Patent Publication Number: US-2023159604-A1

Title: Plasmid encoding b-cell activating factor receptor (baff-r) and uses of same in the treatment and prevention of inflammatory diseases in fish

Description:
The present invention is included in the technical sector of aquaculture, specifically in the treatment and prevention of diseases that cause B-cell mediated inflammation. Thus, the invention relates to a plasmid comprising a nucleotide sequence that encodes a fusion protein comprising a signal peptide and the extracellular domain of the B-cell activating factor receptor (BAFF-R), and optionally, an Fc fragment of an immunoglobulin. In particular, the present invention also discloses the use of said plasmid and of compositions comprising it as a medicament, and more specifically, for the treatment and/or prevention of diseases that cause B-cell mediated inflammation in fish, more preferably in salmonids. 
     BACKGROUND OF THE INVENTION 
     The growth of intensive aquaculture has caused an increase in the incidence of inflammatory diseases in fish caused by viruses, bacteria or parasites. In this sense, the incidence of diseases that cause B-cell mediated inflammation, such as Proliferative Kidney Disease (PKD) caused by the myxozoan  Tetracapsuloides bryosalmonae , or the Red Mark Syndrome, is increasingly common in salmonid aquaculture in both Europe and North America (Grabner, D. S., and M. El-Matbouli. Dis. Aquat. Organ. 2010; 90: 197-206; Okamura, B., et al. Freshwater Biol. 2011; 56: 735-753). In Spain, PKD disease has been identified as one of the biggest problems in Rainbow trout ( Oncorhynchus mykiss ) farming, causing significant economic losses in this sector, mainly during the summer months when the water reaches a temperature higher than 15° C., which favours parasite proliferation and infection. 
     The myxozoan  T. bryosalmonae  has a double host cycle which affects different salmonid species and the bryozoan  Fredericella sultana , which is the invertebrate host. Bryozoans infected by this parasite release malacospores into the water that invade the gills of salmonids. Subsequently, the parasite migrates through the vascular system to the kidney and spleen, these organs being the main target organs for their development and proliferation in salmonids. When the water temperature is higher than 15° C., the extrasporogonic proliferation and development of  T. bryosalmonae  in the interstitial tissue of the kidney in salmonids produces chronic inflammation characterized by lymphocytic hyperplasia, the formation of granulomatous lesions, renal atrophy and hypersecretion of immunoglobulins by the B lymphocytes. As a result of immune system deregulation after parasitic infection, fish are more susceptible to secondary infections and mortalities can reach up to 100%. In contrast, when the water temperature is lower than 15° C., the host develops a moderate immune response against the parasite, associated with fewer clinical symptoms and low mortality. 
     Eukaryotic expression plasmids that encode a vaccine antigen have been used in fish as vaccines. In this sense, international patent application WO2006035084 describes gene constructs comprising the coding sequence for immunogenic peptides of pathogens of aquatic animals and the use of said constructs as vaccines for the prevention of infectious diseases in aquatic animals. Furthermore, international patent application WO2008077413A1 describes the use, method and formulation of inclusion of DNA vaccines in food compositions for livestock animals, in particular in aquacultural systems. Patent ES2321789B1 also describes the use of an expression vector as an immunostimulant or adjuvant for DNA vaccines and to prevent infection of fish by a rhabdovirus, such as Viral Hemorrhagic Septicemia Virus (VHSV) and infectious Hematopoietic Necrosis Virus (IHNV). Furthermore, international patent application WO2014041189A1 also describes nucleic acids, as well as vaccines comprising said nucleic acids, and the use of same against the agents that cause Salmon Pancreas Disease (SPD) caused by a salmonid alphavirus (SAV). 
     It is important to mention that, until 2017, the injection of expression vectors or plasmids for the prevention of diseases in aquaculture was not allowed in fish intended for human consumption in the European Union, but it was precisely in that year when the European Commission approved the use of a DNA vaccine for intramuscular injection in aquaculture. Specifically, it involves the injection of the pUK-SPDV-poly2#1 plasmid, marketed as Clynav, Elanco GmbH, Germany, to protect Atlantic salmon ( Salmo salar ) against SPD caused by salmonid alphavirus subtype 3 (SAV3) (CVMP assessment report for CLYNAV (EMEA/V/C/002390/0000). Salmon pancreatic disease vaccine (recombinant DNA plasmid)). 
     On the contrary, there are no commercial treatments to prevent and/or treat PKD or the red mark syndrome in salmonids, since the use of effective compounds against the parasite  T. bryosalmonae , such as malachite green and fumagillin, are not registered in the European Union for use in aquaculture due to their harmful effects on the health of humans who consume said animals. To that end, taking into account that the incidence of this disease is increasing, in the state of the art there is a need to develop useful compounds for the treatment of pathologies that cause B-cell mediated inflammation, such as, for example, the PKD disease caused by the myxozoan  T. bryosalmonae  or the red mark syndrome, in salmonids. Said compounds and/or prevention and/or treatment systems should be, advantageously, easy and cost-effective in terms of production and administration. 
     DESCRIPTION OF THE INVENTION 
     To solve the aforementioned technical problem, the inventors have used the strategy of inhibiting the signaling mediated by the cytokine BAFF (B-cell activating factor), which is involved in processes of maturation, activation of B lymphocytes, and development and activation of lymphoid organs. This cytokine performs important regulatory functions by inducing pleiotropic responses through its interaction with three receptors: TACI, BCMA and BAFF-R, whose expression is fundamentally restricted to B and T lymphocytes. To carry out this strategy, eukaryotic expression plasmids have been designed which comprise sequences that encode for each of the soluble regions of each BAFF receptor, specifically BAFF-R, TACI and BCMA, independently. Thus, the administration of said plasmids, by intramuscular route, is capable of inhibiting the inflammatory response, and therefore, they are useful in the treatment of infections that cause B-cell mediated inflammation, such as, for example, PKD and the red mark syndrome, in salmonids, preferably in Rainbow trout. 
