EHRLICHIA CANIS 120-KDA IMMUNODOMINANT ANTIGENIC PROTEIN AND GENE

Abstract: The present invention provides a 120-kDa protein gene of Ehrlichia canis, amplified by PCR using primers derived from the DNA sequences flanking the Ehrlichia chaffeensis 120-kDa protein gene. The recombinant E. canis 120-kDa protein contains 14 tandem repeat units with 36 amino acids each. The repeat units are hydrophilic and predicted to be surface-exposed. Also disclosed is that the recombinant E. canis 120-kDa protein is antigenic and reacts with sera from dogs convalescent from canine ehrlichiosis.

Description: EHRLICHIA CANIS 120-KDA IMMUNODOMINANT ANTIGENIC PROTEIN AND GENE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to the fields of molecular biology of parasitic bacteria, particularly agents of rickettsia type diseases and ehrlichial bacteria. More specifically, the present invention relates to molecular cloning and characterization of Ehrlichia canis 120-kDa immunoreactive protein gene. Description of the Related Art Ehrlichia spp. are obligately intracellular gram negative bacteria which reside in the endosome of hematopoietic cells and infect various animal hosts including humans, domestic and wild canidae, deer, horses, sheep, cattle, and wild rodents . Each member of tribe Ehrlichieae has its own particular target cell tropism. Most species of Ehrlichia are either monocytotropic (E. canis, E. chaffeensis, E. sennetsu, E. risticii, and E. muris) o r granulocytotropic (human granulocytic ehrlichia, E. equi, E. phagocytophila, and E. ewingii) with the exceptions of Cowdria ruminantium which grows in the endothelial cells of the host an d Anaplasma marginale, a red blood cell parasite. Although ehrlichiae were described in the early part of this century, they received very little attention because they w ere considered pathogens of only veterinary importance in the United States until this decade. The renewed interest in ehrlichiae is d u e to the emergence of ehrlichioses affecting humans. In the last decade two new human Ehrlichia pathogens (E. chaffeensis and a human E. phagocytophilia-li e organism) were discovered in th e United States (Bakken JS 1994, Chen SM. 1994. JCM, Fishbein DB, 1987, Maeda, K., 1987). Ehrlichia canis, the prototype species of the genus, is the etiologic agent of canine ehrlichiosis, actually only one of five Ehrlichia species that naturally infect dogs. Canine ehrlichiosis is a worldwide disease transmitted by the brown dog tick, Rhipicephalus sanguineus (Groves MG, 1975. Lewis GE Jr, 1977). Ehrlichia canis causes a mild transient acute febrile illness and may progress to severe illness and a fatal syndrome (tropical canine pancytopenia) (Buhles WC, 1974, Greene CE and J.W. Harvey. 1984, Walker, JS 1970). Each year it costs millions of dollars for treating companion and working dogs infected with E canis worldwide. Moreover, E. canis also poses a public health threat. Ehrlichia canis, or an antigenically indistinguishable organism, was isolated from a human recently (Perez M. 1996). Understanding the genetic and antigenic composition of E. canis is essential for studying the pathogenesis of canine ehrlichiosis and developing an effective vaccine. Ehrlichia canis is closely related to E. chaffeensis genetically and antigenically (Anderson BE, 1991 , 1992, Chen SM, 1994. Am J Trop Med Hyg). Therefore, canine ehrlichiosis may be an appropriate model for studying the pathogenesis of monocytotropic Ehrlichia spp. including E. chaffeensis. The prior art is deficient in the lack of cloning and characterization of immunoreactive gene of Ehrlichia canis . Further, the prior art is deficient in the lack of recombinant protein of such immunoreactive gene of Ehrlichia canis. The present invention fulfills this long-standing need and desire in th e art. SUMMARY OF THE INVENTION In one embodiment of the present invention, there i s provided a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis. Preferably, the protein has an amino acid sequence of SEQ ID NO: 8 and the gene has a nucleic acid sequence of SEQ ID NO: 7. In a preferred embodiment of the present invention, there is provided an expression vector comprising a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis and wherein the vector is capable of expressing the gene when the vector i s introduced into a cell. In another embodiment of the present invention, there is provided a recombinant protein comprising an amino acid sequence of SEQ ID NO: 8. Preferably, the amino acid sequence is encoded by a nucleic acid sequence of SEQ ID NO: 7. Preferably, the recombinant protein comprises 14 tandem repeat units w i th 36 amino acids each. More preferably, the repeat units are hydrophilic. Still more preferably, the recombinant protein is a n antigen. In a preferred embodiment of the present invention, there is provided a method of producing the recombinant protein, comprising the steps of obtaining a vector that comprises a n expression region comprising a sequence encoding the amino acid sequence of SEQ ID NO: 8 operatively linked to a promoter; transfecting the vector into a cell; and culturing the cell u nder conditions effective for expression of the expression region. The invention may be described in certain embodiments as a method of inhibiting Ehrlichia canis infection i n a subject comprising the steps of: identifying a subject suspected of being exposed to or infected with Ehrlichia canis; a n d administering a composition comprising a 120 kDa antigen of Ehrlichia canis in an amount effective to inhibit an Ehrlichia canis infection. The inhibition may occur through any means such as, e.g., the stimulation of the subject' s humoral or cellular i mmu n e responses, or by other means such as inhibiting the normal function of the 120 kDa antigen, or even competing with th e antigen for interaction with some agent in the subject' s body. Other and further aspects, features, and advantages of the present invention will be apparent from the following description of the presently preferred embodiments of th e invention given for the purpose of disclosure. BRIEF DESCRIPTION OF THE DRAWINGS So that the matter in which the above-recited features, advantages and objects of the invention, as well as others which will become clear, are attained and can be understood in detail, more particular descriptions of the invention briefly summarized above may be had by reference to certain embodiments thereof which are illustrated in the appended drawings. These drawings form a part of the specification. It is to be noted, however, that the appended drawings illustrate preferred embodiments of th e invention and therefore are not to be considered limiting in their scope. Figure 1 shows DNA sequences and positions of oligonucleotide primers derived from the E. chaffeensis 1 20-kDa protein gene (open box) and the DNA sequences flanking the gene (line). The positions of primers are indicated as minus and plu s for DNA sequences upstream and downstream of the 120-kDa protein gene, respectively. Nine pairs of primers were formed b y combining each forward primer (SEQ ID NOs: 1-3) with each reverse primer (SEQ ID NOs: 4-6) and were used to amplify the E canis 120-kDa protein gene by PCR. Figure 2 A shows agarose gel electrophoresis of th e repeat units of the E. canis 120-kDa protein gene. pCA120 w as first digested with EcoR I to release the insert and then digested with Spe I at various time points. Figure 2B shows a Southern blot determining the number of repeats. DNA digested for 3 5 minutes with Spe I from the gel in panel A was transferred to a nylon membrane and hybridized with a digoxigenin-labeled oligonucleotide probe which anneals to the DNA sequences upstream of the repeat region of the E. canis 120-kDa protein gene. ND = Nondigested DNA Figure 3 shows the DNA sequence of the E. canis 1 20 - kDa gene (SEQ ID NO: 7) and the deduced amino acids (SEQ ID NO: 8). The nucleic acids of repeats 1 , 3, 5, 7, 9, 11 , and 13 are underlined. Figure 4 shows phylogenetic tree of the repeat units of the E. canis 120-kDa gene. The scale represents % difference i n DNA sequence. Figure 5 shows the alignment of the amino acid sequences of the 120-kDa proteins of E. canis (SEQ ID NO: 10) and E. chaffeensis (SEQ ID NO: 9). Bars represent identical amino acids. Colons indicate conserved replacements. Figure 6 shows surface probability an d hydrophobicity of the 120-kDa proteins of E. canis and E chaffeensis. The region between the arrowheads represents th e repeat domain. All the repeats in both proteins are hydrophilic and surface-exposed. The second repeat unit of the E. canis 1 20 kDa protein and a peak of the first repeat unit of E. chaffeensis 120 kDa protein are present between the two arrows and are magnified in Figure 7. Figure 7 shows a comparison of surface-exposed amino acids in a repeat unit of the 120-kDa proteins of E. canis and E. chaffeensis. Figure 7A shows the surface probability of amino acids. The corresponding regions were indicated by tw o arrowheads in Figure 6. Bold letters indicated the conserved amino acids between E. canis and E. chaffeensis. Figure 7B show s an alignment of the amino acid sequence shown in panel A (SEQ ID NOs: 11-12). Bars represent identical amino acids. Dots represent conserved replacements. Figure 8 shows agarose gel electrophoresis of the E canis 120-kDa gene from all E. canis strains partially digested wi th Spe I. The recombinant pCR2.1 plasmids were first digested wi th EcoR I to release the insert from the vector and then digested w i th Spe I partially. Non-digested: Oklahoma strain 120 kDa gene DNA was digested with EcoR I, but not with Spe I to show the size of the insert. Figure 9 A shows SDS-PAGE of E. cσ/7-expressed E canis 120-kDa protein. 