Recombinant antigen for diagnosis and prevention of murine typhus

Abstract

The invention relates to a recombinant immunogenic composition from Rickettsia typhi. The invention also relates to a method for the use of the recombinant proteins in detection and diagnostic assays and as a component in formulations for the induction of an anti-R. typhi immune response.

Description

SEQUENCE LISTING

I hereby state that the information recorded in computer readable form is identical to the written sequence listing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a gene and protein which can be used for vaccination against and/or for the detection and identification of R. typhi infection. More particularly, the invention relates to a specific nucleotide sequence encoding a highly specific and immunogenic portion of the gene encoding the protective OmpB antigen of Rickettsia typhi and the polypeptide products of this gene. The polypeptide sequence can be utilized in diagnostic and detection assays for murine typhus and as an immunogen useful as a component in vaccine formulations against murine typhus.

2. Description of the Prior Art

Murine (endemic or flea-borne) typhus, caused by infection with Rickettsia typhi, is a zoonosis that involves rats (Rattus rattus and R. norvegicus) as the main reservoir and the oriental rat flea (Xenopsylla cheopis) as the main vector [1,2]. The infection is primarily caused by scratching the flea bitted site and self-inoculating the R. typhi-laden feces, or directly by infected flea bite [3]. The symptoms of murine typhus include fever, headache, enlarged local lymph nodes and rashes on the trunk. These clinical manifestations are non-specific and resemble many other diseases such as viral infections, typhoid fever, leptospirosis, epidemic typhus and scrub typhus [3,10]. As a result, murine typhus is frequently misdiagnosed and its incidence is probably grossly underestimated.

Murine typhus is one of the most widely distributed arthropod-borne diseases of humans and occurs in a variety of environments from hot and humid lowlands to semi-arid highlands including Australia [6], Spain [7], Indonesia [8], and southwestern United States [9] in addition to previously reported countries including China, Thailand, Kuwait, Israel, and Vietnam [3,5]. It is often found in international port cities and costal regions where rodents are common [3-5].

The diagnosis of murine typhus relies mainly on serological methods [11]. The old serological assay, Weil-Felix test, is based on the detection of antibodies to Proteus vulgaris OX-19 that contains cross reactive epitopes of Rickettsia [12, 13]. However, determination of R. typhi infection by the Weil-Felix test requires a qualitative determination and therefore somewhat subjective. Additionally, because the Weil-Felix reaction requires specialized reagents, many facilities especially in rural areas or in developing countries often may not be capable of performing the laboratory diagnosis.

Other techniques include immuno-fluorescence assay (IFA) and complement fixation (CT) tests were adapted for the detection of antibodies specific for rickettsiae [14-16]. Current serodiagnostic assays such as the ELISA, Dip-S-Ticks (DS), indirect immunofluorescent antibody (IFA) and indirect peroxidase assays [17,18] require the propagation of rickettsiae in infected yolk sacs of embryonated chicken eggs or cell cultures to prepare the antigens used in these assays. However, only a few specialized laboratories have the ability to culture and purify rickettsiae, which requires Biosafety level three (BSL-3) containment facilities. Additionally, because the organism is required for the assay, in addition to potential biosafety hazards associated with the assay, these assay methods also suffer from refrigerated storage requirements, and the problem of reproducibility associated with frequent production of rickettsial antigens.

In addition to antibody-based assays, polymerase chain reaction (PCR) amplification of rickettsial protein antigen genes has been demonstrated as a reliable diagnostic method, and genotypes can be determined without isolation of the organism [19,20]. However, gene amplification requires sophisticated instrumentation and reagents generally not available in most medical facilities especially those far forward. Based on these considerations, production of recombinant antigens of R. typhi is a logic direction for the development of serological assays and vaccine candidates for murine typhus.

R. typhi has a monomolecular layer of protein arranged in a periodic tetragonal array on its surface [21]. This crystalline layer, representing 10 to 15% of the total protein mass of the rickettsia, was identified as the immunodominant species-specific surface protein antigen OmpB. It has been isolated, purified, and biochemically characterized [22-25]. The earliest and dominant immunological responses in mice, guinea pigs, rabbits, and humans, following infection with R. typhi, are directed against Omp B [17, 4, 25]. We have shown that purified native typhus OmpB induces strong humoral and cell mediated immune responses. Protective immunity was elicited by typhus OmpB in guinea pig and mouse protection models [26-29].

Based on these observations, therefore, OmpB is a particularly advantageous target for developing diagnostic reagents. R. prowazekii, the etiologic agent of epidemic typhus, also belongs to the typhus group of rickettsiae and its OmpB exhibits similar antigenic and chemical structures to those of R. typhi. Therefore, cross-reactivity of antibody to OmpB between these two species is inevitable. Cross absorption of test serum is needed to distinguish between them these to species [10].

