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

Lymphatic filariasis, onchocerciasis and loiasis are widespread neglected tropical diseases causing serious public health problems and impacting the socio-economic climate in endemic communities. More than 100 million people currently suffer from filarial infections but disease-related symptoms and infection-induced immune mechanisms are still ambiguous. Although most infected individuals have dominant Th2 and regulatory immune responses leading to a homeostatic regulated state, filarial-induced overt pathology like lymphedema, dermal pathologies or blindness can occur. Interestingly, besides dominant Th2 and regulatory T cell activation, increased Th17-induced immune responses were associated with filarial infection and overt helminth-induced pathology in humans. However, the immunological mechanisms of Th17 cells and the release of IL-17A during filarial infections remain unclear. To decipher the role of IL-17A during filarial infection, we naturally infected IL-17A−/− and wildtype C57BL/6 mice with the rodent filariae Litomosoides sigmodontis and analysed parasite development and immune alterations. Our study reveals that infected IL-17A-deficient C57BL/6 mice present reduced worm burden on days 7 and 28 p.i. but had longer adult worms on day 28 p.i. in the thoracic cavity (TC), the site of infection. In addition, infiltration of CD4+ T cells, CD4+Foxp3+ regulatory T and functional CD4+Rorγt+pStat3+ Th17 cells in the TC was reduced in IL-17A-deficient mice accompanied by reduced eotaxin-1 and CCL17 levels. Furthermore, mediastinal lymph node cells isolated from IL-17A−/− mice showed increased filarial-specific IFN-γ but not IL-4, IL-6, or IL-21 secretion. This study shows that Th17 signalling is important for host immune responses against filarial infection but appears to facilitate worm growth in those that reach the TC.Electronic supplementary materialThe online version of this article (10.1007/s00436-018-5959-7) contains supplementary material, which is available to authorized users.

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