1, GST fusion protein; 2, E. canis recombinant 120-kDa protein cleaved from the GST fusion protein by thrombin. Figure 9B shows agarose gel electrophoresis of a pGEX plasmid expressing the E. canis 120-kDa protein. The insert is indicated by an arrow. Figure 1 0 shows a Western immunoblot of mouse anti-JE". canis 120-kDa recombinant protein sera reacted with E canis antigen (lane 1 ) and 120-kDa recombinant protein (lane 2, arrow) . Figure 1 1 shows Western blotting of c anine convalescent serum reacting with recombinant 120 kDa protein of E. canis. DETAILED DESCRIPTION OF THE INVENTION In accordance with the present invention there may b e employed conventional molecular biology, microbiology, a n d recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Maniatis, Fritsch & Sambrook, "Molecular Cloning: A Laboratory Manual (1982); "DNA Cloning: A Practical Approach," Volumes I and I I (D.N. Glover ed. 1985); "Oligonucleotide Synthesis" (M.J. Gait ed . 1984); "Nucleic Acid Hybridization" [B.D. Hames & SJ. Higgins eds. (1985)] ; "Transcription and Translation" [B.D. Hames & S.J. Higgins eds. (1984)]; "Animal Cell Culture" [R.I. Freshney, ed. ( 1986)] ; "Immobilized Cells And Enzymes" [IRL Press, ( 1986)]; B. Perbal, "A Practical Guide To Molecular Cloning" (1984). Therefore, if appearing herein, the following term s shall have the definitions set out below. The amino acid described herein are preferred to be i n the "L" isomeric form. However, residues in the "D" isomeric form can be substituted for any L-amino acid residue, as long as th e desired functional property of immunoglobulin-binding is retained by the polypeptide. NH2 refers to the free amino group present a t the amino terminus of a polypeptide. COOH refers to the free carboxy group present at the carboxy terminus of a polypeptide. In keeping with standard polypeptide nomeclature, / Biol. Chem., 243 :3552-59 ( 1969), abbreviations for amino acid residues are shown in the following Table of Correspondence: TABLE OF CORRESPONDENCE SYMBOL AMINO ACID 1 -Letter 3 -Letter Y Tyr tyrosine G Gly glycine F Phe Phenylalanine M Met methionine A Ala alanine S Ser serine I lie isoleucine L Leu leucine T Thr threonine V Val valine P Pro proline K Lys lysine H His histidine Q Gin glutamine E Glu glutamic acid W Trp tryptophan R Arg arginine D Asp aspartic acid N Asn asparagine C Cys cysteine It should be noted that all amino-acid residue sequences are represented herein by formulae whose left a n d right orientation is in the conventional direction of amino- terminus to carboxy-terminus. Furthermore, it should be noted that a dash at the beginning or end of an amino acid residue sequence indicates a peptide bond to a further sequence of one o r more amino-acid residues. The above Table is presented to correlate the three-letter and one-letter notations which m a y appear alternately herein. A "replicon" is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo ; i.e., capable of replication under its o w n control. A "vector" is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A "DNA molecule" refers to the polymeric form of deoxyribonucleotides (adenine, guanine, thymine, or cytosine) i n its either single stranded form, or a double-stranded helix. This term refers only to the primary and secondary structure of th e molecule, and does not limit it to any particular tertiary forms . Thus, this term includes double-stranded DNA found, inter alia, in linear DNA molecules (e.g., restriction fragments), viruses, plasmids, and chromosomes. In discussing the structure herein according to the normal convention of giving only the sequence i n the 5' to 3' direction along the nontranscribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). An "origin of replication" refers to those DNA sequences that participate in DNA synthesis. A DNA "coding sequence" is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a s tart codon at the 5' (amino) terminus and a translation stop codon a t the 3' (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences, cDNA from eukaryotic mRNA, genomic DNA sequences from eukaryotic (e.g., mammalian) DNA, and even synthetic DNA sequences. A polyadenylation signal and transcription termination sequence will usually be located 3 ' to the coding sequence. Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers , polyadenylation signals, terminators, and the like, that provide for the expression of a coding sequence in a host cell. A "promoter sequence" is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3' direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3' terminus by the transcription initiation site and extends upstream (5' direction) to include the minimum number of bases or elements necessary to initiate transcription a t levels detectable above background. Within the promoter sequence will be found a transcription initiation site, as well a s protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Eukaryotic promoters often, b u t not always, contain "TATA" boxes and "CAT" boxes. Prokaryotic promoters contain Shine-Dalgarno sequences in addition to the - 1 0 and -35 consensus sequences. An "expression control sequence" is a DNA sequence that controls and regulates the transcription and translation of another DNA sequence. A coding sequence is "under the control" of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then translated into the protein encoded by th e coding sequence. A "signal sequence" can be included near the coding sequence. This sequence encodes a signal peptide, N-terminal to the polypeptide, that communicates to the host cell to direct th e polypeptide to the cell surface or secrete the polypeptide into th e media, and this signal peptide is clipped off by the host cell before the protein leaves the cell. Signal sequences can be found associated with a variety of proteins native to prokaryotes an d eukaryotes . The term "oligonucleotide", as used herein in referring to the probe of the present invention, is defined as a molecule comprised of two or more deoxyribonucleotides, preferably more than three. Its exact size will depend upon many factors which, i n turn, depend upon the ultimate function and use of the oligonucleotide. The term "primer" as used herein refers to a n oligonucleotide, whether occurring naturally as in a purified restriction digest or produced synthetically, which is capable of acting as a point of initiation of synthesis when placed u nder conditions in which synthesis of a primer extension product, which is complementary to a nucleic acid strand, is induced, i.e., in th e presence of nucleotides and an inducing agent such as a DNA polymerase and at a suitable temperature and pH. The primer may be either single-stranded and must be sufficiently long to prime the synthesis of the desired extension product in th e presence of the inducing agent. The exact length of the pri m er will depend upon many factors, including temperature, source of primer and use of the method. For example, for diagnostic applications, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. The primers herein are selected to be "substantially" complementary to different strands of a particular target DNA sequence. This means that the primers must be sufficiently complementary to hybridize with their respective strands . Therefore, the primer sequence need not reflect the exact sequence of the template. For example, a non-complementary nucleotide fragment may be attached to the 5' end of the primer, with the remainder of the primer sequence being complementary to the strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the primer, provided that th e primer sequence has sufficient complementarity with th e sequence or hybridize therewith and thereby form the template for the synthesis of the extension product. As used