WO2011045056A1

WO2011045056A1 - Methods for prrsv detection

Info

Publication number: WO2011045056A1
Application number: PCT/EP2010/006274
Authority: WO
Inventors: Sebastian Ulbert; Matthias Giese; Samiya Al-Robaiy; N Delaroque; Mathias Salomo
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2009-10-13
Filing date: 2010-10-13
Publication date: 2011-04-21
Also published as: DK2488872T3; EP2488872B1; EP2488872A1

Abstract

The present invention relates to methods for detecting PRRSV antibodies, wherein at least two PRRSV proteins or fragments thereof are provided and contacted with serum under conditions that allow the binding of PRRSV antibodies in the serum to the PRRSV proteins. In an additional step, PRRSV antibodies bound to the PRRSV proteins are detected. Furthermore, a system and a kit for the detection of PRRSV antibodies are provided.

Description

METHODS FOR PRRSV DETECTION

FIELD OF THE INVENTION The present invention relates to the field of PRRSV detection. In particular, the present invention discloses methods for the detection of PRRSV antibodies in serum. The present invention furthermore discloses systems and kits that may be used in the disclosed methods.

BACKGROUND OF THE INVENTION

Arteriviruses are well-known to play a key role in various diseases associated with livestock. The family Arteriviridae generally comprises four enveloped, positive-stranded RNA viruses having an icosahedral core: equine arteritis virus, lactate dehydrogenate elevating virus of mice, porcine reproductive and respiratory syndrome virus, and simian hemorrhagic fever virus.

Porcine reproductive and respiratory syndrome virus (PRRSV), also known as Blue-Ear Pig Disease, is of particular importance since it causes a disease in pigs, called porcine reproductive and respiratory syndrome (PRRS). This economically devastating pandemic disease causes reproductive failure in breeding stock and respiratory tract illness in young pigs. Initially referred to as "mystery swine disease" and "mystery reproductive syndrome," this disease was first reported in North America and Central Europe in 1987. PRRS is the economically most important pig- related disease, with estimated losses in the United States swine industry amounting to $460 million annually. Given the commercial implications of PRRS, a means for the fast and reliable detection of PRRSV infections is of paramount importance to, for example, preventing and containing the spread of this infection as quickly as possible. However, while there are a few known detection systems suitable for screening for the presence of a PRRS infection, these detection systems generally suffer from insufficient sensitivity and/ or specificity allowing for a quick and reliable viral detection, thus leading to an unacceptably high occurrence of false-negative or false-positive screening results.

Thus, there exists a need for improved methods and kits for the detection of PRRSV infections and systems and kits useful for carrying out these methods. SUMMARY OF THE INVENTION

It is an object of the invention to provide methods for the detection of PRRSV antibodies, including the steps of providing at least two PRRSV proteins or fragments thereof; contacting the at least two PRRSV proteins with serum under conditions to allow the binding of PRRSV antibodies comprised by the serum to the PRRSV proteins; and detecting the PRRSV antibodies bound to the PRRSV proteins.

Advantageously, the present invention is based on the unexpected finding that a combination of more than one PRRSV protein provides an improved and reliable detection of PRRSV antibodies in a serum to be tested. Thus, an improved monitoring of the presence and progress of a PRRSV infection or an effective PRRSV vaccination is possible. Unexpectedly, it also discovered that using more than a single PRRSV protein provides a more sensitive and specific method for detecting PRRSV. A higher sensitivity naturally results in a larger number of correcdy deter- mined PRRSV positives, while a higher specificity naturally results in a lower number of PRRSV false-positives. In one embodiment, a combination of three PRRSV proteins is utilized, comprising both GP5 and NSp7, and an additional protein selected from the group consisting of the M- protein, the E-protein, GP3 and the N-protein. The inventors have found that unexpectedly, a particularly high sensitivity can be achieved when a combination of the three PRRSV proteins NSp7, GP5 and GP3 protein is used for the detection of PRRSV. An ELISA assay according to one embodiment of the present invention comprising these three proteins shows a superior sensitivity when compared with other, commercially available ELISA test systems for PRRSV, and is especially well suited also for the detection of PRRSV in highly diluted samples.

The inventors have found that the antigens that are capable of eliciting the highest antibody signal in a PRRSV ELISA assay are GP5, GP3 and NSP7. Unexpectedly, a clear synergistic effect is seen with a combination of the individual antigens is analysed in a PRRSV ELISA according to one embodiment of the present invention, comprising antigens of the SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7).

In a large series of PRRSV-positive pig sera, the inventors could demonstrate that the antigens that elicit the strongest signal in a PRRSV ELISA vary significantly between different serum samples. It has been found that in some sera, NSP 7 is the dominant antigen, whereas in other sera, GP3 or GP5 are the antigens that are predominandy recognized. Therefore, to detect all the PRRSV-positive sera correctly as positive, the presence of all three of these antigens is highly beneficial. Thus, the highest sensitivity was observed when an ELISA comprising NSP7 (SEQ ID NO: 19 and 25), GP3 (SEQ ID NO:5 and 13) and GP5 (SEQ IDNO:l and 9) was utilized. Hence, due to the high degree of variability in the anti-PRRSV antibody responses of pigs, a clear synergistic effect is seen by using the mixture of antigens according to one embodiment of the present invention, namely NSP7, GP5 and GP3 of both genotypes. The presence of all three of these antigens is essential to detect the maximum numbers of PRRSV infections. This leads to a significant advantage over available technologies relying only on one antigen or on antigen mixtures different from the one used in the present invention. Furthermore, the ELISA com- prising SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7) proved to be more sensitive than ELISA assays comprising different combinations of structural and non-structural PRRSV proteins.

In another embodiment, the at least one of the PRRSV proteins is derived from the European PRRSV strain Lelystad or the US PRRSV strain VR-233 and/or further comprises an MBP-tag and/ or a His-tag.

