Every animal has points of contact with the environment. Those points of contact are defensive barriers against pathogens and external aggressions and are equipped with specialized cells and tissues that form the mucosal immune system. In humans and birds, mucosal immune systems include well-organized immune structures such as the tonsils in our nasopharyngeal passages. Currently, organized mucosal immune structures are thought to be present in modern vertebrates only. The investigator proposes for the first time that primitive tonsils first appeared in lobe-finned (sarcopterygian) fishes such as the African lungfish. African lungfish represent the transition of vertebrates from water to land and are the closest living ancestor to all tetrapods. Investigating the immune cells and molecules that drive the formation of organized lymphoid (immune) structures in the mucosal surfaces of lungfish will reveal the primordially conserved features of organized mucosal immune systems in vertebrates. The project will not only use African lungfish as an animal model in the laboratory but also as an educational tool for school children to understand evolution, adaptation, physiology and immunity.
The evolution of adaptive immunity in vertebrates illustrates how some key innovations have resulted in more efficient mechanisms to fight pathogens. Amongst these innovative processes, the organization of lymphocytes in specific anatomical areas, particularly mucosal surfaces, has proven a very effective way for mammals to produce high affinity class-switched antibodies. The high level of organization found in mammalian tonsils or Peyer's patches must have evolved from some sort of primitive organized lymphoid structure. Currently, many investigators see the lymphoid aggregates in the gut of amphibians as the forerunners of mammalian organized mucosa-associated lymphoid tissues (MALT). However, prior to amphibians, the sarcopterygian fishes reveal that the evolutionary origin of organized MALT is actually older and it coincides with the transition of vertebrates from water to land. Thus, lungfishes, the closest ancestors of all tetrapods, may be the first animals to have innovated by grouping lymphocytes in specific lymphoid structures. It has been proposed that the diversification of the tumor necrosis factor (TNF) family of genes from bony fish to mammals has been instrumental for the acquisition of organized lymphoid tissues in higher vertebrates. To test this hypothesis, the investigator proposes the study of the newly discovered lymphoid aggregates (LAs) present in mucosa of the African lungfish (Protopterus sp.). The goals of this project are 1) to characterize the TNF family members in African lungfish and complete the phylogeny of this superfamily using deep sequencing transcriptome databases generated by Dr. Irene Salinas (PI) and Dr. Chris Amemiya (Senior Personnel); 2) to characterize lungfish LAs at the molecular, cellular and functional levels using histology, laser capture microdissection (LCM), RT-PCR and FISH; 3) to establish the role of TNF genes and microbial exposure in the formation and organization of lungfish LAs.