The whole ORF of OmpB codes for a polypeptide of 1642 amino acids. The native matured protein does not contain the leader peptide at the N-terminus and the β-sheet peptide at the C-terminus. The expression of the intact OmpB protein (135 kDa) has been attempted. However, the full-length product was shown to be toxic to Escherichia coli and rapidly degraded. Moreover, due to its large size and high constant of β-sheet structure, refolding of the full-length gene product was not successful.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention are methylated and unmethylated recombinant polypeptides encompassing immunologically active regions of OmpB of Rickettsia typhi.

Another object of the invention is a method of using the methylated or unmethylated recombinant OmpB fragments in antibody-based assays for the detection of exposure to Rickettsia typhi.

A still further object of the invention is the use of OmpB or the OmpB fragments as an immunogen.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. Open reading frame of OmpB and location of Fragments A, K and AN.

FIG. 2. Western blot analysis of IgG and IgM reactivity to fragment AN.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Evaluation of Rickettsia typhi proteins has led to the identification of OmpB is an exceptionally promising candidate as a reagent for use in diagnostic and detection assays as well as components in vaccine formulations. The species-specific surface protein antigen OmpB (SEQ ID No. 3, encoded by nucleotide sequence of SEQ ID No. 4) of R. typhi was identified as the immunodominant. The earliest and dominant immunological anti-protein responses of mice, guinea pigs, rabbits, and humans following infection with R. typhi are directed against this Omp B antigen. These observations suggested OmpB as an appropriate target for developing diagnostic reagents.

Central to the development of improved detection and diagnostic immunoassay methods and standardization is the development of more effective antigens for use in existing antibody-based methods. In order to improve the antigenicity and potential immunogenicity of the OmpB, specific regions of OmpB were evaluated for sera reactivity. Western blot analysis of partially digested OmpB revealed that all the reactive fragments were larger than 20 kDa [31]. One reactive fragment was located at the N-terminus and another located at the C-terminus. Along these lines, efforts have been made to identify immunodominant fragments of OmpB proteins. Accordingly, two highly sera-reactive protein fragments (Fragment A and Fragment K) have been identified. FIG. 1 illustrates the location of these fragments within the OmpB molecule. The location Fragment AN, which has the amino acid sequence of SEQ ID No. 1 and is encoded by nucleotide sequence SEQ ID No. 2, is also illustrated in FIG. 1.

Fragment AN was successfully cloned, expressed, purified, and refolded. The fragment has been shown to be recognized by different patient sera and can be used to replace whole cell antigens and/or native OmpB as a diagnostic marker and a potential vaccine candidate.

Construction of recombinant R. typhi protein AN Fragment was carried out by first producing a cDNA copy of the gene sequence by polymerase chain reaction. A primer pair (SEQ ID No. 5 and 6) was designed using the nucleotide sequence of the ORF of R. typhi OmpB.

The coding sequence was amplified by PCR using DNA from R. typhi Wilminton strain. Amplification was conducted in a mixture of 400 mM each of deoxynucleotide triphosphate, 1 μM of each primer, 1.5 U of Taq polymerase (Perkin Elmer-Cetus, Norwalk Conn.) in 10 mM Tris-HCl buffer, pH 8.3, 1.5 mM MgCl₂, and 50 mM KCl. The PCR reaction was started with 5 min at 94 C, and followed by 30 cycle of 94 C for 50 second, 55 C for 1 min and 72 C for 2 min. the last cycle was extended for 10 min at 72 C. the amplified gene fragment was digested with Nde I (New England BioLabs, Beverly, Mass.) and BamHI (GIBCO-BRL Life Technology, Gaithersburg, Md.) and ligated with doubly digested expression vector pET28a.

Fragment AN was expressed as inclusion body in E. coli BL21. The inclusion bodies were extracted with 0.1×BUG BUSTER™ (Novagene (EMD), San Diego, Calif.) three times. The final pellet was dissolved in 8 M urea and purified over a nickel column then refolded by sequential dialysis in decreasing concentrations of urea. The chemical methylation of fragment A was performed according to the procedures described by Taralp and Kaplan (J. Prot. Chem. 16, 183-193, 1997).

