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 F4/80intermediate macrophages from infected Il-4rα-/-/Il-5-/- mice failed to mature into resident F4/80high 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.

S100A8/S100A9 deficiency increases neutrophil activation and protective immune responses against invading infective L3 larvae of the filarial nematode Litomosoides sigmodontis.

Neutrophils are essentially involved in protective immune responses against invading infective larvae of filarial nematodes. The present study investigated the impact of S100A8/S100A9 on protective immune responses against the rodent filarial nematode Litomosoides sigmodontis. S100A9 forms with S100A8 the heterodimer calprotectin, which is expressed by circulating neutrophils and monocytes and mitigates or amplifies tissue damage as well as inflammation depending on the immune environment. Mice deficient for S100A8/A9 had a significantly reduced worm burden in comparison to wildtype (WT) animals 12 days after infection (dpi) with infective L3 larvae, either by the vector or subcutaneous inoculation, the latter suggesting that circumventing natural immune responses within the epidermis and dermis do not alter the phenotype. Nevertheless, upon intradermal injection of L3 larvae, increased total numbers of neutrophils, eosinophils and macrophages were observed within the skin of S100A8/A9-/- mice. Furthermore, upon infection the bronchoalveolar and thoracic cavity lavage of S100A8/A9-/- mice showed increased concentrations of CXCL-1, CXCL-2, CXCL-5, as well as elastase in comparison to the WT controls. Neutrophils from S100A8/A9-/- mice exhibited an increased in vitro activation and reduced L3 larval motility more effectively in vitro compared to WT neutrophils. The depletion of neutrophils from S100A8/A9-/- mice prior to L. sigmodontis infection until 5dpi abrogated the protective effect and led to an increased worm burden, indicating that neutrophils mediate enhanced protective immune responses against invading L3 larvae in S100A8/A9-/- mice. Interestingly, complete circumvention of protective immune responses in the skin and the lymphatics by intravenous injection of L3 larvae reversed the phenotype and resulted in an increased worm burden in S100A8/A9-/- mice. In summary, our results reveal that lack of S100A8/S100A9 triggers L3-induced inflammatory responses, increasing chemokine levels, granulocyte recruitment as well as neutrophil activation and therefore impairs larval migration and susceptibility for filarial infection.

IL-4 receptor dependent expansion of lung CD169+ macrophages in microfilaria-driven inflammation.

Lung disease is regularly reported in human filarial infections but the molecular pathogenesis of pulmonary filariasis is poorly understood. We used Litomosoides sigmodontis, a rodent filaria residing in the pleural cavity responsible for pleural inflammation, to model responses to human filarial infections and probe the mechanisms. Wild-type and Th2-deficient mice (ΔdblGata1 and Il-4receptor(r)a-/-/IL-5-/-) were infected with L. sigmodontis. Survival and growth of adult filariae and prevalence and density of microfilariae were evaluated. Cells and cytokines in the pleural cavity and bronchoalveolar space were characterized by imaging, flow cytometry and ELISA. Inflammatory pathways were evaluated by transcriptomic microarrays and lungs were isolated and analyzed for histopathological signatures. 40% of WT mice were amicrofilaremic whereas almost all mutant mice display blood microfilaremia. Microfilariae induced pleural, bronchoalveolar and lung-tissue inflammation associated with an increase in bronchoalveolar eosinophils and perivascular macrophages, production of mucus, visceral pleura alterations and fibrosis. Inflammation and pathology were decreased in Th2-deficient mice. An IL-4R-dependent increase of CD169 was observed on pleural and bronchoalveolar macrophages in microfilaremic mice. CD169+ tissue-resident macrophages were identified in the lungs with specific localizations. Strikingly, CD169+ macrophages increased significantly in the perivascular area in microfilaremic mice. These data describe lung inflammation and pathology in chronic filariasis and emphasize the role of Th2 responses according to the presence of microfilariae. It is also the first report implicating CD169+ lung macrophages in response to a Nematode infection.