 9 post-inoculation (the worms were young 4th stage larvae) and day 330 p.i. (worms were old adults). Only the very young adult filarial worms had red blood cells in their gut. Haematophagy was observed between days 25 and 56 p.i. and peaked between day 28 and day 30 p.i. in female worms. In males, haematophagy was less frequent and intense. Similar kinetics of haematophagy were found in BALB/c mice, but frequency and intensity tended to be lower. Haematophagy seems useful to optimize adult maturation. These observations suggest that haematophagy is an important step in the life-cycle of L. sigmodontis. This hitherto undescribed phenomenon might be characteristic of other filarial species including human parasites.

Increased early local immune responses and altered worm development in high-dose infections of mice susceptible to the filaria Litomosoides sigmodontis

The relationship between the number of larvae inoculated and filarial infection outcome is an important fundamental and epidemiological issue. Our study was carried out with BALB/c mice infected with the filaria Litomosoides sigmodontis. For the first time, an immunological analysis of infection with various doses was studied in parallel with parasitological data. Mice were inoculated with 200, 60 or 25 infective larvae (third stage larvae, L3), and monitored over 80 days. At 60 h post-inoculation the immune response was stronger in the 200 L3 group than the 25 L3 group. Cells from lymph nodes draining the site of inoculation proliferated intensely and produced large amounts of IL-5 and IL-4. In the pleural cavity, leukocyte populations accumulated earlier and in larger quantities. IgG1, IL-4 and IL-10 serum concentrations were transiently higher. During the first 10 days the worm recovery rates were identical in all groups, but decreased thereafter in the 200 L3 group. In this group, the development of the worms was altered, with reduced lengths, diminished intra-uterine production of microfilariae and abnormalities of male copulatory organs. Whereas mice inoculated with 25 L3 became microfilaraemic, only one third reached patency in the 200 L3 group. However, detrimental effects of high numbers of worms are not seen in studies using different inoculation protocols. This suggests that the very early events determine subsequent immune response and infection outcome rather than competitive interactions between the worms.

Resistance and susceptibility to filarial infection with Litomosoides sigmodontis are associated with early differences in parasite development and in localized immune reactions

In order to understand natural resistance to filariasis, we compared Litomosoides sigmodontis primary infection of C57BL/6 mice, which eliminate the worms before patency, and BALB/c mice, in which worms complete their development and produce microfilariae. Our analysis over the first month of infection monitoredmigration of the infective larvae from the lymph nodes to the pleural cavity, where the worms settle. Although immune responses from the mouse strains differed from the outset, the duration of lymphatic migration (4 days) and filarial recovery rates were similar, thus confirming that the proportion of larvae that develop in the host species upon infection is not influenced by host genetic variability. The majority of worms reached the adult stage in both mouse strains; however, worm growth and molting were retarded in resistant C57BL/6 mice. Surprisingly, the only immune responses detected at 60 h postinfection occurred in the susceptible mice and only upon stimulation of cells from lymph nodes draining the inoculation site with infective larva extract: massive production of interleukin-6 (IL-6) and IL-5 (the latter cytokine was previously suspected to have an effect on L. sigmodontis growth). However, between days 10 and 30 postinfection, extraordinarily high levels of type 1 and type 2 cytokines and expansion of pleural leukocyte infiltration were seen in the resistant C57BL/6 mice, explaining the destruction of worms later. Our results suggest that events early in the infection determine susceptibility or resistance to subsequent microfilarial production and a parasite strategy to use specific immune responses to its own benefit.

Synergism of gamma interferon and interleukin-5 in the control of murine filariasis

There has been a prevailing perception that Th1 and Th2 immune responses induce antagonistic immune effector mechanisms during an infection. We investigated the role of the Th1 cytokine gamma interferon (IFN-gamma) and the Th2 cytokine interleukin-5 (IL-5) in murine filariasis infections with the rodent filarial nematode Litomosoides sigmodontis with regard to immune responses to the parasite. Earlier data showed an important role for IL-5 and IFN-gamma in effective immune responses to filarial infection. Therefore, in this study it was asked whether IL-5 and IFN-gamma act synergistically or antagonistically. Indeed, IL-5 as well as IFN-gamma knockout (KO) mice show a higher worm load than the wild-type controls. IFN-gamma/IL-5 double-KO mice had a significantly higher worm load than any of the single-KO mice, suggesting a synergism between IFN-gamma and IL-5 in controlling worm infection. Neutrophils are known to play an important role for the containment and encapsulation process of the worms. In infected IFN-gamma KO, IL-5 KO, and IFN-gamma/IL-5 double-KO mice, neutrophils were significantly reduced in chemotactic activity levels compared to controls. In addition, the level of phagocytosis activity of neutrophils from IFN-gamma/IL-5 double-KO mice was further decreased in comparison to that of the single-KO mice. Levels of tumor necrosis factor alpha, which is an important factor for neutrophil activation, were found to be reduced in macrophages from KO mice. In conclusion, these results argue for immune effector mechanisms in murine filarial infection that are dependent on both IFN-gamma and IL-5. Synergistic effects of the two cytokines may be mediated, at least in part, by neutrophils for the control of adult worms.