     In particular, the present invention relates to the synthesis of a fusion protein comprising an amino acid sequence comprising the soluble region (extracellular domain) of BAFF-R, or the soluble region of TACI, or the soluble region of BCMA, from Rainbow trout, bonded immediately after an amino acid sequence comprising the Rainbow trout interleukin-2 (IL-2) signal peptide; and when said fusion protein is administered to the Rainbow trout, preferably comprised in a plasmid, it is able to block the activity of the cytokine BAFF, thus being useful in the treatment of infections that cause B-cell mediated inflammation, such as, for example, PKD and the red mark syndrome. In particular, PKD is associated with a large increase in the expression levels of the cytokine BAFF and is caused by infection with the parasite  T. bryosalmonae . Thus, through said fusion proteins, included independently in a plasmid that is administered to infected animals, the pathology associated with said infection is reduced, and therefore, mortality it entails is reduced, especially in summer when the water temperature is higher than 15° C. 
     As mentioned earlier, the treatment of infections in aquaculture has been mainly based on the use of vaccines comprising antigens of the pathogen; in contrast, the present invention is aimed at obtaining molecules, specifically plasmids or vectors, comprising nucleotide sequences that encode endogenous peptide-based fusion proteins of the host, with the purpose of blocking immunological pathways that are altered when the animal has been infected by a pathogen, therefore being useful in the treatment and/or prevention of said infections. 
     As shown in the examples included herein, the administration of the plasmid comprising the nucleotide sequence that encodes the aforementioned fusion protein in trout naturally infected with the parasite  T. bryosalmonae  reduces the degree of kidney inflammation and the mortality of said animals (Example 4), as well as the parasitic load in animals treated with the plasmid of the invention (Example 5). 
     Thus, in a first aspect, the present invention relates to a fusion protein, hereinafter called fusion protein of the invention, comprising a first amino acid sequence having at least 95%, 96%, 97%, 98%, 99% identity to an amino acid sequence as shown in SEQ ID NO: 2, fused to a second amino acid sequence, wherein said second amino acid sequence has at least 95%, 96%, 97%, 98%, 99% identity to an amino acid sequence as shown in SEQ ID NO: 4. 
     The first amino acid sequence that forms the fusion protein of the invention corresponds to a sequence that encodes a signal peptide; preferably it is an amino acid sequence that encodes the interleukin-2 (IL-2) signal peptide. For purposes of the present invention, the IL-2 signal peptide comes from Rainbow trout and corresponds to amino acids 1 to 20 of the protein with accession number CAM12545.1 in the NCBI (National Center for Biotechnology Information) database. 
     For purposes of the present invention, the term “signal peptide”, “signal sequence” or “localization signal peptide”, used interchangeably herein, refers to a short peptide (5-30 amino acids in length) present at the N-terminus which directs protein transport into the secretory pathway. The signal peptide directs translocation into the endoplasmic reticulum of the protein to which it is bonded. During or after translocation, the signal peptide is cleaved by a signal peptidase, generating a free signal peptide and a secreted mature protein. Signal peptides suitable for use in the present invention include, but are not limited to, signal peptides, capable of directing a protein to the cell membrane, to the nucleus, to the nuclear membrane, to the mitochondrial matrix, to the mitochondrial membrane, to the endoplasmic or sarcoplasmic reticulum, to the cytoplasm, to the Golgi complex, to the chloroplast, to the apoplast or to the peroxisome. In a preferred embodiment, the signal peptide of the fusion protein of the invention is the Rainbow trout IL-2 signal peptide (SEQ ID NO: 2). 
     The second amino acid sequence of the fusion protein of the invention corresponds to the extracellular domain of the BAFF receptor (BAFF-R) from Rainbow trout (SEQ ID NO: 4). The extracellular domain of BAFF-R corresponds to amino acids 21 to 75 of the protein with accession number AQZ26593 in the NCBI database. BAFF-R is a membrane receptor belonging to the tumour necrosis factor (TNF) receptor and ligand superfamily and it is responsible for regulating B-cell survival, proliferation and differentiation. 
     In a preferred embodiment, the fusion protein of the invention comprises a first amino acid sequence such as SEQ ID NO: 2 and a second amino acid sequence such as SEQ ID NO: 4. In another more preferred embodiment, the fusion protein of the invention comprises SEQ ID NO: 8. 
     In another preferred embodiment, the fusion protein of the invention further comprises an amino acid sequence of an Fc domain of an immunoglobulin (Ig). In a more preferred embodiment, the Fc domain belongs to a mouse immunoglobulin G (IgG), selected from IgG1, IgG2, IgG3 and IgG4 isotypes, as well as any allotype within each group of isotypes. The Fc domain of an IgG in the fusion protein of the invention allows for the detection and purification of said fusion protein. In a more preferred embodiment, the Fc domain of a mouse IgG1 comprises an amino acid sequence having at least 95, 96, 97, 98 and 99% identity to SEQ ID NO: 6. In another even more preferred embodiment, the Fc domain of the mouse IgG1 is SEQ ID NO: 6. 
     In another preferred embodiment, the fusion protein of the invention comprises an amino acid sequence with at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 10. More preferably, the fusion protein of the invention comprises SEQ ID NO: 10. 
