Developmental regulation of an Eimeria bovis mRNA encoding refractile body-associated proteins

Characterization of the antigen SO7 during development of Eimeria tenella

The developmental expression of the antigen SO7, which has been previously shown to protect chickens against infection by several Eimeria species, was investigated. Using RT-PCR, mRNA for SO7 was found to be restricted primarily to unsporulated oocysts (0 hr). Western blot (WB) analysis with an antibody to recombinant SO7 (rbSO7) revealed expression of the protein from 6 to 72 hr (fully sporulated) of sporulation and in sporozoites (SZ). SO7 was absent in host-derived second-stage merozoites (MZ) and was present in culture-derived first-stage MZ but at a level of only 25% of that exhibited by SZ. During invasion of Madin-Darby bovine kidney (MDBK) cells by SZ in vitro, the level of SO7 within cells, as determined by WB analysis, remained relatively constant until 48 hr of development and then decreased by about 40% at the next time point (72 hr). The SO7 secreted into the culture media during in vitro development increased to a relative maximum at 48 hr and then decreased to about 20% of maximum at 72 hr. Immunostaining with anti-rbSO7 indicates that SO7 is highly concentrated in both refractile bodies (RB) of SZ, with some limited distribution in the apical complex. Anti-rbSO7 intensively stained the intracellular parasites and the first-stage schizonts during in vitro development of E. tenella in MDBK cells. Upon release from the schizonts, the first-stage merozoites stained with 1 or 2 bright spots typically at each end. The results suggest that SO7 is closely associated with the SZ RB and is developmentally regulated but may not play a direct role in cellular invasion.

Molecular phylogeny of the other tissue coccidia: Lankesterella and Caryospora

Nearly complete sequences were obtained from the 18S rDNA genes of Eimeria falciformis (the type species of the genus), Caryospora bigenetica, and Lankesterella minima. Two clones of the rDNA gene from C. higenetica varied slightly in primary structure. Parsimony-based and maximum likelihood phylogenetic reconstructions with a number of other apicomplexan taxa support 2 major clades within the Eucoccidiorida, i.e., the isosporoid coccidia (consisting of Toxoplasma, Neospora, Isospora [in part], and Sarcocystis spp.) and a second clade containing Lankesterella and Caryospora spp., as well as the eimeriid coccidia (Cyclospora, Isospora [in part], and Eimeria spp.). Our observations suggest that Caryospora spp. may not belong in the family Eimeriidae but rather may be allied with the family Lankesterellidae with which they share molecular and life history similarities. This may be a third lineage of coccidian parasites that has independently evolved a unique heteroxenous transmission strategy.

Mapping and expression of microneme genes in Eimeria tenella

Microneme organelles are located at the apical tip of invading stages of all apicomplexan parasites and they contain proteins that are critical for parasite adhesion to host cells. In this paper, we have utilised the process of oocyst sporulation in Eimeria tenella to investigate the timing of expression of components of the microneme organelle, at both mRNA and protein levels. Two time-course studies showed that there is a high level of synchrony in the sporulation process, especially during the time period when sporozoites are formed. Western blotting showed that the expression of five microneme proteins (EtMIC1-5) is differentially regulated and highly co-ordinated during sporulation with the proteins being detected only towards the end of the process, as the sporozoites matured within the sporocysts. In contrast, mRNA for all five of these microneme proteins was detected some 10-12 h earlier in sporulation than when the corresponding proteins were seen. Overall these data suggest that the expression of proteins destined for the microneme is regulated both at the transcriptional and translational level. The single copy genes encoding EtMIC1-5 are not clustered on the genome, but are found on four different chromosomes.

Responses of chickens to a recombinant refractile body antigen of Eimeria tenella administered using various immunizing strategies