In yet another embodiment, the detection step is carried out immunologically and/ or the detection PRRSV antibodies indicate a PRRSV infection or an effective PRRSV vaccination.

The present invention further contemplates a system for the detection of PRRSV antibodies, comprising a support, wherein immobilized on the support are at least two PRRSV proteins or fragments thereof. In another aspect of the invention, a kit comprising at least two PRRSV proteins or fragments thereof is provided.

In both the systems and kits described herein, the provided proteins may be selected from the group consisting of a fragment of the GP5 protein, preferably SEQ ID NO: 1 or 9, the NSp7 protein, preferably SEQ ID NO: 19 or 25, and a fragment of the GP3 protein, preferably SEQ ID NO: 5 or 13. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is particularly pointed out and distinctly claimed throughout the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

FIG. 1 shows the plasmid map of the pMAL Tag2 vector (SEQ ID NO: 27) FIG. 2 depicts the results of an ELISA assay utilizing an expressed GP5 protein.

FIG. 3 depicts the results of an ELISA assay using multiple PRRSV proteins and a comparison with an IDEXX ELISA. FIG. 4 depicts the result of an ELISA assay using multiple PRRSV proteins (FIG. 4 A) and of IDEXX ELISA (FIG. 4 B)

FIG. 5 depicts the results of an ELISA assay using multiple PRRSV proteins DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is based on the unexpected finding that a combination of more than one PRRSV protein, or fragments thereof, provides for an efficient and reliable detection of PRRSV antibodies in a serum to be tested. Unexpectedly, it was discovered that, due to the inclusion of more than a single protein according to the present methods, higher antibody signals, and thus more reliable results, can be obtained compared to those methods where only a single protein is present. Advantageously, the method according to the invention furthermore allows for a safer distinction between the various possible PRRSV strains infecting an animal, as well as for providing valuable information about the state of a detected PRRSV infection.

PRRSV is a small, enveloped RNA virus. It contains a single-stranded, positive-sense, RNA genome with a size of approximately 15 kilobases. The genome codes for nine open reading frames. PRRSV is a member of the genus Arterivirus, family Arteriviridae, and the order nidovi- rales. An example for a North American strain is VR-2332 and the European strain is known as the Lelystad Virus (LV). The European and North American PRRSV strains have been described to manifest similar clinical symptoms, but they represent two distinct viral genotypes whose genomes diverge by approximately 40%. The genetic variation among the viruses isolated from different locations further increases the difficulty of reliably detecting an infection caused by the respective strains.

The term "PRRSV antibody" as used herein, refers to any antibody that recognizes PRRSV, that is, at least one component of PRRSV, as an epitope. In other words, the term "PRRSV antibody" refers to an anti-PRRSV antibody, for example, produced by a pig in response to an infec- tion with PRRSV. Preferably, the PRRSV antibody recognizes GP5, GP3, the M-protein, the E- protein, the N-protein, NSP7 of PRRSV and/ or fragments thereof as an epitope. Especially preferred fragments of the European strain proteins are aa 33-201 (SEQ ID NO: 1) or aa 33-66 of the GP5 protein (SEQ ID NO: 3), aa 55-238 of the GP3 protein (SEQ ID NO: 5), aa 100-173 of the M protein (SEQ ID NO: 7). Especially preferred fragments of the North American strain are aa 66-200 (SEQ ID NO: 9) or aa 26-67 (SEQ ID NO: 11) of the GP5 protein, aa 61-201 of the GP3 protein (SEQ ID NO: 13), or aa 97-174 of the M protein (SEQ ID NO: 15).

Whenever reference is made to a PRRSV protein, the full-length protein and the fragment of said protein are both referred to, unless explicitly stated otherwise. For example, the term "GP5 protein" designates both the full-length PRRSV GP5 protein and a fragment of said GP5 protein that is sufficiendy recognized as an epitope by a PRRSV antibody. Furthermore, unless designated otherwise, proteins of both strains, the European and the North American are referred to.

The sequences of the full-length proteins ORF7, NSp7 and E protein of the European strain are set forth in SEQ ID NO: 17, 19 and 21, respectively. The sequences of the full-length proteins ORF7 and NSp7 of the North American strain are set forth in SEQ ID NO: 23 and 25, respectively.

A "fragment of a PRRSV protein" as used herein, refers to any portion of a PRRSV protein that is recognized by the immune system of the PRRSV host, i.e. which is recognized as an epitope, preferably by the porcine immune system. In other embodiments, the fragment of the PRRSV protein has a length of > 5 continuous amino acids, > 10 continuous amino acids, > 15 continuous amino acids, > 20 continuous amino acids, 25 continuous amino acids, > 30 continuous amino acids or > 35 continuous amino acids. In more preferred embodiments, the fragment of the PRRSV protein has a length of > 30 and < 40, 34, or 35 continuous amino acids. In other embodiments of the invention, the fragment is a fragment of a protein of the European strain and selected from the group consisting of aa 33-201 of the GP5 protein (SEQ ID NO: 1), aa 33- 66 of the GP5 protein (SEQ ID NO: 3), aa 55-238 of the GP3 protein (SEQ ID NO: 5), and/or aa 100-173 of the M protein (SEQ ID NO: 7). In other embodiments of the invention, the fragment is a fragment of a protein of the North American strain and selected from the group consisting of aa 66-200 of the GP5 protein (SEQ ID NO: 9), aa 26-67 of the GP5 protein (SEQ ID NO: 11), aa 61-201 of the GP3 protein (SEQ ID NO: 13), and/or aa 97-174 of the M protein (SEQ ID NO: 15).

