Influence of host immunity on parasite diversity in Theileria parva

East Coast Fever Carrier Status and Theileria parva Breakthrough Strains in Recently ITM Vaccinated and Non-Vaccinated Cattle in Iganga District, Eastern Uganda

East Coast fever (ECF) is a tick-borne disease of cattle that hinders the development of the livestock industry in eastern, central and southern Africa. The 'Muguga cocktail' live vaccine, delivered by an infection and treatment method (ITM), remains the only immunisation strategy of controlling ECF. However, there are challenges of the live vaccine inducing ECF carrier status in immunised animals and the possibility of lack of protection from parasite strains that are antigenically different from the vaccine strains. In Uganda, there are insufficient data regarding the ECF carrier status and T. parva genetic diversity in vaccinated and associated non-vaccinated cattle to assess the effectiveness of ITM vaccination. Blood was collected from recently ECF vaccinated (98) and non-vaccinated (73) cattle from Iganga district in Eastern Uganda at 120 days post-vaccination. The p104 gene nested PCR was used to screen for T. parva DNA, 11 minisatellite and 3 microsatellite markers (SSR) were used for genotyping. Two minisatellite markers (MS7 and MS19) were used to determine whether ECF carrier status was due to the T. parva vaccine or local strains. The prevalence of T. parva based on p104 nPCR was 61.2% (60/98) (RR 2.234, 95% CI 1.49-3.35, p-value < 0.001) among recently vaccinated cattle and 27.4% (20/73) (RR 1.00) among associated non-vaccinated cattle. The Muguga cocktail vaccine strains were responsible for carrier status in 10 (58.8%) by MS7 and 11 (64.7%) by MS19 in vaccinated cattle. Genotypes of T. parva with different-sized alleles to the vaccine strains that could be potential 'breakthroughs' were detected in 2 (11.8%)) and 4 (23.5%) isolates from vaccinated cattle based on MS7 and MS19 minisatellite markers, respectively. Using 14 SSR markers, T. parva diversity was higher in vaccinated (Na = 2.214, Ne = 1.978, He = 0.465) than associated non-vaccinated (Na = 1.071, Ne = 1.048, He = 0.259) cattle. The principal component analysis (PCA) showed isolates from vaccinated cattle were closely related to those from non-vaccinated cattle. The analysis of molecular variance (AMOVA) revealed high genetic variation (96%) within T. parva isolates from vaccinated and non-vaccinated cattle but low variation (4%) between vaccinated and non-vaccinated cattle. This study reveals the role of ITM in inducing the carrier status and higher T. parva genetic diversity in vaccinated cattle. The low genetic variation between T. parva isolates in both vaccinated and non-vaccinated cattle may be suggestive of the protective role of vaccine strains against genetically related local strains in the study area.

Construction of a genetic map for Theileria parva: identification of hotspots of recombination

The tick-borne protozoan parasite Theileria parva is the causal agent of East Coast Fever (ECF), a severe lymphoproliferative disease of cattle in eastern, central and southern Africa. The life cycle of T. parva is predominantly haploid, with a brief diploid stage occurring in the tick vector that involves meiotic recombination. Resolved genetic studies of T. parva are currently constrained by the lack of a genome-wide high-definition genetic map of the parasite. We undertook a genetic cross of two cloned isolates of T. parva to construct such a map from 35 recombinant progeny, using a genome-wide panel of 79 variable number of tandem repeat markers. Progeny were established by in vitro cloning of cattle lymphocytes after infection with sporozoites prepared from Rhipicephalus appendiculatus ticks fed on a calf undergoing a dual infection with the two clonal parental stocks. The genetic map was determined by assigning individual markers to the four chromosome genome, whose physical length is approximately 8309 kilobasepairs (Kb). Segregation analysis of the markers among the progeny revealed a total genetic size of 1683.8 centiMorgans (cM), covering a physical distance of 7737.62 Kb (∼93% of the genome). The average genome-wide recombination rate observed for T. parva was relatively high, at 0.22 cM Kb(-1) per meiotic generation. Recombination hot-spots and cold-spots were identified for each of the chromosomes. A panel of 27 loci encoding determinants previously identified as immunorelevant or likely to be under selection were positioned on the linkage map. We believe this to be the first genetic linkage map for T. parva. This resource, with the availability of the genome sequence of T. parva, will promote improved understanding of the pathogen by facilitating the use of genetic analysis for identification of loci responsible for variable phenotypic traits exhibited by individual parasite stocks.

