The fine structural relationship between Theileria schizonts and infected bovine lymphoblasts from cultures

Cytoskeleton of apicomplexan parasites

The Apicomplexa are a phylum of diverse obligate intracellular parasites including Plasmodium spp., the cause of malaria; Toxoplasma gondii and Cryptosporidium parvum, opportunistic pathogens of immunocompromised individuals; and Eimeria spp. and Theileria spp., parasites of considerable agricultural importance. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion. This review summarizes our current understanding of the cytoskeletal elements, the properties of cytoskeletal proteins, and the role of the cytoskeleton in polarity, motility, invasion, and replication. We discuss the unusual properties of actin and myosin in the Apicomplexa, the highly stereotyped microtubule populations in apicomplexans, and a network of recently discovered novel intermediate filament-like elements in these parasites.

Measuring tubulin content in Toxoplasma gondii: a comparison of laser-scanning confocal and wide-field fluorescence microscopy

Toxoplasma gondii is an intracellular parasite that proliferates within most nucleated cells, an important human pathogen, and a model for the study of human and veterinary parasitic infections. We used a stable yellow fluorescent protein-alpha-tubulin transgenic line to determine the structure of the microtubule cytoskeleton in T. gondii. Imaging of living yellow fluorescent protein-alpha-tubulin parasites by laser-scanning confocal microscopy (LSCM) failed to resolve the 22 subpellicular microtubules characteristic of the parasite cytoskeleton. To understand this result, we analyzed sources of noise in the LSCM and identified illumination fluctuations on time scales from microseconds to hours that introduce significant amounts of noise. We confirmed that weakly fluorescent structures could not be imaged in LSCM by using fluorescent bead standards. By contrast, wide-field microscopy (WFM) did visualize weak fluorescent standards and the individual microtubules of the parasite cytoskeleton. We therefore measured the fluorescence per unit length of microtubule by using WFM and used this information to estimate the tubulin content of the conoid (a structure important for T. gondii infection) and in the mitotic spindle pole. The conoid contains sufficient tubulin for approximately 10 microtubule segments of 0.5-microm length, indicating that tubulin forms the structural core of the organelle. We also show that the T. gondii mitotic spindle contains approximately 1 microtubule per chromosome. This analysis expands the understanding of structures used for invasion and intracellular proliferation by an important human pathogen and shows the advantage of WFM combined with image deconvolution over LSCM for quantitative studies of weakly fluorescent structures in moderately thin living cells.

TaCRK3 encodes a novel Theileria annulata protein kinase with motifs characteristic of the family of eukaryotic cyclin dependent kinases: a comparative analysis of its expression with TaCRK2 during the parasite life cycle

The TaCRK3 gene from the bovine apicomplexan parasite Theileria annulata, encodes a 46 kDa polypeptide with strong homology to the eukaryotic family of cyclin-dependent kinases. TaCRK3 does not show significant alignment with any particular CDK group, other than the Pfmrk kinases from the related apicomplexans Plasmodium falciparum and Plasmodium yoelii. It has a putative bipartite nuclear localization signal and is located to parasite nuclei by IFAT. Protein levels are constitutive throughout differentiation of the intra-lymphocytic macroschizont. This contrasts with the expression pattern of TaCRK2 (Kinnaird et al., 1996, Mol. Microbiol., 22, 293-302) which is closely related to the eukaryotic CDK1 /2 families involved in regulation of cell cycle progression. TaCRK2 is also located to the parasite nuclei but has no nuclear localization signal and exhibits transient up-regulation in protein levels during mid-merogony. However compared to TaCRK3, it shows down-regulation near the end of merogony. We predict that TaCRK3 may have a role in regulation of gene transcription while TaCRK2 is more likely to be involved in control of parasite nuclear division.

Characterization of species and strains of Theileria

A variety of methods is now available for characterizing species and strains of Theileria. For many practical purposes involving field control of theileriosis, characterization on a broad basis may be sufficient, but in other areas much more precise characterization is required. Such precision can be usefully exploited only when cloned parasite populations are involved, and methods to improve parasite characterization and parasite cloning should be developed concurrently. The current methods of immunization against theileriosis involve the use of live parasite populations which are generally poorly defined and, in addition, have the capacity to undergo biological change (by selection, mutation or genetic recombination) within hosts and vectors. Such changes may be difficult to define and identify, but could have profound effects on immunization strategies. Improved methods of parasite characterization and selection, which are now becoming available, will enable parasite stocks for immunization to be identified and selected more precisely, and any biological changes that occur can be monitored. Improved methods of parasite characterization will also open the way to a better understanding of Theileria genetics and the mechanisms of heritability, which appear to differ in some fundamental ways from patterns of Mendelian inheritance. Controlled matings between selected and defined populations of parasites can be envisaged, with the aim of producing hybrid parasites for immunization. In addition, the prospects of modifying the theilerial genome by genetic manipulation become very real: transfection vectors tailored by restriction enzymes could be used to insert or modify gene sequences to develop parasites with appropriate sets of characters. It may also be possible to identify parasite genes which trigger the cytotoxic response which is so important in immunity (Eugui and Emery, 1981; Emery et al., 1981; Preston et al., 1983). Such genes might then be transfected into bovine host lymphocytes to generate immunity against the whole parasite (Iams, 1985). The gene products which are responsible for stimulating immune responses could also be synthesized artificially and used for vaccination. Methods of characterizing Theileria range from Giemsa's staining to DNA hybridization; all have a role to play, and by judicious selection of appropriate methods for particular circumstances, it is becoming possible to characterize theilerial parasites very precisely. Improved methods of characterization can, in turn, lead to a better understanding of parasite biology and to the development of improved methods of immunization and control.

Ultrastructural characteristics of in vitro parasite-lymphocyte behaviour in invasions with Theileria annulata and Theileria parva

Host-parasite relationships have been studied by electron microscopy using glutaraldehyde-OsO4-fixed pellets of lymphoid cultures infected in vitro by Theileria annulata and T. parva. Intracellular presence of the parasite resulted in a progressive and marked lymphoblastoid transformation. The schizont stage periodically provoked the formation of, and adopted an intimate association with, cytoplasmic annulate lamellae in the interphase cell. Annulate lamellae developed from the outer nuclear membrane of the host cell by a delamination process and were taken into the cytoplasmic matrix of the schizont by phagotrophy through the cytostome. Schizont nuclei themselves were seen to divide at the prometaphase stage of host cell mitosis, the division being characterized by the development of intranuclear spindle microtubules anchored in spindle pole bodies. A hypothesis is propounded that Theileria parasites, consequent on interiorization, provoke the blastoid transformation and the formation of annulate lamellae through the influence of components of their genomic material on host cell deoxyribonucleic acid (DNA) and that the annulate lamellae represent a species of messenger ribonucleic acid (mRNA) and serve as a monitoring device for the schizont, facilitating the accurate timing of the host cell cyclical events.