Abstract:
Construction of a recombinant gene fusion encoding a human IgG1 heavy chain constant region and a single-chain variable fragment antibody of 1A4A1 monoclonal antibody is disclosed. The recombinant antibody of the present invention confers human immune effector functions on murine antibodies. After expression in bacteria as inclusion bodies, the recombinant antibody was purified and refolded in vitro. The recombinant soluble antibody retains high antigen-binding affinity to VEE and possesses some human IgG crystallizable fragment domain functions. On non-reducing gel electrophoresis analysis, disulfide bond formation was found in the hinge region of the recombinant antibody. The present invention shows that the recombinant antibody is in a native, functionally active form and it provides the basis to characterize the recombinant antibody for efficacy in vivo.

Description:
FIELD OF THE INVENTION 
     This invention relates to the construction of a recombinant gene fusion encoding a human IgG1 heavy chain constant region and a single-chain variable fragment antibody of 1A4A1 monoclonal antibody. 
     BACKGROUND OF THE INVENTION 
     List of Prior Art Literatures 
     
         
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     Venezuelan equine encephalitis virus (VEE), a member of alphavirus genus of the family Togaviridae, is an important pathogen of epidemic diseases in humans and of epizootics in rodents, horses, donkeys, and mules in the Americas (Johnston and Peters, 1996). VEE causes a spectrum of human diseases ranging from subclinical infection to acute encephalitis (Franck and Johnson, 1970; Johnson et al., 1968). Neurological disease appears in four to 14% of cases (Johnson and Martin, 1974; Walton and Grayson, 1988). The incidence of human infection during equine epizootics could be up to 30% (Groot, 1972). VEE is a potential biological warfare agent of concern. However, there are no antiviral drugs available that are effective against VEE. Although live-attenuated and inactivated vaccines against VEE have been developed, these products are far from satisfactory. Approximately 20% of live-attenuated TC-83 vaccine recipients fail to develop neutralizing antibodies (Abs), while another 20% exhibit reactogenicity (Pittman et al., 1996). A formaldehyde-inactivated vaccine, C-84, is well tolerated, but requires multiple immunization, periodic boosts, and fails to provide protection against aerosol challenge in some rodent models (Jahrling and Stephenson, 1984). 
     VEE complex is a group of antigenically related, but distinct, viruses divided into six subtypes (Calisher, 1994). VEE virions are composed of an icosahedral nucleocapsid, which is surrounded by a lipid envelope containing two structural glycoproteins, E1 and E2 (Schlesinger and Schlesinger, 1996). Epitopes on E1 and E2 are the targets of neutralizing Abs. Studies have shown that the viral neutralizing epitopes are mainly located on the E2 protein, and that the E2 C  epitope appears to be the hub of the neutralization epitopes (France et al., 1979; Roehrig et al., 1982; Roehrig et al., 1991; Roehrig and Mathews, 1985). Monoclonal antibody (MAb) 1A4A1 is specific for E2 C . This MAb has been shown to be efficient in protecting animals from a lethal peripheral challenge with virulent VEE (Roehrig and Mathews, 1985). 
     Murine MAbs, however, have serious disadvantages as therapeutic agents in humans (Breedveld, 2000; Schroff et al., 1985). They induce human anti-mouse antibodies (HAMA). Re-treatment may result in rapid clearance of the murine MAbs and anaphylaxis. Limitations in the use of murine MAbs in clinical applications led to the development of single-chain variable fragment (scFv) Abs (Winter, 1991). ScFv Abs have several advantages compared with the mouse parental MAbs from which they are generated. They generally retain the same specificity and similar affinity to antigens (Kuroki et al., 2000), and demonstrate decreased immunogenicity as compared with the parental mouse MAbs (Colcher et al., 1998). Furthermore, scFv Abs can be produced economically and in a short period of time in bacteria or yeast (McGregor et al., 1994; Verma et al., 1998), and can be manipulated by genetic engineering to form novel proteins by fusion with other molecules, such as metal-binding proteins (George et al., 1995), cytokines (Boleti et al., 1995), toxins (Wels et al., 1992), or T cell receptors (Eshhar et al., 1993). Although scFvs have immense potential in immunodiagnostics (Colcher et al., 1990; Milenic et al., 1991), they have limited utility in immunotherapy, probably due to their monovalent nature, size and the fact that they are rapidly cleared from body circulation. Furthermore, they are unable to recruit effector functions due to the lack of an Ab constant region. 