In one aspect, the present invention is directed to a method for the detection of PRRSV antibodies comprising the steps of: a) providing at least two PRRSV proteins or fragments thereof; b) contacting the at least two PRRSV proteins or fragments thereof with serum under conditions sufficient to allow the binding of PRRSV antibodies in the serum to the PRRSV proteins; and c) detecting the PRRSV antibodies that are bound to the PRRSV proteins.

In a first step according to the inventive methods, at least two PRRSV proteins are provided in order to detect PRRSV antibodies. Preferably, the PRRSV proteins are provided in an immobilized form, which has an advantage that the detection of PRRSV antibodies bound to said pro- teins can be accomplished more readily. In one preferred embodiment, the at least two PRRSV proteins are immobilized on a solid support, wherein any solid support suitable for this purpose and known to the skilled person can be utilized, such as a membrane, or a gel matrix. Most preferably, the solid support is a plate, for example, a 96-well microtiter plate, as commonly used in cell culture or with enzyme-linked immunosorbent assays (ELISA) techniques. This embodiment has the advantage that the plate can be readily coated with the PRRSV proteins by the skilled person, and that the binding of PRRSV antibodies to PRRSV proteins can be easily detected using standard laboratory equipment, such as an ELISA plate reader.

In one embodiment of the invention, the wells contained within the well plate are coated with the at least two PRRSV proteins at different concentrations, wherein preferably a serial dilution is carried out. In another embodiment, the wells are coated with PRRSV proteins from the different PRRSV strains, for example, one well contains at least two PRRSV proteins from only the European Lelystad strain. This arrangement advantageously allows a swift differentiation between the various PRRSV strains. In a next step according to the invention, the at least two PRRSV proteins are brought into contact with serum under conditions that allow the binding of PRRSV antibodies, which are present in the serum, upon infection of the host with PRRSV. Conditions affecting the binding of anti- bodies to proteins are known to the skilled person, as are standard protocols for facilitating the binding between the two components. Such conditions can include the temperature, duration of contacting, presence and concentration of blocking agents and/ or detergents and so on. In one embodiment, standard ELISA techniques are employed. In the following, a specific, yet generally applicable embodiment of the invention is described:

In a first step, the PRRSV proteins and a negative control, preferably having the same amount of Maltose Binding Protein (MBP), are diluted to appropriate concentrations in a buffer, preferably the buffer is a 50 mM Na₂C0₃ buffer. 100 ul volumes of such diluted antigen are then dispensed into the wells of an ELISA microplate, preferably a flat-bottomed, high protein binding ELISA microplate (Polysorp, Nunc 475094). Even more preferably, 2 wells per test serum are used as well as 2 wells for the negative control reaction. The microplate is then sealed and incubated overnight at 4°C. Thereafter the plates are washed, preferably 3 times with lx PBS-Tween 0.05%.

In an additional step step, 200 ul blocking buffer, preferably PBS with 5% low-fat milk, is added. The microplates are then sealed and incubated, preferably under movement, for example on an on an orbital shaker. This incubation is preferably carried out for 2 h at room temperature. In a further step, the plates are washed, preferably for 3 times with lx PBS-Tween 0.05%.

In turn, the sera to be tested are diluted, for example 50 times, in the blocking buffer and dispensed in each test well, wherein a volume of 100 ul is preferred. The microplates are again incubated, likewise preferably under movement, for example, on an orbital shaker for 1 hour at room temperature.

Following the incubation step, the plates are washed, preferably 3 times with lx PBS-Tween 0.05%. In a next step, the working dilution of a secondary antibody, preferably a rabbit anti-pig immunoglobulin horseradish peroxidise conjugate (1 :10,000 Sigma A5670), is prepared in blocking buffer and 100 ul are added to each test well. In turn, the microplates are likewise, preferably under movement, for example, on an orbital shaker for 1 hour at room temperature.

After incubation, the plates are washed as described above and immediately re-filled with substrate solution, preferably 100 ul TMB One Solution (Promega G7431) followed by incubation, preferably at room temperature for 30 min in a dark environment without shaking. 50 ul of stopping solution, preferably consisting of 1 M sulphuric acid is then added.

Finally, the resulting absorbance values are measured, preferably utilizing an ELISA reader at 450 nm and 520 nm as a reference.

The serum may be any serum sample suitable for the detection of PRRSV antibodies and gener- ally encompasses any liquid that is derivable from an individual to be tested. Preferably, the serum sample is selected from the group consisting of blood, serum, sputum, saliva and/ or milk. More preferably, the serum sample is porcine serum. More preferably, the serum sample has been isolated from the animal prior to carrying out the method of the present invention. While the isolated serum may be subjected to further purification and/or treatment steps in order to improve the binding and/or detection of the PRRSV antibodies, it has been found that such further steps are not necessary in order to achieve reliable results. Therefore, the present invention advantageously offers the possibility of carrying out the present detection method On the spot', i.e. directly at the site of a suspected PRRSV infection. In a further step according to present invention, the PRRSV antibodies bound to the PRRSV proteins are detected. Detection of said PRRSV antibodies can be carried out by any means known to the skilled person. Preferably, the PRRSV antibodies are detected by immunological means, e.g. via conjugated secondary antibodies or functional fragments thereof, such as F_ab fragments. Such conjugated secondary antibodies are directed against the antibody molecules of the host, for example, against porcine antibodies. Generally such antibodies are generated in a different species, e.g. in rabbit, and recognize the species-specific portion(s) of the target antibodies as an epitope. Such secondary antibodies may be conjugated to an enzyme, such as horseradish peroxidase, to enable conversion of a substrate resulting in chemoluminescence or leading to a change in color. Likewise they may be labeled, e.g. by an isotope and/ or any other detectable label known to the skilled person. Advantageously, both PRRSV proteins present in the method according to the invention will allow the binding of PRRSV antibodies. In turn, the signal generated by the secondary antibodies binding to the primary antibodies will be amplified, thus increasing the sensitivity of the method according to the invention.

