Neutropenic Mice Provide Insight into the Role of Skin-Infiltrating Neutrophils in the Host Protective Immunity against Filarial Infective Larvae

Our knowledge and control of the pathogenesis induced by the filariae remain limited due to experimental obstacles presented by parasitic nematode biology and the lack of selective prophylactic or curative drugs. Here we thought to investigate the role of neutrophils in the host innate immune response to the infection caused by the Litomosoides sigmodontis murine model of human filariasis using mice harboring a gain-of-function mutation of the chemokine receptor CXCR4 and characterized by a profound blood neutropenia (Cxcr4+/1013). We provided manifold evidence emphasizing the major role of neutrophils in the control of the early stages of infection occurring in the skin. Firstly, we uncovered that the filarial parasitic success was dramatically decreased in Cxcr4+/1013 mice upon subcutaneous delivery of the infective stages of filariae (infective larvae, L3). This protection was linked to a larger number of neutrophils constitutively present in the skin of the mutant mice herein characterized as compared to wild type (wt) mice. Indeed, the parasitic success in Cxcr4+/1013 mice was normalized either upon depleting neutrophils, including the pool in the skin, or bypassing the skin via the intravenous infection of L3. Second, extending these observations to wt mice we found that subcutaneous delivery of L3 elicited an increase of neutrophils in the skin. Finally, living L3 larvae were able to promote in both wt and mutant mice, an oxidative burst response and the release of neutrophil extracellular traps (NET). This response of neutrophils, which is adapted to the large size of the L3 infective stages, likely directly contributes to the anti-parasitic strategies implemented by the host. Collectively, our results are demonstrating the contribution of neutrophils in early anti-filarial host responses through their capacity to undertake different anti-filarial strategies such as oxidative burst, degranulation and NETosis.

受限于寄生线虫生物学特性带来的实验阻碍，以及缺乏选择性预防或治疗药物，目前学界对丝虫（filariae）诱导的发病机制的认知与调控手段仍较为有限。本研究拟探究中性粒细胞在宿主固有免疫应答中的作用，针对西奥多罗斯鼠丝状线虫（Litomosoides sigmodontis）构建的人类丝虫病小鼠模型，采用携带趋化因子受体CXCR4（chemokine receptor CXCR4）功能获得性突变、表现为重度血液中性粒细胞减少症的CXCR4+/1013小鼠开展实验。我们通过多维度证据证实，中性粒细胞在调控皮肤内感染早期进程中发挥核心作用。其一，当以皮下途径接种丝虫感染期幼虫（infective larvae, L3）时，CXCR4+/1013小鼠体内的丝虫寄生成功率显著降低。该保护效应与突变小鼠皮肤中本就存在的中性粒细胞数量多于野生型（wild type, wt）小鼠相关。事实上，无论是通过耗竭包括皮肤组织在内的中性粒细胞池，还是通过静脉注射L3以绕过皮肤感染途径，CXCR4+/1013小鼠的寄生成功率均恢复至野生型小鼠水平。其二，将上述观察拓展至野生型小鼠后，我们发现皮下接种L3可诱导皮肤内中性粒细胞数量增加。最后，存活的L3幼虫可在野生型与突变小鼠体内均触发呼吸爆发（oxidative burst）反应，并促使中性粒细胞胞外陷阱（neutrophil extracellular traps, NET）的释放。这种适配L3感染期幼虫大体积特性的中性粒细胞反应，可能直接参与宿主实施的抗寄生虫防御策略。综上，本研究结果证实，中性粒细胞可通过介导呼吸爆发、脱颗粒及中性粒细胞胞外陷阱形成（NETosis）等多种抗丝虫防御策略，在宿主早期抗丝虫免疫应答中发挥关键作用。