TOXOPLASMA GONDII ANTIGEN Tg20

Abstract: The present invention relates to isolated and pure Toxoplasma gondii antigenic fragments, recombinant polypeptides, nucleic acids encoding them, primers and probes derived from the same, as well as the use of these polypeptides, nucleic acids, primers and probes in methods and kits for the diagnosis and prevention of T. gondii infection in mammals (humans and animals).

Description: T0X0PLASMA GONDII ANTIGEN Tg20 The present invention relates to isolated and pure Toxoplasma gondii antigenic fragments, recombinant polypeptides, nucleic acids encoding them, primers and probes derived from the same, as well as the use of these polypeptides, nucleic acids, primers and probes in methods and kits for the diagnosis and prevention of T. gondii infection in mammals (humans and animals). Toxoplasma gondii is an ubiquitous intracellular protozoan parasite which infects mammals and birds. Although toxoplasmosis is in general clinically asymptomatic in healthy individuals, it may cause severe complications in pregnant women and immunocompromised patients. If primary infection occurs during pregnancy, transplacental transmission can lead to abortion or neonatal malformations (for reviews see Remington and Krahenbuhl, 1982; Hughes, 1985). In AIDS patients, Toxoplasma is recognized as a major opportunistic pathogen. In such immunodeficient individuals, rupture of cysts which persist in the tissues of the host after a primary' infection and release of proliferative forms of the parasite may cause severe disseminated toxoplasmosis and /or encephalitis. In approximately 30 percent of Toxoplasma-antibodv-positive patients with AIDS, toxoplasmic encephalitis will develop due to reactivation of their latent infection. The fetus and the newborn are very sensitive to toxoplasmosis. Infection of the mother during pregnancy and transmission to the fetus can lead to miscarriage, birth of abnormal children (especially with ocular and cerebral lesions), or birth of apparently normal children who will develop grave sequelae months or years later (blindness, mental retardation). Current estimates indicate that 0.1 to 0.9% of newborns are afflicted with congenital toxoplasmosis. Besides its negative impact on human health, the parasite is also detrimental in sheep and pig farming since abortions resulting from the infection lead to relatively important economic losses (Beverly. 1976). In the absence of efficient profylactic measures, there is a high necessity for early, sensitive and specific diagnosis. In response to infection, immunocompetent hosts mount an immune response, which involves both humoral and cellular components (Remington and Krahenbuhl, 1982). The immune response confers protection against subsequent infection of the host: women showing serological evidence of previous infection before the onset of pregnancy are at no risk of transmitting toxoplasmosis to their fetus. In-vitro and in-vivo studies have indicated that cell- mediated immunity plays an essential role in protection and have identified interferon-gamma (IFN-gamma) as the major mediator of resistance (Frenkel, 1967; Nathan et al. , 1984; Pfefferkorn. 1984; Sethi et al.. 1985: Suzuki and Remington, 1988; Suzuki et al.. 1988; Suzuki and Remington, 1990; Gazinelli et l.. 1991). The humoral component underlies the methods generally used in toxoplasmosis diagnosis. Like in other infectious diseases IgM class antibodies appear before IgG class antibodies. This difference is used to know whether a pregnant woman has an acute or chronic phase infection, only the former being dangerous for fetal transmission. However, in a number of cases IgM class antibodies remain high for a longer than usual time period, making a differential diagnosis difficult. The reference tests in Toxoplasma diagnosis are the Toxoplasma lysis test (TLT) and the immunofluorescence test (IF). The detection of antibodies against Toxoplasma is most often carried out by an Enzyme Linked Immunoassay (ELISA) or tests based on the same principle. The specificity and sensitivity of these tests is not always optimal and depend on the quality of the antigen preparation used. Most often the antigen used is a fraction of a total cell lysate, lacking sufficient specificity. The use of a selection of well characterised Toxoplasma gondii recombinant antigens could furnish better tools for a Toxoplasma diagnostic assay. Several antigens of Toxoplasma have already been cloned and expressed as recombinant antigens. Several excreted/secreted antigens have been shown to be recognized by sera from patients: GRA1 (23 kDa) (Cesbron-Delauw et al,1989; EP-A-0 346 430), GRA2 (28.5 or 28 kDa) (Mercier et al,1993; S.F. Parmley et al, 1993; WO 93/25689), GRA6 (Lecordier et al, 1995). as well as some other cellular antigens, like SAG2 (P22) (Parmley et al, 1992), SAG1 (P30) (Kim et al, 1994), ROP2 (54 kDa) (Van Gelder et al, 1993) and a number of T. gondii antigenic fragments as described e.g. in EP-A-0 431 541 (Behringwerke). A vaccine for controlling this infectious agent would be of great value and the feasibility of its development is suggested by the fact that primary infection with Toxoplasma results in specific and long lasting immunity against reinfection (Remington and Kranenbuhl, 1982). However, no effective and safe vaccine is currently available against toxoplasmosis in humans. A live temperature-sensitive mutant of the highly virulent RH strain, can induce protective immunity in mice and hamsters. This mutant, named ts-4 does not persist in the host, as it cannot form bradyzoites and cysts (Waldeland and Frenkel, 1983; Mc Leod et aL, 1988; Suzuki and Remington.1990). Since 1988, a live vaccine is available for sheep. It consists of T. gondii tachyzoites of the S48 incomplete strain grown on tissue culture (Toxovax, Ministry of Agriculture and Fisheries, New Zealand). By passing through laboratory mice the strain lost the ability to develop bradyzoites (cysts). This vaccine protects naive sheep against an infection with T. gondii (Buxton, 1991 , 1993). The aim of the present invention is to provide new polypeptides and peptides useful in the diagnosis and/or profylaxis of Toxoplasma gondii infection in mammals. It is more paπicularly an aim of the invention to provide polypeptides and peptides useful in the serodiagnosis of T. gondii infection, and possibly enabling discrimination between chronic and acute infection. It is in addition an aim of the present invention to provide polypeptides and peptides useful in a diagnostic assay for T_. gondii infection based on the cellular immune response of the infected host. It is moreover an aim of the present invention to provide for polypeptides and peptides useful in a vaccine preparation against T. gondii infection. It is a specific aim of the present invention to provide purified and isolated Tg20 antigenic fragments. More specifically, it is an aim of the invention to provide recombinant Tg20 antigenic fragments. It is moreover an aim of the invention to provide the amino acid sequence of Tg20 antigenic fragments, and nucleic acid sequences coding for the same. It is also an aim of the present invention to provide for monoclonal and polyclonal antibodies specifically reacting with antigenic fragments of the Tg20 protein. It is also an aim of the present invention to provide for primers specifically amplifying Tg20 nucleic acid sequences, as well as probes specifically hybridizing with Tg20 nucleic acid sequences. It is another aim of the invention to provide for diagnostic methods and/or kits for T. gondii infection, using the above-mentioned Tg20 polypeptides, peptides, antibodies, primers and/or probes as one of the active principles. It is in particular an aim of the present invention to provide for a serodiagnostic method or kit for T. gondii infection, whereby the active principle comprises the Tg20 polypeptides or peptides of the invention in combination with other I. gondii antigens, more paπicularly, in combination with the Tg34 antigen (=Rop2 antigen), or fragments thereof, as described by Van Gelder et al. (1993). It is finally an aim of the present invention to provide for a vaccine composition for providing protective immunity against T. gondii infection in mammals, more paπicularly in humans and/or domesticated animals. All of the above-mentioned aims have been achieved by the following embodiments of the invention. As described in more detail in the examples section, the current invention describes the identification and sequencing of an antigenic fragment of T. gondii. Said antigenic fragment was selected from a λgtl l expression library of 1. gondii (clone Tg20), on the basis of its reactivity with a number of sera from T. gondii infected patients, which were previously shown not to react with other T. gondii antigens, like the Tg34 (Rop2) antigen, or fragments thereof (Van Gelder et al. 1993). As described above, there is a current need to replace crude T. gondii antigen preparations used in actual diagnostic assays, by a selection of well-defined isolated antigenic (poly)peptides. The Tg20 polypeptides and peptides of the current invention are ideal candidates to be included in such "new generation" diagnostic assays, due to their high specificity and sensitivity in detecting T. gondii infected individuals, as described further in the examples section. Moreover, the Tg20 polypeptides have been selected such that they show a complementary reactivity pattern with other T. gondii antigenic fragments, which have already been described to be useful in diagnostics of toxoplasmosis, like the Tg34 antigen (Van Gelder et al. 1993). Tg20 and Tg34 antigenic polypeptides are therefore paπicularly suitable to be used in combination with each other in a diagnostic assay. The invention thus relates to a polypeptide or peptide containing (1) an amino acid sequence extending from amino acid position x to amino acid y in the sequence as shown in Fig. lb (SEQ ID NO 2), with x = 1 and y = 196, and/or x = 1 and y = 160, and/or x = 1 and y = 145, and/or x = 95 and y = 145, and/or (2) any fragment of (1), with said fragment comprising a stretch of at least 7 contiguous amino acids of the sequence as defined in (1), and/ or (3) any equivalents of (1) or (2) originating from the substitution of one or several amino acids in the sequence of (1) or (2), with said polypeptides or peptides containing a reactive epitope important in the humoral and/or cell-mediated immune response of the T. gondii infected host organism. Fragments of the above-mentioned polypeptides should contain at least 7 contiguous amino acids, possibly also a stretch of 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 30, 50 or more contiguous amino acids selected from the amino acid sequence extending from position 1 to position 196 as represented in Fig. lb (SEQ ID NO 2). Polypeptide fragments containing an epitope important in the serological response of an individual infected by T. gondii may be selected from a hydrophylicity plot of the antigen, according to principles known to the person skilled in the an. The hydrophylicity plot of the Tg20 antigen of the invention (SEQ ID NO 2) is represented in Fig. 6. The experimental data in the examples section show that the most immunodominant epitopes of the Tg20 antigen are located in the aminoterminal part of the antigen, i.e. from amino acid position 1 to amino acid position 196, and more paπicularly in the region extending from position 95 to position 145 of the sequence as represented in figure 1. From the hydrophylicity plot of this Tg20 antigen (Fig. 6) it is clear