Toxoplasma gondii possesses a significant genetic and phenotypic diversity and in most of the studies, its virulence has been defined in laboratory mice using parasite strains adapted to laboratory conditions. T. gondii clonal types II and III are the predominant strains in Europe and have been described as low or non-virulent. Although a relative correspondence between the virulence of T. gondii in mice and other species has been traditionally assumed, latest studies evidenced that the virulence in mice was not a reliable predictor of virulence in other target species such as pregnant sheep. In addition, different degrees of virulence among type II and type III isolates in in vitro and in vivo models have been reported. Therefore, to learn more about T. gondii virulence mechanisms is important to investigate the parasite phenotypic traits in target cell lines from a relevant livestock species such as sheep and using parasite isolates not adapted to the laboratory. For this, we have evaluated the interaction of a low in vitro passage of the isolate TgShSp1 with two ovine target cells of T. gondii infection: placental trophoblast cell line (AH-1) and monocyte-derived macrophages. Preliminary results in AH-1 cells, which is a target cell during Toxoplasma transplacental transmission, were similar to those obtained with the TgShSp1 isolate in other established cell lines, showing that this isolate had low invasion rate and a poor tachyzoite yield. In case of monocyte-derived macrophages, one of the most important immune target cells during T. gondii infection, preliminary results of the TgShSp1 proliferation kinetics showed that there was a notable exponential growth at 48 hpi. Further characterizations in both cells are necessary to finish the TgShSp1 phenotypic characterization. In addition, we are starting the in vitro characterization of other recently obtained isolates in both cell types to deepen in how the genetic background of the different isolates impacts in an isolate-specific manner in these ovine target cells. We believe that the standardization of an in vitro methodological approach using target cells for T. gondii studies could be an alternative to animal experimentation following the 3R principle. Moreover, the expected results will be extremely relevant for the One Health strategy, considering that T. gondii is an important cause of reproductive failure in sheep and humans.

*Funded by the Seal of Excellence ISCIII Health (IHMC22/00014).