Amphibians carry a gene that may be central to one of the immune system’s most important balancing acts, according to a new evolutionary study published in Immunogenetics. Researchers Rebecca A. Clemons, Chase H. Smith and Kelly R. Zamudio report that FOXP3, the transcription factor widely recognized as the master regulator of mammalian regulatory T cells, is present across all three major amphibian lineages: frogs and toads, salamanders and newts, and the less familiar limbless caecilians. The finding suggests that a molecular mechanism for suppressing excessive immune responses has been conserved across a remarkably broad span of vertebrate evolution.
The immune system must perform two apparently opposing tasks. It needs to recognize and eliminate pathogens, including viruses, bacteria, fungi and parasites, but it must also avoid attacking the body’s own tissues. Adaptive immunity makes this challenge especially complex because lymphocytes generate highly diverse antigen receptors. Some of those receptors inevitably recognize self molecules. Without mechanisms of immune tolerance and suppression, immune defenses could become destructive, producing chronic inflammation or autoimmune disease. Regulatory T cells, commonly known as Tregs, help control this danger by restraining other immune cells after activation and by preventing inappropriate responses to self.
FOXP3 is a key molecular switch in that process. The gene encodes a forkhead-box transcription factor, a DNA-associated regulatory protein that influences the expression of many other genes. In mammals, FOXP3 is essential for the development, identity and suppressive activity of regulatory T cells. Mutations in the human gene cause IPEX syndrome, a severe disorder involving immune dysregulation, polyendocrinopathy and enteropathy. In laboratory mice, loss of Foxp3 produces a similarly catastrophic failure of immune tolerance. These observations have made FOXP3 one of the defining markers of the regulatory T-cell lineage—and one of the most important genes for understanding how adaptive immunity is controlled.
Despite its importance in mammals, FOXP3 has been comparatively neglected outside a small number of model organisms. Much of immunology has been shaped by research on humans and laboratory rodents, leaving major gaps in knowledge about how immune regulation operates in other vertebrates. Amphibians are particularly valuable for filling those gaps. They occupy an evolutionary position between fishes and amniotes such as reptiles, birds and mammals, and they possess sophisticated adaptive immune systems while also undergoing dramatic developmental transitions. Their immune biology can change during metamorphosis, when animals move from aquatic larvae to terrestrial or semi-terrestrial adults, potentially altering how immune regulation is deployed.
Clemons and colleagues investigated whether FOXP3 could be detected throughout the amphibian tree of life by mining publicly available genomic and transcriptomic resources. Genomic data provide DNA sequences, while transcriptomic data capture RNA molecules produced when genes are actively expressed in tissues or cells. The researchers searched these datasets for sequences matching FOXP3, compared candidate genes with known vertebrate FOXP3 sequences and assessed their evolutionary relationships. Their central result was consistent across the dataset: FOXP3 is present in all major amphibian clades rather than being restricted to a few familiar laboratory species such as Xenopus frogs.
That distribution is significant because the three living amphibian groups have followed very different evolutionary paths. Anurans include frogs and toads, many of which pass through a free-living tadpole stage. Caudates include salamanders and newts, several of which retain juvenile features or live highly aquatic lives as adults. Gymnophionans, or caecilians, are elongated, mostly subterranean amphibians whose biology remains far less studied than that of frogs and salamanders. Finding FOXP3 across these lineages indicates that the gene likely originated before their diversification or was retained independently under strong functional constraints. Either interpretation points to an ancient role for FOXP3-like immune regulation in amphibian biology.
The study also examined whether natural selection acting on FOXP3 has changed in intensity among the major amphibian groups. Evolutionary geneticists can investigate this question by comparing the rates at which protein-altering substitutions and synonymous substitutions accumulate along different branches of a phylogenetic tree. A surplus of amino-acid-changing substitutions can indicate positive selection, whereas an excess of synonymous changes is often consistent with purifying selection, which removes mutations that damage an essential function. The authors used comparative evolutionary analyses to test whether selection on FOXP3 had intensified or relaxed in frogs, salamanders or caecilians. The broader purpose was to determine whether differences in ecology, life history or immune pressures were reflected in the gene’s evolutionary trajectory.
