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

Development of adult Dirofilaria immitis worms in the Rag2/Il-2rγ-/- mouse model

Dirofilaria immitis is the causative agent for one of the major parasitic infections in dogs. It is currently not possible to reliably diagnose the infection before the development of fertile adult female worms and the presence of microfilariae which takes six to 7 months. However, at this point adult worms already reside in the pulmonary arteries and can cause significant damage. Novel in vivo models may facilitate the development of new diagnostic tools and improve treatment options for both the early and late stages of D. immitis infections. In this paper, we aimed to increase the capabilities of recently published mouse models in which severely immune-deficient mice were shown to be susceptible to D. immitis. Our data shows that D. immitis may grow into fully developed mature male and female worms in C57BL/6 Rag2/Il-2rγ-/- mice with comparable growth rates to the natural canine host. The adult worms of D. immitis were shown to migrate into body cavities as well as the heart in this model. However, the presence of adult worms inside the heart of infected mice led to the development of caval syndrome in 36% of infected mice after five to 6 months. Overall, the current study complements recently published efforts to establish a D. immitis mouse model by extending the development of D. immitis into mature adult stages and will facilitate further preclinical research.

Eosinophils Are an Endogenous Source of Interleukin-4 during Filarial Infections and Contribute to the Development of an Optimal T Helper 2 Response

INTRODUCTION

Interleukin-4 (IL-4) is a central regulator of type 2 immunity, crucial for the defense against multicellular parasites like helminths. This study focuses on its roles and cellular sources during Litomosoides sigmodontis infection, a model for human filarial infections.

METHODS

Utilizing an IL-4 secretion assay, investigation into the sources of IL-4 during the progression of L. sigmodontis infection was conducted. The impact of eosinophils on the Th2 response was investigated through experiments involving dblGATA mice, which lack eosinophils and, consequently, eosinophil-derived IL-4.

RESULTS

The absence of eosinophils notably influenced Th2 polarization, leading to impaired production of type 2 cytokines. Interestingly, despite this eosinophil deficiency, macrophage polarization, proliferation, and antibody production remained unaffected.

CONCLUSION

Our research uncovers eosinophils as a major source of IL-4, especially during the early phase of filarial infection. Consequently, these findings shed new light on IL-4 dynamics and eosinophil effector functions in filarial infections.

Pericardial and mediastinal fat-associated lymphoid clusters are rapidly activated in an alkane-induced model of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease predominated by auto-antibodies that recognise cellular components. Pleural involvement is the most common SLE-related lung disease. Natural antibodies are rapidly secreted by innate-like B cells following perturbation of homeostasis and are important in the early stages of immune activation. The serous cavities are home to large numbers of innate-like B cells present both within serous fluid and resident within fat-associated lymphoid clusters (FALCs). FALCs are important hubs for B-cell activation and local antibody secretion within the body cavities. Patients with SLE can develop anti-phospholipid antibodies and in rare situations develop alveolar haemorrhage. Utilising delivery of the hydrocarbon oil pristane in C57BL/6 mice as a model of SLE we identify a rapid expansion of pleural cavity B cells as early as day 3 after intra-peritoneal pristane delivery. Following pristane delivery, pericardial B1 B cells are proliferative, express the plasma-cell surface marker CD138, and secrete both innate and class-switched antibodies highlighting that this cavity niche may play an unrecognised role in the initiation of lupus pleuritis.

The efficacy of the benzimidazoles oxfendazole and flubendazole against Litomosoides sigmodontis is dependent on the adaptive and innate immune system

