Recombinant carp IL-4/13B stimulates in vitro proliferation of carp IgM⁺ B cells

Teleost IL-4/13B is a cytokine related to mammalian IL-4 and IL-13, of which hitherto the function had not been studied at the protein level. We identified an IL-4/13B gene in common carp (Cyprinus carpio) and expressed the recombinant protein (rcIL-4/13B). RcIL-4/13B was shown to stimulate proliferation of IgM⁺ B cells, because after four days of stimulation the IgM⁺ fraction of carp kidney and spleen leukocytes had formed many cell colonies, whereas such colonies were not found in the absence of rcIL-4/13B stimulation. After nine days of incubation with rcIL-4/13B these cells had proliferated to more than 3-to-7-fold higher numbers when compared to untreated cells. The proliferating cells contained a majority of IgM⁺ cells but also other cells, as indicated by FACS and RT-PCR analyses. The important conclusion is that in fish not only IL-4/13A has B cell stimulating properties, as a previous publication has shown, but also IL-4/13B.