     The combination of polypeptides to provide a fusion protein can be achieved by several means, for example: chemically by direct coupling or through an intermediate structure; or by molecular biological fusion, through the combination of recombinant nucleic acid molecules comprising nucleic acid fragments capable of encoding the two, such that a single continuous expression product is finally produced. Thus, for purposes of the present invention, the term “fused to” or “bonded to”, used interchangeably herein, refers to, but is not limited to, a polypeptide or fusion protein formed by the expression of a chimeric nucleotide sequence created by combining more than one sequence, generally by cloning a gene in an expression vector in a frame with a second gene such that the two genes encode a continuous polypeptide. 
     The term “identity”, “percent identity” or “sequence identity” between two sequences (nucleic acids or proteins) is understood as designating a percentage of identical nucleotides or amino acids between the two sequences that are compared, which is obtained after the best alignment, said percentage being purely statistical and the differences between the two sequences being distributed randomly and along the entire length thereof. “Best alignment” or “optimal alignment” is understood as designating the alignment by which the percent identity determined, as described below, is the highest. Comparisons between two nucleotide or amino acid sequences are typically carried out by comparing these sequences after they have been optimally aligned, said comparison being carried out by segment or by “comparison window” to identify and compare the local regions of sequence similarity. Optimal alignment of these sequences for their comparison can be performed, in particular, with the help of one of the following algorithms: Smith and Waterman&#39;s local homology algorithm (1981), Neddleman and Wunsch&#39;s local homology algorithm (1970), Pearson and Lipman&#39;s search for similarity method (1988), computer programs that use these algorithms (GAP, BESTFIT, BLASTP, BLASTN, BLASTX, TBLASTX, FASTA and TFASTA in the Wisconsin Genetics Software package (Genetics Computer Group, 575 Science Dr., Madison, Wis.), or internet servers, in particular those of the National Center for Biotechnology Information (NCBI) (http://www.ncbi.nlm.nih.gov), EMBL (http://www.embl.org) and the Ensembl project (http://www.ensembl.org). To obtain the optimal alignment, the BLAST program is preferably used, with the BLOSUM 62 matrix. PAM or PAM250 matrices can also be used, as well as an identity matrix for the nucleotide sequences. 
     In a second aspect, the present invention relates to a nucleic acid, hereinafter called nucleic acid of the invention, which encodes the fusion protein according to the invention. In a preferred embodiment, the nucleic acid of the invention comprises a first nucleotide sequence comprising at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 1, more preferably SEQ ID NO: 1, fused to a second nucleotide sequence comprising at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 3, more preferably SEQ ID NO: 3. In another more preferred embodiment, the nucleic acid of the invention comprises a nucleotide sequence that has at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 7, more preferably the nucleic acid of the invention comprises SEQ ID NO: 7. 
     In another more preferred embodiment, the nucleic acid of the invention further comprises a nucleotide sequence that encodes the Fc domain of mouse IgG1, wherein said nucleotide sequence comprises at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 5, more preferably SEQ ID NO: 5. 
     In another more preferred embodiment, the nucleic acid of the invention comprises a nucleotide sequence comprising at least 95, 96, 97, 98, 99% identity to SEQ ID NO: 9, more preferably comprising SEQ ID NO: 9. 
     In another aspect of the invention, this relates to an expression vector or plasmid, hereinafter vector or plasmid of the invention, which comprises the nucleic acid of the invention, wherein, optionally, said nucleic acid is operably bonded to an expression control sequence suitable for expression in a host cell. 
     The term “expression vector” or “expression plasmid”, used interchangeably herein, refers to a DNA fragment that has the ability to replicate in a given host and can serve as a carrier to carry out the transcription of a sequence of interest that has been inserted into the same. The expression vector or plasmid can also be incorporated into a cosmid, a bacteriophage, a viral vector, without excluding vectors of another type corresponding to the provided definition of vector. 
     For purposes of the present invention, the term “operably bonded”, as used herein, refers to a control sequence, for example, a promoter or operator, which is suitably placed in a position relative to a coding sequence such that the control sequence directs the production of a polypeptide encoded by the coding sequence. 
     An expression vector or plasmid in the context of the present invention may be any suitable vector or plasmid, including chromosomal, non-chromosomal and synthetic nucleic acid vectors (a nucleic acid sequence comprising a suitable set of expression control elements). Examples of such vectors include derivatives of SV40, bacterial plasmids, phage DNA, baculovirus, yeast plasmids, vectors derived from combinations of plasmids and phage DNA and viral nucleic acid vectors (RNA or DNA). 
     Useful expression vectors or plasmids for eukaryotic hosts include, for example, vectors or plasmids comprising expression control sequences of SV40, bovine papilloma virus, adenovirus, adeno-associated viruses, cytomegalovirus and retrovirus. 
     The expression control sequences are designed to control and direct the transcription of genes of interest and the subsequent expression of proteins in various cell systems or sites of interest. The plasmids combine an expressible nucleotide sequence or gene of interest with expression control sequences (i.e., expression cassettes) comprising desirable elements such as, for example, promoters, enhancers, selectable markers, operators, etc. In the expression vector or plasmid of the invention, the nucleic acid molecules that encode the fusion protein of the invention may comprise or be associated with any promoter, enhancer, selectable marker, operator, repressor protein, polyA termination sequences and other expression-facilitating elements. 
