Major basic protein and eosinophil peroxidase support microfilariae motility inhibition by eosinophil ETosis.

Eosinophils are a hallmark of filarial infections. They are primary effector cells and can attack filariae by releasing extracellular traps that contain toxic cationic proteins, such as eosinophil peroxidase and major basic protein. Previous studies demonstrated that the extracellular traps of eosinophils are induced by the microfilariae of Litomosoides sigmodontis and that they inhibit their motility. In this project, we aimed to investigate the role of these cationic proteins during the extracellular trap-mediated immobilization of microfilariae. Our results indicate that extracellular DNA traps from knockout mice that lack eosinophil peroxidase or major basic protein are significantly less able to immobilize and kill microfilariae. Accordingly, the addition of these cationic proteins to in vitro cultures inhibited microfilariae motility in a dose-dependent manner. Moreover, we examined eosinophils from the natural host, the cotton rat Sigmodon hispidus. While eosinophils of cotton rats release DNA after stimulation with PMA and zymosan, microfilariae did not trigger this effector function. Our work shows that eosinophil granule proteins impair the motility of microfilariae and indicate significant differences in the effector functions of eosinophils between the mouse model and the natural host. We hypothesize that the absence of DNA nets released by cotton rat eosinophils in response to microfilariae may explain the higher microfilarial load and longer patency of the natural host.