Parasites and malignancies, a review, with emphasis on digestive cancer induced by Cryptosporidium parvum (Alveolata: Apicomplexa).

The International Agency for Research on Cancer (IARC) identifies ten infectious agents (viruses, bacteria, parasites) able to induce cancer disease in humans. Among parasites, a carcinogenic role is currently recognized to the digenetic trematodes Schistosoma haematobium, leading to bladder cancer, and to Clonorchis sinensis or Opisthorchis viverrini, which cause cholangiocarcinoma. Furthermore, several reports suspected the potential association of other parasitic infections (due to Protozoan or Metazoan parasites) with the development of neoplastic changes in the host tissues. The present work shortly reviewed available data on the involvement of parasites in neoplastic processes in humans or animals, and especially focused on the carcinogenic power of Cryptosporidium parvum infection. On the whole, infection seems to play a crucial role in the etiology of cancer.

[Identification and molecular characterization of a Toxoplasma gondii microneme]. / Mise en évidence et caractérisation moléculaire d'une adhésine (TgMIC3) des micronèmes de Toxoplasma gondii.

Protozoan of the phylum Apicomplexa are of high medical and veterinary importance, causing diseases such as malaria, toxoplasmosis and cryptosporidiosis. Invasive stages of apicomplexans possess organelles named micronemes, which are involved in the invasion process. We have recently characterized a protein in micronemes of Toxoplasma gondii, TgMIC3, which possess adhesive properties to host cell surface. Immunofluorescence analysis of T. gondii tachyzoite invasion showed that TgMIC3 is exocytosed and re-localised on the surface of the parasite during invasion. By being able to bind both the putative host cells and the parasites, TgMIC3 could be involved in invasion by acting as a bridge between the parasite and the host cell. Gene sequence analysis of TgMIC3 has revealed 5 partially overlapping EGF-like domains and a lectin binding-like domain, which can be involved in protein-protein or protein-carbohydrate interactions respectively. TgMIC3 is a homodimer synthetized with a N-terminal propeptide that is cleaved during trafficking to the organelle, presumably in the trans-Golgi network. The processing involves a serine protease and is required for correct binding function of TgMIC3. The exact role of this propeptide remains unexplained. It may be involved in the targetting of the protein to the micronemes by masking the region involved in interaction with membranes to avoid binding of the protein in the trafficking pathway.

Serine protease inhibitors block invasion of host cells by Toxoplasma gondii.

We investigated the effect of protease inhibitors on the asexual development of the protozoan parasite Toxoplasma gondii. Among the inhibitors tested only two irreversible serine protease inhibitors, 3,4-dichloroisocoumarin and 4-(2-aminoethyl)-benzenesulfonyl fluoride, clearly prevented invasion of the host cells by specifically affecting parasite targets in a dose-dependent manner, with 50% inhibitory concentrations between 1 and 5 and 50 and 100 microM, respectively. Neither compound significantly affected parasite morphology, basic metabolism, or gliding motility within the range of the experimental conditions in which inhibition of invasion was demonstrated. No partial invasion was observed, meaning that inhibition occurred at an early stage of the interaction. These results suggest that at least one serine protease of the parasite is involved in the invasive process of T. gondii.

Apical organelles and host-cell invasion by Apicomplexa.

Host-cell invasion by apicomplexan parasites involves the successive exocytosis of three different secretory organelles; namely micronemes, rhoptries and dense granules. The findings of recent studies have extended the structural homologies of each set of organelles between most members of the phylum and suggest shared functions of each set. Micronemes are apparently used for host-cell recognition, binding, and possibly motility; rhoptries for parasitophorous vacuole formation; and dense granules for remodeling the vacuole into a metabolically active compartment. In addition, gene cloning and sequencing have demonstrated conserved domains, which are likely to serve similar functions in the invasion process. This is especially true for microneme proteins containing thrombospondin-like domains, which are likely to be involved in binding to sulphated glycoconjugates. One such protein was recently shown to be required for the motility of Plasmodium sporozoites. These molecules have been shown to be shed on the parasite and/or cell surfaces during the invasion process in Plasmodium, Toxoplasma and Eimeria. For rhoptries and dense granules, the association between exocytosed proteins and the parasitophorous vacuole membrane had been analyzed extensively in Toxoplasma, as these proteins are likely to play a crucial role in metabolic interactions between the parasites and their host cells. The development of parasite transformation by gene transfection has provided powerful tools to analyze the fate and function(s) of the corresponding proteins.