     The present inventors have cloned and characterized several scFv Abs against the alphavirus genus, VEE or Western equine encephalitis virus (WEE) (Alvi et al., 1999; Long et al., 2000; Xu et al., 1999). An anti-VEE A116 scFv Ab was cloned from 1A4A1 MAb, as disclosed by the Applicant in U.S. patent application Ser. No. 10/096,246, herein incorporated by reference. However, in vitro binding assays indicated that A116 scFv Ab had low binding affinity to VEE, in comparison to the parental MAb. Sequence analysis of A116 revealed that three bases were missing in the conserved framework-1 region of the variable region of the light chain (V L ). Polymerase chain reaction-based site-directed mutagenesis was used to introduce the three missing bases, resulting in a repaired A116 scFv Ab, designated mA116 scFv Ab, as disclosed in the same U.S. patent application Ser. No. 10/096,246. This repaired scFv showed an affinity to VEE comparable to that of the parental 1A4A1 MAb. 
     Antibodies have been recognized as one of the most critical components of the immune system. Many infectious diseases can be cured or prevented by specific Abs. All Abs share the same basic structure. The N-terminal domains of heavy and light chains constitute the variable regions that form the antigen-binding site. The other domains, which comprise the constant region, contribute to the activation of host effector functions such as complement activation, stimulation of phagocytosis by macrophages, and antibody-dependent cellular cytotoxicity (ADCC) (Breedveld, 2000). These effector functions are often required for therapeutic efficacy. 
     Antiviral immunity is complex, with several factors involved, such as Ab response, cell-mediated immunity, and induction of interferon. Ab response to viruses includes not only neutralization of infectivity for susceptible host cells, but also host effector functions such as complement-mediated lysis of infected host cells and opsonization. The host effector functions of Abs are attributable to the crystallizable fragment (Fc) Ab regions (Breedveld, 2000). ScFv Abs lack the Fc region, and thus lack effector functions. 
     Accordingly, it is desirable to produce recombinant scFv Abs having the constant region of human Abs with enhanced effector functions against VEE. 
     SUMMARY OF THE INVENTION 
     In the present invention, the inventors have genetically joined a human IgG 1 heavy chain constant region to anti-VEE mA116 scFv Ab, resulting in mA116huFc Ab. An object of the present invention is to confer on the ScFv Ab some of the human-associated effector functions without increasing immunogenicity in humans. The mA116huFc Ab gene was expressed in  E. coli  and refolded in vitro and the resulting product was purified and characterized. 
     Another object of the present invention is to produce mA116huFc Abs which retain antigen-binding affinity to VEE and possess some human IgG1 Fc domain functions, such as protein G and human C1q binding. 
     According to the present invention, it provides an expressed protein mA116huFc Ab constructed from recombinant gene fusion which comprises encoding a human IgG1 heavy chain constant region and a single-chain variable fragment (“scFv”) antibody of 1A4A1 monoclonal antibody. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows pRSmA116huFc construct. MA 116 scFv Ab gene was cloned into the Sfi I and Not I sites of pRS10B5huFc to replace 10B5 scFv Ab gene as described under “Materials and Methods”. 
         FIG. 2  shows the DNA sequence (SEQ ID NO. 1) and translated amino acid sequence (SEQ ID NO. 2) of the mA116huFc Ab. 
