Kudoa septempunctata was Recognized by Toll-like Receptor 2

Introduction: In Japan, food poisoning outbreaks associated with the consumption of raw olive flounder have increased, with more than 50 cases reported per year. The causative agent of these outbreaks is Kudoa septempunctata, a myxosporean parasite. K. septempunctata dwells in the trunk muscles of olive flounder. The lag phase of disease caused by K. septempunctata is 1-20 h and symptoms include transient but severe diarrhea and emesis. One characteristic of this food-borne disease is a rapid recovery, and the disease has a good prognosis. The rapid recovery following ingestion of the parasite is thought to be associated with the immunity of the host, particularly innate immunity. Our previous studies showed that when macrophages were incubated with K. septempunctata, macrophages strongly secreted TNF-α and several chemokines, such as IP-10, MIP-1β, and MIP-2. However, the receptor on macrophage that recognize K. septempunctata remains unknown.

Purpose: Macrophages express Toll-like receptors (TLRs), and each TLR has a specific ligand. The ligands of TLRs are conserved motifs that are found predominantly in microorganisms, and TLR signaling enables macrophages to recognize pathogens and subsequently produce cytokines. The aim of this study was to identify the TLR receptor that recognize K. septempunctata.

Methods: To identify the macrophage receptor of K. septempunctata, the activation of HEK 293 cells expressing each TLR (from TLR1 to TLR9) was measured using  NF-κB-dependent reporter assay.

Results: TLR2-expressing HEK 293 cells were strongly activated following stimulation with K. septempunctata.

Significance: Our study demonstrated that K. septempunctata was recognized by TLR2. This result suggests the mechanism that K. septempunctata is recognized by TLR2 on macrophages and TLR2-signaling leads to the production of cytokines.