Innate immune receptors in carp: recognition of protozoan parasites

This PhD thesis reports on pattern recognition receptors involved in the immune responses of common carp (Cyprinus carpio) to two protozoan parasites Trypanoplasma borreli and Trypanosoma carassii. The immune responses of carp are fundamentally different when comparing these two extracellular blood parasites. T. borreli induces a characteristically high production of nitric oxide by macrophages, whereas T. carassii parasites seem to preferentially induce an alternative state of macrophage activation. These differences could be driven by differences in the initial engagement of pattern recognition receptors on carp macrophages with either of the two types of parasites. Based on known host-parasite interactions in mammalian vertebrates, Toll-like receptor 2 (TLR2) and TLR9 were selected as candidate receptors for parasite recognition by receptors carp macrophages. Transfection of human cell cultures with carp TLR2 and overexpression of TLR2 in carp macrophages, corroborated the ability of this receptor to bind peptidoglycan from Gram-positive bacteria and glycosylphosphatidylinositol anchors from protozoan parasites. The parasite T. carassii, in particular, induced a TLR2-mediated formation of the cytokine IL-23, leading to a Th17-like immune response in fish infected with T. carassii. Transfection of human cell cultures with carp TLR9 indicated this receptor recognizes bacterial DNA, but not protozoan DNA, and studies in carp macrophages indicated this recognition to be protease-dependent. A novel pattern recognition receptor of carp, named Soluble Immune-Type Receptor (SITR), was identified by investigating an enriched cDNA repertoire from macrophages stimulated by T. borreli. SITR is abundantly expressed in carp macrophages and seems to be secreted upon stimulation with T. borreli. Overexpression of SITR in mouse macrophages and knock-down of SITR in carp macrophages provided evidence for the involvement of this receptor in T. borreli-induced production of nitric oxide. The results presented in this PhD thesis have shed light on the evolution of innate immune receptors involved in the recognition of pathogens.