     For purposes of the present invention, the term “promoter” refers to a DNA sequence sufficient to direct the transcription of a DNA sequence to which it is operably bonded, as described previously. Examples of useful expression promoters in the present invention are constitutive promoters such as, for example, the human cytomegalovirus (CMV) promoter/enhancer or CMV major IE (CMV-MIE) promoter, as well as the Rous sarcoma virus (RSV) promoter, simian virus 40 (SV40) late promoter, SL3-3 promoters, MMTV, ubiquitin (Ubi), ubiquitin C (UbC) and HIV LTR. 
     In a preferred embodiment, the vector or plasmid of the invention comprises a promoter selected from the group consisting of SV40, CMV, RSV, SL3-3, MMTV, Ubi, UbC and HIV LTR. In another even more preferred embodiment, the promoter is CMV. 
     The nucleic acid molecules of the invention may also be operably bonded to an effective poly(A) termination sequence, an origin of replication for the plasmid product in  E. coli , an antibiotic resistance gene as a selectable marker and/or a convenient cloning site (e.g., a polylinker). The nucleic acids may also comprise a regulatable inducible promoter (inducible, repressible, developmentally regulated) as opposed to a constitutive promoter such as the CMV IE (a person skilled in the art will recognize that said terms are actually descriptors of a degree of gene expression under certain conditions). 
     The selectable markers are well-known elements in the art. Under the selective conditions, only cells that express the appropriate selectable marker can survive. Commonly, selectable marker genes express proteins, usually enzymes, which confer resistance to various antibiotics in cell cultures. Under other selective conditions, cells that express a fluorescent protein marker are made visible and, therefore, are selectable. The embodiments include beta-lactamase (b/a) (beta-lactam antibiotic resistance gene or ampicillin resistance gene or ampR), b/s (blasticidin resistance acetyl transferase gene), bsd (blasticidin-S-deaminase resistance gene), bsr (blasticidin-S resistance gene), Sh ble (Zeocin® resistance gene), hygromycin phosphotransferase (hpt) (hygromycin resistance gene), tetM (tetracycline resistance gene or tetR), neomycin phosphotransferase II (npt) (neomycin resistance gene or neoR), kanR (kanamycin resistance gene) and pac (puromycin resistance gene). 
     In certain embodiments, the vector or plasmid of the invention comprises one or more selectable marker genes selected from the group consisting of b/a, b/s, BSD, bsr, Sh b/e, hpt, tetR, tetM, npt, kanR and pac. In other embodiments, the vector comprises one or more selectable marker genes that encode green fluorescent protein (GFP), enhanced green fluorescent protein (eGFP), cyan fluorescent protein (CFP), enhanced cyan fluorescent protein (eCFP) or yellow fluorescent protein (YFP). 
     In another more particular embodiment, the plasmid of the invention comprises SEQ ID NO: 11. 
     In another aspect, the present invention relates to a host cell comprising the nucleic acid molecule or the vector of the invention. 
     For purposes of the present invention, the term “host cell” includes any type of cell that is susceptible to transformation, transfection, transduction and the like with a nucleic acid construct or expression vector comprising a nucleotide or polynucleotide sequence that encodes the fusion protein of the invention. The choice of a host cell will largely depend on the nucleotide sequence encoding the polypeptide and its source. The host cell may be eukaryotic, such as a mammalian, insect, bird, fish, amphibian, reptilian, plant or fungal cell. The choice of a suitable host cell may also be influenced by the choice of the detection signal. For example, the use of constructs with reporter genes (e.g., lacZ, luciferase, thymidine kinase or green fluorescent protein “GFP”) can provide a selectable signal by activating or inhibiting transcription of the gene of interest in response to a transcriptional regulatory protein. In order to achieve optimal selection or “screening”, the phenotype of the host cell must be considered. 
     A host cell of the present invention includes prokaryotic and eukaryotic cells. Prokaryotes include Gram-negative organisms (e.g.,  Escherichia coli ) or Gram-positive organisms (e.g., bacteria of the genus  Bacillus ). Prokaryotic cells will be used, preferably, for the propagation of the transcriptional control sequence of the vector containing the polynucleotide(s) object(s) of the invention, which will make it possible to achieve a greater number of copies of the vector containing the polynucleotide(s) object(s) of the invention. Prokaryotic host cells suitable for the transformation of this vector include, for example, but are not limited to,  E. coli, Bacillus subtilis, Salmonella typhimurium , and other species within the genera  Enterococcus, Lactococcus, Pseudomonas, Streptomyces  and  Staphylococcus . In a more particular embodiment, the prokaryotic host cell of the invention is an  E. coli  cell. Eukaryotic cells include, inter alia, yeasts, insect cells, mammalian cells, and cells of parasitic organisms (e.g., Trypanosomes). Culture systems with mammalian host cells include established cell lines such as COS cells, L cells, 3T3 cells, Chinese hamster ovary (CHO) cells, embryonic stem cells, with BHK, HeK or HeLa cells as preferred cells. Eukaryotic cells are, preferably, used for the expression of the recombinant gene by applying the transcriptional regulation sequence or the expression vector of the present invention. 
     Another aspect of the present invention relates to a method for obtaining the fusion protein of the invention, hereinafter first method of the invention, comprising:
         a) culturing the host cell of the invention under conditions that allow for the production of the fusion protein of the invention; and   b) recovering and purifying the fusion protein produced in step (a) above.       