Immune response and lack of immune response to Plasmodium falciparum P126 antigen and its amino-terminal repeat in malaria-infected humans.

A parasitophorous vacuole protein of Plasmodium falciparum, p126, is a potential candidate for a malaria vaccine. Its N-terminal region, composed of six repeats of eight amino acids, appears to be involved in the induction of protective immunity against P. falciparum challenge in monkeys. This study evaluated the immune response to p126 and to its N-terminal region (Nt47) in patients (n = 45) living in a malaria-endemic area of Brazil (Colina, Porto Velho, Rondonia). Cellular proliferative responses against Nt47 were low and infrequent. The study of the humoral immune response demonstrated that 95% of the patients had detectable anti-p126 antibodies and 77% had anti-Nt47 antibodies. Analysis of the antibody isotypes specific for Nt47 revealed that all four IgG subclasses were present and individuals with higher levels of anti-Nt47 cytophilic IgG antibody (IgG1 + IgG3/IgG2 + IgG4) had significantly lower parasitemia levels, suggesting that antibodies to the N-terminal region of the p126 protein may contribute to acquisition of immunity to P. falciparum malaria.

Immunogenicity of the Plasmodium falciparum serine repeat antigen (p126) expressed by vaccinia virus.

cDNA encoding the serine repeat antigen (SERA) (also called p126) of Plasmodium falciparum has been isolated from the FCR3 strain and inserted into a recombinant vaccinia virus designated vP870. Expression analysis of vP870-infected Vero cells by immunoprecipitation has demonstrated several intracellular forms of SERA and a single secreted SERA peptide. Endoglycosidase digestion of these immunoprecipitated SERA peptides indicated that the intracellular SERA peptides contain simple, high-mannose N-linked oligosaccharides and that the secreted SERA peptide contains complex N-linked oligosaccharides. Pulse-chase experiments indicate that the multiple intracellular SERA peptides in infected Vero cells represent a trafficking pathway whereby the smallest SERA peptide is converted into larger peptides by co- and posttranslational modifications, including glycosylation, and eventually secreted from the cell with complex N-linked oligosaccharides. To study the immunogenicity of vaccinia virus-expressed SERA, rabbits were immunized with vP870 and their sera were analyzed for reactivity with authentic, parasite-derived SERA protein. The anti-vP870 rabbit sera reacted with P. falciparum-infected erythrocytes by immunofluorescence analysis, recognized authentic SERA from schizonts by both immunoprecipitation and Western blot (immunoblot) analyses, and recognized proteolytically processed fragments of SERA secreted into the culture medium by Western blot analysis. These results indicate that when expressed by vaccinia virus, SERA is glycosylated and secreted from infected cells and that in immunized rabbits, vaccinia virus-expressed SERA can stimulate a humoral immune response against SERA derived from blood-stage parasites.

Immunogenicity and antigenicity of the N-term repeat amino acid sequence of the Plasmodium falciparum P126 antigen.

The P126 protein, a parasitophorus vacuole antigen of Plasmodium falciparum has been shown to induce protective immunity in Saimiri and Aotus monkeys. In the present work we investigated its immunogenicity. Our results suggest that the N-term of P126 is poorly immunogenic and the antibody response against the P126 could be under a MHC restricted control in C57BL/6(H-2b) mice, which could be problematic in terms of a use of the P126 in a vaccine program. However, we observed that a synthetic peptide, copying the 6 octapeptide repeat corresponding to the N-term of the P126, induces an antibody response to the native molecule in C57BL/6 non-responder mice. Moreover, the vaccine-P126 recombinant induced antibodies against the N-term of the molecule in rabbits while the unprocessed P126 did not.