         FIG. 3  shows SDS-PAGE analysis of samples from the purification of mA116huFc Ab. Samples were resolved on 10% polyacrylamide gel and stained with Coomassie blue. Lane 1, molecular weight marker; 2, bacterial lysate; 3, solubilized protein fraction; 4, column flow through fraction; 5, 10 and 20 imidazole eluates; 6, final protein preparation. 
         FIG. 4  shows Western blot analysis of samples from the purification of mA116huFc Ab. Samples were resolved by SDS-PAGE, transferred to Immunobilon-P membranes, and probed with (A) HRP-conjugated anti-human Ig, (B) HRP-conjugated Ni—NTA, or (C) anti-Xpress Ab followed by HRP-conjugated anti-mouse Ig. Lane 1, bacterial lysate; 2, solubilized protein fraction; 3, column flow through fraction; 4, 10 and 20 imidazole eluates; 5, final protein preparation. 
         FIG. 5  shows disulfide bond formation assay. Abs were resolved by SDS-PAGE. under reducing or non-reducing conditions and then stained with Commassie blue. Lane 1, molecular weight marker; 2, mA116huFc Ab in reducing condition; 3, mA116huFc Ab in non-reducing condition; 4, 1A4A1 MAb in reducing condition; 5, 1A4A1 MAb in non-reducing condition; 6, mA116 scFv Ab in reducing condition; 7, mA116 scFv Ab in non-reducing condition. 
         FIG. 6  shows VEE antigen binding assay by ELISA. A. Various concentrations of Abs were added to 96-well plate coated with 10 μg/ml of VEE. B. 10 μg/ml of Abs were added to a 96-well plate coated with various concentrations of VEE. Binding was detected with HRP-conjugated anti-human Ig, anti-mouse Ig, and anti-E-tag Ab, followed by ABTS solution. 
         FIG. 7  shows protein G binding assay. Abs were incubated with protein G agarose and precipitated by centrifugation. The pellets were run on SDS-PAGE with Coomassie blue staining. Lane 1, molecular weight marker; 2, mA116huFc Ab only; 3, 1A4A1 MAb only; 4, mA116 scFv Ab only; 5, mA116huFc Ab precipitated by protein G; 6, 1A4A1 MAb precipitated by protein G; 7, mA116 scFv Ab precipitated by protein G. 
         FIG. 8  shows C1q binding assay. The various concentrations of mA116huFc or mA116 scFv Ab were added to strips coated with human C1q protein. Binding was detected with HRP-conjugated anti-human IgG, followed by ABTS solution. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Materials and Methods 
     Construction of pRSmA116huFc 
     MA116 scFv Ab gene was derived from A116 scFv Ab gene, which was constructed using the Recombinant Phage Antibody System (Amersham Pharmacia Biotech, Baie d&#39;Urfe&#39;, QC), as disclosed in the U.S. patent application Ser. No. 10/096,246. MA116 scFv Ab DNA fragment was separated from vector pCANTAB5E by restriction enzyme digestions with Sfi I/Not I, followed by gel electrophoresis and purification with the gel extraction kit (Qiagen, Chatsworth, Calif.). The fragment was subcloned into the Sfi I/Not I site of the pRS10B5huFc expression vector (Long et al., 2000), replacing the anti-WEE virus 10B5 scFv Ab gene. The fragment was inserted in frame with the 5′ end of a gene sequence encoding a 35.8-kDa human IgG1 constant region and the 3′ end of a fragment containing 6-His tag and Xpress epitope, added for ease of purification and detection. The