herein, the terms "restriction endonucleases " and "restriction enzymes" refer to enzymes, each of which cut double-stranded DNA at or near a specific nucleotide sequence. A cell has been "transformed" by exogenous o r heterologous DNA when such DNA has been introduced inside the cell. The transforming DNA may or may not be integrated (covalently linked) into the genome of the cell. In prokaryotes , yeast, and mammalian cells for example, the transforming DNA may be maintained on an episomal element such as a plasmid . With respect to eukaryotic cells, a stably transformed cell is one i n which the transforming DNA has become integrated into a chromosome so that it is inherited by daughter cells through chromosome replication. This stability is demonstrated by th e ability of the eukaryotic cell to establish cell lines or clones comprised of a population of daughter cells containing th e transforming DNA. A "clone" is a population of cells derived from a single cell or ancestor by mitosis. A "cell line" is a clone of a primary cell that is capable of stable growth in vitro for m a n y generations. Two DNA sequences are "substantially homologous" when at least about 75% (preferably at least about 80%, and mos t preferably at least about 90% or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing th e sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Maniatis et al., supra; DNA Cloning, Vols. I & II, supra; Nucleic Acid Hybridization, supra. A "heterologous' region of the DNA construct is a n identifiable segment of DNA within a larger DNA molecule that is not found in association with the larger molecule in nature. Thus, when the heterologous region encodes a mammalian gene, th e gene will usually be flanked by DNA that does not flank th e mammalian genomic DNA in the genome of the source organism. In another example, coding sequence is a construct where th e coding sequence itself is not found in nature (e.g., a cDNA w here the genomic coding sequence contains introns, or synthetic sequences having codons different than the native gene). Allelic variations or naturally-occurring mutational events do not give rise to a heterologous region of DNA as defined herein. The labels most commonly employed for these studies are radioactive elements, enzymes, chemicals which fluoresce when exposed to untraviolet light, and others. A number of fluorescent materials are known and can be utilized as labels . These include, for example, fluorescein, rhodamine, auramine, Texas Red, AMCA blue and Lucifer Yellow. A particular detecting material is anti-rabbit antibody prepared in goats and conjugated with fluorescein through an isothiocyanate. Proteins can also be labeled with a radioactive element or with an enzyme. The radioactive label can be detected by a n y of the currently available counting procedures. The preferred isotope may be selected from 3 H, i C, 3 2p, 3 5s, 36 Q, 5 1Q-, 57 c 0 , 58 Co, 59 Fe , 9θγ 5 i 25i ? I 3 i ι, and i 86R e . Enzyme labels are likewise useful, and can be detected by any of the presently utilized colorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques. The enzyme is conjugated to the selected particle b y reaction with bridging molecules such as carbodiimides, diisocyanates, glutaraldehyde and the like. Many enzymes which can be used in these procedures are known and can be utilized. The preferred are peroxidase, β-glucuronidase, β-D-glucosidase, β- D-galactosidase, urease, glucose oxidase plus peroxidase a n d alkaline phosphatase. U.S. Patent Nos. 3,654,090, 3,850,752, a nd 4,016,043 are referred to by way of example for their disclosure of alternate labeling material and methods. A particular assay system developed and utilized i n the art is known as a receptor assay. In a receptor assay, t h e material to be assayed is appropriately labeled and then certain cellular test colonies are inoculated with a quantitiy of both th e label after which binding studies are conducted to determine th e extent to which the labeled material binds to the cell receptors. I n this way, differences in affinity between materials can b e ascertained. An assay useful in the art is known as a " cis/trans " assay. Briefly, this assay employs two genetic constructs, one of which is typically a plasmid that continually expresses a particular receptor of interest when transfected into an appropriate cell line, and the second of which is a plasmid that expresses a reporter such as luciferase, under the control of a receptor/ligand complex. Thus, for example, if it is desired to evaluate a compound as a ligand for a particular receptor, one of the plasmids would be a construct that results in expression of the receptor in the chosen cell line, while the second plasmid would possess a promoter linked to the luciferase gene in which the response element to th e particular receptor is inserted. If the compound u nd er test is an agonist for the receptor, the ligand will complex with th e receptor, and the resulting complex will bind the response element and initiate transcription of the luciferase gene. The resulting chemiluminescence is then measured photometrically, and dose response curves are obtained and compared to those of known ligands. The foregoing protocol is described in detail in U.S. Patent No. 4,981 ,784. As used herein, the term "host" is meant to include not only prokaryotes but also eukaryotes such as yeast, plant a n d animal cells. A recombinant DNA molecule or gene which encodes a 120 kDa immunoreactive protein of Ehrlichia canis of th e present invention can be used to transform a host using any of th e techniques commonly known to those of ordinary skill in the art. Especially preferred is the use of a vector containing coding sequences for a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis of the present invention for purposes of prokaryote transformation. Prokaryotic hosts may include E. coli, S. typhimu rium , Serratia marcescens, Mycobacterium vaccae and Bacillus subtilis. Eukaryotic hosts include yeasts such as Pichia pastoris, mammalian cells and insect cells. In general, expression vectors containing promoter sequences which facilitate the efficient transcription of t h e inserted DNA fragment are used in connection with the host. The expression vector typically contains an origin of replication, promoter(s), terminator(s), as well as specific genes which ar e capable of providing phenotypic selection in transformed cells. The transformed hosts can be fermented and cultured according to means known in the art to achieve optimal cell growth. The invention includes a substantially pure DNA encoding a 120 kDa immunoreactive protein of Ehrlichia canis, a strand of which DNA will hybridize at high stringency to a probe containing a sequence of at least 15 consecutive nucleotides of (SEQ ID NO:6). The protein encoded by the DNA of this invention may share at least 80% sequence identity (preferably 85%, more preferably 90%, and most preferably 95%) with the nucleic acids listed in Figure 3 (SEQ ID NO: 7). More preferably, the DNA includes the coding sequence of the nucleotides of SEQ ID NO:8, o r a degenerate variant of such a sequence. The probe to which the DNA of the invention hybridizes preferably consists of a sequence of at least 2 0 consecutive nucleotides, more preferably 40 nucleotides, e v e n more preferably 50 nucleotides, and most preferably 1 00 nucleotides or more (up to 100%) of the coding sequence of th e nucleotides listed in SEQ ID NO: 8 or the complement thereof. Such a probe is useful for detecting expression of the 120 kDa immunoreactive protein gene of Ehrlichia canis in a human cell by a method including the steps of (a) contacting mRNA obtained from the cell with the labeled hybridization probe; and (b ) detecting hybridization of the probe with the mRNA. This invention also includes a substantially pure DNA containing a sequence of at least 15 consecutive nucleotides (preferably 20, more preferably 30, even more preferably 50, and most preferably all) of the region from the nucleotides listed in (SEQ ID NO: 8). By "high stringency" is meant DNA hybridization and wash conditions characterized by high temperature and low salt concentration, e.g., wash conditions of 65°C at a salt concentration of approximately 0.1 x SSC, or the functional equivalent thereof. For example, high stringency conditions may include hybridization at about 42°C in the presence of about 50% formamide; a first wash at about 65°C with about 2 x SSC containing 1 % SDS; followed by a second wash at about 65°C with about 0.1 x SSC. By "substantially pure DNA" is meant DNA that is no t part of a milieu in which the DNA naturally occurs, by virtue of separation (partial or total purification) of some or all of th e molecules of that milieu, or by virtue