A novel ligand for chicken Toll-like receptor 5 from Eimeria maxima: dihydrolipoyl dehydrogenase drives M2-like polarization and Treg-like responses.

Avian coccidiosis is a major parasitic disease that poses a significant threat to the poultry industry. Elucidating the regulatory mechanisms of key molecules involved in the interplay between Eimeria and the host immune system is of great importance for the development of novel control strategies. Toll-like receptor 5 (TLR5) is a critical pattern recognition receptor in host innate immunity; however, research on its ligands has primarily focused on bacterial-derived molecules, and Eimeria-derived ligands along with their regulatory mechanisms remain largely uncharacterized. Using immunoprecipitation coupled with proteomics analysis, we identified a novel chicken TLR5 (chTLR5) ligand, E. maxima dihydrolipoyl dehydrogenase (EmDLD). In an in vitro macrophage model, recombinant EmDLD (rEmDLD) polarized chicken primary bone marrow-derived macrophages toward an M2-like phenotype. This polarization was characterized by the upregulation of TGFB1 and CD206, alongside the downregulation of TNFA, IL1B, and IL6, without affecting the production of nitric oxide (NO) or reactive oxygen species (ROS). Unexpectedly, in the chicken macrophage cell line HD11, rEmDLD activated the canonical NF-κB and MAPK signaling pathways. Furthermore, rEmDLD promoted maturation of primary bone marrow-derived dendritic cells in vitro, which enhanced T-cell proliferation and increased the proportion of CD4