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

Abstract

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.

Bacterial Infections in Sea Turtles

“…Antibiotic-resistant bacteria should be taken into particular account as a major health concern of sea turtles because antibiotic treatment of hospitalized turtles may be unsuccessful [ 102 , 110 , 114 , 116 , 130 , 158 ]. However, evolution and severity of the infections, also those caused by opportunistic pathogens, are strongly related to sea turtles’ immune system status; the immune function is sensitive to fluctuations in temperature and to pollutants in the environment; and rapid changes in the marine environment are considered responsible for higher vulnerability to infections and diseases [ 159 ].…”

Sequence and structural analysis of adaptors of Toll-like receptor 4 sheds light on the evolutionary trajectory and functional emergence

“…But unlike Aves, Crocodylus porosus has both Myristoylation motif and TRAF6 binding motif in TRAM protein and conserved serine in pLxIS motif of TRIF. TRAM in Chelonia mydas also lacks the CD14 and has viral RNA sensing by TLR3 mediated signalling [32]. Also, like Crocodylus porosus, it has conserved motifs and key residues.…”

Evidence of chelonid herpesvirus 5 infection in green turtle (Chelonia mydas) indicated a possible tumorigenesis activation by transcriptome analysis

“…was suggested to be a possible vector of ChHV5 transmitting between sea turtles (Greenblatt et al, 2004). Nowadays, several studies suggested that FP in sea turtles is mainly caused by the interaction of ChHV5 and environmental factors such as water quality and autoimmunity (Aguirre et al, 1995;Alfaro-Nunez et al, 2016;Bruno et al, 2021;James et al, 2021;Sposato et al, 2021;Nash & Ryan, 2023). Sea turtles with FP were shown to have immunosuppression and reduced activity of natural killer cells (Work et al, 2001;Li and Chang, 2020;Perrault et al, 2021).…”