Fragment K coding sequence from amino acid 745 to 1353 was amplified by PCR from DNA isolated R. typhi Wilminton strain. The fragment K gene was amplified in a 50 ul mixture of 150 mM each of deoxynucleotide triphosephate, 0.8 μM of each primer, 2.5 U of Taq Gold polymerase (Perkin Elmer-Cetus, Norwalk Conn.) in 10 mM Tris-HCl buffer, pH 8.3, 1.5 mM MgCl₂, and 50 mM KCl. The PCR reaction was started with 10 min at 94 C, and followed by 30 cycle of 94 C for 30 second, 55 C for 30 second and 72 C for 2 min. the last cycle was extended for 7 min at 72 C. The ligation of the amplified fragment K in to pET11a was the same as for fragment A.

Fragment K was over-expressed in BL21 cells by induction with 1 mM IPTG for 4 hr. The over-expressed K was primarily in the inclusion body and was extracted with 4 M urea. The solubilized K in 4 M urea was further purified with HPLC using two gel filtration columns in tandem (TSK-G3000-SW and TSK-G4000-SW) followed by an anion exchange column using a NaCl gradient (50-100 mM in 30 minutes). A greater than 95% purity as demonstrated by SDS-PAGE. The purified K was refolded by dialysis in 2 M urea at 4° C. with two changes of dialysis solution in the presence of reduced glutathione (1 mM), followed by dialysis in buffer without urea. Expression of Fragment AN was accomplished by inserting the encoding DNA into a suitable expression system, such as pET 28a. The R. typhi recombinant protein antigen can be utilized as an antigen either as an unpurified E. coli lysate or purified by any number of methods and subsequently used as antigen in detection or diagnostic assays.

Table 1 illustrates the immuno-reactivity of fragment AN from Richettsia typhi in enzyme-linked immunosorbent assay (ELISA). Serum from R. typhi exposed patients were reacted to either fragment AN or whole cell. As shown in Table 1, serum antibodies that did not exhibit sero-reactivity to whole cell antigen also did not react to fragment AN. However, sera positive to whole cell antigen was also reactive to fragment AN. In Table 1, “0” indicates negative titer and therefore negative serum reactivity.

TABLE 1
Antibody reacitivty against fragment AN or whole cell antigen in ELISA
		ELISA (ANt)		ELISA (whole cell)
	Serum	O.D.		titer
	65	0.481	+	1/100
	107	0.313	+	1/100
	113	0.456	+	1/100
	163	0.255	−	0
	186	0.227	−	0
	219	0.161	−	0
	224	0.411	+	1/400
	243	0.368	+	1/100
	245	0.458	+	1/100
	250	0.206	−	0
	255	0.386	+	1/100
	256	0.333	+	1/100
	278	0.195	−	0
	319	0.071	−	0
	376	0.922	+	1/400
	453	0.14	−	0
	459	0.221	−	0
	527	0.08	−	0

FIG. 2 further illustrates the immunogenicity and immune reactivity of fragment AN. FIG. 2 shows western blot analysis of AN or OmpB against either negative or serum from patients previously exposed to R. typhi. In FIG. 2, OmpBt is the native antigen OmpB of R. typhi and serves as the positive control for the sero-reactivity of recombinant antigen AN. As illustrated in the figure, both IgG as well as IgM antibody isotypes were highly reactive to fragment AN.

Based on these results, protein fragment AN will be a valuable antigen in detection and diagnostic assays either alone or in assays incorporating other R. typhi recombinant proteins, for example Fragment K (SEQ ID. No. 8). Standardization of antigen will improve assay diagnostic performance and provide early and more accurate treatment regimens. Improved sensitivity can be achieved by combination of protein fragments containing a greater number of epitopes well represented in serum antibody repertoires.

Accordingly, an aspect of this invention is the recombinant expression of an immunodominant fragment, fragment AN, derived from the outer membrane protein OmpB. This recombinant protein fragment can be used as antigen in a method for the detection of R. typhi infection and murine typhus. Recombinant versese whole cell antigen will confer improved standardization and concomitant assay reproducibility and potentially sensitivity in assays for the detection and diagnostic assays.

The following examples are provided to further illustrate the use of the invention.

Example 1

Use of OmpB fragment AN as Diagnostic Reagent

Assays using the recombinantly produced proteins include antibody-based assays such as enzyme-linked immunosorbent assays. As previously mentioned, antigen for the assay can be in the form of unpurified E. coli lysate. However, for increased assay sensitivity and reduced background, purified recombinant R. typhi proteins can be used. The general method is to use the recombinant polypeptide as antigen in order to detect anti-R. typhi antibodies. As such, recombinant AN polypeptide, or AN fragments containing B cell epitopes, is exposed to patient antibody. Antibody binding is, therefore, detected by any number of means including enzyme-linked immunosorbant assay, indirect florescent assay (i.e., IFA), plasmon resonance or of a number of other means for detecting antigen-antibody interaction. Additionally, for increased accuracy of the assay, AN, or fragments thereof can be used in tandem with other recombinant proteins such as OmpB, Fragments A or K.

