Macrophages control parasite burden

Macrophages control parasite burden in vivoTo analyze the interactions between type I Toxoplasma and macrophages in depth, we imaged infected mpeg1:G/U:mCherry larvae with Toxoplasma-GFP at 6hpi using confocal microscopy and 3D CLEM. In this case, the majority of intact type I parasites contained within macrophages are single tachyzoites inside PVs (as judged by host mitochondria association to the membrane surrounding the parasites) (Sup. Fig. 3A). This suggests that macrophages may prevent parasite replication. To follow the fate of type I parasites engulfed by macrophages in real-time, mpeg1:G/U:mCherry larvae infected with Toxoplasma-GFP were imaged by time-lapse confocal microscopy. In this case, we frequently observed the engulfment of parasites by macrophages followed by loss of GFP fluorescence, suggesting active parasite degradation (Fig. 4A, Movie 3). Consistent with this, 3D CLEM showed parasite degradation inside macrophages, as identified by fragmentation of tachyzoite organelles (Fig. 4B, Sup. Fig. 3B).To test the role of macrophages in Toxoplasma infection in vivo, mpeg1:G/U:mCherry larvae were pre-treated with control (DMSO) or metronidazole (Mtz) to ablate macrophages (Sup. Fig. 3C and D). In the absence of macrophages, infected larvae showed 100% survival (Sup. Fig. 3E). However, parasite burden is significantly increased, suggesting macrophages are responsible for parasite clearance in vivo (Fig. 4C and D, Sup. Fig. 3F). Similar results are observed with type II and III strain infection of macrophage-ablated larvae (Sup. Fig. 3G). To analyze the viability of parasites that are cleared by macrophages, we performed vacuole volume quantification and show that DMSO and Mtz-treated larvae are comprised of equally replicating Toxoplasma tachyzoites (Fig. 4E). These data suggest that macrophages have a dominant role in clearing healthy viable parasites rather than supporting the parasite’s replicative niche。