IL-4 is required to prevent filarial nematode development in resistant but not susceptible strains of mice

The murine Litomosoides sigmodontis model of filarial infection provides the opportunity to elucidate the immunological mechanisms that determine whether these nematode parasites can establish a successful infection or are rejected by the mammalian host. BALB/c mice are fully susceptible to L. sigmodontis infection and can develop patent infection, with the microfilarial stage circulating in the bloodstream. In contrast, mice on the C57BL background are largely resistant to the infection and never produce a patent infection. In this study, we used IL-4 deficient mice on the C57BL/6 background to address the role of IL-4 in the development of L. sigmodontis parasites in a resistant host. Two months after infection, adult worm recovery and the percentage of microfilaraemic mice in infected IL-4 deficient mice were comparable with those of the susceptible BALB/c mice while, as expected, healthy adults were not recovered from wild type C57BL/6 mice. The cytokine and antibody responses reveal that despite similar parasitology the two susceptible strains (BALB/c and IL-4 deficient C57BL/6) have markedly different immune responses: wild type BALB/c mice exhibit a strong Th2 immune response and the IL-4 deficient C57BL/6 mice exhibit a Th1 response. We also excluded a role for antibodies in resistance through infection of B-cell deficient C57BL/6 mice. Our data suggest that the mechanisms that determine parasite clearance in a resistant/non-permissive host are Th2 dependent but that in a susceptible/permissive host, the parasite can develop in the face of a Th2 dominated response.

Litomosoides sigmodontis cystatin acts as an immunomodulator during experimental filariasis

During chronic filariasis, parasite-specific cellular responsiveness is profoundly down-regulated. Cystatins, a group of cysteine protease inhibitors, have been implicated in this suppressive activity. In an attempt to investigate the effects of cystatins in vivo, we isolated and expressed a 14 kDa protein of the rodent filaria Litomosoides sigmodontis with substantial homologies to cystatins from human pathogenic filariae. Cystatin was detected in antigen preparations of several developmental stages of L. sigmodontis, as well as in the supernatants of in vitro cultured adult worms. On closer examination, L. sigmodontis cystatin (Ls-Cystatin) migrated as two separate bands at 14 and 15 kDa. When cystatin was introduced into the peritoneal cavity of C57BL/6 mice via micro-osmotic pumps, the production of nitric oxide was profoundly reduced upon microfilarial challenge and, at the same time, synthesis of TNF-alpha mRNA became up-regulated. Furthermore, antigen-specific proliferative response of spleen cells to circulating L. sigmodontis microfilariae was significantly diminished in the presence of cystatin, whereas the antibody production was not suppressed. In vaccination trials, using the L. sigmodontis/BALB/c mouse model of filariasis, L. sigmodontis cystatin did not generate protective effects in terms of adult worm recovery, however, lower numbers of patent infections, i.e. less infections with microfilaraemia were observed in vaccinated animals. These results suggested that cystatin acts as an immunomodulatory molecule during the course of a filarial infection, and its neutralisation might contribute to generate protective immune responses.

B-cell deficiency suppresses vaccine-induced protection against murine filariasis but does not increase the recovery rate for primary infection

To establish the role of B cells and antibodies in destroying filariae, mice lacking mature B cells and therefore unable to produce antibodies were used. Litomosoides sigmodontis offers a good opportunity for this study because it is the only filarial species that completes its life cycle in mice. Its development was compared in B-cell-deficient mice (BALB/c muMT mice) and wild-type BALB/c mice in two different in vivo situations, vaccination with irradiated larvae and primary infection. In all cases, mice were challenged with subcutaneous inoculation of 40 infective larvae. Vaccine-induced protection was suppressed in B-cell-deficient mice. In these mice, eosinophils infiltrated the subcutaneous tissue normally during immunization; however, their morphological state did not change following challenge inoculation, whereas in wild-type mice the percentage of degranulated eosinophils was markedly increased. From this, it may be deduced that the eosinophil-antibody-B-cell complex is the effector mechanism of protection in vaccinated mice and that its action is fast and takes place in the subcutaneous tissue. In primary infection, the filarial survival and growth was not modified by the absence of B cells. However, no female worm had uterine microfilariae, nor did any mice develop a patent infection. In these mice, concentrations of type 1 (gamma interferon) and type 2 (interleukin-4 [IL-4], IL-5 and IL-10) cytokines in serum were lower and pleural neutrophils were more numerous. The effects of the muMT mutation therefore differ from those in B1-cell-deficient mice described on the same BALB/c background, which reveal a higher filarial recovery rate and microfilaremia. This outlines B2-cell-dependent mechanisms as favorable to the late maturation of L. sigmodontis.

