GRA7, an excretory 29 kDa Toxoplasma gondii dense granule antigen released by infected host cells.

Monoclonal antibody (mAb) TxE2, reactive with Toxoplasma gondii excretory products, detects an acidic 29 kDa protein (p29) which, in 2D gel electrophoresis, exhibits a migration pattern distinct from those of the toxoplasmic excretory proteins described so far. The sequence of seven peptides from tryptic digestion of isolated p29 allowed the design of primers to obtain the coding DNA sequence. The full-length gene was amplified from genomic DNA of T. gondii strain BK and the sequence was identical with that of the corresponding cDNA, providing evidence for an intron-free gene structure. A single mRNA transcript of 1.3 kb was detected by Northern blot analysis. The deduced 236 amino acid protein contains a putative N-terminal signal peptide, one site of potential N-linked glycosylation, and, close to the C-terminus, a further hydrophobic, putative transmembrane domain. With synthetic peptides spanning the sequence of p29, the epitope for mAb TxE2 was mapped adjacent to the putative signal sequence. The antigen, which represents almost 0.5% of T. gondii protein, is expressed in strains of all three intraspecies subgroups, and is associated with the parasite dense granules as demonstrated by immunoelectron microscopy. In tachyzoite-infected cells, p29 accumulates within the parasitophorous vacuole and co-localizes with its delimiting membrane. In bradyzoite-infected cells, p29 is present within the host cell cytoplasm as detected by immunofluorescence staining, and, furthermore, in the supernatant of cyst-bearing cell culture lacking extracellular parasites as shown by enzyme-linked immunosorbent assay (ELISA). Thus, p29 which is named dense granule protein (GRA)7 may indicate the presence of intracellular toxoplasma.

Towards the Toxoplasma gondii proteome: position of 13 parasite excretory antigens on a standardized map of two-dimensionally separated tachyzoite proteins.

High resolution two-dimensional separation of Toxoplasma gondii tachyzoite lysate revealed up to 224 distinct protein spots in Coomassie-stained gel. Computional matching of 14 digitized gels yielded a standard two-dimensional proteome map. The excretory T. gondii dense granule proteins GRA1-GRA8, S16/acid phosphatase, nucleoside triphosphate hydrolase, and H4 were identified by Western blotting of both total gel and isolated protein spots. In addition, two excretory antigens defined by parasite-specific monoclonal T cells, p36 and p40, were mapped by a novel T-cell blotting technique based on electroeluting single protein spots and testing the eluates for antigenic activity against the T-cell clones. In summary, these results represent a first step in Toxoplasma proteome analysis.

Detection of a novel 40,000 MW excretory Toxoplasma gondii antigen by murine Th1 clone which induces toxoplasmacidal activity when exposed to infected macrophages.

To analyse target molecules of the CD4+ T-cell response to toxoplasma infection, a panel of Toxoplasma gondii-specific murine CD4+ T-cell clones has been established. Clone 3Tx15, belonging to the T helper 1 (Th1) subtype, abolished intracellular parasite growth when co-cultured with macrophages and live toxoplasma at a ratio of 2:2:1. This effect results from macrophage toxoplasmicidal activity induced upon parasite-dependent cellular interaction, an irrelevant Th1 clone failed in this three-party system. Clone 3Tx15 detects its corresponding antigen in the supernatant of infected cells and also reacts with a host cell-free preparation of T. gondii-excreted/secreted antigens. T-cell blot analysis of two-dimensionally separated toxoplasma lysate revealed a molecular weight of about 40,000 for the fractions stimulating clone 3Tx15. As checked in parallel enzyme-linked immunosorbent assay, the 40,000 MW T-cell antigen co-migrates with the excretory protein GRA4, the sole 40,000 MW T. gondii antigen hitherto known to be recognized by T lymphocytes. Nevertheless, neither recombinant GRA4 nor immunoaffinity-purified natural GRA4 was stimulatory for clone 3Tx15. Our findings thus demonstrate that Th1 clone 3Tx15 which induces toxoplasmicidal activity during antigenic interaction with infected macrophages defines a new 40,000 MW excretory T. gondii antigen.