Role of interleukin-6 during infection with the filarial nematode <em>Litomosoides sigmodontis</em>

Abstract

Lymphatic filariasis is a neglected tropical disease that is caused by parasitic helminths and affects 120 million people worldwide, while 1.3 billion people live in endemic areas. Three species of tissue dwelling nematodes cause this disease: <i>Wuchereria bancrofti</i>, <i>Brugia malayi</i> and <i>Brugia timori</i>. Endosymbiotic <i>Wolbachia</i> bacteria are implicated in disease pathology as they activate the host‘s immune system and induce the production of pro-inflammatory cytokines like IL-6.<br /> In this study, I investigated the role of IL-6 using the rodent filarial nematode <i>Litomosoides sigmodontis</i> model that closely resembles immune responses caused during human filarial infection. <br /> IL-6-deficient (IL-6^-/-) and immunocompetent control BALB/c mice were infected via with <i>L. sigmodontis</i> the natural vector and parasitological and immunological analysis were performed at different time points post infection. Infectious third stage larvae (L3) are transferred by infected mites during the blood meal, pass the skin barrier and subcutaneous tissue and migrate via the lymphatics to the pleural cavity where they develop into adult worms and reside. The worm burden in the pleural cavity of IL-6^-/- mice was found to be increased at 7 and 15 days post infection (dpi), but not after the molt to adult worms (30 dpi) and during chronic infection. This increased worm recovery during early infection in IL-6^-/- mice correlated with a reduced relative but not absolute number of total B and B2 cells in the pleural cavity, whereas the number of regulatory T cells and Th17 cells was not altered by IL-6 deficiency during <i>L. sigmodontis</i> infection. <br /> Eosinophils that are known to be involved in the clearance of filarial infection were increased in the pleural cavity of infected IL-6^-/- mice during those early time points. This increase in eosinophil numbers did not mediate the enhanced clearance of <i>L. sigmodontis</i> larvae after 14 dpi in IL-6^-/- mice, as depletion of IL-5 and therefore reduction of eosinophil numbers, did not lead to the maintenance of an increased worm burden after the molt to adult worms. Increased vascular permeability as induced by augmented mast cell degranulation (e.g. release of histamine) that might allow a better worm migration to the pleural cavity in IL-6^-/- mice, was not the mechanism that resulted in an inceased worm burden in IL-6^-/- mice at an early time point of infection. Thus, blocking of histamine receptors did not reduce the worm burden in IL-6^-/- mice and measurement of vascular permeability in response to parasite antigens revealed no difference between IL-6^-/- and BALB/c mice. However, stabilizing mast cells using cromolyn sodium salt reduced the worm burden partially in IL-6^-/- indicating that mast cells of IL-6^-/- mice may facilitate worm migration, although not via the release of histamine or by increasing the vascular permeability. Those results indicate that protective immune responses that are impaired by the IL-6-deficiency are likely to occur before the entrance of the infectious larvae into the vascular system and the pleural cavity. Accordingly, bypassing the skin barrier by inoculating infectious L3 subcutaneously resulted in a worm recovery at 15 dpi that was comparable between the BALB/c and IL-6^-/- mice. This suggests that during natural infection, protective immune responses in the skin against infectious L3, e.g. neutrophil and eosinophil responses are potentially impaired by the lack of IL-6 and facilitate the migration of the larvae to the pleural cavity. This hypothesis is now being tested by local infections and analysis of the cell recruitment to the site of infection.