Determinants for resistance and susceptibility to microfilaraemia in Litomosoides sigmodontis filariasis

Filarial infections of humans are chronic diseases. Despite an ongoing immune response, adult filariae continuously produce their offspring, the microfilariae (Mf), which are able to persist in sufficient numbers to ensure transmission. In this study, host- and parasite-derived factors, which contribute to persistence of Mf, were investigated using the filariasis model of Litomosoides sigmodontis in mice. Different strains of mice were found to differ widely in their capability to eliminate circulating Mf. Studies of congenic mouse strains showed that early and rapid clearance of Mf was mediated by activation pathways relevant to innate immunity, whereas late or delayed clearance of Mf was pre-determined by MHC-related factors. Genetic knock-out of genes for the MHC class-II molecules totally abrogated resistance. Most interestingly, the presence of only I adult female, but not male worms, renders all mice susceptible, irrespective of the genetic background, enabling Mf to circulate for extended periods of time. Such prolonged microfilaraemia was also observed in L. sigmodontis-infected animals challenged with heterologous Mf of Acanthocheilonema viteae. The use of cytokine gene knock-out mice showed that persistence of L. sigmodontis Mf was facilitated by IL-10, but not by IL-4 or IFN-gamma. In conclusion, irrespective of a resistant or susceptible host genetic background, survival of Mf of L. sigmodontis in mice is decisively regulated by the presence of adult female L. sigmodontis which will skew and exploit immune responses to facilitate the survival and persistence of their offspring in the infected host.

Interleukin-4 is essential for the control of microfilariae in murine infection with the filaria Litomosoides sigmodontis

Litomosoides sigmodontis is the only filaria which develops from infective larvae into microfilaria-producing adults in immunocompetent laboratory mice. In this study we report that interleukin-4 knockout (IL-4 KO) mice have an up to 100-fold-higher and a significantly prolonged microfilaremia compared to wild-type BALB/c mice, as well as 20 times more microfilariae in the thoracic cavity, the site of infection. While worm development and adult worm persistence were equivalent in IL-4 KO and wild-type mice, the fertility and length of adult female worms in IL-4 KO mice was clearly enhanced. The high susceptibility to microfilariae in IL-4 KO mice required the presence of adult worms in a full infection cycle since microfilariae loads did not differ much between IL-4 KO and wild-type mice when purified microfilariae were injected into mice. In addition, we found that eosinophilia was diminished and immunoglobulin E (IgE) was absent in IL-4 KO mice. IgE, however, does not seem to be the essential factor for microfilarial containment since microfilaremia was not elevated in B-cell KO mice. In conclusion, IL-4 is shown for the first time to be essential for the control of microfilarial loads but not of adult worm loads in a fully permissive murine filarial infection. IL-4 dependent effector pathways seem to operate on adult worms rather than directly on microfilariae.

Lack of interferon-γ confers impaired neutrophil granulocyte function and imparts prolonged survival of adult filarial worms in murine filariasis

We investigated the role of IFN-gamma in host defense during murine filariasis. Using the fully permissive infection of BALB/c mice with the rodent filaria Litomosoides sigmodontis, we show that interferon (IFN)-gamma is essential for encapsulation of adult filarial worms in inflammatory nodules and for normal worm clearance. IFN-gamma knockout (KO) mice had only one third of the nodules of wild-type mice but displayed a more than twofold increase in worm burden and increased microfilaremia. Neutrophil granulocytes, but not macrophages or eosinophils, appear to directly control worm load and nodule formation. Neutrophils, which we showed earlier to be essential for the encapsulation process in the thoracic cavity, where the worms reside, were diminished at this location in IFN-gamma KO compared to wild-type mice; they also displayed strongly reduced chemotactic and phagocytic activity compared to neutrophils of controls. This argues for a distinct defect in neutrophil activation accounting for the low formation of inflammatory nodules. Tumor necrosis factor-alpha, a major neutrophil-activating cytokine expressed by macrophages in the thoracic cavity around the worms, was highly induced in wild-type but absent in KO mice. Diminished activation of neutrophils seems to be a general hallmark of IFN-gamma KO mice, since neutrophils from uninfected KO mice also showed a reduction in chemotactic and phagocytic activity when induced by casein. In conclusion, these data are the first to define an IFN-gamma-dependent immune effector mechanism in murine filarial infection, i.e. neutrophil-mediated control of the adult worm load.