Genotypic diversity, a survival strategy for the apicomplexan parasite Theileria parva

The tick-borne protozoan parasite Theileria parva causes East Coast fever (ECF), a severe lymphoproliferative disease of cattle that is a major constraint to the improvement of livestock in eastern, central and southern Africa. Studies in cattle experimentally infected with T. parva have shown that the protective cytotoxic T lymphocyte (CTL) response is tightly focused, with individual animals recognizing only one or two dominant antigens, the identity of which varies with MHC class I phenotype. It is well known that cross-protection between T. parva stocks is limited, but precise evaluation of genetic diversity in field populations of the parasite has been hampered by a lack of molecular markers spanning the genome. A recently described panel of satellite markers has provided evidence for substantial genotypic diversity and recombination but does not provide cover for large segments of the genome. To address this deficiency, we undertook to identify additional polymorphic markers covering these regions and we report herein 42 newly identified PCR-RFLP markers distributed across the 4 T. parva chromosomes, as well as 19 new satellite markers for chromosomes 1 and 2. This brings the total number of available polymorphic markers to 141 for the 8.5 Mb genome. We have used these markers to characterise existing parasite stabilates and have also shown that passage of the parasite through naïve cattle and ticks can lead to substantial changes of parasite populations in resulting stabilates. These markers have also been used to show that passage of mixed parasites through an immunised calf results in the removal of the immunising genotype from the parasite population produced by ticks fed on this animal.

Bovine immunity - a driver for diversity in Theileria parasites?

Theileria parva and Theileria annulata are tick-borne parasites of cattle that infect and transform leukocytes, causing severe and often fatal parasitic leukoses. Both species provoke strong immunity against subsequent infection. However, considerable diversity is observed in field populations of each parasite and protection is only assured against homologous challenge. The life cycles of these parasites are complex and involve prolonged exposure to host and vector defence mechanisms. Although the relevant vector mechanisms are poorly defined, protective responses of cattle seem to be tightly focused and variable in their specificity between individuals. This review considers whether bovine immunity acts as a driver for diversity in T. parva and T. annulata and explores other factors that might underlie genetic variation in these parasites.

Expression analysis of the Theileria parva subtelomere-encoded variable secreted protein gene family

Background

The intracellular protozoan parasite Theileria parva transforms bovine lymphocytes inducing uncontrolled proliferation. Proteins released from the parasite are assumed to contribute to phenotypic changes of the host cell and parasite persistence. With 85 members, genes encoding subtelomeric variable secreted proteins (SVSPs) form the largest gene family in T. parva. The majority of SVSPs contain predicted signal peptides, suggesting secretion into the host cell cytoplasm.

Methodology/Principal Findings

We analysed SVSP expression in T. parva-transformed cell lines established in vitro by infection of T or B lymphocytes with cloned T. parva parasites. Microarray and quantitative real-time PCR analysis revealed mRNA expression for a wide range of SVSP genes. The pattern of mRNA expression was largely defined by the parasite genotype and not by host background or cell type, and found to be relatively stable in vitro over a period of two months. Interestingly, immunofluorescence analysis carried out on cell lines established from a cloned parasite showed that expression of a single SVSP encoded by TP03_0882 is limited to only a small percentage of parasites. Epitope-tagged TP03_0882 expressed in mammalian cells was found to translocate into the nucleus, a process that could be attributed to two different nuclear localisation signals.

Conclusions

Our analysis reveals a complex pattern of Theileria SVSP mRNA expression, which depends on the parasite genotype. Whereas in cell lines established from a cloned parasite transcripts can be found corresponding to a wide range of SVSP genes, only a minority of parasites appear to express a particular SVSP protein. The fact that a number of SVSPs contain functional nuclear localisation signals suggests that proteins released from the parasite could contribute to phenotypic changes of the host cell. This initial characterisation will facilitate future studies on the regulation of SVSP gene expression and the potential biological role of these enigmatic proteins.

Live immunization against East Coast fever--current status

The infection-and-treatment method (ITM) for immunization of cattle against East Coast fever has historically been used only on a limited scale because of logistical and policy constraints. Recent large-scale deployment among pastoralists in Tanzania has stimulated demand. Concurrently, a suite of molecular tools, developed from the Theileria parva genome, has enabled improved quality control of the immunizing stabilate and post-immunization monitoring of the efficacy and biological impact of ITM in the field. This article outlines the current status of ITM immunization in the field, with associated developments in the molecular epidemiology of T. parva.

Life history determines genetic structure and evolutionary potential of host-parasite interactions

Measures of population genetic structure and diversity of disease-causing organisms are commonly used to draw inferences regarding their evolutionary history and potential to generate new variation in traits that determine interactions with their hosts. Parasite species exhibit a range of population structures and life-history strategies, including different transmission modes, life-cycle complexity, off-host survival mechanisms and dispersal ability. These are important determinants of the frequency and predictability of interactions with host species. Yet the complex causal relationships between spatial structure, life history and the evolutionary dynamics of parasite populations are not well understood. We demonstrate that a clear picture of the evolutionary potential of parasitic organisms and their demographic and evolutionary histories can only come from understanding the role of life history and spatial structure in influencing population dynamics and epidemiological patterns.