Dark side illuminated: imaging of Toxoplasma gondii through the decades

Abstract

In the more than 100 years since its discovery, our knowledge of Toxoplasma biology has improved enormously. The evolution of molecular biology, immunology and genomics has had profound influences on our understanding of this ubiquitous bug. However, it could be argued that in science today the adage "seeing is believing" has never been truer. Images are highly influential and in the time since the first description of T. gondii, advances in microscopy and imaging technology have been and continue to be dramatic. In this review we recount the discovery of T. gondii and the contribution of imaging techniques to elucidating its life cycle, biology and the immune response of its host.

Figure 1

T. gondii stages visualized by light microscopy. A. Extracellular tachyzoites from the peritoneum of a mouse seven days post infection, stained with H&E. (63x) B. Replicating parasites inside a monocyte, stained with H&E. (63x) C. Bradyzoites found in the brain of a chronically infected mouse, stained with H&E. (40x) D. Unstained cyst from brain homogenate (40x).

Figure 2

Tissue cyst visualized by EM. A. Myeloid cell (pink) intimately associated with a neuron (yellow), infected and stretched thin by a large cyst (red) containing hundreds of bradyzoites (orange). B. 3D reconstruction of infected neuron pictured in A. C. Synaptic vesicles in the presynaptic neuron (grey) is evidence the infected neuron (yellow) is still functioning. D. Large lysosomal bodies, characteristic of myeloid cells (pink), are present in close association to the infected neuron (yellow).

Figure 3

Imaging T. gondii with parasite-specific antibodies, reporter parasites, and reporter hosts. A. Anti-Toxoplasma antibodies used to visualize bradyzoites (red) escaping from a cyst that is next to a CD8+ T cell (green) in the brain of an infected mouse. Nuclei are stained with DAPI (blue). (40x) B. Pru-GFP, a T. gondii strain engineered to fluoresce green, is shown after having invaded a cultured human foreskin fibroblast (HFF) (actin stained in red) (63x) C. Me49-RFP cysts and GFP + leukocytes (green) are visualized in live brain tissue in a DPE-GFP mouse using two-photon microscopy. D. OTI-GFP T cells (green) migrating along a fibrous network (blue) in a live brain infected with Pru-OVA, visualized by two-photon microscopy.

Figure 4

Movies record cell behavior in response to parasite infection. A. Three frames from a movie recording fluorescein labeled macrophage mediated destruction of an RFP + cyst in vitro. B. Three frames from a movie depicting GFP + T cell motility along a fiber visualized by second harmonic generation in a living brain.