however that additional epitopes may be located in the C-terminal pan of the protein, e.g. in the region extending from amino acid position 197 to amino acid position 235, more paπicularly in the region extending from position 205 to position 220. The term "polypeptide" designates a linear series of amino acids connected one to the other by peptide bonds between the alpha-amino and carboxy groups of adjacent amino acids. Polypeptides with a length of twenty five amino acids or less can also be named "peptides". Polypeptides can show a variety of lengths, either in their natural (uncharged) forms or in forms which are salts, and either free of modifications such as glycosylation, side chain oxidation, or phosphorylation or containing these modifications. It is well understood in the an that amino acid sequences contain acidic and basic groups, and that the paπicuiar ionization state exhibited by the peptide is dependent on the pH of the surrounding medium when the protein is in solution, or that of the medium from which it was obtained if the protein is in solid form. Also included in the definition are proteins modified by additional substituents attached to the amino acids side chains, such as glycosyl units, lipids, or inorganic ions such as phosphates, as well as modifications relating to chemical conversions of the chains, such as oxidation of sulfhydryl groups. It is to be understood that the polypeptides and peptides as described in the current invention can also be extended at one or both of their extremities by so called "linker" sequences, consisting of one or several amino acids (or other molecules, like e.g. biotine), and providing additional physicochemical propenies to the (poly)peptide, which may be paπicularly desirable for certain processes, like e.g. purification, coating and presentation of the (poly)peptide. A special type of "linker" sequence is the addition of a biotin molecule to the (poly)peptides of the invention. The biotin molecule may be incorporated in the polypeptide sequence either during or after polypeptide synthesis, and may be located N-terminal, or C- terminal or internal of the polypeptide sequence. The biotinylation is especially advantageous when the above-mentioned polypeptides are used in serodiagnostic assays: the presence of biotin in the (poly)peptide molecule facilitates its adhesion and presentation on membranes or plates coated with a biotin binding molecule (like e.g. streptavidin). Thus, "polypeptide" or its equivalent terms is intended to include the appropriate amino acid sequence referenced, subject to those of the foregoing modifications which do not destroy its functionality. The polypeptides of the invention, and paπicularly the peptides, can be prepared by classical chemical synthesis. The synthesis can be carried out in homogeneous solution or in solid phase. For instance, the synthesis technique in homogeneous solution which can be used is the one described by Houbenweyl in the book entitled "Methoden der Organischen Chemie" (Methods of Organic Chemistry) edited by E. Wunsh, vol. 15-1 et II. THIEME, Stuttgart 1974. The polypeptides of the invention can also be prepared in solid phase according to the methods described by Athenon and Shepard in their book entitled "Solid phase peptide synthesis" (IRL Press, Oxford, 1989). The polypeptides according to this invention can also be prepared by means of recombinant DNA techniques as described by Maniatis et al. , Molecular Cloning: A Laboratory Manual, New York, Cold Spring Harbor Laboratory, 1982). The term "recombinant polypeptides" refers to a polypeptide produced by genetic engineering, through transcription and translation of a corresponding DNA sequence under the appropiate regulation elements within an efficient cellular host. The polypeptides of the invention can also be prepared by isolation and purification from their naturally occurring environment e.g. by purification from T. gondii lysates or extracts. The techniques to be used for purification of the polypeptides of the invention are known in the current art of protein purification. The term "epitope" or "antigenic determinant" refers to a specific region of an antigenic molecule that binds to an antibody (B-cell epitope) or to a T-cell receptor (T-cell epitope). The term "humoral immune response" refers to the immune response mediated by antibodies, while "cell mediated immune response" is mediated by T-lymphocytes. In the case of T. gondii infection, most diagnostic assays are based upon the measurement of the humoral immune response (e.g. serodiagnosis), while protection towards the disease is basically mediated by the cell-mediated immune response. Therefore, it can be said that B-cell epitopes will be important components of a diagnostic assay, while T-cell epitopes will be indispensable in the composition of a vaccine. As shown in the Examples section, the above-mentioned polypeptides and peptides are selected such that they show a reactivity with sera from T. gondii infected individuals and/or stimulate T. gondii reactive T-cells. The term "equivalents" (or "muteins") as specified above, may be defined as polypeptides and peptides containing substitutions of one or several amino acids, provided that said equivalents have retained the immunogenic/antigenic properties of the original polypeptide. An overview of the amino acid substitutions which could form the basis for such equivalents is shown in Table 1. It should be evident that such equivalents may have a slightly different molecular weight from the original sequence as determined by SDS-PAGE. Said "muteins" may be the result of strain to strain variations of the Toxoplasma gondii Tg20 antigen, or may be the result of modifications introduced in the original polypeptide sequences, said modifications bringing about a desirable side effect to the polypeptide molecule (e.g. better physicochemical properties, more efficient purification, more efficient coating characteristics, more stable etc .). In a more specific embodiment, the invention relates to a polypeptide or peptide containing (1) the arnino acid sequence extending from position 95 to position 145 of the sequence in Fig. lb (SEQ ID NO 2), and/or (2) any fragments of (1), with said fragments comprising a stretch of at least 7 contiguous amino acids of the sequence in (1), and/or (3) at least one of the amino acid sequences represented by GluProAspGluGlnGluGluValHisPheArgLysArgGlyValArgSerAspAlaGlu (SEQ ID NO 3), or ArgLysArgGlyValArgSerAspAlaGluValThrAspAspAsnlleTyrGluGluHis (SEQ ID NO 4), or ValThrAspAspAsnlleTyrGluGluHisThrAspArgLysValValProArgLysSer (SEQ ID NO 5), or ThrAspArgLysValValProArgLysSerGluGlyLysArgSerPheLysAspLeuLeu (SEQ ID NO 6), or GluGlyLysArgSerPheLysAspLeuLeuLysLysLeuAlaLeuPro (SEQ ID NO 7) and/or (4) any equivalents of (1), (2) or (3) originating from the substitution of one or several amino acids in the sequences of (1). (2) or (3), with said polypeptides and peptides containing a reactive epitope important in the humoral and/or cell-mediated immune response of the T. gondii infected host organism. Preferably, the above-mentioned polypeptides and their fragments and/or equivalents are reactive with sera from X. gondii infected individuals. Another embodiment of the invention encompasses a fusion protein consisting of one of the polypeptide sequences as defined above, linked to a heterologous polypeptide sequence. The term "heterologous polypeptide sequence" refers to any polypeptide sequence other than the Tg20 sequence itself, and may originate from T. gondii or from another organism. A fusion protein may have advantageous effects over the non-fusion polypeptides, said effects being e.g. - a more efficient expression level in a recombinant host (e.g. mTNF fusion protein as exemplified further) and/or, - a more easy purification system (e.g. polyhistidine tail as exemplified further) and/or, - a combination of several epitopes, originating from different T. gondii antigens and/or, - a more efficient presentation of the epitopes. Said fusion protein may be made by recombinant DNA technology, whereby different coding regions are operably linked to each other and expressed in a suitable host cell. Fusion proteins may however also be prepared according to other methods, like in vitro coupling techniques, or chemical synthesis. The invention also relates to nucleic acids containing (1) a polynucleic acid sequence encoding any of the polypeptides as defined above, and/or (2) a polynucleic acid sequence extending from position x to position y of the polynucleic acid sequence as represented in fig. lb (SEQ ID NO 1) with x = 1 and y = 589, and/or x = 1 and y = 480, and/or x = 1 and y = 434, and/or x = 287 and y = 434, and/or (3) a polynucleic acid sequence which is degenerate as a result of the genetic code to the polynucleic acid sequence of (1) or (2), and which still encodes any of the polypeptides as defined above, and/or (4) a polynucleic acid sequence which hybridizes with any of the polynucleic acid sequences as defined in (1) to (3). and/or (5) a fragment of any of the polynucleic acid sequences defined in (1) to (4), said fragment containing a stretch of at least 10, and preferably 11 , 12, 13, 14. 15 or more contiguous nucleotides from any of the polynucleic acid sequences as defined in (1) to (3). The term "nucleic acid" or "polynucleic acid" coπesponds to either double-stranded or single-stranded cDNA or genomic DNA or RNA, containing at least 10, 20, 30, 40 or 50 contiguous nucleotides. A nucleic acid which is smaller than 100 nucleotides in length is often also refeπed to as an oligonucleotide. Single stranded polynucleic acid sequences are always represented in the current invention from the 5' end to the 3' end. It is to be understood however that the complementary sequences, and double stranded sequences are of course also encompassed by the formula used for presentation. The term "hybridize to" refers to preferably stringent hybridization conditions, allowing hybridization between sequences showing at least 70%, 80%, 90%, 95% or more homology with each other. The term "degenerate" refers to possible variations in the polynucleic acid sequence encoding the same protein, due to the possible occurrence of different codons for the same amino acid. According to another embodiment, the present invention relates to an oligonucleotide comprising in its sequence at least 10 contiguous nucleotides which form part of any of the polynucleic acid sequences as defined above, for use as a specific hybridization probe for detecting the polynucleic acids of the invention. The term "probe" refers to single stranded sequence-specific oligonucleotides which have a sequence which is sufficiently complementary to hybridize to the target sequence to be detected. Probes according to this aspect of the present invention may be chosen according to any of the techniques known in the art. Preferably, these probes are about 10 to 50 nucleotides long, more preferably from about 15 to 25 nucleotides. It should be understood that the probe sequences of the invention may be slightly modified, e.g. by adding or deleting one or a few nucleotides at the extremities (3 ' or 5') or by substituting some non-essential nucleotides by others (including modified nucleotides and inosine), on condition that these modifications do not alter the specificity of the probes. These modifications may be necessary or desirable e.g. to obtain a higher sensitivity (e.g. because of strain variability of the sequences) or to obtain a sufficient specificity under modified hybridization conditions (buffer, temperature, salt