Productive infection of Piscirickettsia salmonis in macrophages and monocyte-like cells from rainbow trout, a possible survival strategy.

Piscirickettsia salmonis is the etiologic agent of the salmonid rickettsial septicemia (SRS), an endemic disease which causes significant losses in salmon production. This intracellular bacterium is normally cultured in salmonid epithelial cell lines inducing characteristic cytopathic effects (CPEs). In this study we demonstrate that P. salmonis is able to infect, survive, replicate, and propagate in the macrophages/monocytes cell line RTS11 derived from rainbow trout spleen, without inducing the characteristic CPEs and the host cells showing the same expression levels as non-infected control cell. On the other hand, bacteria were capable of expressing specific proteins within infected cells. Infected macrophages cease proliferation and a fraction of them detached from the plate, transform to non-adhesive, monocyte-like cells with proliferative activity. Productive infection of P. salmonis into salmonid macrophage/monocyte cells in culture provides an excellent model for the study of host-pathogen interactions, almost unknown in the case of P. salmonis. Our results suggest that the infection of cells from the salmonid innate immune system without inducing an important cell death response should lead to the persistence of the bacteria and consequently their dissemination to other tissues, favoring the evasion of the first line of defense against pathogens. (c) 2009 Wiley-Liss, Inc.