In a further embodiment of the invention, the at least two PRRSV proteins are selected from the group consisting of GP5, GP3, the M-protein, the E-protein, the N-protein, and NSP7.

In another embodiment of the invention, at least one of the PRRSV proteins employed is a re- combinant protein. Preferably, more than one, two, three, four, five and/or all of the PRRSV proteins are recombinant proteins. Recombinant expression of proteins is a technique commonly known to the skilled person. Various expression systems and/or hosts are known, e.g. recombinant expression in yeast, mammalian cells or bacteria. Thus, the provision of the at least two PRRSV proteins according to the method of the invention may furthermore comprise a step of recombinandy expressing and/or purifying the PRRSV protein(s). In one embodiment, the PRRSV protein(s) is/are efficiendy expressed in bacteria as N-terrninal fusion proteins with the Maltose Binding Protein (MBP) from E. colt. Preferably, the PRRSV protein(s) additionally comprise a C-terminal His-tag. This tag confers an advantage that the expressed proteins can be readily purified, and to extremely high grades, preferably using both tags in a tandem-protocol. It is possible to rapidly remove the MBP-tag by digestion with recombinant Factor X. However, unexpectedly it has been found that MBP is completely inert and is not recognized by porcine serum, hence the presence of MBP in the reaction vessel does not lead to false positive results. The skilled person will be aware of the fact that further different tagging systems are effective for achieving a purification of the PRRSV proteins.

In another embodiment of the invention, at least one of the PRRSV proteins is derived from the European PRRSV strain Lelystad (Genbank accession number: M96262) or a North American PRRSV strain e.g. VR-2332 (Genbank accession number: EF536003). Preferably, all of the at least two PRRSV proteins are derived from the same PRRSV strain.

The GP5 protein or sequence can be the GP5 sequence or protein of Acc No. AF095507 (strain 18027), ADN42860 (strain GDENP/1 /08), of ACU56734 (strain 08-1665-YAS).

The NSP7 sequence or protein can be the NSP7 sequence or protein from strain VR2332 or ABY66100 (strain rV63), ACP43262 (strain clone 20).

The GP3 sequence or protein can be the GP3 sequence or protein from VR2332 or ACY29601 In another embodiment of the invention, one of the at least two PRRSV proteins is the GP5 protein. Unexpectedly, it has been found that GP5 may result in a higher antibody signal compared to the N protein. Thus, if GP5 is used as one of the at least two PRRSV proteins, a higher sensitivity of the assay advantageously results.

In a further preferred embodiment of the invention, one of the at least two PRRSV proteins is NSp7. Similar to GP5, it has been found that the use of NSp7 may result in a higher antibody signal compared to that associated with the N protein. Thus, if NSp7 is used as one of the at least two PRRSV proteins, a higher sensitivity of the assay advantageously results.

In another embodiment of the invention, the at least two PRRSV proteins comprise GP5 as well as NSp7. As explained above, both proteins result in a higher sensitivity of the assay. It was found that if both proteins are combined together in a detection assay, a beneficial synergetic effect will occur.

In another embodiment of the invention, at least three PRRSV proteins are employed in the detection method. While any combination of three proteins selected from the group consisting of GP5, GP3, the M-protein, the E-protein, the N-protein, and NSP7 is contemplated by the present invention, a combination of three or four proteins comprising GP5 and NSp7 as well as at least one protein selected from the group consisting of GP3, the M-protein, the E-protein, and the N-protein is preferred.

More preferably, a combination of three or four proteins comprising GP5 and NSp7 as well as at least one protein selected from the group consisting of M-protein, the E-protein, GP3 and the N-protein is utilized. Such a combination of three PRRSV proteins is more preferred than a combination of four proteins. Even more preferably, a combination of the three PRRSV proteins GP5, NSp7 and GP3 is used. The GP5 protein is preferably selected from the group consisting of a fragment of the GP5 protein of the European or North American strain, aa 33-201 of the GP5 protein of the European strain (SEQ ID NO: 1) and aa 26-200 of the GP5 protein of the North American strain (SEQ ID NO: 9). The GP3 protein is preferably selected from the group consisting of a fragment of the GP3 protein of the European or North American strain, aa 55-238 of the GP3 protein of the European strain (SEQ ID NO: 5) and aa 61-201 of the GP3 protein of the North American strain (SEQ ID NO: 13). Additional combinations of four, five or six of said PRRSV proteins are likewise encompassed by the present invention.

In a further embodiment of the present invention, the detection of PRRSV antibodies indicates the presence of a PRRSV infection or an effective PRRSV vaccination. Thus, if PRRSV antibodies are detected in the serum, a PRRSV infection may be actually present or may have been present in the past. In a preferred embodiment of the present methods, pieces of information regarding the state of the PRRSV infection are derivable. This has the advantage that it becomes possible to differentiate whether a PRRSV infection is in an onset phase, has manifested itself, or is already subsiding. This differentiation may be carried out taking into account the various Ig- subclasses. In addition, it has unexpectedly been found that the antibodies directed against GP5, preferably directed against the C-terminus of GP5, and more preferably those directed against the fragment as set forth in SEQ ID NO: 1 and/ or 9, are among the first to be produced by an infected individual, and furthermore are among the first to stop being produced by the individ- ual. In comparison, antibodies directed against the N protein likewise are produced quickly, but tend to be produced for an extended period of time during the infection. Thus, detection of antibodies against GP5 indicates a recent occurrence of a PRRSV infection.

In a further aspect, the present invention is directed to a system for the detection of PRRSV antibodies, wherein the system comprises a support, and immobilized thereon at least two PRRSV proteins or fragments thereof.