concentration ...). Also, changing the amount (concentration) of probe used may be beneficial to obtain more specific hybridization results. It should be noted in this context, that probes of the same length, regardless of their GC content, will hybridize specifically at approximately the same temperature in TMAC1 solutions (Jacobs et al. , 1988). The latter implies that variant probes contemplated within this aspect of the present invention can be defined as probes hybridizing with the same specificity as the probe they are derived from under stringent hybridization and wash conditions, which may be different or the same as the conditions used for the original probes. The term "complement" refers to a nucleotide sequence which is exactly complementary to an indicated sequence and which is able to hybridize to the indicated sequences. According to another embodiment, the present invention relates to an oligonucleotide, comprising in its sequence at least 10 contiguous nucleotides which form part of any of the polynucleic acid sequences as defined above, for use as a primer for specifically amplifying any of the polynucleic acid sequences of the invention. The term "primer" refers to a single stranded DNA oligonucleotide sequence capable of acting as a point of initiation for synthesis of an extension product which is complementary to the nucleic acid strand to be copied. The length and the sequence of the primer must be such that they allow to prime the synthesis of the extension products. Preferably the primer is about 10-50 nucleotides long, more preferably 10-30 nucleotides long. Specific length and sequence will depend on the complexity of the required DNA or RNA targets, as well as on the conditions of primer use such as temperature and ionic strength. The fact that amplification primers do not have to match exactly with the corresponding template sequence to warrant proper amplification is amply documented in the literature (Kwok et al.. 1990). The amplification method used can be either polymerase chain reaction (PCR; Saiki et al.. 1988), ligase chain reaction (LCR; Landgren et al. , 1988; Wu & Wallace, 1989; Barany, 1991), nucleic acid sequence-based amplification (NASBA; Guatelli et al. , 1990; Compton, 1991). transcription-based amplification system (TAS; Kwoh et al. , 1989), strand displacement amplification (SDA; Duck, 1990; Walker et al. , 1992) or amplification by means of Qβ replicase (Lizardi et al. , 1988: Lomeli et al.. 1989) or any other suitable method to amplify nucleic acid molecules using primer extension. During amplification, the amplified products can be conveniently labelled either using labelled primers or by incorporating labelled nucleotides. Labels may be isotopic ( 32 P. 35 S. etc.) or non-isotopic (biotin, digoxigenin, etc.). The amplification reaction is repeated between 20 and 80 times, advantageously between 30 and 50 times. The oligonucleotides used as primers or probes may also contain or consist of nucleotide analoges such as phosphorothioates (Matsukura et al., 1987), alkylphosphoro-thiates (Miller et al. , 1979) or peptide nucleic acids (Nielsen et al. , 1991; Nielsen et al., 1993) or may contain intercalating agents (Asseline et al. , 1984). As most other variations or modifications introduced into the original DNA sequences of the invention, these variations will necessitate adaptions with respect to the conditions under which the oligonucleotide should be used to obtain the required specificity and sensitivity. However, the eventual results of hybridization will be essentially the same as those obtained with the unmodified oligonucleotides. The introduction of these modifications may be advantageous in order to positively influence characteristics such as hybridization kinetics, reversibility of the hybrid-formation, biological stability of the oligonucleotide molecules, etc. Another embodiment of the invention provides for recombinant nucleic acids comprising any of the above-mentioned nucleic acids for cloning and/or expression purposes. The term "recombinant nucleic acid" refers to a nucleic acid molecule which has been made in vitro by recombinant techniques, and which may contain nucleic acids originating from different sources ligated to each other. In particular, the invention provides for a recombinant vector into which one of the above-mentioned nucleic acids has been inserted for cloning and/or expression purposes. The term "vector" refers to an agent (virus or plasmid) used to transmit genetic material to a cell or organism. A vector can be a "cloning vector" used to carry a fragment of DNA into a recipient cell for the purpose of gene cloning, or an "expression vector" used to carry a DNA sequence into a suitable host cell and to direct the synthesis of the protein encoded by the DNA sequence. In the current invention, the most suitable vectors are plasmids, i.e. small circular DNA molecules that replicate independently of the genome of the host cell. However, other vectors like cosmids, viruses and phages may also be used, depending on the host cell used. The present invention relates in particular to a recombinant vector into which the coding sequence for any of the polypeptides of the invention is operably linked to a control sequence capable of providing the expression of the coding sequence by the specific host. The expression "operably linked to" refers to a juxtaposition where the components are configured so as to perform their usual function. Thus, regulatory sequences operably linked to a coding sequence are capable of directing the expression and eventually secretion of the coding gene. The term "control sequence" refers to those sequences involved in the expression of a gene to protein i.e. its transcription and/or translation and/or regulation and/or maturation. Control sequences may thus comprise a promoter sequence, including possible promoter regulation sequences, a ribosome binding site, a sequence coding for a secretion signal, etc. Another example may be that the coding sequence of the polypeptides of the invention is operably linked to another, heterologous coding sequence, being part of the vector sequence, and resulting in the expression of a fusion protein. Thus, in its broadest wording, the term "control sequence" may also comprise the coding sequence of a heterologous protein to which the polypeptides of the invention are fused. According to yet another embodiment, the present invention relates to a host cell transformed by a recombinant vector comprising any of the polynucleic acid sequences of the invention. Preferably, said host cell is a bacterial host cell, most preferably E. coli, but it may also include eukaryotic cells like yeast, plant, insect or animal cells. The techniques for carrying out expression of the polypeptides of the invention in E. coli are demonstrated further in the Examples section. The techniques for carrying out expression in any of the other host cells are well known in the art of recombinant expression technology. According to a subsequent embodiment, the invention provides for a recombinant polypeptide produced by - growing a culture of a transformed host cell as defined above, under conditions which allow the expression and possibly secretion of the encoded polypeptide. and - recovering the expressed polypeptide from the culture. According to a possible embodiment, said recombinant polypeptide may be a fusion protein, consisting of the polypeptide of the invention fused in frame to a heterologous (poly)peptide. As described above, a fusion protein may bring about possible desired effect to the polypeptides of the invention. Another embodiment of the invention provides for an antibody, monoclonal or polyclonal, reacting specifically with the polypeptides of the invention. Preferably, said antibody is different from the monoclonal antibody BATO 214 which has been described earlier by Saavedra et al. (1990). Antibodies according to this preferred embodiment of the invention include specific polyclonal antisera prepared against the J_. gondii polypeptides of the invention, and showing no cross-reactivity to other I. gondii proteins. It also includes monoclonal antibodies prepared against the T. gondii polypeptides of the invention. The monoclonal antibodies of the invention can be produced by any hybridoma formed according to classical methods known in the art, i.e. by fusion of splenic cells of an animal, particularly of a mouse or rat, infected with T. gondii or immunized against the polypeptides according to the invention defined above on the one hand, and of cells of a myeloma cell line on the other hand, and selected by the ability of the hybridoma to produce monoclonal antibodies recognizing the polypeptides which have been initially used for immunization of the animals. The monoclonal antibodies according to a preferred embodiment of the invention may be humanized versions of the mouse monoclonal antibodies made by means of recombinant DNA technology, departing from the mouse and/or human genomic DNA sequences coding for H and L chains or from cDNA clones coding for H and L chains. Also fragments derived from these monoclonal antibodies such as Fab, F(ab)' 2 and ssFv ("single chain variable fragment"), providing they have retained the original binding properties, form part of the present invention. Such fragments are commonly generated by, for instance, enzymatic digestion of the antibodies with papain, pepsin, or other proteases. It is well known to the person skilled in the art that monoclonal antibodies, or fragments thereof, can be modified for various uses. The antibodies involved in the invention can be labelled by an appropriate label of the enzymatic, fluorescent, or radioactive type. The invention also relates to the use of the proteins of the invention, muteins thereof, or fragments thereof, for the selection of recombinant antibodies by the process of repertoire cloning (Perrson et al. , 1991). According to a preferred embodiment of the present invention, an antibody, or an antigen-binding fragment F(ab') 2 , Fab, single chain Fv and all types of recombinant antibodies, as defined above are further characterized in that they can inhibit the infection by Toxoplasma gondii of the specific cell type which it infects in vivo. According to another embodiment, the present invention relates to a monoclonal antibody as defined above, obtainable by a process comprising at least the following steps: fusing the splenocytes from mice infected with Toxoplasma gondii together with myeloma cells, and - selecting the anti-T. gondii hybridomas by means of ELISA and subsequent limiting dilution, selecting the hybridomas producing a monoclonal antibody, specifically directed against any of the T. gondii polypeptides of the invention by means of ELISA, and, - recovering the monoclonal antibodies from ascites fluid or from a culture of the selected hybridomas. The present invention also relates to a hybridoma producing any of the monoclonal antibodies as defined above. The present invention further relates to an anti-idiotype antibody raised against any of the antibodies as defined above. The term "anti-idiotype antibodies" refers to monoclonal antibodies raised against the antigenic determinants of the variable region of monoclonal antibodies themselves raised against the Toxoplasma Tg20 polypeptides. These antigenic determinants of immunoglobulins are known as idiotypes (sets of idiotopes) and can therefore be considered to be the "fingerprint" of an antibody (for review see de Preval, 1978; Fleishmann and Davie, 1984). The methods for production of