construct was named pRSmA116huFc ( FIG. 1 ). The recombinant plasmid DNA was transferred into  Escherichia coli  ( E. coli ) BL-21 (DE3) pLys S competent cells, containing a genomic source of T7 RNA polymerase under lacUV5 promoter control (Invitrogen, Carlsbad, Calif.). The recombinant plasmid, containing the mA116 scFv Ab gene insert, was confirmed by restriction digestion and DNA sequencing. Six primers for DNA sequencing were made on an Oligo 1000 DNA synthesizer (Beckman Instruments, Fullerton, Calif.) and were as follows: SC-1 (5′-CATCATCATCATCATCAT-3′) (SEQ ID NO. 3), SC-9 (5′-CAGCAGCCAACTCAGCTT-3′) (SEQ ID NO. 4), SC-13 (5′-ACTCCTGACATCCTGTCG-3′) (SEQ ID NO. 5), HL-2 (5′-TCTAACGGGACCAGAGACAAC-3′) (SEQ ID NO. 6), HL-3 (5′-GGTAAGTTCAGGGACAGG-3′) (SEQ ID NO. 7), HU-4 (5′-TGCAAGGCCAGTCAGGATGTG-3′) (SEQ ID NO. 8). Sequencing reactions were performed using a Big Dye Terminator™ cycle sequencing kit (PE Biosystems, Mississauga, ON). The reaction products were purified by Centri-Sep™ columns (Princeton Separations, Adelphia, N.J), and then run on an ABI 310 genetic analyzer system (Applied Biosystem, Fullerton, Calif.). Sequences were assembled and analyzed using Lasegene DNA software (DNA star, Madison, Wis.). 
     Expression Purification and Refolding of the Recombinant mA116huFc Ab 
     Recombinant protein, expressed in  E. coli  as inclusion bodies, was solubilized, purified, and refolded, as previously described, with minor modification (Long et al., 2000). Briefly, after induction for 3 hr by isopropyl β-D-thiogal actopyranoside (IPTG),  E. coli  transformants harboring the pRSmA116huFc expression plasmid were harvested by centrifugation. The cell pellet was resuspended in 5 mM borate sodium, pH 9.3, and 4 M urea, and cell lysate was prepared by sonication (three cycles of 10 sec; amplitude 10 μm; 15 sec cooling on ice), using a MSE Soniprep 150-probe sonicator (Wolf Laboratories, Pocklington, UK). The sonicates were centrifuged (13,000 g for 10 min) and pellets were resuspended in 5 mM borate sodium, pH 9.3, 8 mM urea, and 100 mM sodium chloride. Purification of the recombinant protein was performed on Talon™ metal affinity resin (Clontech, Palo Alto, Calif.). The solution of 5 mM borate sodium, pH 9.3, 8 M urea, and 100 mM sodium chloride was used as wash buffer. Bound fractions were eluted with 10-250 mM imidazole and 1 M arginine (final concentration) was added as cosolvent to encourage the correct folding of the protein molecules. The recombinant protein was refolded by removal of 8 M urea, by dialyzing against 5 mM borate sodium, pH 9.3, and 1 M arginine; the cosolvent was then removed by dialyzing against 5 mM borate sodium, pH 9.3. The purity was checked by sodium dodcyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Coomassie brilliant blue R-250 (Bio-Rad Laboratories, Mississauga, ON) staining after concentration by ultrafiltration using Centricon™ YM-30 (Millipore Corp., Bedford, Mass.). 
     SDS-PAGE and Western Blot Analysis 
     Protein samples were electrophoresed in 10% SDS-PAGE gels, on a Mini Protein II apparatus (Bio-Rad Laboratories), under reducing (5% 2-mercaptoethanol) or non-reducing conditions. The bands were visualized by Coomassie blue staining. 