It will be readily apparent to the skilled person that the features described above in connection with the methods of the present invention can be easily adapted into features of the system ac- cording to the invention. This observation is particularly true for the PRRSV proteins immobilized on the support. Therefore, several preferred embodiments of the invention are explicitly stated for the system below:

In one embodiment, one of the at least two PRRSV proteins immobilized on the support is NSp7. In a preferred embodiment, the at least two PRRSV proteins immobilized on the support comprise GP5 and NSp7. As explained above, both proteins yield an increased sensitivity of the assay. It was found that if both proteins are combined in an assay, an advantageous synergetic effect will occur. In another embodiment of the invention, at least three PRRSV proteins are immobilized on the support. While any combination of three proteins selected from the group consisting of GP5, GP3, the M-protein, the E-protein, the N-protein, and NSP7 is possible, and is contemplated by the present invention, a combination of three proteins comprising GP5 and NSp7 as well as at least one protein selected from the group consisting of GP3, the M-protein, the E-protein, and the N-protein is preferred.

In one preferred embodiment according to the invention, a combination of three proteins comprising GP5 and NSp7, as well as at least one protein selected from the gtoup consisting of M- protein, the E-protein, GP3 and the N-protein, is utilized. More preferably, a combination of the three PRRSV proteins GP5, NSp7 and GP3 is used. The GP5 protein is preferably selected from the group consisting of a fragment of the GP5 protein of the European or North American strain, aa 33-201 of the GP5 protein of the European strain (SEQ ID NO: 1) and aa 26-200 of the GP5 protein of the North American strain (SEQ ID NO: 9). The GP3 protein is preferably selected from the group consisting of a fragment of the GP3 protein of the European or North American strain, aa 55-238 of the GP3 protein of the European strain (SEQ ID NO: 5) and aa 61-201 of the GP3 protein of the North American strain (SEQ ID NO: 13). Additional combinations of four, five or six of said PRRSV proteins are likewise encompassed by the present invention.

The support can be any suitable support known to the skilled person. Preferably, the support is suitable for the use in immunological techniques, such as an ELISA technique. In one embodiment of the invention, the support is a solid support, preferably a microtiter plate. In a further embodiment of the invention, the system additionally comprises a microtiter plate reader. Such readers are commonly know to the skilled person and readily available.

In another aspect, the present invention is directed to a kit for the detection of PRRSV antibodies, wherein the system comprises at least two PRRSV proteins or fragments thereof.

As explained above, it will be readily apparent to the skilled person that the features described above in connection with the method or system of the present invention can be easily adapted into features of the kit according to the invention. This observation is particularly true for the PRRSV proteins contained in the kit. Therefore, some of the preferred embodiments applicable for the kit are as follows:

In one embodiment, one of the at least two PRRSV proteins comprised by the kit is NSp7. In a preferred embodiment the at least two PRRSV proteins comprised by the kit comprise GP5 and NSp7. More preferably, at least three PRRSV proteins are comprised by the kit, most preferably a combination of three proteins comprising GP5 and NSp7 and at least one protein selected from the group consisting of GP3, the M-protein, the E-protein, and the N-protein is preferred. More preferably, a combination of the three PRRSV proteins GP5, NSp7 and GP3 is contained in the kit. The GP5 protein is preferably selected from the group consisting of a fragment of the GP5 protein of the European or North American strain, aa 33-201 of the GP5 protein of the European strain (SEQ ID NO: 1) and aa 26-200 of the GP5 protein of the North American strain (SEQ ID NO: 9). The GP3 protein is preferably selected from the group consisting of a fragment of the GP3 protein of the European or North American strain, aa 55-238 of the GP3 protein of the European strain (SEQ ID NO: 5) and aa 61-201 of the GP3 protein of the North American strain (SEQ ID NO: 13). Additional combinations of four, five or six of said PRRSV proteins are likewise encompassed by the kit of the present invention.

In a further embodiment of the invention, the kit additionally comprises a support. The support can be any suitable support known to the skilled person. Preferably, the support is suitable for the use in immunological techniques, such as the ELISA technique. In one embodiment of the invention, the support is a solid support, preferably a microtiter plate.

In another embodiment of the invention, the kit additionally comprises reagents used for immu- nological techniques. Preferably, such reagents include buffers, secondary antibodies, preferably labeled and/or conjugated and substrates for the detection reaction, such as TMB or DAB.

Further advantageous embodiments of the present invention are provided in the dependent claims, the Figures and/ or the Examples provided below. EXAMPLES

EXAMPLE 1 : Expression of recombinant PRRSV proteins Chemically competent E. coli cells (Rosetta DE3) were transformed with the vector pMal Tag2 containing the PRRSV gene of interest. The vector pMal Tag2 (SEQ ID NO: 27) is a modification of the pMal™-c2X plasmid (New England Biolabs) such that a C-terminal His-tag is added to the MBP-fusion protein. FIG. 1 shows a plasmid map of the vector. The sequence of MBP is known (E.coli; GenBank accession AAB86559). For reasons of convenience, it is presently set forth herein at SEQ ID NO: 28.

TABLE 1 summarizes those DNA sequences that were cloned into the pMal Tag2 vector at the indicated positions for expressing the various PRRSV proteins. The first column designates the name of the expressed PRRSV protein: Proteins /cDNA derived from the European PRRSV strain are designated with an "EU", proteins/ cDNA derived from the North American PRRSV strain are designated with a "VR". The second column designates the position and restriction sites of the pMal Tag2 vector used to insert the PRRSV cDNA. The third column designates whether the PRRSV protein was expressed as a full-length protein or as a fraction thereof. The remaining columns designate the SEQ ID NOs of the expressed protein sequence and the cDNA sequence cloned into the vector, respectively.