A refractile body antigen (designated SO7') found in sporozoites of Eimeria tenella was administered to chickens in various immunizing forms to assess its ability to protect against virulent challenge. These included native antigen from the parasite (viable oocysts; per os), recombinant antigen (CheY-SO7'; s.c.), and naked DNA immunization (pcDNA3-SO7'; i.m.). Male White Leghorn chicks were inoculated with oocysts or injected with one of the forms of SO7' antigen at 1 wk of age, boosted at 3 wk of age with the same treatment, and subsequently challenged at 5 wk of age with oocysts of Eimeria tenella USDA Strain 80. Seven days postchallenge, chicks were killed, and weight gains and lesion scores were determined and compared with those of the controls. Significant protection against cecal lesions and weight loss was noted in birds inoculated with oocysts or injected with pcDNA3-SO7' (25 microg). Birds injected with other doses of pcDNA3-SO7' (12.5, 50, 60, and 100 microg) showed no reduction in cecal lesions and maintained poor rates of growth similar to controls. The recombinant antigen (CheY-SO7') was also nonprotective. These results offer promise to the use of DNA immunization strategies for controlling avian coccidiosis and show for the first time naked DNA vaccination with a single coccidial antigen that protects chickens (as measured by reduced lesion scores and high rate of growth) against cecal coccidiosis.

Methods to prepare RNA and to isolate developmentally regulated genes from Eimeria

Coccidians represent a large class of important intracellular parasites that traverse multiple developmental stages that are distinct and required to complete the life cycle. The biochemical details underlying the regulation of transformation from one developmental form to the next are limited and the study of such details presents unique obstacles. However, the genetic program is critical and may provide a basis for understanding the biology of these organisms in addition to the opportunity to suppress development and infection. We provide a basic overview of several strategies, including previously unpublished results, used by this laboratory to isolate stage-specific genes from Eimeria bovis. Additionally, we have included detailed discussions that summarize the associated advantages and disadvantages of each as applied to coccidia and potentially to other parasites in the phylum Apicomplexa. Given that the purification of sufficient quantities of high-quality RNA is vital, we have included detailed protocols for the isolation of RNA from various parasite stages. Also included is a detailed protocol to apply mRNA differential display to investigate stage-specific developmental regulation.

Sporozoites of Toxoplasma gondii lack dense-granule protein GRA3 and form a unique parasitophorous vacuole

The invasion of host cells by sporozoites of Toxoplasma gondii leads to the formation of parasitophorous vacuoles that are distinctly different from those surrounding tachyzoites. In sporozoite-infected cells, the fluid-filled space surrounding the sporozoite is many times larger in volume than the sporozoite, essentially lacks granular or tubular structures, and has no detectable continuous parasitophorous vacuolar membrane when prepared by conventional electron microscopic methods. Consistent with the ultrastructural differences, dense-granule protein GRA3, which associates with the parasitophorous vacuolar membrane of tachyzoites, was not detected by indirect immunofluorescence in sporozoite-infected cells 2-12 h post-inoculation or by Western blot analysis of sporozoite extracts. Western blots incubated with the alpha ROP/DG antiserum, which recognizes tachyzoite rhoptry and dense-granule proteins, revealed numerous other antigenic differences between sporozoites and tachyzoites. Cell cultures inoculated with sporozoites were monitored at various intervals for the expression of GRA3 and the developmentally-regulated tachyzoite surface protein SAG1. Expression of SAG1 and GRA3 was first observed in 30% of the sporozoite-infected cells at 12 and 15 h post-inoculation, respectively, and in all intracellular parasites at 24 h. Parasite replication was only observed in sporozoite-infected cells that were positive for GRA3 and SAG1. Thus, these data indicate that sporozoites and their interaction with host cells differ substantially from tachyzoites and the expression of tachyzoite-specific proteins is likely required for parasite replication.

Identification of Eimeria bovis merozoite cDNAs using differential mRNA display

Differences in gene expression between Eimeria bovis sporozoites and first-generation merozoites were analyzed using the technique of differential mRNA display. Approx. 5% of the sequences detected in first-generation merozoites appear to be unique relative to sporozoites. Several of the bands corresponding to merozoite-specific gene expression were isolated and cloned. Northern blot analysis revealed that the cDNA fragments DMZ-7, DMZ-8 and NMZ-6 hybridized to mRNAs expressed at > 50-fold higher levels in merozoites relative to sporozoites. A fourth cDNA fragment, NMZ-4, hybridized to a mRNA expressed at 3-fold higher levels in merozoites. Further characterization demonstrated that expression of DMZ-8 in E. bovis-infected bovine cells begins as early as 12 h after sporozoite invasion and continues throughout the entire 14 days of first-generation schizogony. Sequence analysis of each of the four merozoite cDNAs failed to identify any significant similarity to any entries in the GenBank database, suggesting that these developmentally regulated genes may be unique to coccidian parasites.