Its transcription in the head-kidney was increased by bacterial and virus infections and in vitro incubation of sole leucocytes with recombinant EBI3 protein resulted in increased respiratory burst and production of IL-1β, IL-8, IFN-stimulated gene 15 (ISG15), chemokines, MyD88 and CD28 as well bactericidal activity. Here, CD28 is linked to viral infectious disease.