Conformational dependence and conservation of an immunodominant epitope within the babesia equi erythrocyte-stage surface protein equi merozoite antigen 1

Development of an Indirect ELISA to Detect Equine Antibodies to Theileria haneyi

The apicomplexan parasite Theileria haneyi is one of two known causative agents of equine theileriosis. It causes milder clinical disease than its more virulent counterpart, Theileria equi, in experimentally infected horses, and can superinfect T. equi-positive horses. The current equi merozoite antigen 1 (EMA1)-based competitive enzyme-linked immunosorbent assay (ELISA)used in the U.S. to detect equine theileriosis detects T. equi but not T. haneyi, and the complexity of molecular assays precludes widespread use for epidemiologic studies. In order to facilitate urgently needed studies on the prevalence of T. haneyi, the goal of this study was to develop a sensitive and specific serologic assay for the diagnosis of T. haneyi based on the equi merozoite antigen 11 (ThEMA11). To achieve this objective, ThEMA11 was recombinantly expressed in eukaryotic cells and its antigenicity assessed using sera from T. haneyi-experimentally infected horses. Confirmation of sera reactivity enabled design and optimization of an indirect ELISA. Specificity of the ELISA for T. haneyi was assessed using a cohort of sera from horses experimentally infected and confirmed PCR-positive for either T. equi or T. haneyi. Data from field samples further demonstrate that the ThEMA11 ELISA is capable of identifying T. haneyi antibodies in horses from multiple continents around the world.

Comparative genomic analysis and phylogenetic position of Theileria equi

Background

Transmission of arthropod-borne apicomplexan parasites that cause disease and result in death or persistent infection represents a major challenge to global human and animal health. First described in 1901 as Piroplasma equi, this re-emergent apicomplexan parasite was renamed Babesia equi and subsequently Theileria equi, reflecting an uncertain taxonomy. Understanding mechanisms by which apicomplexan parasites evade immune or chemotherapeutic elimination is required for development of effective vaccines or chemotherapeutics. The continued risk of transmission of T. equi from clinically silent, persistently infected equids impedes the goal of returning the U. S. to non-endemic status. Therefore comparative genomic analysis of T. equi was undertaken to: 1) identify genes contributing to immune evasion and persistence in equid hosts, 2) identify genes involved in PBMC infection biology and 3) define the phylogenetic position of T. equi relative to sequenced apicomplexan parasites.

Results

The known immunodominant proteins, EMA1, 2 and 3 were discovered to belong to a ten member gene family with a mean amino acid identity, in pairwise comparisons, of 39%. Importantly, the amino acid diversity of EMAs is distributed throughout the length of the proteins. Eight of the EMA genes were simultaneously transcribed. As the agents that cause bovine theileriosis infect and transform host cell PBMCs, we confirmed that T. equi infects equine PBMCs, however, there is no evidence of host cell transformation. Indeed, a number of genes identified as potential manipulators of the host cell phenotype are absent from the T. equi genome. Comparative genomic analysis of T. equi revealed the phylogenetic positioning relative to seven apicomplexan parasites using deduced amino acid sequences from 150 genes placed it as a sister taxon to Theileria spp.

Conclusions

The EMA family does not fit the paradigm for classical antigenic variation, and we propose a novel model describing the role of the EMA family in persistence. T. equi has lost the putative genes for host cell transformation, or the genes were acquired by T. parva and T. annulata after divergence from T. equi. Our analysis identified 50 genes that will be useful for definitive phylogenetic classification of T. equi and closely related organisms.

Expression of equi merozoite antigen 2 during development of Babesia equi in the midgut and salivary gland of the vector tick Boophilus microplus

Equi merozoite antigens 1 and 2 (EMA-1 and EMA-2) are Babesia equi proteins expressed on the parasite surface during infection in horses and are orthologues of proteins in Theileria spp., which are also tick-transmitted protozoal pathogens. We determined in this study whether EMA-1 and EMA-2 were expressed within the vector tick Boophilus microplus. B. equi transitions through multiple, morphologically distinct stages, including sexual stages, and these transitions culminate in the formation of infectious sporozoites in the tick salivary gland. EMA-2-positive B. equi stages in the midgut lumen and midgut epithelial cells of Boophilus microplus nymphs were identified by reactivity with monoclonal antibody 36/253.21. This monoclonal antibody also recognized B. equi in salivary glands of adult Boophilus microplus. In addition, quantification of B. equi in the mammalian host and vector tick indicated that the duration of tick feeding and parasitemia levels affected the percentage of nymphs that contained morphologically distinct B. equi organisms in the midgut. In contrast, there was no conclusive evidence that B. equi EMA-1 was expressed in either the Boophilus microplus midgut or salivary gland when monoclonal antibody 36/18.57 was used. The expression of B. equi EMA-2 in Boophilus microplus provides a marker for detecting the various development stages and facilitates the identification of novel stage-specific Babesia proteins for testing transmission-blocking immunity.