TABLE 1

The transformed cells were plated out on LB agar containing ampicillin (100 μg/ml) and cultivated over night at 37°C. On the following day, a single colony from the plate was resuspended in 5 ml of LB(Amp)-medium and incubated overnight at 37°C at 200 rpm. Subsequendy, LB (Amp) -medium was inoculated with this culture in a ratio of 1 :100. Again, the culture was incubated at 37°C and 200 rpm until an OD₆₀₀ of 0.6-0.8 was reached. At this point, the protein expression was induced by adding IPTG to a final concentration of 1 mM. Protein expression was allowed to take place over night at the above-mentioned conditions.

EXAMPLE 2: Protein preparation and purification

The E. colt cultures described above were centrifuged for 20 min at 4000 x g. The supernatant was discarded and the pellet resuspended in lysis buffer (50 mM Tris pH 7.5, 2 mM EDTA) to a theoretical OD₆₀₀ of 50. The cells were sonicated on ice for 4 x 1 min and centrifuged afterwards (15 min at 10000 g). The supernatant was dialyzed against a His-Tag binding buffer (20 mM Na₂HP0₄/NaH₂P0₄, pH 7.4, 500 mM NaCl, 20 mM Imidazole) at a ratio of 1 :200.

This crude extract was subsequently loaded onto a GE Healthcare HisTrap column. Unbound protein was separated by a washing step using 10 column volumes (CV) of HisTag-binding buffer. In turn, the elution of the bound proteins was carried out using 10 CV of HisTag-Elution buffer (20 mM Na₂HP0₄/NaH₂P0₄, pH 7.4, 500 mM NaCl, 500 mM Imidazole). The chromatography run was analyzed via SDS-PAGE and fractions containing the desired protein were pooled and dialyzed over night against Maltose Binding Protein (MBP) binding buffer (20 mM Tris, pH 7.4, 200 mM NaCl, 1 mM EDTA) at a ratio of 1:200.

A column packed with Amylose Resin (New England Biolabs) was loaded with the dialyzed pro- tein solution from the first purification step. Again, the unbound proteins were washed away with 5 CV MBP binding buffer. In the subsequent step the purified protein was eluted with 5 CV of MBP elution buffer (20 mM Tris, pH 7.4, 200 mM NaCl, 1 mM EDTA, 10 mM Maltose).

Finally, the protein identity and purity was confirmed via SDS-PAGE gel electrophoresis and the concentration of the purified proteins was determined by a BCA protein assay (Pierce).

EXAMPLE 3: PRRSV ELISA utilizing expressed GP5 protein

Two expressed GP5 protein fragments (SEQ ID NOs: 1 and 3) were used in an immunological detection of PRRSV antibodies and compared to expressed ORF7 protein (SEQ ID NO: 17). The ELISA was carried out with four sera in total. Two of the sera were known to be PRRSV positive, i.e. they were known to comprise PRRSV antibodies, and the remaining two sera were PRRSV negative, i.e. derived from a subject that had not been infected by PRRSV. In a first step, the recombinant proteins and the negative control MBP are diluted to the appropriate concentration in 50 mM Na₂C0₃ buffer. The final concentration of PRRSV proteins and MBP was 20 ng/well. Other concentrations like 90 ng/well may also be used.

In a next step, 100 ul volumes of the diluted antigen are dispensed into an appropriate number of wells of a flat-bottomed, high protein binding ELISA microplate (Polysorp, Nunc 475094) using 2 wells per test serum as well as 2 wells for the negative control. Subsequently, the plate was sealed and incubated overnight at 4°C.

On the following day, the plates were washed 3 times with lx PBS-Tween 0.05% and 200 ul blocking buffer (PBS with 5% low fat milk) was added. After sealing the plates were again incubated on an orbital shaker for 2 h at room temperature. Following incubation, the plates were washed 3 times with lx PBS-Tween 0.05%. In a next step, the sera were diluted 50 times in the blocking buffer and 100 ul were dispensed in each test well. Consequendy, the plates were incubated on an orbital shake for 1 hour at room temperature and afterwards washed 3 times with lx PBS-Tween 0.05%. The working dilution of rabbit anti-pig immunoglobulin horse radish peroxidase conjugate in blocking buffer (1:10,000 Sigma A5670) was prepared and 100 μΐ were added to each test well. In turn, the plates were incubated on an orbital shaker for 1 hour at room temperature, washed as described before and immediately re-filled with 00 ul TMB One Solution (Promega G7431) as substrate. After incubation at room temperature for 30 min in a dark place without shaking, 50 ul of stopping solution (1 M sulphuric acid) were added.

The resulting absorbance values were measured on an ELISA reader at 450 nm and 520 nm as a reference. As can be observed from the results presented in FIG. 2, the recombinant GP5 proteins lead to improved antibody signals compared to the N protein (here, ORF7). The sensitivity of the immunological PRRSV detection is significandy increased, while the specificity is not negatively affected. EXAMPLE 4: PRRSV ELISA with multiple PRRSV proteins and comparison to IDEXX ELI- SA

A PRRSV ELISA utilizing multiple PRRSV antigens of the European strain was carried out. The proteins were individually coated on 96-well plates and analyzed in an ELISA using ten PRRSV- positive and negative sera each (N1-N10; P1-P10). The sera had previously been analyzed by the commercial available IDEXX ELISA to determine whether a given serum was PRRSV-positive or PRRSV-negative.

In a first step, the recombinant proteins and the negative control (MPB) were diluted to the ap- propriate concentration in 50 mM Na₂C0₃ buffer.

