Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

Robust adaptive immune response against Babesia microti infection marked by low parasitemia in a murine model of sickle cell disease

The intraerythrocytic parasite Babesia microti is the number 1 cause of transfusion-transmitted infection and can induce serious, often life-threatening complications in immunocompromised individuals including transfusion-dependent patients with sickle cell disease (SCD). Despite the existence of strong long-lasting immunological protection against a second infection in mouse models, little is known about the cell types or the kinetics of protective adaptive immunity mounted following Babesia infection, especially in infection-prone SCD that are thought to have an impaired immune system. Here, we show, using a mouse B microti infection model, that infected wild-type (WT) mice mount a very strong adaptive immune response, characterized by (1) coordinated induction of a robust germinal center (GC) reaction; (2) development of follicular helper T (TFH) cells that comprise ∼30% of splenic CD4+ T cells at peak expansion by 10 days postinfection; and (3) high levels of effector T-cell cytokines, including interleukin 21 and interferon γ, with an increase in the secretion of antigen (Ag)-specific antibodies (Abs). Strikingly, the Townes SCD mouse model had significantly lower levels of parasitemia. Despite a highly disorganized splenic architecture before infection, these mice elicited a surprisingly robust adaptive immune response (including comparable levels of GC B cells, TFH cells, and effector cytokines as control and sickle trait mice), but higher immunoglobulin G responses against 2 Babesia-specific proteins, which may contain potential immunogenic epitopes. Together, these studies establish the robust emergence of adaptive immunity to Babesia even in immunologically compromised SCD mice. Identification of potentially immunogenic epitopes has implications to identify long-term carriers, and aid Ag-specific vaccine development.

Antibodies to in silico selected GPI-anchored Theileria parva proteins neutralize sporozoite infection in vitro

East Coast fever (ECF) caused by Theileria parva kills cattle in East, Central and Southern Africa leading to significant economic losses. Vaccination is used as a control strategy against ECF and is presently dependent on deliberate infection with live sporozoites and simultaneous treatment with a long-acting oxytetracycline. Although effective, this method has serious limitations; the immunity is parasite strain specific and immunized cattle can become life-long asymptomatic carriers of the parasite, posing risk for the spread of the disease. In efforts to develop a subunit vaccine, the role of antibodies in the neutralization of T. parva sporozoites infection of host cells has been investigated and a circumsporozoite protein, p67, is able to induce such neutralizing antibodies. However, the p67 protein only protects a proportion of immunized cattle against T. parva challenge and such protection might be improved by inclusion of additional parasite antigens that neutralize sporozoite infection. In an attempt to identify such antigens, we searched the re-annotated T. parva genome for genes predicted to contain GPI anchor signals, since they are likely to be located on the cell surface, and expressed fragments of six of the selected genes in E. coli. The recombinant proteins were used to raise antisera in mice. Antisera to two proteins, TpMuguga_01g00876 and TpMuguga_01g00939, neutralized sporozoite infectivity to a high degree, while antisera to two additional proteins, TpMuguga_01g00095 and TpMuguga_04g00437, exhibited moderate neutralizing capacity. We conclude that these four antigens are potential vaccine candidates, which should be evaluated further in cattle.

Characterization of the Theileria parva sporozoite proteome

•

2007 Theileria parva proteins expressed in the sporozoite were identified.

•

Proteins include known T. parva antigens targeted by antibodies and cytotoxic T cells.

•

Proteins predicted to be orthologs of Plasmodium falciparum sporozoite surface molecules were identified.

•

Proteins predicted to be orthologs of P. falciparum invasion organelle proteins were identified.

•

Proteins that may contribute to the phenomenon of bovine lymphocyte transformation were identified.

East Coast fever is a lymphoproliferative disease caused by the tick-borne protozoan parasite Theileria parva. The sporozoite stage of this parasite, harboured and released from the salivary glands of the tick Rhipicephalus appendiculatus during feeding, invades and establishes infection in bovine lymphocytes. Blocking this initial stage of invasion presents a promising vaccine strategy for control of East Coast fever and can in part be achieved by targeting the major sporozoite surface protein p67. To support research on the biology of T. parva and the identification of additional candidate vaccine antigens, we report on the sporozoite proteome as defined by LC–MS/MS analysis. In total, 4780 proteins were identified in an enriched preparation of sporozoites. Of these, 2007 were identified as T. parva proteins, representing close to 50% of the total predicted parasite proteome. The remaining 2773 proteins were derived from the tick vector. The identified sporozoite proteins include a set of known T. parva antigens targeted by antibodies and cytotoxic T cells from cattle that are immune to East Coast fever. We also identified proteins predicted to be orthologs of Plasmodium falciparum sporozoite surface molecules and invasion organelle proteins, and proteins that may contribute to the phenomenon of bovine lymphocyte transformation. Overall, these data establish a protein expression profile of T. parva sporozoites as an important starting point for further study of a parasitic species which has considerable agricultural impact.