of alteration of sequences that flank the claimed DNA. The term therefore includes, for example, a recombinant DNA which is incorporated into a vector, into an autonomously replicating plasmid or virus, or into th e genomic DNA of a prokaryote or eukaryote; or which exists as a separate molecule (e.g., a cDNA or a genomic or cDNA fragment produced by polymerase chain reaction (PCR) or restriction endonuclease digestion) independent of other sequences. It also includes a recombinant DNA which is part of a hybrid gene encoding additional polypeptide sequence, e.g., a fusion protein. Also included is a recombinant DNA which includes a portion of the nucleotides listed in SEQ ID NO: 8) which encodes a n alternative splice variant of a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis. The DNA may have at least about 70% sequence identity to the coding sequence of the nucleotides listed i n SEQ ID NO:8, preferably at least 75% (e.g. at least 80%); and most preferably at least 90%. The identity between two sequences is a direct function of the number of matching or identical positions. When a subunit position in both of the two sequences is occupied by the same monomeric subunit, e.g., if a given position is occupied by an adenine in each of two DNA molecules, then th e y are identical at that position. For example, if 7 positions in a sequence 10 nucleotides in length are identical to th e corresponding positions in a second 10-nucleotide sequence, th e n the two sequences have 70% sequence identity. The length of comparison sequences will generally be at least 50 nucleotides, preferably at least 60 nucleotides, more preferably at least 7 5 nucleotides, and most preferably 100 nucleotides. S equence identity is typically measured using sequence analysis software (e.g., Sequence Analysis Software Package of the Genetics Computer Group, University of Wisconsin Biotechnology Center, 1710 University Avenue, Madison, WI 53705). The present invention comprises a vector comprising a DNA sequence which encodes a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis and said vector is capable of replication in a host which comprises, in operable linkage: a) an origin of replication; b) a promoter; and c) a DNA sequence coding for said protein. Preferably, the vector of th e present invention contains a portion of the DNA sequence show n in SEQ ID No: 8. A "vector" may be defined as a replicable nucleic acid construct, e.g., a plasmid or viral nucleic acid. Vectors m a y be used to amplify and/or express nucleic acid encoding a 1 20 kDa immunoreactive protein of Ehrlichia canis. An expression vector is a replicable construct in which a nucleic acid sequence encoding a polypeptide is operably linked to suitable control sequences capable of effecting expression of the polypeptide in a cell. The need for such control sequences will vary depending upon the cell selected and the transformation method chosen. Generally, control sequences include a transcriptional promoter and/or enhancer, suitable mRNA ribosomal binding sites, an d sequences which control the termination of transcription and translation. Methods which are well known to those skilled in th e art can be used to construct expression vectors containing appropriate transcriptional and translational control signals. See for example, the techniques described in Sambrook et al., 1989 , Molecular Cloning: A Laboratory Manual (2nd Ed.), Cold Spring Harbor Press, N.Y. A gene and its transcription control sequences are defined as being "operably linked" if the transcription control sequences effectively control the transcription of the gene. Vectors of the invention include, but are not limited to, plasmid vectors and viral vectors. Preferred viral vectors of the invention are those derived from retroviruses, adenovirus, adeno- associated virus, SV40 virus, or herpes viruses. By a "substantially pure protein" is meant a protein which has been separated from at least some of those components which naturally accompany it. Typically, the protein is substantially pure when it is at least 60%, by weight, free from the proteins and other naturally-occurring organic molecules w i th which it is naturally associated in vivo. Preferably, the purity of the preparation is at least 75%, more preferably at least 90%, a n d most preferably at least 99%, by weight. A substantially p ure 120 kDa immunoreactive protein of Ehrlichia canis may b e obtained, for example, by extraction from a natural source; b y expression of a recombinant nucleic acid encoding a 120 kDa immunoreactive protein of Ehrlichia canis; or by chemically synthesizing the protein. Purity can be measured by a n y appropriate method, e.g., column chromatography such a s immunoaffinity chromatography using an antibody specific for a 120 kDa immunoreactive protein of Ehrlichia canis, polyacrylamide gel electrophoresis, or HPLC analysis. A protein is substantially free of naturally associated components when it is separated from at least some of those contaminants which accompany it in its natural state. Thus, a protein which is chemically synthesized or produced in a cellular system different from the cell from which it naturally originates will be, b y definition, substantially free from its naturally associated components. Accordingly, substantially pure proteins include eukaryotic proteins synthesized in E. coli, other prokaryotes, o r any other organism in which they do not naturally occur. In addition to substantially full-length proteins, th e invention also includes fragments (e.g., antigenic fragments) of the 120 kDa immunoreactive protein of Ehrlichia canis (SEQ ID No: 7). As used herein, "fragment," as applied to a polypeptide, will ordinarily be at least 10 residues, more typically at least 2 0 residues, and preferably at least 30 (e.g., 50) residues in length, but less than the entire, intact sequence. Fragments of the 1 20 kDa immunoreactive protein of Ehrlichia canis can be generated by methods known to those skilled in the art, e.g., by enzymatic digestion of naturally occurring or recombinant 120 kDa immunoreactive protein of Ehrlichia canis, by recombinant DNA techniques using an expression vector that encodes a defined fragment of 120 kDa immunoreactive protein of Ehrlichia canis, or by chemical synthesis. The ability of a candidate fragment to exhibit a characteristic of 120 kDa immunoreactive protein of Ehrlichia canis (e.g., binding to an antibody specific for 120 kDa immunoreactive protein of Ehrlichia canis) can be assessed b y methods described herein. Purified 120 kDa immunoreactive protein of Ehrlichia canis or antigenic fragments of 120 kDa immunoreactive protein of Ehrlichia canis can be used to generate new antibodies or to test existing antibodies (e.g., as positive controls in a diagnostic assay) by employing standard protocols known to those skilled in the art. Included in this invention are polyclonal antisera generated by using 120 kDa immunoreactive protein of Ehrlichia canis or a fragment of 120 kDa immunoreactive protein of Ehrlichia canis as the immunogen in, e.g., rabbits. Standard protocols for monoclonal and polyclonal antibody production known to those skilled in this art are employed. The monoclonal antibodies generated by this procedure can be screened for the ability to identify recombinant Ehrlichia canis cDNA clones, and to distinguish them from known cDNA clones. Further included in this invention are fragments of th e 120 kDa immunoreactive protein of Ehrlichia canis which are encoded at least in part by portions of SEQ ID NO: 7, e.g., products of alternative mRNA splicing or alternative protein processing events, or in which a section of the sequence has been deleted. The fragment, or the intact 120 kDa immunoreactive protein of Ehrlichia canis, may be covalently linked to another polypeptide, e.g., which acts as a label, a ligand or a means to increase antigenicity. The phrase "pharmaceutically acceptable" refers to molecular entities and compositions that do not produce a n allergic or similar untoward reaction when administered to a human. The preparation of an aqueous composition that contains a protein as an active ingredient is well understood in the art. Typically, such compositions are prepared as injectables, either a s liquid solutions or suspensions; solid forms suitable for solution in, or suspension in, liquid prior to injection can also be prepared. The preparation can also be emulsified. A protein may be formulated into a composition in a neutral or salt form. Pharmaceutically acceptable salts, include the acid addition salts (formed with the free amino groups of th e protein) and which are formed with inorganic acids such as, for example, hydrochloric or phosphoric acids, or such organic acids a s acetic, oxalic, tartaric, mandelic, and