A comparative study on intestinal immune cells in two elasmobranchs species

The immune system of fish is of great relevance as it provides information on the evolution of immunity in vertebrates. All sharks, skates and rays are cartilaginous fish and together form a group called the elasmobranchs. Elasmobranchs have a low incidence of disease and their immune cells have been identified as possible sources of novel tumour cell inhibitors. The basic anatomical structure of the elasmobranch gut is similar to that of other vertebrates, with a striking exception of the presence of a spiral intestine that provides an enlarged surface area for digestion and absorption of food by means of spiral folds. During recent years, considerable number of studies have described the immune system of teleosts, whereas little effort has been directed towards studying immunity in elasmobranchs. The lack of knowledge on immune cells in the intestine of elasmobranchs prompted us to carry out present study. During 2018, 13 blackmouth catshark Galeus melastomus and 15 specimens of the thornback ray Raja clavata were provided by a trawl fleet from the Gulf of Asinara (Sardinia, western Mediterranean Sea). Histochemical, immunohistochemical and ultrastructural observations were conducted on a subsample of spiral intestine of these fish species. Regarding R. clavata, three types of granular cells were identified; type I in epithelium, types II and III in lamina propria-submucosa, with each of them containing cytoplasmic granules with distinct ultrastructural characteristics. Immunostaining of intestinal sections showed the reactivity of granular cells: type I cells were positive for lysozyme, mast cell tryptase and tumor necrosis factor-ɑ (TNF-ɑ based on antibody staining; type III cells were immune-reactive to anti-interleukin 6 (IL-6) antibody, whilst type II cells were negative to all the antibodies used. Additionally, in G. melastomus, our studies identified unique, large granular cell type in intestinal epithelium. Electron microscopy showed that the epithelial granular cell type made intimate contact, by means of junctional complexes, with adjacent epithelial and mucous cells. Histochemical staining showed the cytoplasmic granules to be strongly eosinophilic and stained positively to periodic acid-schiff and Alcian blue. Immunostaining of intestinal sections revealed immunoreactivity of the granular cell to TNF-ɑ antibody. However, no reactivity to inducible-nitric oxide synthesis, IL-6, interleukin IL-1β, lysozyme, serotonin 5-HT antibodies was detected. Comparison of each granular cell type in both elasmobranch species with immune cells of teleosts or mammals and a hypothesis on their nature and function would be described