monoclonal anti-idiotypic antibodies have been described by Gheuens and McFarlin (1982). Monoclonal anti-idiotypic antibodies have the property of forming an immunological complex with the idiotype of the monoclonal antibody against which they were raised. In this respect the monoclonal antibody is often referred to as Abl , and the anti-idiotypic antibody is referred to as Ab2. These anti-idiotype Ab2s may be used as substitutes for the polypeptides of the invention or as competitors for binding of the polypeptides of the invention to their target. The present invention further relates to antisense peptides derived from the Toxoplasma polypeptides as defined above. More particularly, the term "antisense peptide" is reviewed by Blalock (1990) and by Roubos (1990). In this respect, the molecular recognition theory (Blalock, 1990) states that not only the complementary nucleic acid sequences interact but that, in addition, interacting sites in proteins are composed of complementary amino acid sequences (sense ligand with receptor or sense ligand with antisense peptides). Thus, two peptides derived from complementary nucleic acid sequences in the same reading frame will show a total interchange of their hydrophobic and hydrophilic amino acids when the amino terminus of one is aligned with the carboxy terminus of the other. This inverted hydropathic pattern might allow two such peptides to assume complementary conformations responsible for specific interaction. The antisense peptides can be prepared as described in Ghiso et al. (1990). By means of this technology it -is possible to logically construct a peptide having a physiologically relevant interaction with a known peptide by simple nucleotide sequence analysis for complementarity, and synthesize the peptide complementary to the binding site. The present invention still further relates to a method for in vitro diagnosis of T. gondjj infection comprising at least the step of contacting a sample possibly containing anti-T. gondii antibodies, T. gondii antigens and/or T. gondii nucleic acids, with: - a polypeptide or peptide as defined above, under conditions allowing the formation of an immunological complex, or, a probe as defined above, under conditions allowing the formation of a hybridization complex, with said nucleic acids of said sample being possibly amplified prior to hybridization, using a primer as defined above, or, - an antibody specifically directed against a polypeptide as defmed above, under conditions allowing the formation of an immunological complex, or, an anti-idiotype antibody as defined above, under conditions allowing the formation of an antibody-anti-idiotypic complex, or, an antisense peptide as defined above, under conditions allowing the formation of an antigen-antisense peptide complex, and subsequently detecting the complexes formed. The term "sample" may refer to any biological sample (tissue or fluid) containing T. gondii nucleic acids, antibodies or polypeptides. In a preferential embodiment, the invention relates to a method for detection of anti-J\ gondii antibodies and the preferred sample in that case is serum or plasma. In a more specific embodiment, the invention relates to a method for detecting antibodies to T. gondii present in a biological sample, comprising: contacting the biological sample to be analysed with any of the polypeptide as described above, under conditions allowing the formation of an immunological complex, and - detecting the immunological complex formed between said antibodies and said polypeptide. Conditions allowing the formation of an immunological complex are known to the person skilled in the art. In a special embodiment, the polypeptides being used in the above-described method for detection of anti-T. gondii antibodies, can be replaced by anti-idiotype antibodies as described above, acting as their equivalents. Conditions allowing the formation of an antibody-anti-idiotypic complex are known in the art. The invention further relates to a method for detecting the presence of T. gondii antigens in a biological sample, comprising: contacting the biological sample to be analysed with an antibody according to the invention, under conditions allowing the formation of an immunological complex, and detecting the immunological complex formed between said antigens and said antibody. In a special embodiment, the antibodies being used in the above-described method for detection of 1. gondii antigens, may be replaced by anti-sense peptides as described above, acting as their equivalents. Conditions allowing the formation of an antigen-antisense peptide complex are known in the art. Design of immunoassays is subject to a great deal of variation, and many formats are known in the art. Protocols may, for example, use solid supports, or immunoprecipitation. Most assays involve the use of labeled antibody or polypeptide; the labels may be, for example, enzymatic, fluorescent, chemiluminescent, radioactive, or dye molecules. Assays which amplify the signals from the immune complex are also known; examples of which are assays which utilize biotin and avidin or streptavidin, and enzyme-labeled and mediated immunoassays, such as ELISA assays. An advantageous embodiment provides for a method for detection of anti-T . . gondii antibodies in a sample, whereby the (poly)peptides of the invention are immobilized on a solid support, eventually on a membrane strip. Different (poly)peptides of the invention may be immobilized together or next to each other on distinct locations (e.g. in the form of parallel lines). The polypeptides of the invention may also be combined in the same assay with other antigens from Toxoplasma gondii or from other organisms. The invention thus also relates to a solid support onto which the (poly)peptides of the invention, possibly in combination with other (poly)peptides, have been immobilized. Another embodiment of the invention provides for a method for detecting the presence of T. gondii polynucleic acids present in a biological sample, comprising: possibly extracting the polynucleic acids contained in the sample, amplifying the T. gondii polynucleic acids with at least one primer as described above, - detecting the amplified nucleic acids, possibly after hybridization with a probe as described above. Conditions allowing hybridization are known in the an and e.g. exemplified in Maniatis et al. (1982). However, according to the hybridization solution (SSC, SSPE, etc.), the probes used should be hybridized at their appropriate temperature in order to attain sufficient specificity (in some cases differences at the level of one nucleotide mutation are to be discriminated). Amplification of nucleic acids present in a sample prior to detection in vitro may be accomplished by first extracting the nucleic acids present in the sample according to any of the techniques known in the art. In case of extraction of RNA. generation of cDNA is necessary; otherwise cDNA or genomic DNA is extracted. The amplification methods are detailed above. Suitable assay methods for purposes of the present invention to detect hybrids formed between oligonucleotide probes according to the invention and the nucleic acid sequences in a sample may comprise any of the assay formats kown in the an. For example, the detection can be accomplished using a dot blot format, the unlabelled amplified sample being bound to a membrane, the membrane being incubated with at least one labelled probe under suitable hybridization and wash conditions, and the presence of bound probe being monitored. Probes can be labelled with radioisotopes or with labels allowing chromogenic or chemilumeniscent detection such as horse-radish peroxidase coupled probes. An alternative is a "reverse" dot-blot format, in which the amplified sequence contains a label. In this format, the unlabelled oligonucleotide probes are bound to a solid suppon and exposed to the labelled sample under appropriate stringent hybridization and subsequent washing conditions. It is to be understood that also any other assay method which relies on the formation of a hybrid between the nucleic acids of the sample and the oligonucleotide probes according to the present invention may be used. According to an advantageous embodiment, the process of detecting T. gondii nucleic acid sequences contained in a biological sample comprises the steps of contacting amplified fragments of the polynucleic acids of the invention, with a solid support onto which probes as defined above, have been previously immobilized. In a very specific embodiment, the probes have been immobilized on a membrane strip in the form of parallel lines. This type of reverse hybridization method is specified further as a Line Probe Assay (LiPA). The invention thus also relates to a solid support onto which the polynucleotides of the invention have been immobilized. According to another embodiment, the present invention relates to a method using at least one of the polypeptides or peptides of the invention, possibly in combination with other J. gondii polypeptides, for measuring a cellular immune response in an individual which has been in contact with the T. gondii pathogen, said cellular immune response being measured either in vivo, such as a delayed type hypersensitivity reaction upon subcutaneous injection of the polypeptides of the invention, or in vitro, such as stimulation of periferal blood lymphocytes or secretion of interferon-gamma, upon addition of the polypeptides of the invention to a sample of periferal blood lymphocytes under conditions allowing recognition of the polypeptides by the cells responsive for the immune response, conditions which are known to the person skilled in the art. The present invention also relates to a kit for detecting anti-T. gondii antibodies, or T. gondii nucleic acids or T. gondii antigens according to any of the methods as defined here above, comprising at least one of the T. gondii polypeptides or peptides as defined above, or any of the I. gondii nucleic acids as defined above, more specifically any of the probes or primers as defined above, or any of the antibodies as defined above, or any of the anti- idiotypic antibodies as defined above, or any of the antisense-peptides as defined above. According to this embodiment, the detection of antibodies against the T. gondii polypeptides of the invention is preferred over the detection of nucleic acids or polypeptides. The present invention relates more particularly to a kit for determining the presence of anti-T. gondii polypeptide antibodies as defined above (= a serodiagnostic kit/assay) in a biological sample, comprising: at least one polypeptide or peptide as defined above, possibly in combination with other polypeptides or peptides from T. gondii. with said polypeptides being preferentially immobilized on a solid substrate, a buffer, or components necessary for producing the buffer, enabling a binding reaction between the polypeptides of the invention and the antibodies possibly present in the biological sample, - means for detecting the immune complexes formed in the preceding binding reaction, possibly also including an automated scanning and interpretation device for inferring the presence of T. gondii antibodies in the sample. The kit or method according to this aspect of the present invention may comprise in addition to peptide or polypeptide antigens according to the invention, also other T. gondii antigenic proteins or peptides known in the art (such as outer membrane protein (OMP) proteins or peptides), or other bacterial antigenic proteins or peptides in general. The combination