Filarial nematodes can cause debilitating diseases such as lymphatic filariasis and onchocerciasis. Oxfendazole (OXF) is one promising macrofilaricidal candidate with improved oral availability compared to flubendazole (FBZ), and OXF is currently under preparation for phase 2 clinical trials in filariasis patients. This study aimed to investigate the immune system's role during treatment with OXF and FBZ and explore the potential to boost the treatment efficacy via stimulation of the immune system. Wild type (WT) BALB/c, eosinophil-deficient ΔdblGata1, IL-4r/IL-5^-/-, antibody-deficient μMT and B-, T-, NK-cell and ILC-deficient Rag2/IL-2rγ^-/- mice were infected with the rodent filaria Litomosoides sigmodontis and treated with an optimal and suboptimal regimen of OXF and FBZ for up to 5 days. In the second part, WT mice were treated for 2-3 days with a combination of OXF and IL-4, IL-5, or IL-33. Treatment of WT mice reduced the adult worm burden by up to 94% (OXF) and 100% (FBZ) compared to vehicle controls. In contrast, treatment efficacy was lower in all immunodeficient strains with a reduction of up to 90% (OXF) and 75% (FBZ) for ΔdblGata1, 50 and 92% for IL-4r/IL-5^-/-, 64 and 78% for μMT or 0% for Rag2/IL-2rγ^-/- mice. The effect of OXF on microfilariae and embryogenesis displayed a similar pattern, while FBZ's ability to prevent microfilaremia was independent of the host's immune status. Furthermore, flow cytometric analysis revealed strain-and treatment-specific immunological changes. The efficacy of a shortened 3-day treatment of OXF (-33% adult worms vs. vehicle) could be boosted to a 91% worm burden reduction via combination with IL-5, but not IL-4 or IL-33. Our results suggest that various components of the immune system support the filaricidal effect of benzimidazoles in vivo and present an opportunity to boost treatment efficacy.

The design and development of a study protocol to investigate Onchocerca volvulus, Loa loa and Mansonella perstans-mediated modulation of the metabolic and immunological profile in lean and obese individuals in Cameroon

BACKGROUND

Life-style metabolic diseases are steadily rising, not only in developed countries, but also in low- and middle-income countries, presenting a global health problem. Metabolic disorders like type 2 diabetes and cardiovascular diseases are among the ten leading causes of death defined by the WHO in 2019. Results from animal and observational human studies suggest a connection between the decline in human helminth infections and rise of life-style-associated metabolic diseases in developing regions. This trial was designed to investigate filarial infections and their impact on metabolic diseases in Cameroon. We hypothesize that the induction of regulatory immune responses during filarial infection reduces obesity-induced low-grade inflammatory immune responses and thereby improves metabolic parameters, whereas anthelmintic treatment abolishes this protective effect.

METHODS/DESIGN

Participants infected with Mansonella perstans, Onchocerca volvulus and/or Loa loa being lean (BMI <25), overweight (BMI >25 and <30) or clinically obese (BMI ≥30) from Littoral regions of Cameroon will be evaluated for their parasitological, immunological, metabolic and biochemical profile before and after treatment of their parasitic infections. Anthropomorphic measurements and a detailed questionnaire will complement our analysis. The investigation will assess blood immune cell populations, serum adipokines and cytokines that could be influenced by the parasite infection and/or metabolic diseases. Further, parameters like blood glucose, homeostatic model assessment of insulin resistance (HOMA-IR), circulating lipids and circulating makers of liver function will be monitored. Parameters will be assessed before treatment, 12 and 18 months after treatment.

CONCLUSION

The focus of this study is to obtain a comprehensive metabolic profile of the participants in rural areas of Cameroon and to investigate the relationship between filarial immunomodulation and metabolic diseases. This study will elucidate the effect of anti-filarial treatment on the metabolic and immunological parameters that partake in the development of insulin resistance, narrowing in on a potential protective effect of filarial infections on metabolic diseases.

TRIAL REGISTRATION

doi.org/10.1186/ISRCTN43845142, ISRCTN43845142 February 2020 Trial title Effects of filarial parasite infection on type 2 diabetes Issue date: 27.10.22, V.1.

