Acetylcholine mediates protective immune responses against the rodent filarial nematode <em>Litomosoides sigmodontis</em>

Abstract

Tropical infectious diseases are caused by parasitic helminths, such as lymphatic filariasis and onchocerciasis (river blindness). Filariae have the ability to modulate the immune response of their host in order to be tolerated for many years. For a better understanding of the protective immune response against filariae and their ability to modulate immune mechanisms of the host, the rodent filarial nematode <em>Litomosoides sigmodontis</em> was used. Previous studies showed that the immune system and the nervous system interact with each other to ensure proper regulation of inflammatory processes. The aim of this thesis was to investigate the contribution of acetylcholine (ACh) signalling through muscarinic receptors in regulating the immune response against <em>L. sigmodontis</em>. <br /> ACh was produced by immune cells such as CD4⁺ T cells, B cells and neutrophils, which are essential for the protective immune responses against filariae. Cholinergic inhibition during natural infection with <em>L. sigmodontis</em> resulted in an increased worm recovery at day 9 when L3 larvae reached the thoracic cavity. Circumventing the migration of L3 larvae through the skin by an intravenous injection led to a comparable worm burden in mice treated with or without muscarinic inhibitor, indicating that the protective immune responses within the skin were compromised by the inhibition of ACh signalling. <br /> Confirmation of the involvement of ACh in protective immune responses against <em>L. sigmodontis</em> was further provided in muscarinic receptor type 3 (M3R) knockout mice. M3R^-/- mice had a delayed arrival of L3 larvae in the thoracic cavity with lower worm numbers at day 9, but a higher worm recovery at 15 and 37 days post natural infection in comparison to wild-type mice. The reduced worm burden was abolished by an intravenous injection, which was accompanied by a lower histamine release 30 min after L3 injection. Furthermore, the vascular permeability was investigated as it is known that histamine induces vasodilation, which facilitates larval migration due to an increased vascular permeability. Basophils are the main producers of histamine in the plasma. Vascular permeability was examined for confirmation, however a significantly increased vascular permeability in the M3R deficient mice was noticed. <em>In vitro</em> analysis further revealed that activation is impaired in neutrophils and basophils derived from M3R^-/- mice. These data suggest that ACh is involved in protective immune responses against filariae and associated with granulocyte activation and recruitment.