Afterwards, 100 ul volumes of the diluted antigen were dispensed into the wells of a flat- bottomed, high protein binding ELISA microplate (Polysorp, Nunc 475094) using 2 wells per test serum as well as 2 wells for the negative control. The final PRRSV-antigen concentrations used for the coating of the plates were: SEQ ID NO: 17 20 ng/well; SEQ ID NO: 19 15 ng/well; SEQ ID NO:l 20 ng/well; and SEQ ID NO:7 100 ng/well. The plate was sealed and incubated overnight at 4°C. On the following day, the plates were washed 3 times with lx PBS-Tween 0.05% and 200 ul blocking buffer (PBS with 5% low fat milk) was added. After sealing, the plates were incubated on an orbital shaker for 2 h at room temperature and subsequently washed 3 times with 1 x PBS- Tween 0.05%. The sera were diluted 50 times in blocking buffer and 100 μΐ were dispensed in each test well. The plates were then incubated on an orbital shaker for 1 hour at room temperature and washed 3 times with lx PBS-Tween 0.05%.

The working dilution of rabbit anti-pig immunoglobulin horse radish peroxidase conjugate (1:10,000 Sigma A5670) was prepared in blocking buffer and 100 μΐ were added to each test well. In turn, the plates were incubated on an orbital shaker for 1 hour at room temperature.

Following this period, the plates were washed as before and immediately re-filled with 100 ul TMB One Solution (Promega G7431) as a substrate and incubated at room temperature for 30 min in a dark environment without shaking. 50 ul of stopping solution consisting of 1 M sulphuric acid was then added.

The resulting absorbance values are measured on an ELISA reader at 450 nm and 520 nm as a reference.

As can be observed from the results depicted in FIG. 3, an ELISA assay combining multiple PRRSV proteins, according to the invention, yields markedly more specific and sensitive PRRSV antibody detection then the commercially available IDEXX ELISA. All of the samples P1-P10 tested positive and likewise tested positive using the ELISA assay according to the invention, and partially with even higher absorption readings. Notably, of the ten samples, N1-N10 tested negative using the IDEXX ELISA, yet samples N9 and N10 were clearly positive in the ELISA assay according to the invention. These results were further confirmed by an ELISA assay, according to the invention, utilizing SEQ ID NO: 3 and/or SEQ ID NO: 21 as a PRRSV antigen. To summarize, the ELISA assay provided by the present invention provided for the detection of two PRRSV positive sera that were tested as false-negatives using the commercially available IDEXX ELISA assay. EXAMPLE 5: PRRSV ELISA with PRRSV proteins of a North American strain

The ELISA assays described in Examples 3 and 4 herein were repeated with the corresponding PRRSV proteins of a North American strain. Thus, SEQ ID NOs: 1, 3, and 17 of Example 3 were exchanged with SEQ ID NOs: 9, 11, and 23. Furthermore, SEQ ID NOs: 17, 19, 1, and 7 of Example 4 were exchanged with SEQ ID NOs: 23, 25, 9, 15.

The ELISA assay according to the present invention utilizing PRRSV proteins from a North American strain were shown to yield similar results as those obtained using the European strain as described above (Lelystad Virus (LV)).

EXAMPLE 6: PRRSV ELISA with the three PRRSV proteins GP5. NSp7 and GP3

ELISA assays employing various combinations of the PRRSV proteins, GP5, NSp7 and GP3 were tested. The ELISA assays were carried out as described in Examples 3-5 above.

The NSp7 protein used was SEQ ID NO: 19. The GP5 protein used was either the GP5 fragment set forth in SEQ ID NO: 1 or SEQ ID NO: 3. The GP3 protein used was the GP3 fragment set forth in SEQ ID NO: 5.

Concentration of each individual protein was 30 ng/well, hence 90 in total ng/well.

While all of the above combinations were observed to work extremely well, a combination of SEQ ID NO 19, SEQ ID NO: 1 and SEQ ID NO: 5 yielded the best, most effective results.

The ELISA assays described above were repeated with the corresponding PRRSV proteins of a North American strain and yielded similar results. Thus, SEQ ID NOs: 19, 1, and 3 were exchanged for SEQ ID NOs: 25, 9 and 11. EXAMPLE 7: Comparison of an ELISA according to the present invention with the commercially available PRRSV-ELISA systems IDEXX. INGENASA and Hipra

To compare the performance of a PRRSV ELISA according to a preferred embodiment of the present invention with other available assays, eighteen porcine serum samples from a proficiency testing scheme (PTS) for PRRSV were analyzed with three different commercially available PRRSV-ELISAs (IDEXX, INGENASA and Hipra) and with an ELISA comprising antigens of the SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7). The samples had a defined infection status, for six of them with a defined PRRSV genotype.

The ELISA assays were carried out as described in Examples 3-6 above or according to the manufacturer's instructions.

FIG. 4 A illustrates the optical density-results for the PRRSV ELISA comprising antigens of the SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7). The optical density-results are depicted separately for US and EU antigens. FIG. 4 B depicts the optical density-results obtained for the IDEXX HerdCheck PRRS X3 ELISA. Cut-Off values were calcu- la ted according to the manufacturer's instructions or by adding three standard deviations to the internal negative control (neg) for the PRRSV ELISA according to the present invention. For INGENASA and HIPRA, results were obtained from the final report of the PTS trial (Wellen- berg et al., GD BV Animal Health Service Ltd., Deventer, The Netherlands, 2010) All test systems failed to detect antibodies in the negative pig serum (sample PTS3, FIG. 4, Table 2). However, the PRRSV ELISA according to a preferred embodiment of the present invention performed best in correctly recognizing the other sera (Table 2). It was the most sensitive assay, as it detected the highly diluted samples 14 and 17. Two samples (PTS12 and PTS14) were only detected as positive by the PRRSV ELISA according to the present invention. Furthermore, the PRRSV ELISA according to the present invention was most successful in extinguishing PRRSV infections with American from infections with the European genotype (Table 3). For instance, antibodies present in samples PTS5 and PTS12 (both infected with an US genotype virus) only recognized the US antigen-mix.