Potential of Nucleic Acid Receptor Ligands to Improve Vaccination Efficacy against the Filarial Nematode Litomosoides sigmodontis

More than two-hundred-million people are infected with filariae worldwide. However, there is no vaccine available that confers long-lasting protection against filarial infections. Previous studies indicated that vaccination with irradiated infective L3 larvae reduces the worm load. This present study investigated whether the additional activation of cytosolic nucleic acid receptors as an adjuvant improves the efficacy of vaccination with irradiated L3 larvae of the rodent filaria Litomosoides sigmodontis with the aim of identifying novel vaccination strategies for filarial infections. Subcutaneous injection of irradiated L3 larvae in combination with poly(I:C) or 3pRNA resulted in neutrophil recruitment to the skin, accompanied by higher IP-10/CXCL10 and IFN-β RNA levels. To investigate the impact on parasite clearance, BALB/c mice received three subcutaneous injections in 2-week intervals with irradiated L3 larvae in combination with poly(I:C) or 3pRNA prior to the challenge infection. Vaccination with irradiated L3 larvae in combination with poly(I:C) or 3pRNA led to a markedly greater reduction in adult-worm counts by 73% and 57%, respectively, compared to the immunization with irradiated L3 larvae alone (45%). In conclusion, activation of nucleic acid-sensing immune receptors boosts the protective immune response against L. sigmodontis and nucleic acid-receptor agonists as vaccine adjuvants represent a promising novel strategy to improve the efficacy of vaccines against filariae and potentially other helminths.

Macrophages show up in style when Th2 lymphocytes organize their homecoming

Monocytes can differentiate into tissue-resident pleural macrophages, but the mechanisms underlying this process are not yet fully understood. In this issue of Immunity, Finlay et al.1 show that Th2 cytokines promote this differentiation in resistant mice infected with Litomosoides sigmodontis.

T helper 2 cells control monocyte to tissue-resident macrophage differentiation during nematode infection of the pleural cavity

The recent revolution in tissue-resident macrophage biology has resulted largely from murine studies performed in C57BL/6 mice. Here, using both C57BL/6 and BALB/c mice, we analyze immune cells in the pleural cavity. Unlike C57BL/6 mice, naive tissue-resident large-cavity macrophages (LCMs) of BALB/c mice failed to fully implement the tissue-residency program. Following infection with a pleural-dwelling nematode, these pre-existing differences were accentuated with LCM expansion occurring in C57BL/6, but not in BALB/c mice. While infection drove monocyte recruitment in both strains, only in C57BL/6 mice were monocytes able to efficiently integrate into the resident pool. Monocyte-to-macrophage conversion required both T cells and interleukin-4 receptor alpha (IL-4Rα) signaling. The transition to tissue residency altered macrophage function, and GATA6+ tissue-resident macrophages were required for host resistance to nematode infection. Therefore, during tissue nematode infection, T helper 2 (Th2) cells control the differentiation pathway of resident macrophages, which determines infection outcome.

Parasitic worms affect virus coinfection: a mechanistic overview

Helminths are parasitic worms that coevolve with their host, usually resulting in long-term persistence through modulating host immunity. The multifarious mechanisms altering the immune system induced by helminths have significant implications on the control of coinfecting pathogens such as viruses. Here, we explore the recent literature to highlight the main immune alterations and mechanisms that affect the control of viral coinfection. Insights from these mechanisms are valuable in the understanding of clinical observations in helminth-prevalent areas and in the design of new therapeutic and vaccination strategies to control viral diseases.

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.

Eosinophils in filarial infections: Inducers of protection or pathology?

Filariae are parasitic roundworms, which can cause debilitating diseases such as lymphatic filariasis and onchocerciasis. Lymphatic filariasis, also known as elephantiasis, and onchocerciasis, commonly referred to as river blindness, can lead to stigmatizing pathologies and present a socio-economic burden for affected people and their endemic countries. Filariae typically induce a type 2 immune response, which is characterized by cytokines, i.e., IL-4, IL-5 and IL-13 as well as type 2 immune cells including alternatively activated macrophages, innate lymphoid cells and Th2 cells. However, the hallmark characteristic of filarial infections is a profound eosinophilia. Eosinophils are innate immune cells and pivotal in controlling helminth infections in general and filarial infections in particular. By modulating the function of other leukocytes, eosinophils support and drive type 2 immune responses. Moreover, as primary effector cells, eosinophils can directly attack filariae through the release of granules containing toxic cationic proteins with or without extracellular DNA traps. At the same time, eosinophils can be a driving force for filarial pathology as observed during tropical pulmonary eosinophilia in lymphatic filariasis, in dermatitis in onchocerciasis patients as well as adverse events after treatment of onchocerciasis patients with diethylcarbamazine. This review summarizes the latest findings of the importance of eosinophil effector functions including the role of eosinophil-derived proteins in controlling filarial infections and their impact on filarial pathology analyzing both human and experimental animal studies.

