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The malaria parasites
Published in David A Warrell, Herbert M Gilles, Essential Malariology, 2017
Robert E Sinden, Herbert M Gilles
In the normal developing exo-erythrocytic schizont, the cytoplasm of the parasite becomes subdivided and the ensuing invasive stages (merozoites) develop on the expanded cell surface (analogous to sporozoite formation in the oocyst). The emergent merozoites are small spherical/ovoid cells approximately 1 μm in diameter. Consistent with their small size and brief life span, the merozoites are the ‘simplest’ of the invasive stages. The microtubule component of the cytoskeleton is reduced to a very few microtubules (typically around two) attached to the apical rings. The secretory organelles are represented by a paired rhoptry, the constituent proteins of which have been extensively characterized and include apical merozoite antigen 1 (AMA-1, a potential vaccine candidate), a few micronemes, and microspheres – which are released post-invasion and are responsible for changes in the structure of the parasitophorous vacuole. Other organelles are similar to those described for the sporozoite. The plasma membrane surface of these merozoites is dominated by the aggregated peptides of merozoite surface 1 protein (MSP-1).
Malaria
Published in F. Y. Liew, Vaccination Strategies of Tropical Diseases, 2017
Michael J. Lockyer, Anthony A. Holder
The paired rhoptry organelles of merozoites contain a number of proteins synthesized during schizogony.6 Since invasion is associated with the discharge of rhoptry contents onto the erythrocyte membrane, there is interest in the possible use of rhoptry antigens as vaccine components. Immunization with a P. falciparum, 41-kDa rhoptry antigen in FCA partially protected Saimiri monkeys against challenge.52 An affinity-purified, 140-kDa rhoptry antigen partially protected one out of three Aotus monkeys against challenge.47 Immunization of mice with an affinity-purified P. yoelii, 235-kDa rhoptry antigen protected them against challenge.46 The Mab against this protein was also protective on passive transfer, indicating that the protective response was at least partially antibody mediated. Further characterization of rhoptry antigens will be necessary before their potential efficacy in a vaccine can be assessed.
The Ecology of Parasitism
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2015
Eric S. Loker, Bruce V. Hofkin
In the face of such specificity, how are the tachyzoites of T. gondii able to infect such a broad spectrum of host cells? Does the parasite use a ubiquitously expressed host cell receptor, does it insert its own receptor onto the host cell, or does it have multiple mechanisms of cell entry? Although we do not yet have a definitive answer, molecular parasitologists have learned a great deal about host-cell penetration in this species. The parasite engages in an active, multi-step penetration process, with each step increasing the parasite’s grip on the cell. Moreover, we are beginning to zero in on particular genes, such as the rhoptry kinase gene ROP18, the products of which seem to be capable of broadly inactivating intracellular host defenses. We return to this topic in Chapter 10.
Immunogenicity and protection efficacy of enhanced fitness recombinant Salmonella Typhi monovalent and bivalent vaccine strains against acute toxoplasmosis
Published in Pathogens and Global Health, 2021
Fei-Kean Loh, Sheila Nathan, Sek-Chuen Chow, Chee-Mun Fang
In order to improve the overall protective efficacy, different antigen combinations shall be selected. Rhoptry proteins (ROPs) have emerged as one of the most highly explored and immunogenic T. gondii antigen. As reviewed, ROP16 delivered by canine adenovirus type 2 (CAV2) has conferred the highest protection among the T. gondii DNA vaccine candidates in previous five years and is characterized by high levels of proinflammatory cytokines IFN-ɣ and IL-2 [22,50]. Apart from that, the S. Typhi bivalent strains can be potentially developed as a combination toxoplasmosis and typhoid fever vaccine to improve its commercial value. For instance, the two most established licensure combination vaccines – diphtheria, tetanus, and pertussis (DTP) vaccine and measles, mumps, and rubella (MMR) vaccine have formulated multiple antigens as a single product to facilitate fewer immunizations with broader disease coverage [61]. Hence, the selection of T. gondii antigens that are conserved with phylogenetically closely related parasites such as Neospora, may lead to vaccines developed for both parasitic diseases. Preliminary study reported that mice immunized with plasmid-expressed T. gondii MIC3 showed lower parasite burden following T. gondii and N. caninum challenge [62]. The future construction of multivalent S. Typhi strains can also include antigens from closely related intracellular pathogens which require the induction of similar immune responses.
Induction of Th1 type-oriented humoral response through intranasal immunization of mice with SAG1-Toxoplasma gondii polymeric nanospheres
Published in Artificial Cells, Nanomedicine, and Biotechnology, 2018
Huma Naeem, Madiha Sana, Saher Islam, Matiullah Khan, Farooq Riaz, Zunaira Zafar, Haroon Akbar, Wasim Shehzad, Imran Rashid
In recent era, the immunogenic recombinant proteins have been extensively used as a tool for the development of a successful vaccine against toxoplasmosis [53]. Certain antigens such as SAG1, SAG2, ROP2, and GRA2 have been frequently studied for the development of a recombinant protein vaccine. Surface antigen-1 is highly conserved in different strains of Toxoplasma gondii, therefore is best characterized as a recombinant protein candidate for vaccine development [18]. Rhoptry protein-2 is another desirable antigen as it expresses in all the stages of this intracellular protozoan [54]. Likewise, dense granule antigen-2 proves to be highly immunogenic [55] and has been used to induce long-term activation of helper T cells response against toxoplasmosis [56]. However, various studies have revealed that the purified proteins generally appeared to be weak immunogenic and require effective adjuvants for inducing strong immune response [28]. Hence, the development of the most suitable vaccine against T. gondii direly needs the most appropriate adjuvant that must have the tendency to boost the protective immune response in the host [57].
Cerebral toxoplasmosis in HIV-infected patients: a review
Published in Pathogens and Global Health, 2023
Sofiati Dian, Ahmad Rizal Ganiem, Savira Ekawardhani
Diagnostic testing of aptamers-based assay are being developed, including for toxoplasmosis. An Enzyme-linked aptamer assay (ELAA) was developed using two aptamers to detect total antigen from RH strain and recombinant protein of Rhoptry protein 18 (ROP18). One study showed significant association between the ELAA test for human serum samples and severe congenital toxoplasmosis [47]; while another study showed that the aptamer against surface antigen 1 (SAG1) protein of T. gondii used in a new direct enzyme-linked aptamer assay (DELAA) had higher specificity and sensitivity than the ELISA for T. gondii’s circulating agent when used on human sera that had been verified for Toxoplasma DNAs by PCR method.