of different antigens in one single detection method or kit as described above, may have certain advantages, such as for example: - achieving a higher test sensitivity: e.g. by combining several antigenic determinants from T. gondii. the number of correctly identified positive sera may be greater, and/or - enabling differentiation between chronic and acute T. gondii infection. In a preferential embodiment, the invention relates to a serodiagnostic method and/or kit as described above, whereby the polypeptides and peptides of the invention are combined with the Tg34 antigen of T. gondii. or fragments thereof, as described by van Gelder et al. 1993. Another particular case of combination, also in included in the current invention, is the combination of the polypeptides of the current invention with antigenic fragments located in the C-terminal part of the Tg20-protein, i.e. sequences located in the region from amino acid position 197 to amino acid position 235 of the sequence represented in Fig. lb (SEQ ID NO 2). As described further in the examples section, the most immunodominant epitopes of the Tg20 protein reside in the region extending from amino acid position 1 to amino acid position 197, more particularly in the region extending from position 95 to position 145 as represented in Fig. lb. However, it may be necessary, in certain instances, to combine the polypeptides of the invention with (poly)peptide fragments originating from the less antigenic C-terminal part of Tg20. Said combination of antigens in one assay may be accomplished in different ways, depending on the application or test format. For example, antigenic fragments of several polypeptides (e.g. Tg20 of the invention and Tg34) may be immobilized together on a solid surface (microtiter plate or membrane...), in the same location or in distinct locations. A prefeπed example of the latter test format is a Line Immuno Assay (LIA, INNOGENETICS) where different antigenic fragments are immobilized as parallel lines on a membrane strip, thus allowing to monitor the differential reactivities with the different antigenic fragments. Alternatively, the combination of the polypeptides of the invention with other T. gondii antigenic determinants, may be made on the molecule level, i.e. by the creation of a "superantigen" for the detection of T. gondii infection, combining different antigenic epitopes in one molecule. Said "superantigen" may be produced by recombinant DNA techniques, known in the art, e.g. by linking operably together the different DNA sequences encoding the different antigenic sites and expressing them in a suitable host. It is to be understood that the above-described combination is not exclusively to be used in serodiagnostic assays, but may also prove to be useful in other types of diagnostic assays, and/or in vaccine compositions. In a very specific embodiment the invention relates to a kit for the detection of anti-T. gondii antibodies in a biological sample as described above, whereby the polypeptides of the invention are replaced by the anti-idiotype antibodies as described above. The invention further relates to a diagnostic kit for the detection of antigens of T. gondii present in a biological sample, said kit comprising an antibody as described above, with said antibody being preferably bound to a solid support. In a very specific embodiment, the invention relates to a diagnostic kit for the detection of antigens of T. gondii present in a biological sample, whereby the antibody as described above is replaced by an antisense peptide. The invention further also relates to a diagnostic kit for the detection of T_. gondii polynucleic acids present in a sample, said kit comprising a probe as described above and/or a primer as described above. According to a preferred embodiment, the present invention relates to a kit or method for diagnosis of T. gondii infection as defined above, further characterized in that said polypeptides, peptides, polynucleic acids, antibodies, anti-idiotypic antibodies or anti-sense peptides are particularly useful for differentiating in vitro T. gondii chronically infected individuals from acutely infected individuals. According to another embodiment, the present invention relates to a vaccine composition which provides protective immunity against T. gondii infection comprising as an active principle one or more of the T. gondii polypeptides according to the invention, or one or more of the polynucleic acid sequences or recombinant vectors according to the invention, said active principle being combined with a pharmaceutically acceptable carrier. According to a special embodiment, the vaccine composition as described above may comprise as an active principle one of the anti-idiotype antibodies as described above. Besides the T. gondii proteins according to the invention, said vaccine composition may also comprise any other Toxoplasma immunogenic components (such as Rop2 (Tg34), Gral , Gra2, Sagl . Sag2 antigens e.o.) or any other bacterial or other immunogenic components in general. In a specific embodiment, polynucleic acid sequences coding for any of the polypeptides as defined above, are used as a vaccine, either as naked DNA or as part of recombinant vectors. In this case, it is the aim that said nucleic acids which are administered to the individual to be immunized, are expressed into immunogenic protein / peptide in situ and thus confer protection to the vaccinated host (e.g. Ulmer et al. , 1993). The active ingredients of such a vaccine composition may be administered orally, subcutaneously. conjunctivally, intramuscularly, intranasally, or via any other route known in the art including for instance via the binding to earners, via incorporation into liposomes, by adding adjuvants known in the art. etc. The invention also relates to any of the above-mentioned substances (polypeptides, antibodies, polynucleic acids, anti-idiotype antibodies, antisense peptides) for use as a medicament, more particularly for any of the medical (diagnostic or prophylactic) applications as mentioned above. Furthermore, the invention relates to the use of any of the above-mentioned substances (polypeptides, antibodies, polynucleic acids, anti-idiotype antibodies, antisense peptides) for the manufacture of a medicament, more particularly for the preparation af a vaccine or for the preparation of a diagnostic composition. Figure legends Fig. 1 la. Restriction map of the 1260 bp EcoRI fragment of clone Tg20 and position of the different deletion clones analyzed for immune reactivity. lb. Complete nucleic acid (SEQ ID NO 1) and amino acid (SEQ ID NO 2) sequence of the 1260 bp EcoRI fragment of clone Tg20. Relevant restriction sites are indicated. Arrows indicate possible signal peptide cleavage sites. Fig. 2 Map of vector pmTNF.MPH Fig. 3 Map of vector pIGFHl 11 Fig. 4 Amino acid sequence of mTNF.H6.Tg20 fusion protein. Normal characters represent the amino acid sequence originating from the vector. Bold characters correspond to the Tg20 antigen sequence. Fig. 5 Reactivity in Western Blot with Mab BATO 214 of the recombinant Tg20 antigen and its naturally occurring counterpart. Lane 1 : 1. gondii lysate Lane 2: MW markers (Biolabs) with increasing sizes of 6.5, 16.5, 25.0, 32.5, 47.5, 62.0, 83.0, 175.0 kDa. Lane 3: purified recombinant Tg20 fusion protein. Fig. 6 Hydrophylicity plot of the Tg20 antigen (SEQ ID NO 2) Table legends Table 1: Amino acid substitutions which may form the basis for the muteins (equivalents) according to the present invention. Table 2: Reactivity (OD450 values x 1000) of different (human) sera in the classical tests used for Toxoplasmosis diagnosis (IF (IgG and IgM) and Biomerieux ELISA test), and in ELISA using Tg20 antigen of the invention, or a fragment thereof (Tg20BN), in ELISA using the Tg34AR antigen, or combinations of the foregoing antigens. Sera positive in classical testing are numbered from 1 to 95. Sera negative in classical testing are numbered from 206 to 250. Table 3: Comparison of reactivity of some of the sera from table 2 with the full size Tg20 antigen and the Tg20BN fragment. 7300 25 TABLE 1 Amino acids Synonymous groups Ser (S) Ser, Thr, Gly, Asn Arg (R) Arg, His, Lys, Glu, Gin Leu (L) Leu, He, Met, Phe, Val, Tyr Pro (P) Pro, Ala, Thr, Gly Thr (T) Thr, Pro, Ser, Ala, Gly, His, Gin Ala (A) Ala, Pro, Gly, Thr Val (V) Val, Met, He, Tyr, Phe. Leu, Val Gly (G) Gly, Ala, Thr, Pro, Ser He (I) He, Met, Leu, Phe, Val, Tyr Phe (F) Phe. Met, Tyr, lie. Leu, Trp, Val Tyr (Y) Tyr. Phe, Trp, Met, He, Val, Leu Cys (C) Cys. Ser, Thr, Met His (H) His, Gin, Arg, Lys, Glu, Thr Gin (Q) Gin. Glu, His, Lys, Asn, Thr, Arg Asn (N) Asn, Asp, Ser, Gin Lys (K) Lys. Arg, Glu, Gin, His Asp (D) Asp. Asn, Glu, Gin Glu (E) Glu, Gin, Asp, Lys, Asn, His, Arg Met (M) Met. He, Leu, Phe, Val Tg34AR Tg20 Tg34AR Tg34AR Table 2 Serum IF IgG IF IgM BioMer Tg20 Tg20BN Nr trtre titre IgG 1 1/256 1/50 1219 378 524 849 795 2 1/64 <1/50 616 702 518 1105 1101 3 1/64 <1/50 10B5 197 147 286 251 4 1/64 <1/50 337 23 33 107 73 5 1/256 1/50 716 430 119 586 674 6 1/64 <1/50 598 874 99 664 845 7 1/64 <1/50 988 487 103 653 B71 8 1/64 < 1/50 660 183 54 433 381 9 1/1024 < 1/50 809 1473 429 1608 1821 10 1/256 <1/50 952 507 257 685 712 117 252 301 11 1/256 <1/50 1014 201 12 1/64 <1/50 1014 678 344 1048 1074 13 1/256 <1/50 1102 313 375 474 603 14 1/64 876 652 1010 1443 1304 15 1/256 1093 260 983 1049 564 16 1/64 956 213 183 435 439 17 1/64 < 1/50 262 85 136 195 192 18 1/1024 <1/50 1153 1058 1413 1904 1856 19 1/64 < 1/50 510 142 8 533 401 1124 20 1/256 <1/50 1381 678 669 943 21 1/64 <1/50 768 108 249 493 340 22 1/1024 1/50 1361 692 748 1401 1196 23 1/256 <1/50 1 34 861 374 1079 1152 24 1/1024 < 1/50 1262 506 283 812 916 25 1/1024 1/100 1295 1848 723 2030 2096 26 1/256 <1/50 1137 303 357 849 593 27 1/1024 1495 526 649 1344 1087 21C5 1925 28 1/1024 < 1/50 1593 882 2346 274 1430 1377 1199 29 1/256 1415 30 1/1024 <1/50 1391 593 1248 1305 1089 31 1/256 1221 283 1589 1527 1328 32 1/64 <1/50 173 34 127 213 137 33 1/256 <1/50 530 42 58 137 141 34 1/256 <1/50 498 29 342 406 303 35 1/255 <1/50 1273 444 336 921 578 36 1/256 <1/50 673 298 200 384 435 37 1/256 <1/50 1074 748 430 1193 1205 38 1/256 1361 475 1154 1397 1106 39 1/64 <1/50 157 31 48 87 62 40 1/64 <1/50 591 1360 1410 869 428 41 1/64 <1/50 663 175 176 325 378 42 1/256 < 1/50 1388 595 1640 1711 1624 43 1/64 <1/50 697 139 127 369 279 44 1/256 1/200 1323 293 1226 1442 1199 45 1/64 < 1/50 237 42 32 49 37 46 1/256 < 1/50 789 226 39 426 472 504 547 47 1/64 387 115 442 4B 1/1024 <1/50 1224 1244 1115 1765 1940 49 1/1024 <1/50 648 268 366 563 448 50 1/256 <1/50 646 256 492 697 611 51 1/1024 <1/50 1351 348 845 1102 636 52 1/256 <1/50 589 489 280 480 492 53 1/256 <1/50 355 25 75 118 98 54 1/1024 <1/50 1186 1727 1199 1874 1891 55 1/256 < 1/50 1180 736 1840 1836 1723 56 1/1024 1/50 1367 1542 2515 2114 1929 57 1/256 1/50 569 164 1221 972 400 58 1/1024 832 366 421 585 486 Table 2 (continued 2) Table 3 Table 3 (continued) Tg20 Tg20BN EXA PLES Example 1: Materials and Methods Reagents. All reagents were of analytical grade and obtained from Merck (Darmstadt, Germany), Sigma (St. Louis,Mo.) or Bio-Rad Laboratories (Richmond, Calif.). Restriction enzymes and DNA modifying enzymes were purchased from Boehringer Mannheim (Brussels, Belgium) and were used according to the manufacturer's instructions. Protein concentrations were determined by the bicinchoninic acid method (Pierce, Rockford, II.) Monoclonal antibodies. Monoclonal antibody BATO 214 was prepared as described in Saavedra et al. (1990). Anti- TNF monoclonal was produced from hybridoma culture supernatant. Human