Evidence of Trained Immunity in a Fish: Conserved Features in Carp Macrophages

TLDR

Trained immunity, an innate immune memory observed in mammals, has not yet been clearly demonstrated in teleost fish. The study hypothesizes that teleost myeloid cells exhibit trained‑immunity features similar to those seen in mammalian macrophages. An in vitro model using head‑kidney‑derived carp macrophages was trained with a NOD‑specific ligand and β‑glucan, rested for six days, then re‑stimulated. Trained carp macrophages displayed metabolic reprogramming, increased phagocytosis, higher il6 and tnf‑α expression, and, upon re‑stimulation, produced more reactive oxygen and nitrogen species than untrained cells, indicating conserved trained‑immunity traits.

Abstract

Trained immunity is a form of innate immune memory best described in mice and humans. Clear evidence of the evolutionary conservation of trained immunity in teleost fish is lacking. Given the evolutionary position of teleosts as early vertebrates with a fully developed immune system, we hypothesize that teleost myeloid cells show features of trained immunity common to those observed in mammalian macrophages. These would at least include the ability of fish macrophages to mount heightened responses to a secondary stimulus in a nonspecific manner. We established an in vitro model to study trained immunity in fish by adapting a well-described culture system of head kidney-derived macrophages of common carp. A soluble NOD-specific ligand and a soluble β-glucan were used to train carp macrophages, after which cells were rested for 6 d prior to exposure to a secondary stimulus. Unstimulated trained macrophages displayed evidence of metabolic reprogramming as well as heightened phagocytosis and increased expression of the inflammatory cytokines il6 and tnf-α. Stimulated trained macrophages showed heightened production of reactive oxygen and nitrogen species as compared with the corresponding stimulated but untrained cells. We discuss the value of our findings for future studies on trained immunity in teleost fish.