Early intestinal infection kinetics and immune responses to Toxoplasma gondii in pigs

Toxoplasma gondii is an obligate intracellular parasite, able to infect all homeothermic animals mostly through ingestion of food or drinks contaminated with tissue cysts or oocysts. Recently, we showed a T. gondii strain-specific clearance from tissues upon infection in pigs. While the swine-adapted LR strain persisted in muscle tissues, the human-adapted Gangji strain was cleared from these tissues. We hypothesized that intestinal immune responses short after infection might play a role in this strain-specific clearance. To assess this possibility, the parasite load in duodenal, jejunal and ileal lymph node cells and blood immune cells (PBMCs) as well as the IFNγ secretion by these cells were evaluated at 2, 4, 8, 14 and 28 days post oral inoculation of pigs with both strains. Interestingly, at day 4 post inoculation with the LR strain the parasite was only detected by qPCR in the duodenal lymph node cells, while in the jejunal and ileal lymph node cells and PBMCs the parasite was detected from day 8 post inoculation onwards. Although we observed a similar profile upon inoculation with the Gangji strain, the parasite load in the examined cells was much lower. This was reflected in a significantly higher T. gondii-specific serum IgG response in LR than Gangji infected pigs at day 28 post inoculation. Unexpectedly, this was not reflected in the IFNγ secretion upon re-stimulation of the cells. However, the recall test most likely does not pick up the IFNγ production by innate immune cells, which might have been more important for clearance. In conclusion, our results show that T. gondii first enters the host at the duodenum and then probably disseminates from this site to the other host tissues.