Either the entire AN fragment or one or more fragments containing the predicted B-cell epitopes, contained in fragment AN, can be utilized. Table 2 illustrates predicted B-cell epitopes contained in fragment AN and are represented by SEQ ID Nos. 9-29.

TABLE 2
Predicted B-cell epitopes
contained in fragment AN
Amino Acid Amino Acid
position   Sequence
4          VMQYNRTTNAA
40         ITANSNNAITFNTPNGNLNS
82         TNVTKQGN
111        QQAATTKSAQNV
131        AINDNDLSG
157        INPTTQEAP
187        GFVKVSDKTF
231        INFNGRDGTGKLVLVSKNGNATE
305        SVDNGNAAT
345        GGKTNFKTADSKV
390        IGDAKNNGNTAG
410        TLVSGNTDPN
464        NGPVNQNPLVNNNALA
542        IQLTSTQNNIL
558        DVTTDQTGV
570        SSLTNNQTLT
590        NTKTLGRFNVGSSKT
619        ENDGSVHLTHNTY
638        NAANQGK
650        DPINTDT
698        NNITTTDANVGSL

As an illustration, the following procedure is provided, comprising the following steps:

• • 1. Recombinant protein represented by SEQ ID No. 1, or one or more fragments thereof, is immobilized to a solid surface, such as in 96-well plates;
  • 2. Unreacted/unbound antigen is washed off. A preferred embodiment of the inventive method is to wash at least 3 times with wash buffer containing 0.1% polysorbate surfactant such as polyoxyethylene (20) sorbitan monolaurate;
  • 3. Block unreacted sites. In a preferred embodiment, blocking of unreacted sites is accomplished with 5% skim milk in wash buffer)×45 minutes and then rinsed three times.
  • 4. React test sera to the bound antigen. As a further control, intact OmpB (SEQ ID No. 3), can also be immobilized and exposed to patient sera;
  • 5. Plates are washed three times with wash buffer;
  • 6. After incubating the test sera, the bound antibody-antigen is exposed to a probe. In a preferred embodiment, the probe is enzyme-labeled (e.g. peroxidase) anti-human immunoglobulin;
  • 7. detecting bound probe. Detection of bound probe can by any number of methods. In a preferred embodiment, detection is by measurement of enzymatic reaction of added substrate.

The above specific procedural outline is provided to illustrate the general method of using the fragments for the detection R. typhi infection. However, other iterations of the general antibody-based procedure is contemplated. Furthermore, a standard curve can be constructed by conducting the above ELISA procedures with the recombinant proteins but utilizing a range of concentrations of specific antibody to R. typhi. The extent of measured binding of patient serum antibody is compared to a graphic representation of the binding of the R. typhi-specific antibody concentrations.

Example 2

Prophetic Use of Recombinant R. typhi Proteins as a Vaccine Component

Because of its strong immunoreactivity with serum antibody from R. typhi exposed patients, the recombinantly produced polypeptides is an excellent candidate for use as a componented in R. typhi vaccine formulations. Accordingly, Fragment AN (SEQ ID No. 1), or one or more fragments of the R. typhi protein Fragment AN or their respective DNA sequences (SEQ ID No. 2) incorporated into a suitable expression vector system, can be utilized as vaccine components. Fragments of AN containing B-cell epitopes are represented by SEQ ID Nos. 9-29 (see also Table 2). The method for induction of R. typhi immunity contains the following steps:

• • a. administering an immunogenic composition containing the entire or immunogenic fragments of the recombinant polypeptides selected from the group consisting of SEQ ID No. 1 in a unit dose range of 50 μg to 2 mg;
  • b. administration of boosting dose of said immunogenic composition at least 1 week after priming dose with unit dose range of 50 μg to 2 mg in a buffered aqueous solution, wherein an immune response is elicited.

An alternative method of immunizing is to administer DNA sequences encoding Fragment AN, or combinations thereof, inserted into a suitable expression system capable of expressing the fragments in vivo. Suitable expression systems can include viral expression vectors as well as a number of available DNA vector systems.

REFERENCES

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Claims

1. A method of detecting R. typhi infection comprising the steps: a) exposing a composition consisting a recombinant Rickettsia typhi OmpB polypeptide AN fragments containing predicted B cell epitope set forth in selected SEQ ID No. 16 to patient sera; andb) measuring antibody bound to said AN polypeptide fragments.

2. The method of claim 1, wherein said composition is immobilized onto a solid surface.

3. The method of claim 1, wherein said method also includes the steps exposing patient sera to recombinant OmpB polypeptide and measuring antibody bound to said OmpB polypeptide.