     A host cell culture refers to the process of maintaining and growing host cells. Cell cultures need controlled conditions of temperature, pH, percentages of gases (oxygen and carbon dioxide), as well as the presence of suitable nutrients to allow for cell viability and division. Cell cultures can be grown on solid substrates such as agar, or in liquid medium, which allows large numbers of cells to be cultured in suspension. 
     The term “purify” as used in the description refers to the isolation of the fusion protein of the invention and its concentration, of the rest of the polypeptides present in the culture medium, and of the host cell of the invention. The isolation of the polypeptide of the invention can be carried out by differential solubility, chromatography, electrophoresis or isoelectric focusing techniques. Chromatography techniques can be based on molecular weight, ionic charge (based on the ionization state of amino acids under working conditions), the affinity of the protein for certain chromatographic columns or matrices, or by means of purification tags, and it can be carried out in a column, on paper or on a plate. The protein can be isolated, for example, by precipitation with ammonium sulphate, fast protein liquid chromatography (FPLC) or high performance liquid chromatography (HPLC), using automated systems that significantly reduce the purification time and increase purification performance. The expression “purification tag” or “affinity tag”, as used herein, refers to an amino acid sequence that has been incorporated (generally, by genetic engineering) into a protein to facilitate its purification. The tag, which can be another protein or a short amino acid sequence, allows for protein purification, for example, by affinity chromatography. Purification tags known in the state of the art are, for example, but not limited to, calmodulin-binding peptide (CBP), glutathione-S-transferase (GST) enzyme or a tail of histidine residues. 
     In another aspect, the present invention relates to a composition, hereinafter composition of the invention, which comprises the fusion protein, the nucleic acid, the plasmid, or the host cell of the invention, and at least one excipient and/or carrier. 
     In another aspect, the present invention relates to the fusion protein, the nucleic acid, the vector or the composition of the invention for use as a medicament. 
     From here, all information mentioned in relation to the medical uses of the different aspects of the invention: the fusion protein, the nucleic acid, the vector or the composition of the invention, indistinctly refer to all of them, although only the composition of the invention is mentioned. 
     The term “medicament”, as used in this specification, refers to any substance used for the prevention, diagnosis, alleviation, treatment or cure of diseases in humans and animals. For purposes of the present invention, the terms “medicament”, “pharmaceutical composition”, or “veterinary composition” are used as synonyms. 
     In a preferred embodiment, the composition of the invention is a medicament for veterinary use, more preferably for use in aquatic animals, even more preferably for use in fish, and more preferably, in fish of the salmonid family. 
     “Aquatic animal”, as used herein, includes any multicellular organism that lives in water, typically fish. Preferably, said aquatic animal is an animal belonging to a fish species cultivated by means of aquaculture. Illustrative examples of said aquatic animals include teleost fish, such as vertebrate fish, for example, salmonids (e.g., trout, salmon, etc.), carp, turbot, sea bream, seabass, etc. 
     In a preferred embodiment, the aquatic animals are preferably animals belonging to the salmonid family. The term “salmonid family” within the scope of this invention shall be understood to include all representatives of the family Salmonidae, especially of the subfamily Salmoninae and, preferably, the following species: Rainbow trout ( Oncorhynchus mykiss ); Chinook salmon ( Oncorhynchus tshawytscha ); Coho salmon ( Oncorhynchus kisutch ); Atlantic salmon ( Salmo salar ); Common trout ( Salmo trutta ); grayling ( Thymallus thymallus ); whitefish ( Coregonus  spp.); Chum salmon ( Oncorhynchus keta ); Sockeye salmon ( Oncorhynchus nerka ); Lake trout ( Salvelinus namaycush ); Brook trout ( Salvelinus fontinalis ); Arctic char ( Salvelinus alpinus ). In a more particular embodiment, the aquatic animals are preferably Rainbow trout ( Oncorhynchus mykiss ) and Common trout ( Salmo trutta ). 
     The compositions of this invention, as well as the other aspects of the invention mentioned above, fusion protein, nucleic acids that encode the fusion protein of the invention, and the plasmid of the invention, are suitable for treating myxozoic parasitic diseases of economic importance in farmed fish species, including  Kudoa  spp.,  Ceratomyxa  spp.,  Parvicapsula  spp.,  Myxobolus  spp.,  Tetracapsuloides  spp., inter alia; they are preferably suitable for treating parasitic diseases caused by pathogens of the genus  Tetracapsuloids  spp., preferably by the species  T. bryosalmonae . Said compositions are also useful for the treatment of the red mark syndrome. 
     Thus, another aspect of the present invention relates to the composition of the invention for its use in the treatment and/or prevention of inflammatory diseases in aquatic animals, preferably diseases that cause B-cell mediated inflammation, preferably the red mark syndrome or PKD disease, the latter caused by parasites of the genus  Tetracapsuloids , more preferably by  T. bryosalmonae.    
     In a more preferred embodiment, the aquatic animals are preferably fish, more preferably fish of the salmonid family, as described earlier. In a more preferred embodiment, the aquatic animal is the Rainbow trout. 
     In another more preferred embodiment, the parasite of the genus  Tetracapsuloids  spp. is preferably the species  T. bryosalmonae.    
     The term “treatment” as understood in the present invention refers to combating the effects caused as a result of a disease or pathological condition of interest in a subject, preferably an aquatic animal, more preferably, a fish, which includes:
         (i) inhibiting the disease or pathological condition, in other words, stopping its development;   (ii) alleviating the disease or the pathological condition, in other words, causing the remittance of the disease or pathological condition or the symptoms thereof;   (iii) stabilizing the disease or pathological condition.       

