Immunity in Sea Turtles: Review of a Host-Pathogen Arms Race Millions of Years in the Running

Simple Summary Sea Turtles have a unique immune system, which evolved over millions of years in the persistent host-pathogen arms race. As this species occupies a unique evolutionary and environmental niche, they provide an opportunity to gain insight into the evolution of immunity. We present an overview of the turtle immune system, including the cells and organs important for coordinating the immune response to pathogens, with a focus on pathogen recognition and inflammatory mediators, including Interferons. We highlight areas for future study and note which studies have investigated freshwater turtles and are lacking in sea turtles. We particularly focus on the Green Sea turtle (Chelonia mydas) as the juvenile turtles within this species are the most afflicted by the neoplastic tumorous disease, fibropapillomatosis (FP). The immune system of sea turtles is not completely understood. Sea turtles (as reptiles) bridge a unique evolutionary gap, being ectothermic vertebrates like fish and amphibians and amniotes like birds and mammals. Turtles are ectotherms; thus, their immune system is influenced by environmental conditions like temperature and season. We aim to review the turtle immune system and note what studies have investigated sea turtles and the effect of the environment on the immune response. Turtles rely heavily on the nonspecific innate response rather than the specific adaptive response. Turtles' innate immune effectors include antimicrobial peptides, complement, and nonspecific leukocytes. The antiviral defense is understudied in terms of the diversity of pathogen receptors and interferon function. Turtles also mount adaptive responses to pathogens. Lymphoid structures responsible for lymphocyte activation and maturation are either missing in reptiles or function is affected by season. Turtles are a marker of health for their marine environment, and their immune system is commonly dysregulated because of disease or contaminants. Fibropapillomatosis (FP) is a tumorous disease that afflicts sea turtles and is thought to be caused by a virus and an environmental factor. We aim, by exploring the current understanding of the immune system in turtles, to aid the investigation of environmental factors that contribute to the pathogenesis of this disease and provide options for immunotherapy.