the like. Salts formed with the free carboxyl groups can also be derived from inorganic bases such as, for example, sodium, potassium, ammonium, calcium, o r ferric hydroxides, and such organic bases as isopropylamine, trimethylamine, histidine, procaine and the like. Upon formulation, solutions will be administered in a manner compatible with the dosage formulation and in such amount as is therapeutically effective. The formulations are administered in a variety of dosage forms such as injectable solutions. For parenteral administration in an aqueous solution, for example, the solution should be suitably buffered if necessary and the liquid diluent first rendered isotonic with sufficient saline or glucose. These particular aqueous solutions are especially suitable for intravenous, intramuscular, subcutaneous an d intraperitoneal administration. In this connection, sterile aqueous media which can be employed will be known to those of skill i n the art in light of the present disclosure. For example, one dosage could be dissolved in 1 mL of isotonic NaCl solution and either added to lOOOmL of hypodermoclysis fluid or injected at th e proposed site of infusion, (see for example, "Remington's Pharmaceutical Sciences" 15th Edition, pages 1035- 1038 an d 1570- 1580). Some variation in dosage will necessarily occur depending on the condition of the subject being treated. The person responsible for administration will, in any event, determine the appropriate dose for the individual subject. As is well known in the art, a given polypeptide m a y vary in its immunogenicity. It is often necessary therefore to couple the immunogen (e.g., a polypeptide of the present invention) with a carrier. Exemplary and preferred carriers are keyhole limpet hemocyanin (KLH) and human serum albumin . Other carriers may include a variety of lymphokines a n d adjuvants such as IL2, IL4, IL8 and others. Means for conjugating a polypeptide to a carrier protein are well known in the art and include glutaraldehyde, m - maleimidobenzoyl-N-hydroxysuccinimide ester, carbo-diimide a n d bis-biazotized benzidine. It is also understood that the peptide may be conjugated to a protein by genetic engineering techniques that are well known in the art. As is also well known in the art, immunogenicity to a particular immunogen can be enhanced by the use of non-specific stimulators of the immune response known as adjuvants . Exemplary and preferred adjuvants include complete BCG, Detox, (RIBI, Immunochem Research Inc.) ISCOMS and aluminum hydroxide adjuvant (Superphos, Biosector). The preparation of vaccines which contain peptide sequences as active ingredients is generally well understood in th e art, as exemplified by U.S. Patents 4,608,251 ; 4,601 ,903 ; 4,599,231 ; 4,599,230; 4,596,792; and 4.578,770, all incorporated herein by reference. Typically, such vaccines are prepared a s injectables, either as liquid solutions or suspensions: solid forms suitable for solution in, or suspension in, liquid prior to injection may also be prepared. The preparation may also be emulsified. The active immunogenic ingredient is often mixed with excipients which are pharmaceutically acceptable and compatible with th e active ingredient. Suitable excipients are, for example, water, saline, dextrose, glycerol, ethanol, or the like and combinations thereof. In addition, if desired, the vaccine may contain minor amounts of auxiliary substances such as wetting or emulsifying agents, pH buffering agents, or adjuvants which enhance th e effectiveness of the vaccines. The 120-kDa protein is a potential adhesin of Ehrlichia spp. and is differentially expressed on the surface of dense-cored cells of E. chaffeensis. The gene of E. canis was amplified by PCR using primers derived from the DNA sequences flanking the E chaffeensis gene. The E. canis gene was cloned, sequenced, a n d over-expressed in Escherichia coli. The 120-kDa protein of E. canis contains 14 tandem repeat units with 36 amino acids each. The DNA sequences of the repeats are 94% homologous to one other. The repeat units are hydrophilic and by probability analysis ar e predicted to be surface-exposed. The overall amino acid sequence of the E. canis 120-kDa protein is 30% homologous to the E chaffeensis 120-kDa protein. The repeat regions of the 120-kDa proteins of the two species share common amino acid sequences which are predicted to be surface-exposed. The recombinant E canis 120-kDa protein reacted with sera from dogs convalescent from canine ehrlichiosis. The present invention is directed to a gene encoding a 120-kDa immunoreactive protein of Ehrlichia canis and a recombinant protein encoded by such gene thereof. In one embodiment of the present invention, there is provided a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis. Preferably, the protein has an amino acid sequence of SEQ ID NO: 8 and the gene has a nucleic acid sequence of SEQ ID NO: 7. In a preferred embodiment of the present invention, there is provided an expression vector comprising a gene encoding a 120 kDa immunoreactive protein of Ehrlichia canis and capable of expressing the gene when the vector is introduced into a cell. In another embodiment of the present invention, th ere is provided a recombinant protein comprising an amino acid sequence of SEQ ID NO: 8. Preferably, the amino acid sequence is encoded by a nucleic acid sequence of SEQ ID NO: 7. Preferably, the recombinant protein comprises 14 tandem repeat units with 36 amino acids each. More preferably, the repeat units are hydrophilic. Still more preferably, the recombinant protein is a n antigen. In a preferred embodiment of the present invention, there is provided a method of producing the recombinant protein, comprising the steps of obtaining a vector that comprises a n expression region comprising a sequence encoding the amino acid sequence of SEQ ID NO: 8 operatively linked to a promoter; transfecting the vector into a cell; and culturing the cell u nder conditions effective for expression of the expression region. As used herein the term "complement" is used to define the strand of nucleic acid which will hybridize to the first nucleic acid sequence to form a double stranded molecule u nder stringent conditions. Stringent conditions are those that allow hybridization between two nucleic acid sequences with a high degree of homology, but precludes hybridization of random sequences. For example, hybridization at low temperature and/or high ionic strength is termed low stringency and hybridization a t high temperature and/or low ionic strength is termed high stringency. The temperature and ionic strength of a desired stringency are understood to be applicable to particular probe lengths, to the length and base content of the sequences and to th e presence of formamide in the hybridization mixture. As used herein, the term "engineered" o r "recombinant" cell is intended to refer to a cell into which a recombinant gene, such as a gene encoding an Ehrlichia chaffeensis antigen has been introduced. Therefore, engineered cells are distinguishable from naturally occurring cells which d o not contain a recombinantly introduced gene. Engineered cells a re thus cells having a gene or genes introduced through the hand of man. Recombinantly introduced genes will either be in the form of a cDNA gene, a copy of a genomic gene, or will include genes positioned adjacent to a promoter not naturally associated w i th the particular introduced gene. In addition, the recombinant gene may be integrated into the host genome, or it may be contained i n a vector, or in a bacterial genome transfected into the host cell. The following examples are given for the purpose of illustrating various embodiments of the invention and are not meant to limit the present invention in any fashion. EXAMPLE 1 Ehrlichia Ehrlichia canis Oklahoma strain was provided by Dr. Jacqueline Dawson (Centers for Disease Control, Atlanta, GA). E canis Florida strain and three North Carolina isolates (Demon, DJ, and Jake) were provided by Dr. Edward B. Breitschwerdt (College of Veterinary Medicine, North Carolina State University, Raleigh, NC). E. canis Louisiana strain was provided by Dr. R. E. Corstvet (Louisiana State University, Baton Rouge, Louisiana). Ehrlichiae were cultivated in DH82 cells, a canine macrophage-like cell line (Dawson JE, 1991 ). DH82 cells were harvested with a cell scraper when 100% of the cells were infected with ehrlichiae. The cells were centrifuged at 17,400 x g for 20 min. The pellets w ere disrupted with a Braun-Sonic 2000 sonicator at 40 W for 30 sec twice on ice. The cell lysate was loaded onto discontinuous gradients of 42%-36%-30% renografin, and then centrifuged a t 80,000 x g for 60 min. Ehrlichiae in the heavy and light b and s were collected (Weiss E, 1975) and washed by centrifugation wi th sucrose-phosphate-glutamate buffer (SPG, 218 mM sucrose, 3.8 mM KH2P04,, 7.2 mM K2HP04, 4.9 mM glutamate, pH 7.0). EXAMPLE 2 DNA Preparation Ehrlichia canis genomic DNA was prepared from renografin density gradient purified ehrlichiae by using a n IsoQuick nucleic acid extraction kit (ORCA Research Inc., Bothell, WA) according to the instructions of the manufacturer. Plasmid DNA was purified by using a High Pure Plasmid Isolation Kit (Boehringer Mannheim Corp., Indianapolis, IN). The PCR product was purified by using a QIAquick PCR Purification kit (QIAGEN Inc., Santa Clarita, CA). EXAMPLE 3 PCR Amplification of the E. canis 120-kDa Protein Gene Primers were designed based on the DNA sequence of the E. chaffeensis 120-kDa protein gene (SEQ ID NOs: 1-6, Figure 1 ) (Yu, X-J 1996). The E. canis 120-kDa protein gene was amplified by PCR with 30 cycles of 94°C 30 sec, 52°C min., and 72°C 2 min. PCR amplified products were cloned into pCR2.1 TA cloning vector (Invitrogen, Carlsbad, CA). EXAMPLE 4 DNA Sequencing DNA was sequenced with an ABI Prism 377 DNA Sequencer (Perkin-Elmer Applied Biosystems, Foster City, CA). EXAMPLE 5 Unidirectional Deletion of the E. canis 120 kDa Protein Gene The repeat region was deleted from the 5' end of the E canis 120 kDa protein gene by using Spe I restriction endonuclease partial digestion. The pCR120 plasmid was first completely digested with Xba I, which had a unique cleavage site on the plasmid sequence near the 5' end of the E. canis 120 kDa protein gene. Then the plasmid was partially digested with Spe I . Spe I had a unique cleavage site in each repeat of the E. canis 1 20 kDa protein gene, but had no cutting site outside the repeat region including the plasmid vector sequence. To ensure a n appropriately representative partial digestion, an aliquot w as removed from the digestion mixture every 5 min. The digestion was stopped by adding EDTA to a final concentration of 50 m M and heating at 70°C for 1 0 min. After complete digestion with Xba I and partial digestion with Spe I, various numbers of repeat units were removed (deleted) between Xba I and each Spe I cleavage site to generate deleted plasmid DNAs with no n- compatible ends (Xba I at the 3' end and Spe I at 5' the end). The restriction enzyme digested mixture was treated with Klenow fragment to fill in the ends. The restriction mixture was th en separated by electrophoresis on an agarose gel to remove the plasmids from the internal repeats because their molecular sizes differed significantly. The mixture of the deleted plasmids w ere extracted from the gel by using a QIAquick Gel Extraction Kit (QIAGEN Inc., Santa Clarita) and self-ligated by using T4 ligase. The deleted plasmids were transformed into E.coli strain DH5 and selected for sequencing according to their sizes. Alternatively, t h e repeat region of the E. canis 120 kDa protein gene w as unidirectionally deleted from the 3' end by using Exonuclease I I I with the Erase-a-Base System (Promega, Madison, WI) according to the instructions of the manufacturer. EXAMPLE 6 Determining the Number of Repeats in the E. canis 1 20-kDa Protein Gene The PCR amplified 120 kDa protein gene from all E canis were cloned into pCR2.1 TA Cloning vector (Invitrogen). The recombinant plasmid was first digested completely with EcoR I and then digested partially with Spe I as described above. The digestion mixtures were separated in a 1% agarose gel an d vacuum transferred onto a nylon membrane. The DNA bands i n the nylon membrane were hybridized with an oligonucleotide probe which was derived from the sequence upstream of the repeat region of the E. canis 120-kDa protein gene. The DNA probes were labeled using digoxigenin- 1 1 -dUTP with a DIG Oligonucleotide Tailing Kit (Boehringer Mannheim Co., Indianapolis, In) according to the manufacturer's protocol. EXAMPLE 7 Gene Analysis DNA sequence and deduced amino acid sequences were analyzed using the Wisconsin GOG software package (Genetic Computer Group, Inc., Madison, WI) and DNASTAR software (DNASTAR, Inc., Madison, WI). The signal sequence of th e deduced protein was analyzed by using the PSORT program (World Wide Web site at URL: http://psort.nibb.ac.jp), which predicts the presence of signal sequences by the methods of McGeoch (D. J. McGeoch, Virus Research, 3, 271 , 1985) and vo n Heijne (von Heijne G., 1986. Nucl. Acids Res., 14, 4683.) an d detects potential transmembrane domains by the method of Klein et al. (P. Klein, M. Kanehisa, and C. DeLisi, Biochem. Biophys. Acta, 815, 468, 1985). EXAMPLE 8 Expression of the E. canis 120-kDa Protein Gene in E. coli Directly cloning the E. canis 120 kDa protein gene into the pGEX expression vector (Amersham Pharmacia Biotech, Piscataway, NJ) was prevented by the absence of matched restriction endonuclease cleavage sites on both DNA sequences of the 120 kDa protein gene and the multiple cloning site of the pGEX vector. The coding region of the E. canis 120 kDa protein gene from nucleotide 175 to nucleotide 1793 was amplified by using PCR with a forward primer (SEQ ID NO: 13) and a reverse primer (SEQ ID NO: 14). The PCR products were cloned into pCR2.1 TA cloning vector (Invitrogen) to obtain the EcoR I cleavage site o n both ends of the insert. The insert in a recombinant pCR2.1 plasmid was cut by EcoR I and separated from the plasmid DNA in an agarose gel. The insert was extracted from agarose gel b y using a QIAquick Gel Extraction Kit (QIAGEN Inc., Santa Clarita) and cloned into EcoRl digested pGEX vector. The E. canis protein was expressed in E. coli BL21 strain as a GST-fusion protein. The GST-fusion protein was affinity-purified by using Glutathione Sepharose 4B beads (Amersham Pharmacia Biotech, Piscataway, NJ). The E. canis 120-kDa recombinant protein was cleaved from the GST-fusion protein with thrombin. EXAMPLE 9 Immunization of Mice Mice were immunized with recombinant E. canis 1 20 - kDa protein to produce antisera. The recombinant pl20 protein was mixed with an equal volume of Freund's complete adj u vant for the first injection and with Freund' s incomplete adjuvant for the subsequent injections. Mice were immunized intraperitoneally or subcutaneously with 50 μg of the recombinant fusion of th e 120-kDa protein and GST on each of 4 occasions. EXAMPLE 10 Detectin g of the E canis 120- kDa Protein Gene Southern blotting was used to attempt to detect the E canis 1 20-kDa protein gene . A 1 .2 kb DNA fragment amplified from the E. chaffeensis 120-kDa protein gene with PCR primer pair pxcf3b (SEQ ID NO: 3) and pxar4 (SEQ ID NO: 5, Figure 1) was labeled with digoxigenin-d-UTP and used as a probe to detect the homologous gene in E. canis by Southern blot. S outhern blot revealed that the E. chaffeensis 120-kDa protein gene probe failed to hybridize with EcoR I-digested E. canis genomic DNA under conditions in which the probe gave strong hybridization with E. chaffeensis genomic DNA. This result indicated that E canis 120-kDa protein gene differed substantially from th e homologous E. chaffeensis 120-kDa protein gene. Although the overall similarity of the 120-kDa protein genes of E. canis and E. chaffeensis is low, it was conceivable th at they might contain some conserved domains which could be u s ed to design PCR primers that would amplify both 120 kDa protein gene. Therefore, the homologous gene of the 120-kDa protein in E canis (Oklahoma strain) was further amplified by PCR. Primers derived from the E. chaffeensis 120-kDa protein gene and sequences flanking the gene had been used previously for sequencing the E. chaffeensis 120-kDa protein gene (Figure 1 ) . Three forward primers (SEQ ID NOs: 1-3) were paired with 3 reverse primers (SEQ ID NOs: 4-6) to form nine pairs of primers . PCR results demonstrated that E. canis DNA was not amplified w i th primers within the coding region of the E. chaffeensis 120-kDa protein gene. A 2.5 kb DNA fragment was amplified from E. canis genomic DNA by the primer pair, pxcf2-2 and pxar3, derived from the non-coding DNA sequences flanking the 120-kDa protein gene of E. chaffeensis (Figure 1). EXAMPLE 11 Determining the Number of Repeat Units in the E. canis 120-kDa Protein Gene Southern blot was used to demonstrate how m an y repeat units the E. canis 120 kDa protein gene had. Restriction enzyme analysis of the DNA sequence of the 120 kDa protein gene of Oklahoma strain demonstrated that all repeats have a unique Spe I endonuclease cleavage site. The insert in the pCR120 plasmid was partially digested with Spe I. Spe I partial digestion of the DNA of E. canis 120-kDa protein gene produced three kinds of DNA fragments: repeats with the 5' end non-repeat sequence, the internal repeats, and the repeats with 3' end non-repeat sequences. After Spe I partial