Patterns and role of diversifying selection in the evolution of Toxoplasma gondii SAG5 locus

A systematic review of Toxoplasma gondii antigens to find the best vaccine candidates for immunization

At present, there is not any available accepted vaccine for prevention of Toxoplasma gondii (T. gondii) in human and animals. We conducted literature search through English (Google Scholar, PubMed, Science Direct, Scopus, EBSCO, ISI Web of Science) scientific paper databases to find the best vaccine candidates against toxoplasmosis among T. gondii antigens. Articles with information on infective stage, pathogenicity, immunogenicity and characterization of antigens were selected. We considered that the ideal and significant vaccines should include different antigens and been expressed in all infective stages of the parasite with a high pathogenicity and immunogenicity. Evaluation within this systematic review indicates that MIC 3, 4, 13, ROP 2, RON 5, GRA 1, 6, 8, 14 are expressed in all three infective stages and have pathogenicity and immunogenicity. MIC 5, ROM 4, GRA 2, 4, 15, ROP 5, 16, 17, 38, RON 4, MIC 1, GRA 10, 12, 16, SAG 3 are expressed in only tachyzoites and bradyzoites stages of T. gondii with pathogenicity/immunogenicity. Some antigens appeared to be expressed in a single stage (tachyzoites) but have high pathogenicity and induce immune response. They include enolase2 (ENO2), SAG 1, SAG5D, HSP 70, ROM 1, ROM 5, AMA 1, ROP 18, RON2 and GRA 24. In conclusion, current vaccination against T. gondii infection is not satisfactory, and with the increasing number of high-risk individuals, the development of an effective and safe specific vaccine is greatly valuable for toxoplasmosis prevention. This systematic review reveals prepare candidates for immunization studies.

SAG5B and SAG5C combined vaccine protects mice against Toxoplasma gondii infection

Infections with the protozoan parasite Toxoplasma gondii, which are common around the world, can lead to congenital infections in humans. T. gondii surface antigen protein 5B (SAG5B) and SAG5C are potential stimulators of humoral and cellular immune responses. In this study, a multi-antigenic DNA vaccine constructed to express T. gondii SAG5B and SAG5C proteins simultaneously was used to immunize BALB/c mice to evaluate the protective efficacy of the vaccine. IgG antibody and gamma interferon (IFN-γ) cytokine production in the pSAG5B/SAG5C DNA vaccine group were significantly higher (0.853±0.103 and 915.2±106.9, respectively) than in the single DNA vaccine groups (pSAG5B, 0.667±0.109 and 598.3±74.9, respectively; pSAG5C, 0.696±0.092 and 623.7±95.5, respectively). After a lethal challenge with 1×10⁴ RH strain tachyzoites, the survival time of the mice (17days) immunized with pSAG5B/SAG5C was longer than that of the single-gene-immunized mice (12days) or the control mice (6days). Moreover, after intragastric infection with 20 T. gondii PRU (low virulence) strain cysts, the number of brain cysts in the pSAG5B/SAG5C-vaccinated mice was only 25% of the number for the PBS-injected mice. Our findings indicate that, in comparison with the other mouse groups, the multi-antigenic DNA vaccine (pSAG5B/SAG5C) significantly induced immune responses and improved the protection against challenge with T. gondii in the host animals.

Identification of microsporidia host-exposed proteins reveals a repertoire of rapidly evolving proteins

Pathogens use a variety of secreted and surface proteins to interact with and manipulate their hosts, but a systematic approach for identifying such proteins has been lacking. To identify these ‘host-exposed' proteins, we used spatially restricted enzymatic tagging followed by mass spectrometry analysis of Caenorhabditis elegans infected with two species of Nematocida microsporidia. We identified 82 microsporidia proteins inside of intestinal cells, including several pathogen proteins in the nucleus. These microsporidia proteins are enriched in targeting signals, are rapidly evolving and belong to large Nematocida-specific gene families. We also find that large, species-specific families are common throughout microsporidia species. Our data suggest that the use of a large number of rapidly evolving species-specific proteins represents a common strategy for microsporidia to interact with their hosts. The unbiased method described here for identifying potential pathogen effectors represents a powerful approach to study a broad range of pathogens.

Unbiased identification of proteins from pathogens that are exposed to a host can provide insight into host–pathogen interaction. Here, the authors use an enzymatic tagging method and mass spectrometry to identify rapidly evolving Nematocida microsporidia proteins when infecting C. elegans.

Epitope analysis, expression and protection of SAG5A vaccine against Toxoplasma gondii

Bioinformatics approaches were used to identify B-cell epitopes and T-cell epitopes on SAG5A protein. Compared to SAG1, SAG5A with good B-cell epitopes and T-cell epitopes had a potentiality to become a more successful vaccine against Toxoplasma gondii. Thereafter, SAG5A DNA vaccine was constructed successfully and was injected into mice with peptide to evaluate the immunoprotection. Compared to the control groups, the vaccine (DNA/peptide) could induce more effective cellular and humoral immune responses in immunized mice. Furthermore, a significant reduction of brain cyst was detected in the mice vaccinated with peptide (732±160), pSAG5A (815±197), or pSAG5A/peptide (436±174) compared by the mice injected by PBS (1260±241) or pEGFP-C1 (1350±268). The number of cysts in brains was 35% reduced in the mice immunized with DNA/peptide than in the control mice treated by PBS. The results indicated that the DNA vaccine encoding SAG5A significantly induced immune responses and enhanced protection against cysts of PRU strain, especially with the help of peptide.

Alpha-galactosylceramide enhances protective immunity induced by DNA vaccine of the SAG5D gene of Toxoplasma gondii

BACKGROUND

Toxoplasmosis caused by the intracellular parasite Toxoplasma gondii (T. gondii) is a global epidemic parasitic disease. DNA vaccines play an important role in preventing the spread of toxoplasmosis. SAG family genes encoding particular surface proteins of T. gondii are the best candidates of DNA vaccine. As a member of SAG family genes, SAG5 gene has been proved to have better antigenic than SAG1. In addition, alpha-Galactosylceramide (α-GalCer) was used to be an adjuvant in malaria vaccine and received positive results. In this study, the effect of the DNA vaccine enhanced by α-GalCer was evaluated by immunizing BALB/c mice.

METHODS

In the present study, SAG5D gene of T. gondii was cloned, sequenced, and biologically characterized. BALB/c mice were randomly divided into five groups, including three experimental groups (pEGFP-C1-SAG5D, α-GalCer and α-GalCer/pEGFP-C1-SAG5D) and two control groups (PBS and pEGFP-C1), and were immunized intramuscularly three times. The levels of IgG antibodies and cytokine productions in mouse sera were determined by enzyme-linked immunosorbent assays (ELISA). Two weeks after the last immunization, all mice were challenged intraperitoneally with 1 × 10(4) tachyzoites of T. gondii and the survival time of mice was recorded.

RESULTS

A significant level of increase of IgG response against the soluble tachyzoite antigens (STAg) was detected by ELISA in experimental group. It revealed relatively high level of IFN-γ production by the spleen cells. There were higher productions of interleukin-4 (IL-4) in α-GalCer treated groups compared to control groups. Challenge experiment showed a longer survival period (11 days compared with 5 days in control) in SAG5D DNA vaccinated mice was found after a lethal challenge with T. gondii RH strain.

CONCLUSIONS

The present study suggested that T. gondii SAG5D was a novel and positive DNA vaccine candidate against toxoplasmosis. In addition, the adjuvant (α-GalCer) enhanced the body's cellular immune response and prolonged the survival time of mice after challenge.