     The composition or medicament to which the present invention relates is preferably for veterinary use. The medicament or composition for veterinary use is any substance or combination of substances that has curative or preventive properties with respect to animal diseases or that can be administered to an animal for the purpose of restoring, correcting or modifying its physiological functions exerting a pharmacological, immunological or metabolic effect, or establishing a veterinarian diagnosis. “Veterinary medicines” will also be considered “premixes for medicated feed” prepared to be incorporated into a feed. 
     Optionally, the medicament of the present invention comprises, at least, a carrier and/or an acceptable excipient. The term “excipient” refers to a substance that aids the absorption of any of the components of the composition of the present invention, stabilizes said components or aids the preparation of the composition in the sense of giving it consistency or providing flavours that make it more pleasant. Thus, excipients could have the function of holding components together, such as starches, sugars or celluloses, the function of sweetening, the function of colouring, the function of protecting the medicament, for example to isolate it from air and/or moisture, the function of filling a tablet, capsule or any other form of presentation such as dibasic calcium phosphate, the function of disintegrating in order to facilitate the dissolution of components and the absorption thereof in the intestine, without excluding other types of excipients not mentioned herein. Therefore, the term “excipient” is defined as a material that, included in “galenic forms”, is added to active ingredients or to the associations thereof to allow for the preparation and stability thereof, to modify the organoleptic properties thereof or to determine the physicochemical properties of the pharmaceutical or veterinary composition and the bioavailability thereof. 
     The “carrier” or vehicle is preferably an inert substance. The function of the carrier is to facilitate the incorporation of other compounds, allow better dosing and administration or give the pharmaceutical composition consistency and shape. Therefore, the carrier is a substance that is used in the medicament or pharmaceutical or veterinary composition to dilute any of the components thereof to a certain volume or weight; or that, even without diluting said components, it is capable of allowing better dosing and administration or giving the medicament or composition consistency and shape. When the form of presentation is liquid, the pharmaceutically acceptable carrier is the diluent. 
     Moreover, the excipient and the carrier must be pharmacologically or veterinarily acceptable, in other words, the excipient and the carrier are allowed and evaluated so that no harm is caused to the organisms to which it is administered. 
     The composition of the invention will contain a prophylactically or therapeutically effective amount of the fusion protein, nucleic acid or plasmid of the invention, to provide the desired therapeutic effect. As used herein, the term “effective amount” refers to the amount of the fusion protein, nucleic acid, or plasmid of the invention contained in the composition that is capable of producing the desired therapeutic effect. In general, the effective amount to be administered will depend, among other factors, on the subject&#39;s own characteristics, the severity of the disease, the form of administration, etc. For this reason, the doses mentioned in this invention should be taken only as a guide for the person skilled in the art, who should adjust this dose depending on the factors described above. 
     In each case, the form of presentation of the medicament or composition will be adapted to the type of administration used; to that end, the composition of the present invention can be presented in the form of solutions or any other form of administration that is veterinarily permitted and in a therapeutically effective amount. Thus, the composition can be presented in a form adapted for oral, sublingual, nasal, intrathecal, intramuscular, bronchial, lymphatic, rectal, transdermal or inhaled administration, but without being limited to these forms. As understood by a person skilled in the art, sometimes the direct administration of the composition or plasmid of the invention to the site that is to be helped can be advantageous. In this manner, the direct administration of the composition or the plasmid of the invention to the desired organ or tissue can be achieved by direct administration (by injection, etc.) on the external surface of the affected organ or tissue by inserting a suitable device, e.g., an appropriate cannula, by arterial or venous perfusion (including retrograde flow mechanisms) or by other means mentioned in this description or known in the art. 
     In a preferred embodiment, the administration of the composition or plasmid of the invention is an intramuscular administration. 
     The composition of the invention will be formulated according to the chosen form of administration. Thus, the composition of the invention can be prepared in a liquid dosage form, for example, in the form of a solution or suspension, to be injected or perfused to the individual, preferably the aquatic animal as defined above. 
     In another aspect, the present invention relates to a method of treatment and/or prevention of inflammatory diseases in aquatic animals that comprises the administration of a therapeutically effective amount of the fusion protein, nucleic acid, plasmid, or composition of the invention. 
     In a more particular embodiment of the method of treatment and/or prevention of the invention, it is characterized in that the aquatic animals belong to the salmonid family and are selected from the list that consists of: Rainbow trout ( Oncorhynchus mykiss ); Chinook salmon ( Oncorhynchus tshawytscha ); Coho salmon ( Oncorhynchus kisutch ); Atlantic salmon ( Salmo salar ); Common trout ( Salmo trutta ); Grayling ( Thymallus thymallus ); Whitefish ( Coregonus  spp.); Chum salmon ( Oncorhynchus keta ); Sockeye salmon ( Oncorhynchus nerka ); Lake trout ( Salvelinus namaycush ); Brook trout ( Salvelinus fontinalis ) and Arctic char ( Salvelinus alpinus ). In another more particular embodiment, the salmonids are Rainbow trout ( Oncorhynchus mykiss ) and Common trout ( Salmo trutta ). 
     In another more particular embodiment, the inflammatory diseases that can affect salmonid species from the foregoing list comprise Proliferative Kidney Disease, caused by parasites of the genus  Tetracapsuloids , preferably by  T. bryosalmonae , and the red mark syndrome. 
     Effective dosages and administration schedules of the compositions comprising the fusion protein, nucleic acid, plasmid, or composition of the invention described herein can be determined empirically and making such determinations is within the skill in the art. Dosage ranges for the administration of the compositions are those wide enough to produce the desired effect. The dosage should not be so high as to cause adverse side effects, such as unwanted cross-reactions, anaphylactic reactions, and the like. The dosage may vary and can be administered in one or more daily dose administrations, for one or several days. 
     Throughout the description and the claims, the word “comprises” and its variants do not intend to exclude other technical features, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention may be partially deduced from both the description and the embodiment of the invention. The following examples and figures are provided by way of illustration and are not intended to limit the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1   . Transcription levels of the gene that encodes the extracellular domain of the Rainbow trout BAFF receptor (BAFF-R) after intramuscular injection of the pBAFF-R plasmid. At day 7 post-injection, the fish were sacrificed to sample the muscle of the dorsal area and determine the levels of gene expression by real-time PCR. The empty plasmid without the gene construct was used as a negative control. The data is shown as the relative expression of each gene compared to the expression of the endogenous control gene EF-1α (mean+SD; n=3-6). 
         FIG.  2   . Protein corresponding to the extracellular domain of the Rainbow trout BAFF receptor (BAFF-R) detected by the western blot technique using the concentrated supernatant of EPC cells transfected with the pBAFF-R plasmid. The empty plasmid without the gene construct was used as a negative control. M, marker. 
         FIG.  3   . Effect of intramuscular treatment with the pBAFF-R plasmid in Rainbow trout naturally infected with  T. bryosalmonae . (A) Degree of posterior kidney inflammation of trout treated with the pBAFF-R plasmid (n=20) or PBS (negative control; n=24). The individual value plot shows the interquartile range and the median of the data. (B) Percentage of trout affected or killed by PKD after intramuscular injection of the pBAFF-R plasmid or PBS (control). 
         FIG.  4   . Transcription levels of 18S rRNA from  T. bryosalmonae  after intramuscular injection of the plasmid (0.1 and 1 μg) containing the sequence of the extracellular domain of the BAFF receptor (pBAFF-R) in Rainbow trout naturally infected by  T. bryosalmonae . At day 110 post-injection, the fish were sacrificed to sample the posterior kidney and determine the expression levels of the 18S rRNA gene of  T. bryosalmonae  by real-time PCR. The empty plasmid without the gene construct was used as a negative control. The data is shown as the relative expression of each gene compared to the expression of the endogenous control gene EF-1α (mean+SD; n=5-7). 
     