digestion, all DNA fragments w i th the 5' end non-repeat sequence of the gene started from the s ame point (5' end of the gene), but finished within the various repeats . These DNA fragments were separated on agarose gel according to their length which corresponded to their repeat numbers. DNAs were transferred to a nylon membrane and used to hybridize wi th a DIG-labeled oligonucleotide. The oligonucleotide (SEQ ID NO: 1 5 ) was derived from the DNA sequences from nucleotide 38 to 5 9 which were upstream of the repeat region. Therefore, t h e oligonucleotide hybridized only with the DNA fragments which had the 5' end of the gene, but not with the internal repeats or th e repeats with 3' end of the gene. Thus, the number of bands detected by the oligonucleotide probe represents the number of repeats. Southern blotting revealed a ladder of 14 bands with increments of 108 bp in E. canis (Figure 2). These results indicated that E. canis had 14 repeats with 108 bp each. EXAMPLE 12 DNA Sequence Analysis of the E. canis 120-kDa Protein Gene The PCR amplified E. canis DNA fragment was cloned into pCR2.1 vector. The resultant recombinant plasmid w as designated as pCA120. The pCA120 plasmid DNA was used a s template for sequencing the E. canis DNA insert. DNA sequence was obtained initially by utilizing T7 and M13 reverse primers which were complementary to the vector DNA sequence flanking the insert. Subsequent sequencing of DNA was achieved b y primer walking the insert and restriction endonuclease o r exonuclease III unidirectional deletion of the insert in pCA 120 plasmid. DNA sequence analysis demonstrated that the DNA insert contained an open reading frame (ORF) of 2064 nucleotides (SEQ ID NO: 7) which encoded 688 amino acids (SEQ ID NO: 8). This open reading frame was designated as the 120 kDa protein gene of E. canis. No consensus DNA sequences of E. coli promoter near th e 5' end of the gene was found. The N-terminal of the deduced amino acids did not share consensus sequence with E. coli signal peptides. There are 14 tandem repeats in the E. canis 1 20-kDa protein gene. Each repeat consisted of 108 nucleotides which encoded 36 amino acids each (Figure 3). The amino acid homology of all repeats were greater than 94% (Figures 4 ). Preceding th e first repeat there is an incomplete repeat which has a 7 amino acid deletion (Figure 3) and is 70% homologous to the other repeats. The FastA program search of the Genbank database revealed that the E. canis 120-kDa protein gene has no significant homology with any known sequence in the database. The amino acid homology of 120-kDa proteins of E. canis (SEQ ID NO: 10) a nd E. chaffeensis (SEQ ID NO: 9) is 30% (Figure 5). The 120-kDa proteins are more conserved on their N-terminal and in the repeat region. The amino acid homology is 50% for the first 32 amino acids on the N-terminus of the 120-kDa proteins of E. canis and E chaffeensis. The DNA sequence homology is 58% for the 120 kDa protein genes of the two species. The non-coding sequence DNA of 340 bp upstream of the E. canis 120-kDa protein gene w as sequenced and it was found that the non-coding regions adj acent to the 120-kDa protein genes of the two species of Ehrlichia h av e 84% homology. EXAMPLE 13 Predicted Localization and Antigenicity of the E. canis 1 20-kDa Protein The deduced amino acid sequence of the 1 20-kDa protein gene of E. canis was analyzed for hydrophobicity, surface probability, and antigenicity with the Protein program of Lasergene software (DNASTRA Inc., Madison, WI). All repeat units are predicted to be hydrophilic and surface exposed (Figures 6) . Comparison of the 120-kDa proteins of E. canis and E. chaffeensis demonstrated that all repeat units in both proteins are predicted to be surface-exposed. The surface-exposed regions of the repeats have common amino acids in the two ehrlichial species (SEQ ID NOs: 1 1-12, Figure 7). These results suggest that the 120-kDa protein of E. canis is an outer membrane protein. The hydrophilic amino acids in the repeats may be the surface exposed portions of the protein. Analysis of proteins with the Jameson-Wolf method which predicts potential antigenic determinants indicated th at both E. canis and E. chaffeensis 120 kDa proteins were very likely to be highly antigenic (Figure 6). Analysis of the proteins with th e Rothbard-Taylor method, which locates potential T-lymphocyte antigenic determinants, demonstrated that the E. canis 120 kDa protein had a few predicted T-cell epitopes and all of them located on the sequences outside the repeat domain. In contrast, each repeat of the E. chaffeensis 120 kDa protein had two T-cell epitopes. EXAMPLE 14 Homologous Genes in Other Strains of E. canis PCR was used to amplify the 120-kDa protein gene from other strains of E. canis. A 2.5 kb DNA fragment w as amplified from all strains of E. canis including strains Florida, Louisiana, and three North Carolina canine isolates: Demon, DJ, a n d Jake with primers PXCf2-2 and PXAr3. The segments of 120 kDa protein genes of all E. canis strains were sequenced on both the 5' and 3' ends. DNA sequence analysis demonstrated that the DNA sequences up- and downstream of the repeat region w e re identical among all strains of E. canis. The complete repeat region for all E. canis strains was not sequenced because the gene had to be deleted for sequencing. Only the last repeat of all strains a n d the first repeat of DJ strain was sequenced. The sequence of th e first repeat of DJ and Oklahoma strains was identical. The sequence of the last repeat was identical among all strains. The homology of the 120 kDa protein gene from all E. canis strains w as further demonstrated by their identical Spe I restriction physical maps (Figure 8). EXAMPLE 15 SDS-PAGE and Western Blot. The E. canis 120-kDa protein gene was overexpressed in E. coli (Figure 9). The recombinant protein encoded by a 1 620 bp DNA fragment including the entire repeat region of the 1 20 - kDa protein gene was expressed as a GST fusion protein. The estimated molecular size of the fusion protein on SDS gel w a s approximately 140-kDa, which is much larger than the predicted molecular mass of the entire E. canis 120-kDa protein, which i s only 73.6 kDa based on the amino acids deduced from the DNA sequence (Figure 9). Mouse antibodies to the recombinant 1 20 - kDa protein reacted with a 120-kDa protein of E. canis (Figure 10) . Canine anti-i?. canis sera reacted with the recombinant E. canis 120-kDa protein also (Figure 1 1 ). EXAMPLE 16 Discussion Although Ehrlichia spp. are obligately intracellular bacteria, neither the adhesin or the invasin gene of ehrlichiae h as been identified. The 120-kDa protein gene of E. chaffeensis w as previously cloned and sequenced (Yu 1997). The E. chaffeensis 120-kDa protein was recently demonstrated to be an outer membrane protein that is preferentially expressed on the dens e- core ultrastructural form of E. chaffeensis, but not on the reticular cell. A non-invasive, non-adherent strain of E. coli expressing th e 120-kDa protein acquired the ability of adherence and entry into cultured mammalian cells. These results suggest that the 120-kDa protein could be an adhesin or invasin, and therefore it would be a vaccine candidate. Ehrlichia canis and E. chaffeensis are genetically a n d antigenically closely related species. The homologies between th e E. canis and E. chaffeensis are 98% for the 16S rRNA gene and 89% for the nadA gene (Yu 1997). Since the 120-kDa protein appears to be important in the attachment and entry of E. chaffeensis a n d the two Ehrlichia species are closely related, it was hypothesized that an analogue of the 120-kDa protein gene of E. chaffeensis might exist in E. canis and possess similar biological functions. The hypothesis was confirmed by PCR amplification of the E. canis 1 20 kDa protein gene with primers derived from the E. chaffeensis gene. However, the DNA homology of the 120 kDa protein gene of E. canis differed substantially from the homologous gene of E chaffeensis. The homology of the 120-kDa protein genes of E canis and E. chaffeensis is only 30%. The low homology of th e genes of these two Ehrlichia species may explain the lack of hybridization by Southern blotting of E. canis DNA with a E chaffeensis 120-kDa protein gene probe and the lack of PCR amplification of the E. canis gene with primers derived from th e coding region of the E. chaffeensis 120-kDa protein gene. It is surprising that the non-coding sequences flanking the 120-kDa protein genes are more conserved than the coding sequences of the 120-kDa protein genes of E. canis and E. chaffeensis. From a n evolutionary point of view, the coding sequence which is u nd er selection pressure would be expected to be more conserved th an the non-coding sequence in which