The three commercially available ELISAs contain a single antigen, whereas the PRRSV ELISA according to the present invention consists of three different antigens. The data presented here clearly show that there is a significant advantage of the present invention in sensitivity and differentiation of genotypes compared with other, established systems. TABLE 2

positive; -: negative; +/-: half of the tests executed detected this sample as positive

SPF: specific pathogen free; vacc: vaccinated with the US type PRRSV vaccine

* SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7).

TABLE 3

ID Genotype EU/US differentiated by

1 EU Ingenasa (2.5 x more OD *

PRRSV ELISA according to the present invention (4 x more OD)

2 EU Ingenasa (2.5 x more OD)* ELISA** comprisingGP3, GP5 and NSP7 (4 x more OD)*

5 US ELISA** comprisingGP3, GP5 and NSP7

9 US Ingenasa (50%), Hipra

ELISA** comprisingGP3, GP5 and NSP7

12 US ELISA** comprisingGP3, GP5 and NSP7

15 US Hipra (2-3 x more OD)*

16 US Ingenasa (50%)

* OD-value of the test specific for one genotype compared to the test specific for the other genotype ** SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7).

EXAMPLE 8: Synergistic effects between the single antigens of PRRSV ELISA

It is important to test which antigens comprised in the test are the critical ones. These are the antigens eliciting the highest antibody signal. Unexpectedly, a clear synergistic effect is seen with a combination of the individual antigens is analysed in a PRRSV ELISA according to a preferred embodiment of the present invention, comprising antigens of the SEQ IDs 1 and 9 (GP5), SEQ IDs 5 and 13 (GP3) and SEQ IDs 19 and 25 (NSP7). The ELISA assay was carried out as described in Examples 3-7 above.

FIG. 5 shows 18 different PRRSV-positive pig sera, reacting with the individual antigens present in the PRRSV ELISA antigen mixture. The antigens that elicit the strongest signal vary significantly between the serum samples. In some sera, NSP 7 (SEQ ID NO: 19 and 25) is the dominant antigen. Other sera recogni2e GP3 (SEQ ID NO:5 and 13) or GP5 (SEQ IDNO:l and 9) most strongly. Therefore, to detect all the sera as positive, the presence of all three of the antigens is essential.

As examples, sera A and C are only detected as positive because antibodies against GP3 (SEQ ID NO: 13) are present, similarly to NSP7 (SEQ ID NO: 19) in serum O. In serum G, the dominant antibody response is measured against GP3 (SEQ ID NO:5) and GP5 (SEQ ID NO:l). In other examples (L), the presence of two antigens allows for the discrimination between infections by the US and EU-strains. Here, using only US_GP5 (SEQ ID NO:9) would lead to a dif- ference which would be far less significant compared to the addition of the signals elicited by US_GP3 (SEQ ID NO:13). In sera Q and R, addition of the GP3 antigen (SEQ ID NO: 13) to the NSP7 antigen (SEQ ID NO:25) leads to a significantly increased antibody signal and enables a clearer discrimination of the PRRSV US genotype against the EU genotype.

Hence, due to the high degree of variability in the anti-PRRSV antibody responses of pigs, a clear synergistic effect is seen by using the mixture of antigens, namely NSP7, GP5 and GP3 of both genotypes. The presence of all three of these antigens is essential to detect the maximum numbers of PRRSV infections. This leads to a significant advantage over available technologies relying only on one antigen or on antigen mixtures different from the one used according to a preferred embodiment of the present invention.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be advantageously used according to the invention. Any reference signs in the claims should not be construed as limiting scope.

Claims

1. A method for the detection of PRRSV antibodies comprising the steps of:

a) providing at least two PRRSV proteins or fragments thereof;

b) contacting the at least two PRRSV proteins with serum under conditions to allow the binding of PRRSV antibodies in the serum to the PRRSV proteins or fragments; and

c) detecting the PRRSV antibodies bound to the PRRSV proteins.

2. The method according to claim 1, wherein the at least two PRRSV proteins are selected from the group consisting of GP5, GP3, M-protein, E-protein, N-protein, and NSP7.

3. The method according to any of the preceding claims, wherein at least one of the PRRSV proteins is a recombinant protein.

4. The method according to claim 3, wherein at least one of the PRRSV proteins further comprises an MBP-tag and/or a His-tag.

5. The method according to any of the preceding claims, wherein at least one of the PRRSV proteins is derived from a European PRRSV strain Lelystad or a North American PRRSV strain, preferably VR-2332.

6. The method according to any of the preceding claims, wherein one of the at least two PRRSV proteins is the GP5 protein.

7. The method according to any of the preceding claims, wherein one of the at least two PRRSV proteins is the NSp7 protein.

8. The method according to any of the preceding claims, wherein three proteins are provided and are selected from the group consisting of a fragment of the GP5 protein, preferably SEQ ID NO: 1 or 9, the NSp7 protein, preferably SEQ ID NO: 19 or 25, and a fragment of the GP3 protein, preferably SEQ ID NO: 5 or 13.

9. The method according to any of the preceding claims, wherein the serum is selected from the group consisting of blood, serum, sputum, saliva and/ or milk.

10. The method according to any of the preceding claims, wherein the detection is carried out immunologically.

11. The method according to any of the preceding claims, wherein the detection of PRRSV antibodies indicates a PRRSV infection or an effective PRRSV vaccination.

12. A system for the detection of PRRSV antibodies comprising a support and immobilized thereon at least two PRRSV proteins or fragments thereof.

13. A kit comprising at least two PRRSV proteins or fragments thereof.

14. The system according to claim 12 or the kit according to claim 13, wherein three proteins are provided that are selected from the group consisting of a fragment of the GP5 protein, preferably SEQ ID NO: 1 or 9, the NSp7 protein, preferably SEQ ID NO: 19 or 25, and a fragment of the GP3 protein, preferably SEQ ID NO: 5 or 13.