     The separated proteins were also transferred to Immunobilon-P membranes (Millipore Corp, Bedford, Mass.) by use of a Western blot semi-dry transfer apparatus (Bio-Rad Laboratories) with Towbin buffer (25 mM Tris-HCl, pH 8.3, 192 mM glycine, and 20% methanol). Unreacted sites were blocked with blocking buffer (3% non-fat skim milk in phosphate-buffered saline (PBS)). Blots were washed three times for 5 min with PBS containing 0.1% tween-20 (PBST). Some blots were then incubated for 1 hr at room temperature with a 1:5000 dilution of anti-Xpress Ab (Invitrogen). These blots were then washed three times for 5 min with PBST and incubated for 1 hr at room temperature with a 1:3000 dilution of horseradish peroxidase (HRP)-conjugated goat anti-mouse Ig (Caltag Laboratories, Burlingame, Calif.) in TBST. Other blots were incubated directly with either a 1:2000 dilution of HRP-conjugated donkey anti-human Ig (Jackson ImmunoResearch Laboratories, West Grove, Pa.) or a 1:1000 dilution of HRP-conjugated Ni-nitrilotriacetic acid (NTA) (Qiagen, Valencia, Calif.) at room temperature for 1 hr. After three washes for 5 min with PBST, and two washes for 2 min with deionized water, the specific binding was detected by an enhanced chemilumeniscence kit (Amersham Pharmacia Biotech). 
     ELISA 
     The reactivity of purified mA116huFc Ab to VEE antigen was determined by an enzyme-linked immunosorbent assay (ELISA). Nunc maxisorp™ flat-bottomed 96-well plates (Canadian Life Technologies, Burlington, ON) were coated overnight at 4° C. with inactivated, whole VEE (strain TC-83) at a fixed concentration of 10 μg/ml, or various concentrations of 0.01-100 μg/ml, in carbonate bicarbonate buffer, pH 9.6, containing 0.02% sodium azide. The plates were washed five times with PBST and then blocked twice in 2% bovine serum albumin for 1 hr at 37° C. After five washes with PBST, plates were incubated for 1 hr at 37° C. with various concentrations of 0.01-100 μg/ml, or a fixed concentration of 10 μg/ml of purified Ab (mA116huFc Ab, 1A4A1 MAb, or mA116 scFv Ab) diluted in PBST. Following five washes with PBST, plates were incubated for 1 hr at 37° C. with HRP-conjugated Ab diluted 1:3000 in PBST (HRP-donkeyanti-human Ig for detection of mA116huFc Ab, HRP-goat anti-mouse Ig for 1A4A1 MAb, or HRP-anti-E tag for mA116 scFv Ab). Finally, the plates were washed five times with PBST and developed for 30 min at room temperature with a substrate consisting of 2,2′-azino-di-(3-ethyl-benzthiazoline-sulfonic acid) diammonium salt (ABTS) and hydrogen peroxidate (Kirkegaard and Perry Laboratories, Gathersburg, Md.). The reactions were read at an absorbance of 405 nm by a microplate autoreader (Molecular Devices, Sunnyvale, Calif.). 
     Protein G Binding Assay 
     The assay for purified recombinant mA116huFc Ab to bind to Protein G was modified based on an immunoprecipitation protocol (Harlow and Lane, 1999). MA116huFc Ab, 1A4A1 MAb, or mA116 scFv Ab, respectively, was incubated with washed protein G agarose (Life Technologies, Burlington, ON) in radioimmunoprecipitation (RIP) buffer (50 mM Tris-HCl, pH 7.4, 150 mM sodium chloride, 0.1% SDS, and 1% Triton X-100). After 1 hr incubation with rocking at room temperature, the agarose was collected by centrifugation at 13,000 g for 1 min at room temperature. The pellets were washed three times with RIP buffer and centrifuged at 13,000 g for 1 min. The protein G binding complexes were resuspended in Laemmli sample buffer containing 5% 2-mercaptoethanol, and heated in boiling water for 10 min. After centrifugation, the supernatants were run on a 10% SDS-PAGE gel followed by Coomassie blue staining. 