The results reinforce the idea that FOXP3 belongs to a conserved vertebrate immune toolkit. Earlier studies had identified FOXP3-related sequences in fishes and had provided evidence for the gene in birds, but amphibians remained a major gap in the evolutionary record. The new analysis helps connect those discoveries, suggesting that the molecular architecture underlying regulatory immune cells extends across jawed vertebrates. Presence of the gene does not, by itself, prove that every amphibian species possesses mammal-like regulatory T cells or that the protein performs exactly the same cellular functions. However, the repeated recovery of FOXP3 sequences across the amphibian phylogeny provides a strong genomic foundation for testing those questions experimentally.
That foundation matters for viral science because amphibians are facing an expanding series of infectious threats. Ranaviruses, including frog virus 3, can cause severe disease and mass mortality in wild and captive amphibians. These viruses belong to the family Iridoviridae and can infect multiple amphibian species, sometimes producing systemic disease characterized by hemorrhaging, tissue damage and immune disruption. Another major threat is chytridiomycosis, caused by the fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans. Although these pathogens are not viruses, research has shown that amphibian immune responses can be suppressed or manipulated during infection. Understanding how regulatory pathways shape inflammation could help explain why some species tolerate infection while others experience rapid population declines.
FOXP3 could become especially relevant when researchers study the difference between protective immunity and immunopathology in amphibian infections. A response that is too weak may allow a virus or fungus to replicate unchecked, while a response that is too strong may damage the host and accelerate disease. Regulatory T cells can influence this balance by limiting the activation of effector T cells, altering cytokine production and helping terminate immune reactions after a threat has diminished. In an amphibian infected with ranavirus, for example, FOXP3-associated regulation might affect the intensity and duration of antiviral responses. The present study does not test that mechanism directly, but it identifies a conserved gene that can now be investigated in the relevant species and tissues.
The discovery also carries implications for comparative medicine and conservation biology. If FOXP3 has retained a similar role across distant vertebrates, researchers may be able to use amphibians to study fundamental principles of immune tolerance in a wider evolutionary context. At the same time, amphibian-specific differences could reveal how immune regulation has been adapted to metamorphosis, large genome sizes, unusual regenerative abilities and variable environmental conditions. The authors emphasize that their work is a confirmation of gene conservation, not a complete functional description of amphibian Tregs. Future studies will need to determine when and where FOXP3 is expressed, identify the cells that produce it, test whether those cells suppress immune activity and examine how infections alter the pathway.
For now, the study closes an important gap in the history of vertebrate immunity. FOXP3 is not merely a mammalian innovation associated with laboratory mice and human autoimmune disease. Its presence across Amphibia indicates that the genetic logic of immune restraint is far older and more widespread. As amphibian populations continue to encounter emerging pathogens and changing environments, this conserved regulatory system may prove essential for understanding disease resistance, susceptibility and survival. The gene’s evolutionary persistence offers researchers a new molecular entry point into the immune lives of frogs, salamanders and caecilians—and into the biological mechanisms that determine whether an antiviral defense protects an animal or harms it.
Subject of Research: Evolution and conservation of the FOXP3 immune-regulatory gene across amphibians
Article Title: Primary regulatory T cell activator FOXP3 is present across Amphibia
Article References: Clemons, R. A., Smith, C. H. & Zamudio, K. R. “Primary regulatory T cell activator FOXP3 is present across Amphibia.” Immunogenetics 77, Article 15 (2025). https://doi.org/10.1007/s00251-025-01372-0
Image Credits: AI Generated
DOI: 10.1007/s00251-025-01372-0
Keywords: Amphibian, immune conservation, immune suppression, selection, transcription factor, adaptive immunity, regulatory T cells, FOXP3, ranavirus, viral disease
Tags: amphibian immune system geneticsconservation of FOXP3 geneevolution of immune regulation across vertebratesevolutionary biology of Treg cellsimmune response balancing in amphibiansimmune system adaptations in amphibiansimmune tolerance in amphibiansmolecular mechanisms of immune suppressionregulatory T cell activation mechanismsregulatory T cell developmentrole of FOXP3 in autoimmune disease preventionsignificance of FOXP3 in non-mammalian species