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.

ILC2s Control Microfilaremia During Litomosoides sigmodontis Infection in Rag2-/- Mice

Group 2 innate lymphoid cells (ILC2s) are inducers of type 2 immune responses, but their role during filarial infection remains unclear. In the present study, we used the Litomosoides sigmodontis rodent model of filariasis to analyze ILC2s during infection in susceptible BALB/c mice that develop a chronic infection with microfilaremia and semi-susceptible C57BL/6 mice that eliminate the filariae shortly after the molt into adult worms and thus do not develop microfilaremia. ILC2s (CD45⁺ Lineage^- TCRβ^- CD90.2⁺ Sca-1⁺ IL-33R⁺ GATA-3⁺) were analyzed in the pleural cavity, the site of L. sigmodontis infection, after the infective L3 larvae reached the pleural cavity (9 days post infection, dpi), after the molt into adult worms (30dpi) and during the peak of microfilaremia (70dpi). C57BL/6 mice had significantly increased ILC2 numbers compared to BALB/c mice at 30dpi, accompanied by substantially higher IL-5 and IL-13 levels, indicating a stronger type 2 immune response in C57BL/6 mice upon L. sigmodontis infection. At this time point the ILC2 numbers positively correlated with the worm burden in both mouse strains. ILC2s and GATA-3⁺ CD4⁺ T cells were the dominant source of IL-5 in L. sigmodontis-infected C57BL/6 mice with ILC2s showing a significantly higher IL-5 expression than CD4⁺ T cells. To investigate the importance of ILC2s during L. sigmodontis infection, ILC2s were depleted with anti-CD90.2 antibodies in T and B cell-deficient Rag2^-/- C57BL/6 mice on 26-28dpi and the outcome of infection was compared to isotype controls. Rag2^-/- mice were per se susceptible to L. sigmodontis infection with significantly higher worm burden than C57BL/6 mice and developed microfilaremia. Depletion of ILC2s did not result in an increased worm burden in Rag2^-/- mice, but led to significantly higher microfilariae numbers compared to isotype controls. In conclusion, our data demonstrate that ILC2s are essentially involved in the control of microfilaremia in Rag2^-/- C57BL/6 mice.

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.

Concurrent Infection With the Filarial Helminth Litomosoides sigmodontis Attenuates or Worsens Influenza A Virus Pathogenesis in a Stage-Dependent Manner

Filarial helminths infect approximately 120 million people worldwide initiating a type 2 immune response in the host. Influenza A viruses stimulate a virulent type 1 pro-inflammatory immune response that in some individuals can cause uncontrolled immunopathology and fatality. Although coinfection with filariasis and influenza is a common occurrence, the impact of filarial infection on respiratory viral infection is unknown. The aim of this study was to determine the impact of pre-existing filarial infection on concurrent infection with influenza A virus. A murine model of co-infection was established using the filarial helminth Litomosoides sigmodontis and the H1N1 (A/WSN/33) influenza A virus (IAV). Co-infection was performed at 3 different stages of L. sigmodontis infection (larval, juvenile adult, and patency), and the impact of co-infection was determined by IAV induced weight loss and clinical signs, quantification of viral titres, and helminth counts. Significant alterations of IAV pathogenesis, dependent upon stage of infection, was observed on co-infection with L. sigmodontis. Larval stage L. sigmodontis infection alleviated clinical signs of IAV co-infection, whilst more established juvenile adult infection also significantly delayed weight loss. Viral titres remained unaltered at either infection stage. In contrast, patent L. sigmdodontis infection led to a reversal of age-related resistance to IAV infection, significantly increasing weight loss and clinical signs of infection as well as increasing IAV titre. These data demonstrate that the progression of influenza infection can be ameliorated or worsened by pre-existing filarial infection, with the outcome dependent upon the stage of filarial infection.