     Human C1q Binding Assay 
     The capacity of purified recombinant mA116huFc Ab to bind to human C1q was assayed using the circulating immune complexes (CIC)-C1q test kit (QUIDEL Corp., San Diego, Calif.) in accordance with the manufacturer&#39;s instructions. In brief, eight-well strips coated with human C1q protein were rehydrated by wash buffer (0.05% tween-20 and 0.01% thimerosal in PBS). The purified recombinant mA116huFc Ab, or mA116 scFv Ab, was serially diluted from 100 μg/ml in wash buffer, and incubated in the strips for 1 hr at room temperature. After five washes with wash buffer, HRP-conjugated goat anti-human Ab was added to each well and the strips were incubated at room temperature for 30 min, after which the strips were washed five times with wash buffer and incubated with ABTS substrate solution for 30 min. To stop the enzymatic reaction, stop solution containing 250 mM oxalic acid was added to the wells, and the absorbance was measured at 405 nm by the microplate autoreader. 
     Results 
     Construction, Expression and Purification 
     The pRS10B5huFc gene construct, in which the anti-WEE 10B5 scFv Ab gene was linked with human IgG1 heavy chain constant region (Long et al., 2000), and the anti-VEE mA116 scFv Ab gene in pCANTAB5E, were used as source materials to create the mA116huFc Ab gene construct. The 10B5 scFv Ab gene of pRS10B5huFc was replaced by the mA116scFv Ab gene. The resulting plasmid, designated pRSmA116huFc, contained the mA116scFv Ab gene, arranged in variable heavy (V H )-V L  chain orientation via a (Gly 4 Ser) 3  linker, followed by human IgG1 heavy chain constant (CH) regions under the control of T7 promoter ( FIG. 1 ). In addition, there was a 6-His tag sequence for immobilized metal affinity chromatography (IMAC) purification and a Xpress epitope for detection by anti-Xpress Ab. 
     The DNA sequence and translated amino acid sequence are showed in  FIG. 2 . The mA116 scFv Ab gene was 723 bp in length, encoding 241 residues with a molecular weight of 25.7 kDa. The frameworks for VH and VL were well-conserved, as compared with those of mouse anti-guinea pig C5 scFv Ab (Genbank AJ250760), anti-solamargine scFv Ab (Genbank AF332008), and anti-CD30 Ki-3 scFv Ab (Genbank AF280760) (data not shown). The complementarity determining region (CDR) sequences of both VH and VL were different from those of the above-noted scFv Abs (data not shown). Overall, mA116 scFv Ab was 72%, 72%, and 70% homologous to mouse anti-guinea pig C5 scFv Ab, anti-solamargine scFv Ab, and anti-CD30 Ki-3 scFv Ab, respectively. The first 66 bases of CH1 were missing from the human IgG1 CH region. The encoded Fc protein was 322 residues with a molecular weight of 35.8 kDa. The molecular weight of the whole fusion protein, including 6His tag and Xpress epitope, was about 68 Kda. 
     The mA116huFc Ab was expressed in  E. coli  BL-21 cells and purified by IMAC. SDS-PAGE demonstrated that there was a large amount of protein in the bacterial lysate of molecular weight ˜70 kDa, corresponding to the predicted size (68 kDa) of mA116huFc Ab ( FIG. 3 , Lane 2). After centrifugation of the lysate, and dissolution of the pellet in denaturing agent, many of the proteins were removed from the lysate ( FIG. 3 , Lane 3). The solubilized protein fraction was incubated with metal affinity resin and loaded to an empty column. After thoroughly washing, the bound fractions were eluted by an imidazole gradient (10 mM to 250 mM) in elution buffer. The 10 and 20 mM imidazole eluates showed a major band at ˜70 kDa, accompanied by other weak bands ( FIG. 3 , Lane 5), whereas, in the eluates of imidazole concentration 50 mM and greater, only the 70 kDa band was present ( FIG. 3 , Lane 6). In this elution protocol, the expressed protein could be purified to &gt;90%. 
