Abstract
Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) recognizes muramyl dipeptide (MDP) of bacterial cell walls, triggering NFκB-induced pro-inflammation. As most human pathogenic filariae contain Wolbachia endobacteria that synthesize the MDP-containing cell wall precursor lipid II, NOD2’s role during infection with the rodent filaria Litomosoides sigmodontis was investigated. In NFκB reporter-cells, worm-extract containing Wolbachia induced NOD2 and NOD1. NOD2-deficient mice infected with L. sigmodontis had significantly more worms than wildtype controls early in infection. Increased worm burden was not observed after subcutaneous infection, suggesting that protective NOD2-dependent immune responses occur within the skin. Flow cytometry demonstrated that neutrophil recruitment to the skin was impaired in NOD2−/− mice after intradermal injection of third stage larvae (L3), and blood neutrophil numbers were reduced after L. sigmodontis infection. PCR array supported the requirement of NOD2 for recruitment of neutrophils to the skin, as genes associated with neutrophil recruitment and activation were downregulated in NOD2−/− mice after intradermal L3 injection. Neutrophil depletion before L. sigmodontis infection increased worm recovery in wildtype mice, confirming that neutrophils are essential against invading L3 larvae. This study indicates that NOD-like receptors are implemented in first-line protective immune responses against filarial nematodes.
Highlights
Impaired neutrophil recruitment phenotype[17,18]
NOD2-dependent NFκB signaling was induced when HEK293-cells transfected with murine NOD1 or NOD2 genes were stimulated with Wolbachia-containing crude extract of L. sigmodontis adult worms (Fig. 2), while extract from Wolbachiadepleted worms resulted in significantly lower activation of NOD1 and NOD2, respectively
Once described as an intracellular pattern recognition receptors (PRRs) exclusively involved in bacterial infections, several studies have suggested that NOD2 has a broader than expected role in the innate immune system[19,37,38]
Summary
Impaired neutrophil recruitment phenotype[17,18]. A function of NOD2 in murine skin wound healing processes was recently reported, with NOD2 deficiency leading to a substantial defect in wound repair associated with an initial delay in neutrophil recruitment[19]. The in vivo studies revealed that naturally infected C57BL/6 NOD2−/− mice have a higher worm burden during the early and middle phases of infection than wildtype controls. This increased worm burden is due to delayed NOD2-dependent neutrophil recruitment into the skin during the initial phase of the infection. This is the first report demonstrating the impact of an NLR on protective immune responses against a filarial nematode infection, contributing to the still poorly understood protective immune responses within the skin during the initial phase of filarial infection
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