sera. The serum samples used in this study were referred to the clinical biology laboratory for routine screening or diagnosis of toxoplasmosis. These samples were tested with ϊmmuno fluorescence (IF) for IgG and IgM (BioMerieux, Brussels). The positive sera were given numbers from 1 to 95 and the negative sera from 206 to 250. Those sera were tested again with the Toxo-IgG Micro EIA (ELISA) of BioMerieux. Two negative sera showing discrepant results with the IFA test were eliminated from the panel. The reactivities obtained with the panel of sera in the classical reference tests of Biomerieux are represented in table 2. Parasites and lysate. RH and Wiktor strains of Toxoplasma gondii were grown as described by Saavedra et al (1991). Construction of c-DNA library in λgtll, screening and lysogen preparation. Described in Saavedra et al (1991). Gel electrophoresis and western blotting. The total £. c__h extracts or Toxoplasma lysate were analyzed by SDS-PAGE (12.5 %) in the presence of β-mercaptoethanol as described by Laemmli (1970). Eventually, proteins were transferred to nitrocellulose membranes by the wet western blotting technique (Towbin et al ..1979) in carbonate buffer (10 mM NaHCO 3 , 3mM Na 2 CO 3 , 20% (v/v) methanol). The membrane was saturated with 5% fat free milk in TNT (lOmM Tris, 150mMNaCl, O,O5% (v/v)Tween20) for lh, followed by two washes in TNT. The membranes were incubated with monoclonals or sera appropiately diluted in TNT containing 1 %BSA for 90 min. Before use, sera were preabsorbed on ice for 30 min using 10% E.coli lysate in the dilution buffer (TNT+ 1 %BSA). After three washes with TNT, the bands were revealed with rabbit anti-mouse IgG conjugate (Dako, Denmark) or rabbit anti- human IgG conjugate (Dako, Denmark) AP labelled. Conjugates were diluted 1/2000. The AP activity was revealed by using the chromogenic substrate nitroblue tetrazolium-5-bromo-4- chloro-3-indolyl-phosphate in 50mM TrisHCl (pH 9.5), 150mM NaCl, 5mM MgCl 2 buffer (Blake et al. 1984). ELISA with recombinant antigens. ELISA plates (Immuno Plate Maxisorp F96; Nunc, Roskilde, Denmark) were coated by incubation at 37°C for lh with a recombinant antigen solution (lOOμl/well) in carbonate buffer 0.1M pH 9.5 for antigen Tg20 (2μg/ml) or in glycine-HCL 0.2M pH 4 for antigen Tg34AR (6μg/ml). Blocking of the solid phase was carried out by incubation for lh at 37° with phosphate buffered saline (PBS) containing 0.1 % casein (300μl/well). After three washes with PBS containing 0.05% Tween 20 (washing buffer), human sera were added at a 1/100 dilution in sample diluent (PBS 4- 0.1 % casein + 0.01 % Triton X-705 + 1 % E. coli lysate (v/v)). Incubation was done for 1.5h at 37°. The wells were then washed four times and incubated with horseradish peroxidase-labelled goat anti-human IgG (Fc-fragment) (Bethesda Research Laboratories, Gaitersburg, Md.) at a 1/5000 dilution in PBS +0.1 % casein, for lh at 37° . After four washes, the peroxidase activity was detected with H 2 O 2 and 3.3 ' ,5,5'-tetramethyl-benzidine. The reaction was stopped after 30 min by adding lOOμl of IN H SO 4 and the O.D. was read at 450nm. Example 2: Identification and sequencing of clone Tg20 Antigen Tg34AR, a C-terminal fragment of Tg34 (=ROP2), detects 89% of Toxoplasma positive sera in ELISA as described by Van Gelder et al. 1993. Antigen Tg34AR has been retested with the serum samples, described in Example 1, resulting in a test sensitivity of 77% (cut off value = mean of O.D. of negative sera -I- 3 standard deviations (sd), serum dilution 1/100). Sera that were missed out by the Tg34AR ELISA, or that were only borderline positive, were used to screen a number of lambda gtl l lysogens which had previously been described to be moderately reactive with a pool of human and/or murine anti-T. gondii antibodies in WB (Saavedra et al.(199D). This strategy leads to the identification of clones which express an antigen "complementary" to Tg34AR, i.e. which reacts with most of the sera which cannot be detected by the Tg34AR antigen. The following λgtl l clones, belonging to different hybridisation groups, were chosen for screening: Tg6, Tgl3, Tgl8, Tgl9, Tg20, Tg27, Tg34 and Tg46. The lysogens of the clones were induced, producing β-galactosidase fusion proteins, and subsequently analyzed on Western Blot with the above-selected sera. Clones Tgl3, Tgl9 and Tg27 were probed with only three sera (#4, 21 , 33). They showed only a very weak reactivity and were therefore not analysed further. Clones Tg6, Tgl8, Tg20, Tg46, Tg47 and Tg34 were probed with sera # 4, 21 , 33, 17, 39, 53, 68, 45. 63, 79, 83 and 87. Serum #18 was used as a positive control (high titre in ELISA Tg34AR). Clone Tg34 is the previously characterised clone (=Rop2), of which Tg34AR is a fragment. Clone Tg47 produces β galactosidase only, and was used as a negative control. Clone Tg20 reacted with 10/13 sera, clone Tg6 reacted with 8/13 sera, clone Tg46 reacted with 3/13 sera. Sera #17,#45 and #63 did not react with any of the clones. From these results, clone Tg20 was chosen for further characterization and sequencing. The EcoRI fragment of 1260 bp contained in clone Tg20 was transferred from λgtl l to pBluescript KS + , for restriction analysis and sequencing. To facilitate sequencing, one subclone containing the 5 ' EcoRI-Hindlll fragment of 399 bp was ligated into pBluescript KS-I- as well. Primers were designed to elucidate the complete sequence of the clone. Every part of the sequence was at least sequenced on both strands. The sequence of the full EcoRI- insert (1260 bp) is shown in fig. 1 (SEQ ID NO 1). An open reading frame of 705 bp (1 - 705) was identified, coding for a protein with a theoretical calculated molecular weight of 25,695 kDa, the sequence of which is represented in fig. 1 (SEQ ID NO 2). Example 3: Expression in E. coli of recombinant Tg20 antigen 1.Construction of recombinant vector The 1260 bp EςoRI-fragment of clone Tg20 was transferred to vector pmTNFMPH (Irmogenetics) for expression. This vector (see fig. 2) enables expression of recombinant proteins in E. coli as fusion proteins with a short mouse tumor necrosis factor (mTNF) peptide. Moreover, the vector also confers a polyhistidine sequence of six consecutive histidine residues to the fusion protein, allowing fast and efficient purification using immobilized metal affinity chromatography (IMAC). The vector pmTNFMPH was digested with Apal and blunted with T4 DNA polymerase. The Tg20 1260 bp-fragment was recovered from pBluescriptKS +Tg20 by EcoRI digestion and blunting with T4 DNA polymerase. Ligation of both fragments with T4 DNA ligase confers an in frame fusion between the ORF of Tg20 and the mTNF-his6 leader peptide. This fusion protein contains 37 aa provided by the leader peptide and 235 aa encoded by the Toxoplasma gene fragment. The amino acid sequence of the fusion protein is shown in fig. 4 (SEQ ID NO 8). 2. E. coli expression of recombinant Tg20 antigen The transcription of heterologous genes cloned in the expression vector pmTNFMPH is initiated by the early leftward lambda promoter (PI) which is controlled by the Cl repressor. The host cell for expression is E. coli. strain MC1061 [pACl], containing a compatible plasmid which carries the C 1-857 mutant gene, encoding a temperature sensitive variant of the Cl repressor. This allows the initiation of expression of heterologous genes by shifting the temperature from 28 °C to 42 °C. Cells of strain MC1061 [pACl] were transformed with the expression plasmid pmTNFMPH-Tg20 and grown at 28°C. An overnight culture was used to inoculate (1/100) 25 ml LB medium containing tetracyclin (lOμg/ml), which was further grown at 28 °C (275 φm) until the O.D. (at 600nm) reached 0.2. The culture was divided into two equal parts, one of which was shifted to 42 °C, while the second pan was kept at 28 °C. At 1.5h intervals, samples were taken and analysed on SDS-PAGE and western blotting using anti-mTNF monoclonal antibodies. The fusion protein mTNFTg20 was detectable on Coomassie Blue stained gels at a MW of approximately 29 kDa Western blots confirmed the identity of this band, which was only present in the induced cultures. From these experiments the conditions for a large scale fermentation of the strain were determined (see example 4). 3. Characterization of the Tg20 antigen: reaction with monoclonal antibodies and MW of the corresponding natural T. gondii antigen Total lysate of cells expressing mTNF.H6.Tg20 (shortly recombinant Tg20) was run on a polyacrylamide (PA) gel and transferred to a nitrocellulose membrane. A set of monoclonal antibodies reacting with T. gondii proteins with small molecular weight, as described by Saavedra et al (1990). were probed against this WB. Only monoclonal antibody BATO 214 reacted with this antigen at a MW of about 29 kDa. On a 12.5 % PA gel total toxoplasma lysate and purified recombinant Tg20 were run in parallel and transferred to nitrocellulose. The blot was probed with BATO 214 and revealed a MW for the naturally occurring I. gondii antigen of 24 kDa, while the recombinant antigen has a MW of 29 kDa (see Fig. 5). The leader peptide of the recombinant antigen (mTNF) is about 2 kDa, which explains partly the larger size of the fusion protein. In addition, when the sequence of Tg20 is analyzed for possible eukaryotic cleavage sites (cleaving off signal peptides for secretion), three possible cleavage sites are found between aminoacid positions 17- 18, 20-21 and 25-26 (see arrows figure lb). Cleavage at one of these positions would reduce the MW of the recombinant to 24, 23.7 or 23.3 kDa, in agreement with the MW of the natural I. gondii antigen. The presence of a cysteine residue in the possible signal peptide may explain the formation of a dimer of the fusion protein (see band at about 58 kDa), which is absent in the naturally occurring (cleaved) Tg20 protein. Example 4. Purification of recombinant Tg20 antigen A 151 fermentation was performed using an induction time of 3h. The cells were collected by low speed centrifugation and the pellet was stored at -70°C until further use. One third of the cell pellet (9g) was thawed, resuspended in 30 ml lysis buffer (lOmM TrisHCl, lOOmM KC1, 5mM EDTA. 25 mM e-aminocaproic acid, lmM DTT, ImM PMSF) and passed twice through the French press. The lysate was centrifuged at 27000 g 30' ,4 D C. The pellet obtained was then extracted with 20 ml 7M guanidine chloride, 50 mM phosphate buffer pH7.2. The extract was centrifuged at 27000 g 30' ,4°C. The obtained pellet was extracted again with 10 ml of the same buffer. Both extracts were pooled. An IMAC column was prepared with chelating sepharose fast flow (Pharmacia), activated with NiCl 2 as described by the manufacturer, and equilibrated with 6M guanidinium chloride, 50 mM phosphate buffer pH 7.2. The pooled cell extract (30 ml) was loaded on the column. After washing with runmng buffer ( 6M guanidinium chloride, 50mM phosphate buffer pH7.2) a step elution was performed with 30, 60 and 100 mM imidazole in running buffer. Eluted fractions (6 ml) were analyzed on SDS-PAGE and WB. The fractions eluting at 100 mM imidazole contained the recombinant Tg20 protein at a purification degree of about 95 %. From 51 culture, 36 mg purified protein in 29 ml was thus obtained (1.25 mg/ml) or 7.25 mg/1 culture. The purified recombinant protein showed a clear positive reaction in WB with monoclonal antibodies anti-mTNF and BATO 214 and was further evaluated in ELISA with a panel of sera. Example 5: Evaluation of recombinant Tg20 in ELISA. 