Subversion of host cellular functions by the apicomplexan parasites

Rhoptries are club-shaped secretory organelles located at the anterior pole of species belonging to the phylum of Apicomplexa. Parasites of this phylum are responsible for a huge burden of disease in humans and animals and a loss of economic productivity. Members of this elite group of obligate intracellular parasites include Plasmodium spp. that cause malaria and Cryptosporidium spp. that cause diarrhoeal disease. Although rhoptries are almost ubiquitous throughout the phylum, the relevance and role of the proteins contained within the rhoptries varies. Rhoptry contents separate into two intra-organellar compartments, the neck and the bulb. A number of rhoptry neck proteins are conserved between species and are involved in functions such as host cell invasion. The bulb proteins are less well-conserved and probably evolved for a particular lifestyle. In the majority of species studied to date, rhoptry content is involved in formation and maintenance of the parasitophorous vacuole; however some species live free within the host cytoplasm. In this review, we will summarise the knowledge available regarding rhoptry proteins. Specifically, we will discuss the role of the rhoptry kinases that are used by Toxoplasma gondii and other coccidian parasites to subvert the host cellular functions and prevent parasite death.

Identification of a piroplasm protein of Theileria orientalis that binds to bovine erythrocyte band 3

Theileria orientalis infects cattle and causes various disease symptoms, including anaemia and icterus. The erythrocytic stages are responsible for these symptoms but the molecular events involved in these stages have not yet been fully elucidated. In this study, we identified a T. orientalis cDNA that encodes a polypeptide related to identity to the microneme-rhoptry protein of Theileria parva. Analysis of its recombinant product (ToMRP) by indirect fluorescent-antibody test revealed that it is specifically expressed at the early erythrocytic stage after invasion. This expression disappears during the intermediate stages of intra-erythrocytic development. Its expression then reappears at the late stages after the parasite has divided by binary fission into diad or tetrad forms and before these forms are released from the host erythrocyte. In vitro erythrocyte binding assays showed that ToMRP associates with the Triton X-insoluble fraction of erythrocytes membrane but not with intact erythrocytes. Cosedimentation and Western blot analyses revealed that ToMRP binds to band 3, a membrane component of bovine erythrocytes. These observations suggest that ToMRP may be involved in the parasite's egress from and/or invasion into the host erythrocytes by interacting with a protein in the membrane skeleton of the erythrocyte and thereby modifying the structure and function of the cell.

A membrane-anchored Theileria parva cyclophilin with a non-cleaved amino-terminal signal peptide for entry into the endoplasmic reticulum

Recent studies suggest that peptidyl-prolyl isomerases of the cyclophilin family, that access the secretory pathway, can be involved in the interaction of parasitic protozoa with mammalian host cells. The amino acid sequence of a cDNA encoding a cyclophilin family member of the intracellular protozoan parasite of cattle Theileria parva contains a conserved C-terminal domain that exhibits 70% amino acid identity to cyclophilin proteins from other organisms, and a unique 60 amino acid novel N-terminal extension. Cell-free expression of the cDNA revealed a 26kDa amino translation product, indicating expression of the N-terminal domain. The protein-coding region contains three short introns, less than 100 base pairs in length and Northern blot analysis demonstrates expression of a single 0.9 kb transcript in the piroplasm and schizont stages. The transcript is present in high abundance in the intra-lymphocytic schizont stage. The recombinant protein binds to immobilized cyclosporin A, a finding consistent with peptidyl-prolyl cis-trans isomerase function in vivo. A predicted N-terminal signal peptide was functional for entry into the eukaryotic secretory transport pathway in a cell-free in vitro transcription/translation system. The C-terminal cyclophilin domain was translocated across the membrane of the endoplasmic reticulum and the uncleaved signal peptide functioned as a membrane anchor.