Litomosoides sigmodontis in mice: reappraisal of an old model for filarial research

Onchocerciasis and lymphatic filariasis (LF) are major causes of severe morbidity and considerable socio-economic problems throughout the tropics. Vector control and mass chemotherapy have helped to control these infections in some regions, but the temporary success of such measures argues strongly for the development of vaccines. Success in such a venture will require detailed knowledge of protective immune responses in conjunction with the identification of target antigens. By comparison with other important parasitic infections, such as schistosomiasis and leishmaniasis, work on the development of vaccines for onchocerciasis and LF has been constrained because of the difficulties of producing cyclical and patent filarial infection in laboratory mice. Wolfgang Hoffmann and colleagues here outline the opportunities presented by the rodent filaria Litomosoides sigmodontis for filarial research.

A new mechanism for IL-5-dependent helminth control: neutrophil accumulation and neutrophil-mediated worm encapsulation in murine filariasis are abolished in the absence of IL-5

IL-5 production and eosinophilia are features of helminth infections, but results concerning the role of IL-5 and eosinophils (EP) in worm control are contradictory. We describe here a novel, IL-5-dependent mechanism of helminth control in vivo, using a fully permissive murine filariasis model, i.e. infection of BALB/c mice with Litomosoides sigmodontis. Worm control was exerted by the formation of inflammatory nodules around adult filariae which initially remained alive but were eventually killed within several weeks. The cell population essential for inflammatory nodule formation was found to be neutrophils (NP) but not EP. Neutralization of IL-5 led to a failure of both EP and NP accumulation at the site of infection (i.e. the thoracic cavity), resulting in cessation of inflammatory nodule formation around worms and in their survival. The role of NP in this process was confirmed by treatment of mice with anti-granulocyte colony stimulating factor (G-CSF) which also resulted in a lack of inflammatory nodule formation and worm killing albeit in the presence of EP. Since IL-5, due to the absence of IL-5 receptors on NP, does not act on these cells directly, it was investigated if anti-IL-5 altered the production of NP-chemotactic cytokines. In anti-IL-5-treated mice, cytokines known to promote NP accumulation like tumor necrosis factor-alpha, G-CSF and KC (IL-8) were found to be strongly reduced, while NP-deactivating cytokines like IL-10 were increased. In conclusion, IL-5 constitutes a cytokine essential for NP-mediated worm control in filarial infection.

Drastic reduction of a filarial infection in eosinophilic interleukin-5 transgenic mice

In order to establish the role of eosinophils in destroying parasites, transgenic mice have been used in experimental helminthiases but not in filariasis. Litomosoides sigmodontis offers a good opportunity for this study because it is the only filarial species that completes its life cycle in mice. Its development was compared in transgenic CBA/Ca mice overexpressing interleukin-5 (IL-5) and in wild-type mice following subcutaneous inoculation of 40 infective larvae. An acceleration of larval growth was observed in the IL-5 transgenic mice. However, the recovery rate of adult worms was considerably reduced in these mice, as evidenced 2 months postinoculation (p.i.). The reduction occurs between days 10 and 30 p.i. in the coelomic cavities. As early as day 10, spherical aggregates of eosinophils and macrophages are seen attached on live developing larvae (always similarly localized on the worm) in both wild-type and transgenic mice. However, on day 60 p.i., granulomas were found in the transgenic mice only, probably because of the higher density of eosinophils. Furthermore, on day 30 p.i., young filariae are seen trapped in granulomas, some of them surrounded by Splendore-Hoeppli deposits, which illustrates the release of the major basic protein by eosinophils. The high protection rate obtained (65%) is similar to that observed previously in BALB/c mice following vaccination with irradiated larvae. Both protocols have a common factor, the high production of IL-5 and eosinophilia. However, protection occurs later in primary infected transgenic mice because specific antibodies are not yet present at the time of challenge.

