Theileria's Strategies and Effector Mechanisms for Host Cell Transformation: From Invasion to Immortalization

Abstract

One of the first events that follows invasion of leukocytes by Theileria sporozoites is the destruction of the surrounding host cell membrane and the rapid association of the intracellular parasite with host microtubules. This is essential for the parasite to establish its niche within the cytoplasm of the invaded leukocyte and sets Theileria spp. apart from other members of the apicomplexan phylum such as Toxoplasma gondii and Plasmodium spp., which reside within the confines of a host-derived parasitophorous vacuole. After establishing infection, transforming Theileria species (T. annulata, T. parva) significantly rewire the signaling pathways of their bovine host cell, causing continual proliferation and resistance to ligand-induced apoptosis, and conferring invasive properties on the parasitized cell. Having transformed its target cell, Theileria hijacks the mitotic machinery to ensure its persistence in the cytoplasm of the dividing cell. Some of the parasite and bovine proteins involved in parasite-microtubule interactions have been fairly well characterized, and the schizont expresses at least two proteins on its membrane that contain conserved microtubule binding motifs. Theileria-encoded proteins have been shown to be translocated to the host cell cytoplasm and nucleus where they have the potential to directly modify signaling pathways and host gene expression. However, little is known about their mode of action, and even less about how these proteins are secreted by the parasite and trafficked to their target location. In this review we explore the strategies employed by Theileria to transform leukocytes, from sporozoite invasion until immortalization of the host cell has been established. We discuss the recent description of nuclear pore-like complexes that accumulate on membranes close to the schizont surface. Finally, we consider putative mechanisms of protein and nutrient exchange that might occur between the parasite and the host. We focus in particular on differences and similarities with recent discoveries in T. gondii and Plasmodium species.

Keywords: Apicomplexa; Plasmodium; Theileria; Toxoplasma; annulate lamellae; invasion; microtubule; transformation.

FIGURE 1

Punctate localization of Theileria annulata p104 and bovine CLASP1 on the schizont membrane. Structured illumination microscopy (SIM) was performed on T. annulata infected macrophages (TaC12) with anti-p104 (red) and anti-CLASP1 (green) antibodies. Z-stack images were taken at 12.5 nm intervals, one z-stack is shown. Scale bar is 5 μm.

FIGURE 2

Transmission electron microscopy analysis of a Theileria annulata infected macrophage reveals annulate lamellae pore complexes close to the schizont surface. Glutaraldehyde-fixed and Epon-embedded TaC12 cells were analyzed using TEM. Annulate lamellae (AL) and AL pore complexes (ALPC, black arrows) are indicated. The parasite plasma membrane (PPM) is indicated with black arrow heads. The white arrow indicates a putative cytostome. Scale bar is 400 nm. The white box in the inset indicates the magnified area. Inset: overview of the infected cell. Host (N) and parasite (n) nuclei have been colored purple, the host cytoplasm has been colored yellow, host mitochondria are blue, and host derived AL are highlighted brown. The outline of the schizont is indicated with a dashed line.

FIGURE 3

Schematic representation of key host-parasite interactions in Theileria annulata infected leukocytes. Theileria schizonts export proteins into the host nucleus (TashAT, TashHN) or cytoplasm (PIN1, Ta9), where they have the potential to interact with host proteins (FBW7) to modify the host phenotype. Theileria interacts closely with host MTs, mediated in part by a protein complex on the schizont surface comprising bovine CLASP1, CD2AP, EB1 and the schizont proteins p104 and MISHIP. The IKK complex is recruited to the parasite surface, enabling the nuclear localization of NF-kB subunits (p65) to the host nucleus. Porous annulate lamellae that align closely to the schizont membrane are depicted, as are structural elements of the parasite plasma membrane such as dynamic protrusions and a potential cytostome. Fluorescent images show host MTs, labeled with anti-CLASP2 antibodies (green, top panel) and host ALPCs, labeled with anti-RanGAP1 (green, bottom panel) in relation to the schizont membrane, which is labeled with anti-p104 (red). The scale bar is 5 μm.