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 60kDa and with 12-15kDa (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 12-15kDa 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.