Interleukin-5 is essential for vaccine-mediated immunity but not innate resistance to a filarial parasite

The study of protective immune mechanisms effective against filarial nematodes has been hampered by the inability of these important human pathogens to infect laboratory mice. Recently, Litomosoides sigmodontis, a natural parasite of rats, has been developed as a valuable model for the study of filarial infection. BALB/c mice are fully susceptible to infection with L. sigmodontis third-stage larvae and develop patent infection. In contrast, mice on the C57BL background are resistant, and parasites undergo only a single molt and do not mature to adulthood. We used interleukin-5 (IL-5)-deficient mice on the C57BL/6 background to address the role of IL-5 and eosinophils in the innate resistance of C57BL/6 mice. We found no differences in parasite survival between IL-5-deficient and C57BL/6 mice. However, when these mice were used for the analysis of vaccine-mediated immunity, a critical role for IL-5 was elucidated. Mice genetically deficient in IL-5 were unable to generate a protective immune response when vaccinated with irradiated larvae, whereas C57BL/6 mice were fully protected from challenge infection. These studies help to clarify the highly controversial role of eosinophils in filarial infection.

Cellular responses to Loa loa experimental infection in mandrills (Mandrillus sphinx) vaccinated with irradiated infective larvae

In order to shed light on the mechanisms of antifilarial protective immunity, we investigated the course of experimental loaiosis after vaccination in a nonhuman primate host, Mandrillus sphinx. Six vaccinated (V) mandrills received 50 irradiated L3 while six nonvaccinated (NV) received saline solution on days -60, -30 and -15. All animals were challenged with 100 intact L3 (day 0). Parasitological and immunological status were followed for 9 months. Vaccination delayed the appearance and mean peak of microfilaraemia. Five mandrills (Mf-) were never microfilaraemic (one V mandrill) or microfilaraemic on only one occasion (2 V and 2 NV), the other seven having stable microfilaraemia (Mf+). The cytokine response of peripheral blood mononuclear cells to L3 (L3 Ag) was Th2 dominated, while microfilariae (Mf Ag) elicited a Th0-like response. During vaccination, Th2 cytokine production significantly increased in V mandrills against L3 Ag, as well as Mf Ag, whereas Th1 cytokines decreased. On day 60 postinoculation, cellular proliferation was higher in V mandrills in response to L3 and Mf Ags and PHA-L mitogen. At the end of prepatency (on day 130), mandrills with delayed appearance of microfilaraemia exhibited a high, transient IL-2 and IL-4 secretion in response to L3 Ag. Finally, high anti-Mf Th2 cytokine levels characterized Mf-mandrills not only during prepatency, but also (for IL-5) before immunization. However, the presence of a balanced Th1 anti-L3 response during prepatency in the amicrofilaraemic mandrill suggests its importance in protective immunity. Taken together, our data suggest that Th2 cells and also Th1 components of the antifilarial response, especially to larval antigen, may contribute to parasite elimination.

Litomosoides sigmodontis: dynamics of the survival of microfilariae in resistant and susceptible strains of mice

Litomosoides sigmodontis in the BALB/c mouse is the only model of filariasis which allows the observation of the complete development in an immunocompetent mouse. In this study, we injected microfilariae (mf) intravenously, as well as into the pleural cavity, the site of natural release of mf from adult female worms, and followed the kinetics of elimination within the host. In susceptible BALB/c mice, mf circulated at high levels in the blood. In contrast, in C57BL/6 mice, which are refractory to full development, mf were eliminated rapidly from the peripheral blood. However, 6 days after intrapleural injection, viable larvae could be found in the pleural cavity and lung capillaries of both susceptible and resistant strains. The numbers of mf in the pleural cavity and lung capillaries in individual mice were significantly correlated, but not dependent on strain or peripheral microfilaraemia. Thus, although C57BL/6 mice showed enhanced production of nitric oxide by pleural exudate cells and a faster change in the numbers of circulating leukocytes after injection, rapid killing of mf by cell or nitric oxide-mediated mechanisms were not the reason for the different outcome. Furthermore, 3 h after iv injection, only a small percentage of mf could be recovered from the peripheral circulation, indicating the presence of a reservoir for mf containment. In conclusion, injected mf showed disparate dynamics of persistence within susceptible and resistant hosts, which is similar to the disparate outcome of natural infections with L. sigmodontis. This difference became obvious within 1 day after injection. The lung capillary system plays obviously a crucial part in regulation of microfilaremia. Our model also provides a possible means to explain frequent cases of occult infections in human filariasis.

Infection of BALB/c mice with the filarial nematode Litomosoides sigmodontis: role of CD4+ T cells in controlling larval development

Litomosoides sigmodontis is the only filaria which develops from infective larvae into adults in immunocompetent laboratory mice. Depletion of CD4+ T cells from infected BALB/c mice resulted in worm and microfilarial burdens significantly higher than those of infected controls. Th2 cytokines, eosinophilia, and immunoglobulin E, which were strongly induced in infected controls, were diminished in CD4-depleted mice.