The cavefish Triplophysa rosa has a well-developed adaptive immune system: Evidence from histological and comparative genomic analysis

The long-term impact of limited and species-specific antigen exposure on immune system evolution is not well understood. Herein, we conducted histological and comparative genomic analyses using the cavefish Triplophysa rosa in order to investigate how restricted antigen exposure influences the evolutionary path of the vertebrate immune system. The histology and cellular characteristics of mucosa-associated lymphoid tissues (gills, skin, and intestines) and the major immune organs (spleen, head kidney, and thymus) of T. rosa were systematically investigated. Histological studies showed that T. rosa had intact immune organs (tissues) and a well-developed adaptive immune system. Our findings suggested that the immune system might have evolved due to the non-degradation of the thymus, which may be an adaptation to the cave environment. A comparative genomic analysis of T. rosa and surface fish was carried out to explore the evolutionary adaptiveness of their immune systems and the potential genomic changes arising from thymus non-degeneration. The analysis revealed that T. rosa developed an efficient adaptive immune system due to the expansion of genes related to T-cell activation and differentiation, consistent with the histological results. Achievement of well-developed adaptive immunity through a limited number of fixed antigens in a cave environment could confer long-term protection against pathogens, highlighting it as a beneficial survival strategy for cavefish. Our findings could provide new ideas for further research on immunity in cavefish.