The mammalian mucosal immune system is the largest lymphoid compartment, encounters the most diverse antigenic load, and is breached or infected by the great majority of infectious diseases worldwide. Studies of gut associated lymphoid tissues (GALT) in mouse and man have provided much insight into the specialized lymphocyte subsets and immunoglobulin (Ig) isotypes that manage the balance between tolerance to food antigens and commensals and activation against pathogen. However little is known of comparative immunology of GALT. This is especially true in the cold-blooded vertebrates where the system arose. I propose to test the hypothesis that specialized T cell subsets and Ig isotypes in the gut are an ancient and fundamental part of our immune system that were necessary early in the history of adaptive immunity. The shark and frog GALT are the models in which this hypothesis will be tested. In shark the molecular characterization of intestinal lymphocytes will focus on the repertoire of special subsets such as the new NAR-TcR T cell. Tissue staining will map these cells anatomically within the gut epithelia or lamina propria. The second aim will employ the experimental advantages and immunological tools of the African clawed frog system to investigate which isotypes and mechanisms are conserved in the humoral mucosal compartment. Oral immunizations will test route of administration, thymus dependence of class switch, generation of oral tolerance in the periphery and light chain isotype function. GALT study in lower vertebrates will teach us what is phylogenetically basic in mediating defense and tolerance. Additionally, we will gain insight into the early lymphocyte subpopulations and repertoires of the adaptive immune system. By allowing me protected time for this fundamental research as a new Assistant Professor, a stronger application will be possible for my first R01 in three years. This work will seed my independent career investigating the origins and natural history of our immune system to better able the physician to modify repertoires for the amelioration of disease. A unifying theme in the four cornerstones of NIAID's strategic plan is the need for a better comprehension of lymphocyte regulation from the extremes of failure of the repertoire to autoimmunity and allergy, and comparative work studying GALT immunology in model primitive vertebrates is ripe to yield such understanding.