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

## TL;DR

Eosinophils use toxic proteins to immobilize parasitic worm larvae, but this mechanism differs between lab mice and natural host cotton rats.

## Contribution

Demonstrates that major basic protein and eosinophil peroxidase are critical for microfilariae motility inhibition via eosinophil extracellular traps.

## Key findings

- Eosinophils lacking MBP or EPO are less effective at immobilizing microfilariae.
- Adding MBP and EPO in vitro inhibits microfilariae motility in a dose-dependent manner.
- Cotton rat eosinophils do not release DNA traps in response to microfilariae.

## Abstract

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.

Filarial nematodes are thread-like worms that can cause debilitating diseases such as river blindness (onchocerciasis) or elephantiasis (lymphatic filariasis). Similar to most helminth infections, filarial infections lead to eosinophilia, an increase of eosinophil granulocyte numbers in the peripheral blood. Although eosinophils have been shown to contribute actively to the protective immune responses, the underlying mechanisms are not yet fully understood. In the present study, we investigated the protective effect of the eosinophil granule proteins major basic protein (MBP) und eosinophil peroxidase (EPO) against the filarial progeny, the microfilariae, of the rodent filarial nematode Litomosoides sigmodontis. Eosinophils lacking MBP or EPO inhibited microfilariae motility in vitro to a lesser extent than wild-type eosinophils. Consistent with these results, the addition of MBP and EPO inhibited microfilariae motility in vitro. Additionally, we demonstrate that eosinophils from the natural host of L. sigmodontis, the cotton rat, did not inhibit microfilariae motility in the same way, indicating either intrinsic differences among cotton rat and mouse eosinophils or an adaptation of the parasite to its natural host.