     Biochemical Characterization 
     To confirm the presence of intact, expressed mA116huFc Ab, a series of Western blotting experiments was performed, in which the 70 kDa protein was detected by HRP-anti-human Ig, HRP—Ni—NTA, and anti-Xpress epitope followed by HRP-anti-mouse Ig, respectively. As shown in  FIG. 4 , the 70 kDa protein was recognized in Western blots by all three of these Abs. The HRP—Ni—NTA and anti-Xpress Abs also detected a 32 kDa fragment in the crude fractions, however, this fragment was eliminated in the purified fraction ( FIGS. 4 , B and C). 
     The hinge region of an Ab is responsible for disulfide bond formation between two identical Ab heavy chains. Since mA116huFc Ab contained an intact hinge region, interchain disulfide bond formation was examined by comparing the Ab protein under reducing and non-reducing conditions in SDS-PAGE ( FIG. 5 ). Under reducing conditions (addition of 5% 2-mercaptoethanol), 1A4A1 MAb appeared as two bands, of molecular weight 50 kDa and 25 kDa, representing heavy chain and light chain, respectively ( FIG. 5 , Lane 4). Under non-reducing conditions, 1A4A1 Ab appeared as high molecular weight aggregates ( FIG. 5 , Lane 5). Under reducing conditions, mA116huFc Ab migrated as one band of molecular weight ˜70 kDa and, under non-reducing conditions, as a high molecular weight aggregate ( FIG. 5 , Lane 2 and 3). On the other hand, mA116 scFv migrated as only one band of molecular weight ˜30 kDa under both reducing and non-reducing conditions ( FIG. 5 , Lane 6 and 7). 
     Binding Properties to VEE Antigen 
     The immunoreactivity of mA116huFc Ab to VEE antigen was examined by ELISA. When the plates were coated with a fixed concentration of inactivated VEE (10 μg/ml), mA116huFc Ab bound to VEE in a dose-dependent manner, similar to the binding to VEE of parental 1A4A1 MAb and mA116 scFv Ab ( FIG. 6A ). An additional ELISA test was performed in which a concentration gradient of VEE was titrated against a fixed concentration of Abs (10 μg/ml). A similar dose-response relationship was observed ( FIG. 6B ). Furthermore, less than 10 ng/ml VEE could be detected by 10 μg/ml of mA116huFc Ab. 
     Protein G and C1q Binding 
     Although 22 residues were missing in the CH1 of mA116huFc Ab, CH2 and CH3 were intact. CH2 and CH3 are believed to be associated with essential features of the constant region of the Abs, such as binding proteins G and A, and effector functions. To determine the affinity of binding of mA116 huFc Ab to the protein G, mA116huFc Ab was incubated with protein G agarose and then, after thorough washing and centrifugation, analyzed by SDS-PAGE. As shown in  FIG. 7 , the mA116huFc Ab, like its parental 1A4A1 MAb, efficiently bound to protein G agarose, while mA116 scFv Ab did not. To determine whether the mA116huFc Ab could initiate complement activation, one of the important effector functions, an ELISA was performed, in which human C1q was coated on the strips. MA116huFc Ab bound to human C1q in a doze-independent fashion, while mA116 scFv Ab did not bind to human C1q ( FIG. 8 ). 
     Discussion 
     The inventors of the present invention have genetically fused anti-VEE mA116 scFv Ab with a human IgG1 heavy chain constant region, in order to confer some human effector functions, and to reduce immunogenicity of the MAb in humans. DNA sequencing confirmed that DNA cloning was successful. The construct, mA116huFc Ab was expressed in  E. coli  to high levels in the form of insoluble inclusion bodies. The insoluble recombinant mA116huFc Ab was solubilized by denaturing agent, 8 M urea. Inclusion of 6-His tag allowed solubilized recombinant Ab to be purified via IMAC. It has been found that 10 and 20 mM imidazole could minimize nonspecific binding and reduce the amount of contaminating proteins, in spite of some recombinant Ab loss. Accordingly, greater than 90% purity of mA116huFc Ab could be obtained. After purification, arginine was introduced to the recombinant protein solution to direct correct refolding. 
