Delineating cellular interactions between ciliates and fish by co‐culturing Tetrahymena thermophila with fish cells

Abstract

Although several species of Tetrahymena are often described as histophagous and opportunistic pathogens of fish, little is known about ciliate/fish cell interactions, but one approach for studying these is in vitro with cell lines. In this study, T. thermophila, B1975 (wild type) and NP1 (temperature sensitive mutant for phagocytosis) were cultured on monolayers of 3 fish epithelial cell lines, CHSE-214, RTgill-W1, and ZEB2J, and the rabbit kidney epithelial cell line, RK-13. Generally the ciliates flourished, whereas the monolayers died, being completely consumed over several days. The destruction of monolayers required that the ciliates could make contact with the animal cells through swimming, which appeared to dislodge or loosen cells so that they could be phagocytosed. The ciliates internalized into food vacuoles ZEB2J from cell monolayers as well as from cell suspensions. Phagocytosis was essential for monolayer destruction as monolayers remained intact under conditions where phagocytosis was impeded, such as 37°C for NP1 and 4°C for B1975. Monolayers of fish cells supported the proliferation of ciliates. Thus T. thermophila can 'eat' animal cells or be histophagous in vitro, with the potential to be histophagous in vivo.

Some but not All Tetrahymena Species Destroy Monolayer Cultures of Cells from a Wide Range of Tissues and Species

“…Destruction was observed at 20, 22, and 26 °C, but not 37 °C. The stages of destruction and the swimming behavior of T. corlissi when in contact with the monolayer—referred to as “grazing”—was typical of what occurred with T. thermophila , and has been described previously (Pinheiro and Bols ).…”

“…The invertebrate cell line Sf9 was also destroyed and has a similar suspended morphology to RTS‐11. As initially shown (Pinheiro and Bols ), T. thermophila is capable of consistently destroying animal cell monolayers in culture. It was notable that, although both T. thermophila and T. corlissi showed similar behaviors when in contact with monolayers, the former of the two Tetrahymena destroyed the cell lines in a notably shorter period than T. corlissi, with only the insect cell line, Sf9, as the exception to this.…”

“…Each of these ciliate species had a different capability for destroying monolayers when considering their action over all cell lines tested (Table ). An expanded investigation of T. thermophila for its ability to destroy monolayers increased the range of cell lines destroyed from those tested previously (Pinheiro and Bols ), with no undisturbed cell line yet to be found. These included epithelial and fibroblast cell lines from different organs and from four species.…”

“…Tetrahymena corlissi could destroy most fish cells, whereas T. canadensis could not. In addition, there were clear differences in the behavior of each of these species when in coculture with an animal cell monolayer; T. corlissi showing a grazing behavior also seen with other monolayer‐destroying species (Pinheiro and Bols ), T. canadensis showing no apparent attraction to the monolayer surface. These results indicate that only some tetrahymenas have the capacity to destroy fish cells in vitro.…”

“…One way to study ciliate/fish interaction at the cellular level is to use cocultures of ciliates and fish cells using a variety of potential methods (reviewed in Pinheiro and Bols ). This was recently done with Tetrahymena thermophila (Pinheiro and Bols ), which is a species that is widely used as a model organism in cell biology but usually not considered a fish pathogen. Surprisingly, T. thermophila destroyed cell lines from a wide range of fish species and tissues by a mechanism that appeared to involve phagocytosis.…”