Rainbow trout (our fish model), is known to contain at least three immunoglobulin isotypes, IgM, IgD and the recently discovered IgT. Thus far nothing is known about the protein structure and distribution of IgT, and its specific role in immunity. We have generated antibodies against IgT that has led us to the finding of a novel B cell subset expressing exclusively surface IgT. Significantly, IgT+ B cells represent the most prevalent B cell type in the gut of trout. Infection of trout with C. shasta, a gut parasite, revealed a massive infiltration of IgT+ B cells in the gut epithelium and lamina propria of survivor fish. Most notably, we found that IgT-specific titers against the parasite were confined to gut mucus, whereas IgM was the only isotype involved in serum responses. Supporting further the role of IgT in gut homeostasis, we found that a majority of intestinal bacteria were coated with IgT. Collectively, these data indicate that IgT represents the functional equivalent of mammalian IgA. The overarching goal of this proposal is to use our fish-parasite model to gain primordial insights of mucosal immunity that will facilitate the analysis of existing paradigms in human mucosal immunology. To date, the animal models being used to study gut mucosal immunity are limited to mammalian species. Thus, our studies with fish will provide a new phylogenetic dimension into the field. Since fish lack peyer patches and lymph nodes in the gut, a lower spatial and functional complexity of the local mucosal responses should be expected, thereby facilitating the analysis of extrafollicular IgT, and by extension, IgA, responses. Accordingly, data generated in this proposal is likely to elucidate novel mechanisms of intestinal IgT/IgA-mediated immunity that are independent of the presence of follicular structures. Finding extrafollicular pathways of IgT/IgA production will be fundamental to developing novel mucosal vaccination strategies. Thus, the goals of this proposal are: AIM 1: Biochemical characterization, production and transport of IgT in the gut of trout. Here we will complete the characterization of gut IgT and IgT-secreting cells, and we will investigate the mechanisms involved in the transport of IgT into the gut luminal area, through a newly discovered rainbow trout Ig polymeric receptor (TIgPr). AIM 2: Mechanisms and pathways involved in the development of gut IgT immune responses to Ceratomyxa shasta. Here we will investigate the extrafollicular mechanisms and pathways by which IgT and IgT+ B cells respond to Ceratomyxa shasta. To this end, we will perform a temporal-spatial analysis of the IgT response in fish infected with the parasite, and we will investigate pathways of IgT+ B cell homing into the gut of infected fish. AIM 3. To investigate the primary effector functions of gut IgT and IgT+B cells against C. shasta. The main goals of this aim are to establish whether the generated IgT-specific responses induced by the parasite are protective, and to elucidate the effector mechanisms by which IgT and IgT+ B cells contribute to protecting fish from parasite invasion. . PUBLIC HEALTH RELEVANCE: We have demonstrated that IgT, a newly discovered teleost immunoglobulin, is the functional homolog of mammalian IgA. We have established a model system that uses a gut parasite to induce strong IgT-specific responses in the gut of rainbow trout. Thus, the overarching goal of this proposal is to use our fish-parasite model to gain primordial insights of mucosal immunity immunity that will facilitate the analysis of existing paradigms in human mucosal immunology. To date, the animal models being used to study mucosal immunity are limited to mammalian species. Thus, our studies with fish will provide a novel phylogenetic dimension into the field. It should be stressed that the selective forces (i.e. host-microbial pathogenic or mutualistic interactions) that have shaped fish and mammalian mucosal immune systems are similar. Thus, some of the novel immunological solutions driven by these selective forces in fish and mammals are likely to share fundamental structural and mechanistic aspects. In fact, the mammalian intestinal IgA is considered a primitive form of adaptive immunity that regulates microbial communities in the gut. Indeed, the features of mucosal immunity in mammals may have remained primitive precisely because the selective forces and physical constraints of mucosal surfaces of fish and mammals are basically very similar. Since fish lack peyer patches and lymph nodes in the gut, a lower spatial and functional complexity of the local mucosal responses should be expected, thereby facilitating the analysis of extrafollicular IgT, and by extension, IgA, responses. Accordingly, data generated in this proposal is likely to elucidate novel mechanisms of intestinal IgT/IgA-mediated immunity that are independent of the presence of follicular structures. Finding extrafollicular pathways of IgT/IgA production will be fundamental to developing novel mucosal vaccination strategies. In addition, this knowledge will be instrumental to help treating diseases that develop as result of the dysregulation of mucosal immune responses (i.e., inflammatory bowel disease, Crohn's disease). It is worth noting that the beneficial effects of fish diets are well-known (i.e., low fat, low cholesterol). Thus, the consumed of fish diets is growing at a very fast pace. Our studies on the fish immune system will also benefit society by contributing to the fish farming industry in the short and long-term. Among the major problems in fish farming are those related to health and disease issues. Therefore, a better knowledge of the fish immune system is essential to the development of new therapeutic agents and vaccines to prevent fish diseases.