     The results of Western blot analysis confirmed that the refolded recombinant protein was intact, with a molecular weight of ˜70 kDa. A comparison of the SDS-PAGE electrophoretic patterns of mA116huFc Ab, obtained under reducing and non-reducing conditions, was conducted. As expected, mA116huFc Ab migrated as a high molecular weight aggregation under non-reducing conditions, as did the parental 1A4A1 MAb. Under reducing conditions, mA116huFc Ab migrated as one band of molecular weight ˜70 kDa, corresponding in size to the monovalent mA116huFc. These results suggested that there was inter-chain disulfide bond formation in the mA116huFc Ab. Included as a control, mA116 scFv Ab migrated as a single ˜30 kDa band under both reducing and non-reducing conditions. This finding was as expected as the scFv Ab has no disulfide linkages. However, dimers of scFv Ab fragments have been shown to have a longer biological half life and increased avidity (Adams et al., 1998; Colcher et al.,1998; Long et al., 2000). 
     The in vitro binding characteristics of mA116huFc Ab to VEE antigen were assayed by ELISA. The mA116huFc Ab exhibited strong binding activity to VEE, indicating that folding was appropriate for the formation of antigen-binding sites. It is worth noting that 10 μg/ml of mA116huFc could detect VEE down to a concentration of less than 10 ng/ml, suggesting the potential utility of mA116huFc Ab for VEE immunodiagnostics applications. The parental 1A4A1 MAb and mA116 scFv Ab showed similar binding activity to VEE, however, a direct comparison of the binding affinities of the three Abs was not possible by ELISA, since each Ab required a different conjugated secondary Ab. 
     Results of the protein G binding assay indicated that mA116huFc could bind to protein G, thus suggesting that the constant region of mA116huFc Ab was correctly folded. Protein G specifically interacts with CH2 and CH3 constant regions of Ab (Stone et al., 1989). The finding that mA116huFc Ab could bind to protein G suggested that the Fc region must be close in structure to the native profile. The present invention investigated whether the constant region in mA116huFc Ab was folded well enough to activate complement. The classical pathway of complement activation is initiated by the constant region of Ab binding to C1q, a constituent of the first component of complement (Burton, 1985; Hughes-Jones and Garder, 1979). The inventors found that mA116huFc Ab could strongly bind to human C1q in a dose-dependent manner. As expected, mA116 scFv Ab could not bind to C1q due to the lack in Ab structure of the human IgG1 heavy chain constant region. These results indicate that mA116huFc Ab might be active in the recruitment of complement-mediated cell lysis. 
     All Abs are glycoproteins and are glycosylated at characteristic positions according to their isotype. The IgG molecule has one conserved glycosylation site, at Asn 297, within the CH2 domain of each of its two heavy chains (Sutton and Phillips, 1983). The oligosaccharides are thought to stabilize the molecule and to contribute to the tertiary structure of the constant region, which is very important to such effector functions as complement activation, Fc receptor recognition, and ADCC. Although Asn 457 is available for glycosylation in mA116huFc Ab, bacterial expressions systems are not capable of glycosylating proteins. It has been reported that lack of glycosylation of IgG could affect its binding ability to complement binding capacity (Leatherbarrow et al., 1985; Nose and Wigzell, 1983; Tao and Morrison, 1989). However, the present invention indicates little to no effect of the lack of glycosylation on the ability of mA116huFc to bind to complement. 
     In summary, a chimeric Ab consisting of the human IgG1 constant region fused to a mouse scFv Ab to VEE has been engineered. This Ab was shown to be a disulfide-linked homodimer, with demonstrated retention of antigen-binding affinity to VEE antigen. In addition, this Ab was shown to possess some human Ig G1 Fc domain functions, such as the ability to bind protein G and human C1q complement. From these findings, it was concluded that this Ab was in a native, functionally active form. The present invention forms the basis for further investigation to characterize mA116huFc Ab in protection studies. 
     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.