    
    
     EXAMPLES 
     Next, the invention will be illustrated by means of assays carried out by the inventors that demonstrate the effectiveness of the product of the invention. 
     Example 1. Obtaining the pBAFF-R Plasmid (SEQ ID NO: 11) Encoding the IL BAFF-R Fusion Protein (SEQ ID NO: 8) 
     The nucleotide sequence (SEQ ID NO: 3) that encodes the extracellular domain of BAFF-R (SEQ ID NO: 4) from Rainbow trout was fused to the nucleotide sequence (SEQ ID NO: 1) that encodes the Rainbow trout IL-2 signal peptide (SEQ ID NO: 2) giving rise to the IL-2-BAFF-R construct comprising the nucleotide sequence SEQ ID NO: 7 that encodes the IL-2-BAFF-R fusion protein comprising the amino acid sequence SEQ ID NO: 8. 
     To allow further purification of the IL-2-BAFF-R construct, the nucleotide sequence (SEQ ID NO: 5) that encodes the Fc region of the mouse IgG1 immunoglobulin-like domain (SEQ ID NO: 6) was fused thereto, giving rise to the IL-2-BAFF-R-IgG1 construct which comprises the nucleotide sequence SEQ ID NO: 9, which encodes the amino acid sequence SEQ ID NO: 10. 
     Thus, the foregoing construct was cloned into the pcDNA3.1+ eukaryotic expression vector (Invitrogen), previously digested by HindIII/XhoI restriction enzymes. Next, said plasmid, called pBAFF-R (SEQ ID NO: 11), was transformed into JM109  E. coli  competent cell bacteria (Promega) following the manufacturer&#39;s instructions. Transformed colonies were selected on LB agar medium (Invitrogen) supplemented with ampicillin (100 μg/ml) for subsequent plasmid extraction and purification (Invitrogen). 
     Subsequently, the nucleotide sequence of the cloned pBAFF-R plasmid was confirmed by sequencing using the T7 primer (SEQ ID NO: 12; 5′-TAATACGACTCACTATAGGG-3′) derived from the vector. 
     The empty plasmid without the nucleotide sequence that encodes the IL-2-BAFFR-IgG1 gene construct was used as a negative control. 
     Example 2. Study of BAFF-R Transcription after Intramuscular Injection of pBAFF-R (SEQ ID NO: 11) 
     Rainbow trout ( Oncorhynchus mykiss ) of 7 cm supplied by the Cifuentes fish farm (Guadalajara, Spain) were used. To that end, the fish were kept at the Animal Health Research Centre (CISA-INIA) at 14° C. and fed daily with a commercial diet (Skretting, Norway). Before starting the experiments, the fish were acclimatised to laboratory conditions for 2 weeks. 
     The fish were divided into two groups and injected intramuscularly with 1 μg of pBAFF-R plasmid resuspended in 50 μl of sterile saline solution (0.9% NaCl) or with the same amount of empty plasmid (negative control). The fish were sacrificed 7 days post-injection and a sample was taken from the muscle area of each fish where the injection had been made for RNA extraction. 
     The total muscle RNA was extracted using Tri-reagent (Invitrogen), following the manufacturer&#39;s instructions and stored at −80° C. until use. The purified RNA was quantified using the Nanodrop 1000 spectrophotometer (Thermo Scientific). Next, 1 μg of RNA was treated with the DNase I enzyme using the RapidOut DNA Removal Kit (Thermo Scientific) to remove traces of genomic DNA and it was used to synthesise cDNA with the RevertAid Reverse Transcriptase enzyme (Thermo Scientific) and oligo(dT)23VN (1.6 μM), as indicated by the manufacturer. The resulting cDNA was diluted to 1:5 with nuclease-free water and stored at −20° C. until use. 
     The expression levels of the extracellular domain of BAFF-R were analysed by real-time PCR using the LightCycler 96 System instrument (Roche). All amplification reactions were performed in duplicate using the FastStart Essential DNA Green Master reaction mix (Roche) and specific primers (Table 1). The amplification conditions consisted of an initial denaturation step (95° C., 10 minutes), followed by 40 cycles of amplification (95° C. for 10 s, 60° C. for 10 s and 72° C. for 10 s). Moreover, a dissociation curve was obtained by reading the fluorescence signal every degree between the temperatures 60° C. and 95° C. to verify that this signal is due to the amplification of a single product. Negative controls without template DNA and negative reverse transcription (RT-) controls were included in all assays. The expression of the extracellular domain of the BAFF-R receptor was normalised with the expression of the gene that encodes the Rainbow trout elongation factor-1α (EF-1α) (Montero, J., J. et al., J. Virol. 2011; 85: 4046-4056) which is constitutively expressed in all organs to the same extent, using specific primers (Table 1). The expression levels were calculated with the 2-ΔCt method, wherein ΔCt was determined by subtracting the Ct value of EF-1α from the Ct value of the target gene (Livak, K. J., and T. D. Schmittgen. Methods. 2001; 25: 402-408). Statistical analysis was performed using a two-tailed Student&#39;s t-test with Welch&#39;s correction and the differences were considered statistically significant when p&lt;0.05. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 List of primers used in transcriptional studies. 
               
