Rainbow trout (Oncorhynchus mykiss) sIgM− leucocytes secrete an interleukin-2 like growth factor after mitogenic stimulation in vitro

Two secreted proteins were detected in culture supernatants of PHA or PMA stimulated, immunomagnetically separated, sIgM− leucocytes of rainbow trout (Oncorhynchus mykiss) with 60 kDa and with 12–15 kDa (multiple bands). So called conditioned media (CM), containing these proteins, induced significant activation of blood and head kidney leucocytes. Immunomagnetically separated, naive as well as PHA activated sIgM− T lymphocytes and LPS prestimulated sIgM+ B lymphocytes could be identified to be responding to these secreted proteins. Using a monoclonal antibody specific for mouse IL-2 (clone JES6-1A12), one of the multiple 12–15 kDa proteins could be stained in Western blots. It was also shown that the induced proliferation was due to this protein in the CM, as the same anti-IL-2 mab was able to block the CM induced proliferation. Furthermore, survival of the IL-2 dependent mouse cell line HT-2 was enhanced after addition of various concentrations of CM. The data presented show, for the first time, that mitogen stimulated trout sIgM− leucocytes secrete a cytokine like growth factor sharing functional and structural similarities with mammalian IL-2.