Development of a monocyte/macrophage-like cell line, RTS11, from rainbow trout spleen

A new rainbow trout cell line, RTS11, arose spontaneously from a long-term haemopoietic culture of spleen. For routine subculture, RTS11 requires 20–30% fetal bovine serum, a high cell density, and conditioned medium from previous cultures. Through successive passages, RTS11 cultures maintain a balance between small, non-adherent cells that form the majority of cells (referred to as round cells) and a few larger, granular cells that are either adherent or in suspension surrounded by clusters of round cells. The large granular cells appear to be macrophages, and are referred to as macrophage-like cells, because they are phagocytic, take up DiI-acetylated low density lipoprotein and acridine orange and stain for non-specific esterase, whereas the round cells appear to be at an earlier stage of macrophage development. Medium conditioned by RTS11 also contains lysozyme-like activity, an additional indication that they are of the macrophage lineage. In proliferation assays, the non-adherent cells are stimulated strongly by RTS11 conditioned medium and by lipopolysaccharide. Rainbow trout head kidney leucocytes also show a positive proliferative response to RTS11 conditioned medium. Therefore, RTS11 cells appear to both produce and respond to an autocrine growth factor(s).