            
           
           
               
               
               
            
               
                   
                 Forward primer 
                 Reverse primer 
               
               
                 Gene 
                 (5′-3′) 
                 (5′-3) 
               
               
                   
               
               
                 EF-1α 
                 SEQ ID NO: 13 
                 SEQ ID NO: 14 
               
               
                   
                 (gatccagaagga 
                 (ttacgttcgacc 
               
               
                   
                 ggtcacca) 
                 ttccatcc) 
               
               
                   
               
               
                 BAFF-R 
                 SEQ ID NO: 15 
                 SEQ ID NO: 16 
               
               
                   
                 (gacaaactgctc 
                 (atatccagacag 
               
               
                   
                 atcacctgtatc) 
                 ctggactcactg) 
               
               
                   
               
               
                 18S rRNA 
                 SEQ ID NO: 17 
                 SEQ ID NO: 18 
               
               
                 
                   T. 
                 
                 (ggacactgcatg 
                 (ccatgctagaat 
               
               
                 
                   bryosalmonae 
                 
                 tgctgcatagt) 
                 gtccaggcact) 
               
               
                   
               
            
           
         
       
     
     As can be seen in  FIG.  1   , after intramuscular injection of the pBAFF-R plasmid, an increase in BAFF-R mRNA expression was detected in the muscle (56 times higher) compared to the expression shown by the trout treated with the empty plasmid (negative control). 
     Example 3. Obtaining Supernatants with the Extracellular Domain of the Rainbow Trout BAFF Receptor (BAFF-R) 
     To verify that the pBAFF-R plasmid of the invention is capable of effectively transcribing and translating the extracellular domain of Rainbow trout BAFF-R, EPC ( Epithelioma papulosum  cyprinid) cells were transfected with said pBAFF-R plasmid using the 4D-Nucleofector™ kit (Lonza). To that end, the EPC cells maintained in Leibovitz medium (L-15, Life Technologies) supplemented with penicillin (100 units/ml, Life Technologies), streptomycin (100 μg/ml, Life Technologies) and foetal bovine serum (10% FBS, Life Technologies) were trypsinised and transfected with 1 μg of plasmid pBAFF-R using the reagents of the Amaxa P3 Primary cell kit (Lonza). Next, the transfected cells were cultured in 24-well plates at a concentration of 5×10 5  cells/ml. After 24 h of incubation at 20° C., the culture medium was changed using L-15 medium supplemented with antibiotics and 0.1% FBS. After incubating the cells for another 24 h, the supernatants were collected from the wells and concentrated (50×) using 3 kDa molecular-weight cutoff centricons (GE Healthcare Life Sciences). In addition, the culture supernatants of the EPC cells transfected with the empty plasmid (negative control) were also collected. In order to verify the correct transfection of the cells, the pmaxGFP plasmid supplied in the Amaxa kit was used as a positive control for the subsequent visualisation of the green fluorescent protein by fluorescence microscopy (Zeiss Axio Vert.A1). 
     The concentrated supernatants of the EPC cells transfected with the pBAFF-R plasmid of the invention and with the empty plasmid were analyzed by polyacrylamide gel electrophoresis (SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis). To that end, the polyacrylamide gel (12%; Bio-Rad) was loaded with 20 μl of concentrated supernatant under reducing conditions and stained with Coomassie blue. Protein transfer was carried out on a polyvinylidene fluoride membrane (PVDF; Bio-Rad) using the Trans-Blot Turbo kit (Bio-Rad) to identify the protein corresponding to the extracellular domain of the Rainbow trout BAFF receptor (BAFF-R) by Western blot. To that end, the membrane was blocked with 5% skim milk in phosphate-buffered saline (PBS) at room temperature for 1 h. Then, the membrane was incubated with a rabbit anti-mouse IgG primary antibody (Sigma-Aldrich) and prepared in the blocking solution at 4° C. for 16 h. The membrane was then washed three times with 0.1% Tween 20 (Sigma-Aldrich) prepared in PBS for 10 min and incubated with a horseradish peroxidase-conjugated donkey anti-rabbit IgG secondary antibody (GE Healthcare Life Sciences) at room temperature for 1 h. After three washes in 0.1% Tween 20-PBS and a final wash in PBS, the membrane was revealed by peroxidase chemiluminescent reaction using the ECL kit (GE Healthcare Life Sciences). 
     As can be seen in  FIG.  2   , the protein corresponding to the extracellular domain of BAFF-R was detected, showing a specific band of 37 kDa. 
     Example 4. The Administration of the pBAFF-R Plasmid (SEQ ID NO: 11) Reduces the Degree of Posterior Kidney Inflammation and Mortality in Trout Naturally Infected by  T. bryosalmonae    
     Forty-four 30-40 g trout from the Southampton fish farm (United Kingdom) were used, free from infection by  T. bryosalmonae  and divided into two groups (20-24 fish per group). The fish of one of the groups were treated with two intramuscular injections (front and back of the dorsal fin) of the pBAFF-R plasmid (10 μg) dissolved in 20 μl of phosphate buffer (PBS) (20 μg of the pBAFF-R plasmid administered in total per fish). The fish of the second group were treated in the same way, but instead of administering the pBAFF-R plasmid, they were only administered the same volume of phosphate buffer (PBS) without plasmid. 
     Next, both groups of animals were introduced into a fish farm where an outbreak of PKD had been detected (Test Valley Trout), recording at each moment the number of dead fish. This assay was carried out in the middle of May, when the water temperature increased, reaching values higher than 15° C. At 90 days post-injection, when the temperature was still above 15° C. and the outbreak of PKD persisted on the farm, the trout were sacrificed to analyze the level of posterior kidney inflammation and thus determine the degree of infection by the parasite. The statistical analysis was performed using the Mann-Whitney nonparametric test and the differences were considered statistically significant for p&lt;0.05. 
     The results show that the intramuscular administration of the pBAFF-R plasmid in trout produced a decrease in the degree of kidney inflammation caused by PKD infection compared to the degree of kidney inflammation shown by the control group ( FIG.  3 A ). 
     The percentage of mortality presented by both groups of animals was also analyzed, showing that said percentage of mortality was lower in the group of trout treated with the pBAFF-R plasmid (4.76%) compared to the percentage shown by the control group (25%) ( FIG.  3 B ). Moreover, as shown in  FIG.  3 B , the percentage of trout that presented a degree of mortality was greater (52.38%) in the group treated with the pBAFF-R plasmid than in the control group (37.50%). 
     Example 5. The Administration of the pBAFF-R Plasmid (SEQ ID NO: 11) Reduces the Load of  T. bryosalmonae  in Trout Naturally Infected by this Parasite 
     To determine whether the administration of the pBAFF-R plasmid is capable of modifying the long-term parasite load in trout naturally infected with  T. bryosalmonae  and that manage to survive the disease, the pBAFF-R plasmid was injected intramuscularly into the fish to carry out transcriptional studies of the 18S rRNA gene of  T. bryosalmonae . Due to the marked seasonality of this parasitic disease, the treatment with the pBAFF-R plasmid was carried out during the month of July when an outbreak of PKD was detected, when cases of mortality associated with the characteristic symptoms of this disease appeared and, moreover, the temperature of the water at the fish farm was above 15° C. 
     Forty 30-40 g Rainbow trout were used from the Cifuentes fish farm (Guadalajara, Spain) and were divided into four groups (10 fish per group). Each fish was injected intramuscularly with 100 μl of sterile saline solution (0.9% NaCl) containing 0.1 or 1 μg of the pBAFF-R plasmid (two groups), as well as 0.1 or 1 μg of the empty plasmid without the construct in two other groups (negative controls). The fish of each group were sacrificed 110 days post-injection, in the month of November when the water temperature was below 15° C. and the outbreak of PKD had subsided at the fish farm. As expected, inflammation of the kidney was not observed in any case. A posterior kidney sample was taken from each trout for RNA extraction with Tri-reagent and subsequent cDNA synthesis using the methodology explained above in Example 2 was carried out. 
     The results show that the administration of a dose of 0.1 μg of the pBAFF-R plasmid did not induce a decrease in the expression of the 18S rRNA gene of  T. bryosalmonae  in the posterior kidney at 110 days post-injection ( FIG.  4   ). However, when a dose of 1 μg of the pBAFF-R plasmid was administered, a decrease in the parasite load detected in the kidney at the transcriptional level was observed ( FIG.  4   ). 
     Thus, these results indicate that the administration of the plasmid of the invention is capable of favouring the specific immune response of the infected organism against the parasite and its elimination from the organism, as well as controlling B-cell mediated inflammation of the kidney tissue, thus favouring the survival and symptoms of the infection caused by  T. bryosalmonae , in Rainbow trout.