1.ELISA of Tg20 on G-bank sera. Optimal conditions for coating Tg20 antigen on ELISA plates were determined as 2μg/ml (lOOμl/well) antigen and pH9.5 (carbonate buffer). The serum dilution was 1/100 and 1 % E. coli lysate was added to absorb anti- E. coli antibodies. The serum panel (95 positive sera and 43 negative sera) were tested against Tg20 antigen (see table 2). The cutoff value (co.) was calculated as the mean O.D. value (x) of the negative sera, plus 3 standard deviations (s.d.), with the mean value being the sum of the individual values divided by the number of values. Sensitivity and specificity of the assay as compared to IF were 79% and 96% respectively. Furthermore, a correlation was shown between the Immunofluorescence (IF) titre and the sensitivity of the Tg20 ELISA. More specifically, the Tg20 ELISA showed a sensitivity of 100% for sera with a IF-titre > 1/1024. For sera with an IF-titre of 1/256, sensitivity was 85%, while the sensitivity of the Tg20 ELISA reached only 55 % for sera with a IF-titre of 1/64. 2. ELISA of Tg34AR on G-bank sera. The serum samples of the G-bank (95 positive and 43 negative sera) were also tested in ELISA Tg34AR, using the conditions as desribed by Van Gelder et al.(1993) except for the serum dilution that is now 1/100. The results obtained are shown in table 2. Sensitivity and specificity of the Tg34AR assay relative to the IF-test are 77% and 98% respectively. Again, the sensitivity was higher for sera with high IF-titre: sera with IF-titre > 1/1024 were all positive in Tg34AR ELISA (100%) while sensitivity was only 89% and 41 % for sera with IF- titres of respectively 1/256 and 1/64. 3. Combination ELISA with Tg34AR and Tg20 Antigen Tg34AR is usually coated in glycine buffer pH4 (Saavedra et al 1991). The optimal buffer for coating Tg20 antigen is carbonate pH9.5. Several buffers and pH were tested for both antigens, and from this experiment the best compromise for coating both antigens at the same time was determined as carbonate buffer pH 9.5. Both antigens were diluted in carbonate buffer and coated at concentrations of 6μg/ml for Tg34AR and 2μg/ml for Tg20. Again, 95 positive and 43 negative sera from the G-bank were used to test the ELISA. As described in Example 1, 1 % E.coli lysate was added to the diluted serum (1/100). The results obtained with the individual sera are represented in Table 2. Sensitivity and specificity of this combination ELISA in comparison to the IF test are 92.5 % and 98% respectively. In conclusion, it can thus be said that antigen Tg34AR alone detects 77% of the positive sera in this panel. Tg20 detects 79% of the positive sera. Both antigens combined detect 92.5% of the positive sera and thus partially complement each other. Example 6: Production of deletion clones of Tg20 In order to localize the most important epitopes of the Tg20 antigen, deletion clones were made, expressing only parts of the protein. Clone Tg20 contains unique restriction sites at positions 585 (Styl), 434 (Nael) and 287 (Asp700) (Fig. la). The fragments were cut out of pmTNFMPH. Tg20 by digestion with BamHT-Styl (Styl site blunted), BamHI-Nael and BamHI-Asp700 respectively. The fragments were ligated into vector pIGFHl l l or pIGFHlO digested with BamHI-StuI. The vectors pIGFHl l l (see fig. 3) and pIGFHlO (Innogenetics N.V.) are derivatives of pmTNFMPH and have the same basic properties. They contain a larger number of unique restriction sites in the poly linker sequence and the € -enhancer sequence has been inserted in order to give higher expression levels. Plasmid pIGFHlO is the same as pIGFHl 11 except for the Apal unique restriction site in the polylinker sequence which is missing in the former. The principle and protocol to obtain expression of a heterologous gene in pIGFHl l l (and pIGFHlO) is the same as for pmTNFMPH (described in Example 3). Strain SG4044 [pACl] was used. Three different clones, corresponding to the three different restriction fragments, were tested for expression of the corresponding antigen fragments: Tg20BS (BamHI-Styl), Tg20BN (BamHI - Nael) and Tg20BA (BamHI - Asp700). The samples collected from the induction experiments were analysed on Western blots and probed with anti-mouse TNF monoclonal and with BATO 214. All deletion clones reacted with anti-mouse TNF at the expected size, i.e. Tg20BS at approximately 25 kDa, Tg20BN at approximately 20 kDa (with a major degradation band around 15 kDa) and Tg20BA at approximately 14 kDa. Deletion clone Tg20BS gave only very low expression levels. For Tg20BN a lot of "leak" expression was observed, i.e. expression at the non-permissive temperature of 28°C. When probed with BATO 214 only the two largest deletion clones, i.e. Tg20BS and Tg20BN reacted. This gives a good indication for the position of the BATO 214 epitope on antigen Tg20, i.e. between amino acid position 95 (bp 287) and amino acid position 145 (bp 434) on Fig. lb. Testing with human sera indicates that the main epitopes important in the recognition of human sera are also located in this region (see example 7 and 8). Clones Tg20BN and Tg20BA were further purified and evaluated in ELISA (see examples 7 and 8). The expression level of clone Tg20BS was too low for purification purposes. Example 7: Purification of Tg20BN and evaluation in ELISA 1. Purification of Tg20BN antigen fragment Tg20BN was purified on IMAC as described above for Tg20 purification, with some modifications. A 151 fermentation was carried out, but growth and expression were carried out at 28°C only. The "leak" expression level at 28°C was sufficient for purification purposes and it prevents the more pronounced degradation of the recombinant protein seen at the increased temperature of 42°C. After French press lysis of one cell pellet (51) in lysis buffer (lOmM sodium phosphate, lOOmM KC1, lOmM e-amino caproic acid). 2mM PMSF was added. The lysate was centrifuged and the soluble fraction was used for purification. Guanidinium chloride powder was added to reach a final concentration of 6M and the solution was centrifuged again. Imidazole was added to reach a concentration of 20mM. Starting the purification with 20mM imidazole prevents some E. (___i contaminants to bind to the column. An LMAC column was made with chelating sepharose fast flow (Pharmacia), activated with NiCl 2 as described by the manufacterer, and equilibrated with running buffer (6M guanidine chloride, 50mM phosphate, 20mM imidazole, pH 7.2). The sample (50 ml) was loaded and after washing with running buffer, a step elution was performed with 35, 50 and 200mM imidazole in running buffer. Eluted fractions (3ml) were analysed on WB and SDS-PAGE. The 200 mM fractions contained the recombinant purified protein, that was sufficiently pure (95 %). Three fractions (= 9 ml) were pooled, the pool containing 230μg/ml Tg20BN. 2.ELISA with Tg20BN ELISA plates were coated with antigen Tg20 and Tg20BN separately at 2μg/ml in carbonate buffer. To the sample diluent 1 % E. CQH lysate was added and the serum was diluted 1/100. Of the G-bank 50 positive (#1 to #50) and 37 negative (#206 to #245 except #221,#228.#233) sera were tested and the results for Tg20 and Tg20BN were compared (see table 3). At a cutoff value of (mean + 3 s.d.) no false positives were detected with either test (100 % specificity)- As to the sensitivity of both assays, the ELISA with the full Tg20 antigen resulted in a sensitivity of 90%, while the truncated protein Tg20BN yielded a sensitivity of 78%. Most of the sera that are missed by Tg20BN-ELISA are only borderline positivcwith Tg20-ELISA (see table 3), except for serum #35 and #40 which show a clear drop in reactivity when tested with the deletion clone Tg20BN. Since the large majority of the sera are still detected by Tg20BN antigen, it can be concluded that the immunodominant epitopes are located on the N-terminal part (aa 1 to aa 145) of the Tg20 antigen. For a few sera (like #35 and #40) an epitope on the C-terminal part seems to be important. These two sera are thus missed by the Tg20BN ELISA, but it is important to note that they are detected by the Tg34AR ELISA (cfr. Table 2). Example 8: Purification of Tg20BA and evaluation in ELISA 1. Purification of Tg20BA The IMAC-purification of Tg20BA was carried out on small scale with Ni-NTA spin columns (Qiagen). A culture of 100 ml MCI 061 [pACl] transformed with pIGFH10Tg20BA was induced (42°C) for 2h. Cells were harvested and lysed in 6M guanidinium. HCl, 50mM phosphate pH 8.3 (shaking for 45 min.). After centrifiigation a cleared lysate was obtained and this was loaded on the spin column after the column was equilibrated with the same buffer. The column was washed twice with 6M guanidiniumHCl, 50mM phosphate buffer at pH 6.3. Elution was done with 6M guanidinium. HCl, 50mM phosphate pH 4.3. This elution step was repeated. The eluates were checked on WB. The first eluate contains most of the purified recombinant protein and is 95 % pure. It contained 265 μg/ml in 600 μl. 2. ELISA with Tg20BA From the G-bank 31 sera that gave a positive reaction with Tg20BN were tested on the smaller fragment, Tg20BA, including also 8 negative sera. Tg20BA gave no reaction higher than the cutoff value with these sera (results not shown). This indicates that the Tg20BA fragment (aa pos. 1 to aa pos. 95) does not contain any epitopes important for the recognition of human sera from infected individuals. From the differential reactivity of antigen fragment Tg20BN and Tg20BA it can be concluded that the main epitope(s) important in the recognition of the humoral immune response are localized in the fragment extending from aa pos. 95 to aa pos. 145, or at least that this region is important for the construction of an immunoreactive epitope. Further fine mapping of the epitope(s) can be performed using an overlapping set of synthetic peptides. Example 9: The combination of Tg34AR and Tg20BN in ELISA Antigen fragment Tg20BN was combined in ELISA with Tg34AR to compare the results of this combination ELISA with the results obtained earlier with the combination of Tg20 and Tg34AR (see Table 2). Again, the coating concentration of Tg34AR was 6 μg/ml and for Tg20BN a concentration of 2 μg/ml was used. The sensitivity of the combination ELISA with the full size antigen Tg20 (Tg20 + Tg34AR) was only slightly higher (92.5%) as compared to the combination ELISA of the antigen fragment Tg20BN with Tg34AR (sensitivity 91.5 %). From this it can be concluded that the most important B-cell epitopes of the Tg20 antigen are located in the Tg20BN-fragment. Example 10: Localisation of the dominant human epitope on Tg20BN with peptides. In order to localize the most important B-cell epitope(s) more precisely in the Tg20-BN fragment, a series of overlapping peptides are synthezised, covering the sequence region extending from aminoacid position 95 to amino acid position 145 in Figure lb. A biotin molecule was added at the N-terminus during synthesis. The peptides have an overlap of 10 amino acids, and are represented by the following sequences: GluProAspGluGlnGluGluValHisPheArgLysArgGlyVaJArgSerAspAlaGlu (SEQ ID NO 3, aa90- 110) ArgLysArgGlyValArgSerAspAlaGluValThrAspAspAsnlleTyrGluGluHis (SEQ ID NO 4, aalOO-120) ValThrAspAspAsnlleTyrGluGluHisThrAspArgLysValValProArgLysSer (SEQ ID NO 5, aa 110-130) ThrAspArgLysValValProArgLysSerGluGlyLysArgSerPheLysAspLeuLeu (SEQ ID NO 6, aal20-140) GluGlyLysArgSerPheLysAspLeuLeuLysLysLeuAlaLeuPro (SEQ ID NO 7, aal30-146) Microtiter plates are coated with streptavidin by incubating 100 μl/well of a 5 μg/ml streptavidin solution in carbonate buffer (50 mM, pH 9.6) for 1 h at 37 °C. After washing, the biotinylated peptides are added to the wells (100 ng/well), each well (in duplicate) representing a different peptide. Peptides are left to bind with streptavidin for 1 h at 37°C after which washing occurs. The ELISA procedure is then further continued as described for Tg20 ELISA in Example 1. The five peptides were tested with two positive human sera (18,23), one negative serum (210) and with mouse monoclonal BATO 214 and an unrelated monoclonal (BATO 35). Both monoclonal preparations were dilutions (1/2000) from ascites produced in mice. The conjugate used in the ELISA with the monoclonals was rabbit anti mouse conjugate (Sigma, dilution 1/5000). The results with the human sera do not indicate one peptide as the more important, all petides react to a greater or smaller extent. Monoclonal BATO 214 reacts with petides SEQ ID NO 3 and SEQ ID NO 4. This probably defines the epitope for BATO 214 between aa 100 and 110. 