The Immune Functional Study Of B Cells In Nasal Organ And Buccal Mucosa Of Rainbow Trout

Aquaculture is a fast-growing food producing sector,and health management of the cultured species is critical for the sustainable growth of the industry.Fish live in aquatic environments,which provide an ideal setting for the growth of a variety of microorganisms that are in continuous contact with the mucosal surfaces,such as gill,skin and gut.According to this,the importance of mucosal barriers in aquatic animals may be far more than those of their terrestrial counterparts,and thus,over the last decades,efforts have been made by fish scientists to gain a better understanding of mucosal immune system.In order to protect the organisms from the continuous bombardment of microbes and antigens,the fish mucosal barriers have developed an immune system armed with both cellular and humoral defenses.So far,four different teleost MALTs have been described:GALT(gut-associated lymphoid tissue),SALT(skin-associated lymphoid tissue),GIALT(gill-associated lymphoid tissue),and NALT(nasopharynx-associated lymphoid tissue).There is a common feature among them that the presence of a specialized mucosal immunoglobulin class,IgT/Z.As described recently,the abundance of IgT~+B-cells as well as the high concentration of IgT has been discoved in the teleost nasal organ like mammalian,but the molecular mechanism of dedicated mucosal B-cell and immunoglobulin responses is still unknown.Moreover,there are some scientific questions need to be futher studied about whether teleosts contain another mucosal immune tissue and has the same features.The nasal organ of vertebrates receives chemical cues present in the air or water and,at the same time,they are exposed to invading pathogens.Nasal-associated lymphoid tissue(NALT),which serves as a mucosal inductive site for humoral immune responses against antigen stimulation in mammals,is present also in teleosts.IgT in teleosts is responsible for similar functions to those carried out by Ig A in mammals.Moreover,teleost NALT is known to contain B-cells and teleost nasal mucus contains immunoglobulins(Igs).Yet,whether nasal B cells and Igs respond to infection remains unknown.We hypothesized that water-borne parasites can invade the nasal cavity of fish and elicit local specific immune responses.To address this hypothesis,we developed a model of bath infection with the Ichthyophthirius multifiliis(Ich)parasite in rainbow trout,Oncorhynchus mykiss,an ancient bony fish,and investigated the nasal adaptive immune response against this parasite.Critically,we found that Ich parasites in water could reach the nasal cavity and successfully invade the nasal mucosa.Moreover,strong parasite-specific IgT responses were detected in the nasal mucus,and the accumulation of IgT~+B-cells was noted in the nasal epidermis after Ich infection.Strikingly,local IgT~+B-cell proliferation and parasite-specific IgT generation were found in the trout nasal organ,providing new evidence that nasal-specific immune responses were induced locally by a parasitic challenge.Overall,our findings suggest that nasal mucosal adaptive immune responses are similar to those reported in other fish mucosal sites and that an antibody system with a dedicated mucosal Ig performs evolutionary conserved functions across vertebrate mucosal surfaces.The buccal mucosa(BM)is a critical first line of defense in terrestrial animals.In mammals,some mucosal regions within the BC are covered by a keratinized stratified epithelium(gingival,hard palate,outer lips),In contrast,the entire buccal epithelium of fish is non-keratinized.Interestingly,the non-keratinized buccal areas of mammals resemble the overall structure of the fish buccal mucosa(BM)as both contain two main layers,an outer layer of stratified squamous epithelium and an underlying layer of dense connective tissue(lamina propria).To gain further insights into the evolutionary origins and primordial roles of BM in teleosts,here we show that rainbow trout,a teleost fish,contains a diffuse mucosal associated lymphoid tissue(MALT)and mucus secreating cells within its buccal cavity,but no salvary gland.Fish s IgT produced by intraepithelial IgT+B cells is transported via p Ig R-expressing epithelial cells into the outer layer of the buccal epithelium.Upon parasite infection,a fish immunoglobulin specialized in mucosal immunity(s IgT)was induced to a high degree,and parasite-specific s IgT responses were mainly detected in the buccal mucus.Moreover,we show that the trout buccal microbiota is prevalently coated with s IgT.Overall our findings revealed that the MALT is present in the BM of a non-tetrapod species.As fish IgT and mucus-producing cells are evolutionarily unrelated to mammalian Ig A and salivary glands,respectively,our findings indicate that mucosal immune responses in the BM of teleost fish and tetrapods evolved through a process of convergent evolution.