mutation would not be expected to affect survival of the organism. Although the homology of the 120 kDa protein genes of E. canis and E. chaffeensis are not significant, it was believed that the E. canis gene is the homologue of the E. chaffeensis 1 20 - kDa protein considering that they are located in similar positions in the respective genome; they are 30% homologous; a n d especially they share common motifs in the repeat region. However, both the amino acid sequence and the number of repeats in the 120-kDa-protein genes of E. chaffeensis and E. canis are different. The E. chaffeensis 120-kDa protein contains 4 repeats with 80 amino acids each, and the E. canis 1 20-kDa protein contains 14 repeats with 36 amino acids each. However, the repeats in both proteins are hydrophilic and are predicted to be surface-exposed. Even the total number of surface-exposed regions in the repeats of the two proteins is very close in spite of the difference in the repeat number. The repeat units of both proteins share a common motif consisting of identical amino acids that are hydrophilic and form the core of the surface exposed regions of these proteins. The repeat units of both proteins ar e rich in serine and glutamic acid. Serine and glutamic acid each comprise 19% of the amino acids of the E. canis repeat unit. Glutamic acid and serine comprise of 22% and 15% of the amino acids of the E. chaffeensis repeat units, respectively. Although a signal sequence was not found at the N-terminus of the deduced amino acids of the 120-kDa protein of E. canis by the PSORT protein localization prediction program, however, it was believed that the 120-kDa protein of E. canis is a surface protein similar to the E. chaffeensis 120-kDa protein in which the signal sequence is also absent (Yu 1997). Like the E. chaffeensis 120-kDa protein gene, th e predicted molecular mass of the E. canis 120-kDa protein is much larger than the molecular size estimated by the electrophoretic mobility of the protein by SDS-PAGE. The same phenomenon has been reported for other proteins containing repeat domains, including Anaplasma marginale (Allred DR et al 1990) , Plasmodium (Kemp DJ 1987), Staphylococcus aureus (Hollingshead 1986, Signas 1989), and the human granulocytic ehrlichia (HGE) 100-kDa and 130-kDa proteins (Storey JR 1998). The repeat units of the HGE 100- and 130-kDa proteins have sequences in common with those of the E. chaffeensis 120-kDa protein (Storey, 1998 ). The aberrant migration of the 120-kDa proteins of E. canis and E chaffeensis is not caused by the high percentages of certain amino acids since the molecular weight of the protein is larger than th e total molecular weight of the predicted amino acids. The aberrant migration of the 120-kDa protein is possibly related to post- translational modification of the protein such as glycosylation. The post translational modification of the 120-kDa protein of E canis and E. chaffeensis is currently under investigation. Since th e 120-kDa protein of E. chaffeensis was differentially expressed i n different ultrastructural forms of E. chaffeensis, this protein m a y play a role in the pathogenesis of E. chaffeensis infection. Whether or not the E. canis 120-kDa protein is preferentially expressed in the dense core cell of E. canis is under investigation. Although the 120 kDa protein gene of most E. canis strains was no t sequenced completely, it is reasonable to assume that th e sequence of the 120 kDa protein gene is identical among all strains of E. canis based on the fact that the known sequence including the non-repeat regions as well as the first and the last repeats are identical among strains of E. canis and the fact that all E. canis strains have same number of repeats. The highly homology of DNA sequence and identical number of repeats of the 120 kDa protein gene among the strains of E. canis indicated that E. canis strains are genetically less diversified than E. chaffeensis in which the number of repeats of the 120 kDa protein gene differed among strains . The following references were cited herein. I . Allred, D.R., et al., 1990. Proc. Natl. Aca. Sci. USA. 87:3220-4. 2. Anderson, B.E., et al., 1991. J Clin Microbiol 29:2838-2842. 3. Anderson, B.E., et al., 1992. Int J Syst Bacteriol 42:299-302. 4. Bakken, J. S., et al., 1994. JAMA. 272:212-8. 5. Buhles, W.C. Jr., et al., 1974. J Infect. Dis 130:357-367. 6. Chen, S. M., et al., 1994. J Clin Microbiol 32:589-95. 7. Chen, S.M, et al., 1994. Am J Trop Med Hyg 50:52-58. 8. Dawson J. E., et al., 1991. J Infect Dis 163:564-567. 9. Fishbein, D.B., et al., 1987. JAMA. 257:3100-4. 10. Greene C.E., et al., 1984. Canine ehrlichiosis. p545-561. In C.E. Greene (ed), Clinical microbiology and infectious diseases of the dog and cat. The W.B. Saunders Co., Philadelphia. I I . Groves, M.G., et al., 1975. Am J Veterinary Research. 36:937- 40. 12. Hollingshead, S. K, et al., 1986. J Biol Chem 261 : 1677-86. 13. Kemp, D.J., et al., 1987. Annu. Rev. Microbiol. 41 : 181-208. 14. Jameson, B. A., et al., 1988. CABIO, 4: 181-186. 15. Klein, P., et al., 1985. Biochem. Biophys. Acta, 815: 468-76. 16. Lewis, G.E. Jr., et al., 1977. American Journal of Veterinary Research. 38: 1953-5. 17. Maeda, K., N. et al., 1987. N. Engl. J. Med. 316:853-856. 18. McGeoch D. J., 1985. Virus Research 3, 271-86. 19. Perez, M., et al., 1996. J Clin Microbiol 34:2133-9. 20. Rothbard, J. B., et al., 1988. The EMBO J 7:93-100. 21. Signas, C, et al., 1989. Proc. Natl. Acad. Sci. USA 86:699-703. 22. Storey, J. R., et al., 1998. Infect, Immun. 66: 1356-63. 23. von Heijne G. 1986. Nucl. Acids Res. 14: 4683-90. 24. Walker, J. S., et al., 1970. J Am Vet Med Assoc. 157:43-55. 25. Yu, X-J., et al., 1996. Gene. 184: 149-54. 26. Yu X-J., et al., 1997. FEMS Microbiol Letters. 154:53-8. Any patents or publications mentioned in this specification are indicative of the levels of those skilled in the a r t to which the invention pertains. These patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. One skilled in the art will readily appreciate that th e present invention is well adapted to carry out the objects a n d obtain the ends and advantages mentioned, as well as tho se inherent therein. The present examples along with the methods , procedures, treatments, molecules, and specific compounds described herein are presently representative of preferred embodiments, are exemplary, and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art which are encompassed within th e spirit of the invention as defined by the scope of the claims.

Claims: WHAT IS CLAIMED IS: 1. An isolated nucleic acid segment encoding a 120 kDa protein of Ehrlichia canis, wherein said protein is immunoreactive with anti -Ehrlichia canis serum, and wherein said protein has an amino acid sequence of SEQ LD NO: 8. 2. The nucleic acid segment of claim 1 , wherein said segment has the sequence of SEQ ID NO: 7. 3. A vector comprising the nucleic acid segment of claim 1. 4. The vector of claim 3, wherein said vector is an expression vector capable of expressing a polypeptide encoded b y SEQ ID NO: 7 when said expression vector is introduced into a cell. 5. A recombinant protein comprising the amino acid sequence of SEQ ID NO: 8. 6. The recombinant protein of claim 5, wherein said amino acid sequence is encoded by a nucleic acid segment comprising a sequence of SEQ ID NO: 7. 7. The recombinant protein of claim 6, wherein said nucleic acid segment is contained in an expression vector. 8. The recombinant protein of claim 5, comprising 14 tandem repeat units with 36 amino acids each. 9. The recombinant protein of claim 8, wherein said repeat units are hydrophilic. 10. The recombinant protein of claim 5 as an antigen. 1 1. A host cell comprising the nucleic acid segment of claim 1. 12. An antibody immunoreactive with an amino acid sequence of SEQ ID NO:8. 13. A method of inhibiting Ehrlichia canis infection in a subject comprising the steps of: identifying a subject suspected of being exposed to or infected with Ehrlichia canis; and administering a composition comprising a 120 kDa antigen of Ehrlichia canis in an amount effective to inhibit an Ehrlichia canis infection. 14. The method of claim 13, wherein said 120 kDa antigen is a recombinant protein comprising an amino acid sequence of SEQ ID NO: 8. 15. The method of claim 13, wherein said recombinant protein is encoded by a gene comprising a sequence according to SEQ ID NO:7. 16. The recombinant protein of claim 10 dispersed in a pharmaceutically acceptable carrier.

Inventors: WALKER DAVID H, YU XU-JIE, WALKER, DAVID, H., YU, XU-JIE
Applicants: RES DEV FOUNDATION, RESEARCH DEVELOPMENT FOUNDATION
Application year: 1999
Main classification: C12N15
IPC codes: A61K38, A61K39, A61K48, A61P31, A61P37, C07K14, C07K16, C12N1, C12N5, C12N15, C12P21, C12R1