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Fever and adaptive immunity as ancient survival strategyWarm-blooded animals can raise their core body temperature in response to infection, but cold-blooded animals must behaviorally induce a fever by seeking external warmth to raise body temperature.Haiyou Gao, Xiumei Wei, Kang Li, Yi Cao et al. examined the effect of behaviorally induced fevers on T cells, a part of the adaptive immune system that responds to infection in a pathogen-specific, rather than a generalized, manner.The authors observed that Nile tilapia infected with the waterborne bacterial pathogen Edwardsiella piscicida spent time in a section of the holding tank that had the highest water temperature over the course of five days.The brains and spleens of the fish with behaviorally induced fevers showed an increase in gene expression associated with fever induction, compared with uninfected fish.Whereas genes related to T cell activation were not up-regulated, genes associated with T cell longevity were ramped up.Flow cytometry and Western blot analysis showed a reduction in caspase-8 and caspase-3, which are enzymes that target T cell destruction.Spleen lymphocytes from fish with a behavioral fever also showed increased cytotoxicity.Fever in warm-blooded animals is known to affect T cells, but the study suggests that the integration of fever and adaptive immunity is an ancient survival strategy that evolved before the appearance of four-limbed vertebrates, according to the authors.-T.H.