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Claims: CLAIMS 1. A polypeptide or peptide containing (1) an amino acid sequence extending from amino acid position x to amino acid y in the sequence as shown in Fig. lb (SEQ ID NO 2), with x = 1 and y = 196, and/or x = 1 and y = 160, and/or x = 1 and y = 145, and/or x = 95 and y = 145, and/or (2) any fragment of (1), with said fragment comprising a stretch of at least 7 contiguous amino acids of the sequence as defined in (1), and/or (3) any equivalents of (1) or (2) originating from the substitution of one or several amino acids in the sequence of (1) or (2), with said polypeptides or peptides containing a reactive epitope important in the humoral and/or cell-mediated immune response of the 1. gondii infected host organism. 2. A polypeptide or peptide containing (1) the amino acid sequence extending from position 95 to position 145 of the sequence in Fig. lb (SEQ ID NO 2), and/or (2) any fragments of (1), with said fragments comprising a stretch of at least 7 contiguous amino acids of the sequence in (1), and/or (3) at least one of the amino acid sequences represented by GluProAspGluGlnGluGluValHisPheArgLysArgGlyValArgSerAspAlaGlu (SEQ ID NO 3). or ArgLysArgGlyValArgSerAspAlaGluValThrAspAspAsnlleTyrGluGluHis (SEQ ID NO 4), or ValThrAspAspAsnlleTyrGluGluHisThrAspArgLysValValProArgLysSer (SEQ ID NO 5), or ThrAspArgLysValValProArgLysSerGluGlyLysArgSerPheLysAspLeuLeu (SEQ ID NO 6). or GluGlyLysArgSerPheLysAspLeuLeuLysLysLeuAlaLeuPro (SEQ ID NO 7) and/or (4) any equivalents of (1), (2) or (3) originating from the substitution of one or several amino acids in the sequences of (1), (2) or (3), with said polypeptides and peptides containing a reactive epitope important in the humoral and/or cell-mediated immune response of the T. gondii infected host organism. 3. A fusion protein consisting of a polypeptide or fragment thereof according to claims 1 or 2 linked to a heterologous polypeptide sequence. 4. A polynucleic acid containing (1) a polynucleic acid sequence encoding any of the polypeptides as defined above, and/or (2) a polynucleic acid sequence extending from position x to position y of the polynucleic acid sequence as represented in fig. lb (SEQ ID NO 1) with x = 1 and y = 589, and/or x = 1 and y = 480, and/or x = 1 and y = 434, and/or x = 287 and y = 434, and/or (3) a polynucleic acid sequence which is degenerate as a result of the genetic code to the polynucleic acid sequence of (1) or (2), and which still encodes any of the polypeptides of claims 1 or 2, and/or (4) a polynucleic acid sequence which hybridizes with any of the polynucleic acid sequences as defined in (1) to (3), and/or (5) a fragment of any of the polynucleic acid sequences defined in (1) to (4), said fragment containing a stretch of at least 10, and preferably 11 , 12, 13, 14, 15 or more contiguous nucleotides from any of the polynucleic acid sequences as defined in (1) to (3). 5. An oligonucleotide comprising in its sequence at least 10 contiguous nucleotides which form part of any of the polynucleic acid sequences as defined in claim 4, for use as a specific hybridization probe for detecting any of the polynucleic acids as defined in claim 4 or for use as a primer for specifically amplifying any of the polynucleic acid sequences as defined in claim 4. A recombinant polynucleic acid comprising in its sequence a polynucleic acid or a fragment thereof of claim 4 as well as a heterologous polynucleic acid sequence. 7. A recombinant vector into which one of the polynucleic acids as defined in claim 4 have been inserted for cloning and/or expression purposes. 8. A recombinant vector according to claim 7 wherein the coding sequence for any of the polypeptides of claims 1 or 2 is operably linked to a control sequence capable of providing the expression of said coding sequences by the specific host. 9. A host cell transformed with a recombinant vector according to claim 7 or 8. 10. A recombinant polypeptide encoded by at least part of a polynucleic acid according to claim 4, produced by transforming a recombinant vector according to claim 7 or 8 into a suitable host cell, culturing said transformed cellular host under conditions which allow the expression and possibly secretion of the encoded polypeptide, and recovering the expressed polypeptide from the culture. 11. An antibody, monoclonal or polyclonal, capable of reacting specifically with the polypeptides as defined in claims 1 or 2, provided that said antibody is different from the monoclonal antibody BATO 214. 12. An antibody according to claim 11 or an antigen-binding fragment or a recombinant antibody derived thereof which is further characterized in that it is capable of inhibiting the infection by Toxoplasma gondii of the specific cell type which it infects in vivo. 13. A hybridoma producing a monoclonal antibody according to claim 11 or 12. 14. An anti-idiotype antibody raised against a monoclonal antibody of claim 11 or 12. 15. An antisense peptide derived from the polypeptides of claim 1 or 2. 16. A method for detecting antibodies to T. gondii present in a biological sample, comprising: contacting the biological sample to be analysed with at least one polypeptide or peptide according to any of claims 1, 2 or 10 under conditions allowing the formation of an immunological complex, and, detecting the immunological complex formed between said antibodies and said polypeptide(s). 17. A method for detecting T. gondii antigens present in a biological sample, comprising: - contacting the biological sample to be analysed with at least one antibody according to claim 11 or 12, under conditions allowing the formation of an immunological complex, and, detecting the immunological complex formed between said antibodies and said antigen. 18. A method for detecting I. gondii polynucleic acids present in a biological sample, comprising: a. contacting the biological sample to be analysed with at least one probe according to claim 5, under conditions allowing the formation of a specific hybridization complex, with said nucleic acids of said sample being possibly amplified prior to hybridization with said probe, or, b. contacting the biological sample to be analysed with at least one primer according to claim 5, under conditions allowing the formation of a specific hybridization complex, and, c. detecting the specifically amplified polynucleic acids formed in step b. or the specifcally hybridizing polynucleic acids as in a. 19. A method according to claim 16 further comprising also other T. gondii polypeptides or peptides. for measuring a cellular immune response in an individual which has been in contact with the T. gondii pathogen, said cellular immune response being measured either in vivo or in vitro. 20. A kit for detecting anti-1. gondii antibodies in a sample according to the method of claim 16 or 19, said kit comprising: at least one of the T. gondii polypeptides or peptides according to any of claims 1 , 2 or 10, optionally in combination with other polypeptides or peptides from T. gondii. with all said polypeptides or peptides being preferably immobilized on a solid support, optionally a buffer, or components necessary for producing the buffer, enabling a binding reaction to occur between the antibodies and the antigens present in said sample, optionally a means for detecting the immune complexes formed, and, - optionally also an automated scanning and interpretation device for inferring the presence of said antibodies in said sample. 21. A kit for detecting T. gondii antigens in a sample according to the method of claim 17. said kit comprising: - at least one antibody according to claim 11 or 12, with said antibody being preferably bound to a solid support, optionally a buffer, or components necessary for producing the buffer, enabling a binding reaction to occur between the polypeptides and the antibodies present in said sample, - optionally a means for detecting the immune complexes formed, and, optionally also an automated scanning and interpretation device for inferring the presence of said antigens in said sample. 22. A kit for detecting T. gondii polynucleic acids according to a method of claim 17, said kit comprising: at least one of the T. gondii polynucleic acids according to claim 4, more specifically any of the probes or primers according to claim 5, with said probes being preferably immobilized on a solid support. - optionally a buffer, or components necessary for producing the buffer, enabling a hybridization reaction to occur between said polynucleic acids and said probes or said primers, optionally a means for detecting the polynucleic acid complexes formed, and, optionally also an automated scanning and interpretation device for inferring the presence of said polynucleic acids in said sample. 23. A kit according to any of claims 20 to 22 further characterized in that it also allows the detection of any other pathogen besides T. gondii by inclusion of polypeptides, antibodies or polynucleic acids capable of specifically reacting with said other pathogen. 24. A vaccine composition which provides protective immunity against T. gondii infection in mammals comprising as an active principle one or more of the T. gondii polypeptides according to any of claims 1 , 2 or 10, or one or more of the polynucleic acid sequences or recombinant vectors according to claims 4, 6, 7 or 8, and with said active principle being combined with a pharmaceutically acceptable carrier. 25. A vaccine composition according to claim 24 in which also other Toxoplasma immonogenic components or other bacterial or other immunogenic components in general are incorporated. 26. A vaccine composition which provides protective immunity against T. gondii infection in mammals comprising as an active principle one or more of the T. gondii polynucleic acids according to claim 4 or parts thereof as naked DNA or as part of a recombinant vector according to claim 7 or 8, whereby said polynucleic acids which are administered to the individual to be vaccinated are expressed into immunogenic protein or peptide in situ and thus confer protection to the vaccinated host. 27. A polypeptide according to claims 1 or 2 or 10 for use as a medicament. 28. A polynucleic acid according to claims 4, 5 or 6 for use as a medicament. 29. An antibody according to claim 11 or 12 for use as a medicament.

Inventors: JACOBS DIRK, SAMAN ERIC, VAN HEUVERSWYN HUGO, JACOBS, DIRK, SAMAN, ERIC, VAN HEUVERSWYN, HUGO
Applicants: HEUVERSWYN HUGO VAN, INNOGENETICS NV, JACOBS DIRK, SAMAN ERIC, INNOGENETICS N.V., JACOBS, DIRK, SAMAN, ERIC, VAN HEUVERSWYN, HUGO
Application year: 1997
Main classification: C12N15
IPC codes: A61K31, A61K38, A61K39, A61K48, A61P33, C07K7, C07K14, C07K16, C07K19, C12N1, C12N15, C12P21, C12Q1, C12R1, G01N33