Adoptive Transfer of Immune Cells Into RAG2IL-2Rγ-Deficient Mice During Litomosoides sigmodontis Infection: A Novel Approach to Investigate Filarial-Specific Immune Responses

Despite long-term mass drug administration programmes, approximately 220 million people are still infected with filariae in endemic regions. Several research studies have characterized host immune responses but a major obstacle for research on human filariae has been the inability to obtain adult worms which in turn has hindered analysis on infection kinetics and immune signalling. Although the Litomosoides sigmodontis filarial mouse model is well-established, the complex immunological mechanisms associated with filarial control and disease progression remain unclear and translation to human infections is difficult, especially since human filarial infections in rodents are limited. To overcome these obstacles, we performed adoptive immune cell transfer experiments into RAG2IL-2Rγ-deficient C57BL/6 mice. These mice lack T, B and natural killer cells and are susceptible to infection with the human filaria Loa loa. In this study, we revealed a long-term release of L. sigmodontis offspring (microfilariae) in RAG2IL-2Rγ-deficient C57BL/6 mice, which contrasts to C57BL/6 mice which normally eliminate the parasites before patency. We further showed that CD4⁺ T cells isolated from acute L. sigmodontis-infected C57BL/6 donor mice or mice that already cleared the infection were able to eliminate the parasite and prevent inflammation at the site of infection. In addition, the clearance of the parasites was associated with Th17 polarization of the CD4⁺ T cells. Consequently, adoptive transfer of immune cell subsets into RAG2IL-2Rγ-deficient C57BL/6 mice will provide an optimal platform to decipher characteristics of distinct immune cells that are crucial for the immunity against rodent and human filarial infections and moreover, might be useful for preclinical research, especially about the efficacy of macrofilaricidal drugs.

Human filariasis-contributions of the Litomosoides sigmodontis and Acanthocheilonema viteae animal model

Filariae are vector-borne parasitic nematodes that are endemic worldwide, in tropical and subtropical regions. Important human filariae spp. include Onchocerca volvulus, Wuchereria bancrofti and Brugia spp., and Loa loa and Mansonella spp. causing onchocerciasis (river blindness), lymphatic filariasis (lymphedema and hydrocele), loiasis (eye worm), and mansonelliasis, respectively. It is estimated that over 1 billion individuals live in endemic regions where filarial diseases are a public health concern contributing to significant disability adjusted life years (DALYs). Thus, efforts to control and eliminate filarial diseases were already launched by the WHO in the 1970s, especially against lymphatic filariasis and onchocerciasis, and are mainly based on mass drug administration (MDA) of microfilaricidal drugs (ivermectin, diethylcarbamazine, albendazole) to filarial endemic areas accompanied with vector control strategies with the goal to reduce the transmission. With the United Nations Sustainable Development Goals (SDGs), it was decided to eliminate transmission of onchocerciasis and stop lymphatic filariasis as a public health problem by 2030. It was also requested that novel drugs and treatment strategies be developed. Mouse models provide an important platform for anti-filarial drug research in a preclinical setting. This review presents an overview about the Litomosoides sigmodontis and Acanthocheilonema viteae filarial mouse models and their role in immunological research as well as preclinical studies about novel anti-filarial drugs and treatment strategies.

Lessons in type 2 immunity: Neutrophils in Helminth infections

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

IL-33: A central cytokine in helminth infections

IL-33 is an alarmin cytokine which has been implicated in allergy, fibrosis, inflammation, tumorigenesis, metabolism, and homeostasis. However, amongst its strongest roles are in helminth infections, where IL-33 usually (but not always) is central to induction of an effective anti-parasitic immune response. In this review, we will summarise the literature around this fascinating cytokine, its activity on immune and non-immune cells, the unique (and sometimes counterintuitive) responses it induces, and how it can coordinate the immune response during infections by parasitic helminths. Finally, we will summarise some of the ways that parasites have developed to modulate the IL-33 pathway for their own benefit.

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.