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Infection and Immunity, April 2003, p. 2234-2238, Vol. 71, No. 4
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.4.2234-2238.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Sophie Leary, Anthony J. Stagg, E. Diane Williamson, and Richard W. Titball
Defence Science and Technology Laboratory, Porton Down, Wiltshire SP4 OJQ, United Kingdom
Received 6 August 2002/ Returned for modification 26 September 2002/ Accepted 11 December 2002
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At present, protection against plague can be mediated through vaccination or antibiotic treatment. Antibiotics are used both to treat plague victims and as prophylaxis to control the spread of the disease (25). The incidence of antibiotic resistance in Y. pestis is low, but recent plague isolates in Madagascar have been found to have multiple drug resistance, conferred by a transferable plasmid (10, 11). Although the bacteria were resistant to the frontline antibiotics streptomycin and tetracycline, they were susceptible to additional antibiotics.
Existing plague vaccines include killed whole-cell preparations, and efforts to develop new vaccines are in progress (39). Problems associated with whole-cell vaccines include relatively low levels of protection, adverse side effects, slow time to immunity, and a need for regular booster immunizations (30). Although whole-cell vaccines are thought to be effective against the most common form of plague (bubonic plague), which develops following a bite from an infected insect, their efficacy against pneumonic plague has been questioned. Consequently, whole-cell vaccines are no longer licensed for use in the United States. Next-generation plague subunit vaccines, based on the recombinant F1 and V (LcrV) antigen proteins derived from Y. pestis, are being developed. Immunization with either protein provides protection against pneumonic or bubonic disease in animal models of infection (12, 17, 39), but greater-than-additive protection is achieved when F1 and LcrV are combined, with protection against up to 109 median lethal doses (MLD) of Y. pestis reported (40). Such vaccines must be administered prior to exposure, and multiple doses are required. Although strategies to reduce the time to immunity and the number of vaccine doses have shown promise (41), it is unlikely that vaccination will provide postexposure protection against disease. There is therefore a need for alternative fast-acting antiplague treatments to provide rapid protection, particularly to combat drug-resistant strains of Y. pestis.
Because antisera have been used widely to treat a range of diseases caused by other pathogens (15), we considered monoclonal antibodies (MAbs) as a treatment for plague. Previously, F1-04-A-G1, a mouse MAb raised against F1, was shown to protect mice in models of bubonic and pneumonic plagues (1). Also, preliminary studies showed that an LcrV-specific MAb (MAb 7.3) protected mice in a bubonic plague model (13). In this study, we considered the prophylactic and therapeutic properties of MAb 7.3, when administered alone and in combination with F1-04-A-G1, to determine whether antibodies could be used as a postexposure therapy for plague.
MAb 7.3 and F1-04-A-G1 were purified by ammonium sulfate precipitation from hybridoma supernatants. An equal volume of saturated ammonium sulfate solution was added slowly to tissue culture supernatants, followed by overnight stirring at 4°C and then centrifugation at 3,000 x g for 30 min. The pellets were drained and resuspended in phosphate-buffered saline (PBS; GIBCO, Paisley, United Kingdom) in 0.1 volume of the original volume, which was then dialyzed against three changes of PBS. Disposable Econopak columns (BioRad, Hemel Hempstead, United Kingdom) were packed with protein G-Sepharose beads (Sigma, Poole, United Kingdom), and antibody solution was passed through the column. The beads were washed with PBS, and then antibody was eluted with 50 mM glycine (pH 3) and stored in fractions containing 150 µl of Tris HCl (pH 9.1) per 3 ml of eluate. Protein fractions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) on 10 to 15% Phastgels (Pharmacia, Milton Keynes, United Kingdom), and fractions containing antibody were dialyzed against three changes of PBS. Antibody concentration was determined by bicinchoninic acid assay (Perbio, Tattenhall, United Kingdom) with a bovine serum albumin standard as recommended by the manufacturers. Antibody purity was assessed by SDS-PAGE analysis.
Antibodies were tested in murine models of bubonic and pneumonic plagues. Six- to 8-week-old BALB/c mice were used (Charles River, Ltd., Margate, United Kingdom). Animal experiments were performed in accordance with United Kingdom legislation relating to animal experimentation (Animals [Scientific Procedures] Act 1986).
Mice received antibody by intraperitoneal (i.p.) injection in 100 µl of PBS prior to or after infection as indicated. Y. pestis strain GB, a fully virulent human isolate, with an estimated MLD of 1 CFU via the subcutaneous (s.c.) route (30), was used in all challenge experiments. In the bubonic plague model, mice received approximately 10 to 105 MLD resuspended in 100 µl of PBS, by s.c. injection. In the pneumonic plague model, mice were exposed to approximately 100 MLD of airborne bacteria, as described previously (42). Animals were checked at least twice daily, and deaths were recorded over a 14-day period.
MAb 7.3 protection. Mice were treated with purified MAb 7.3 24 h prior to challenge with 9.6 or 96 MLD of Y. pestis. As little as 3.5 µg of antibody protected mice and extended the mean time to death (TTD) of animals that died (Table 1). Greater survival was noted in groups given 10.5 or 35 µg than in those given 3.5 and 0.7 µg of MAb 7.3. The degree of protection was smaller in animals that received 96 MLD than in those injected with 9.6 MLD (50 and 83% were survivors, respectively). Therefore, protection against plague appeared to be proportional to the amount of antibody administered and was dependent on the challenge dose.
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MAb 7.3 was administered -4, +24, +48, or +96 h relative to s.c. Y. pestis challenge. Protection was observed when antibody was given up to 48 h postinfection (Fig. 1A). Also, a statistically significant delay in the TTD was observed in the +96-h treatment group. One mouse in the +96-h treatment group had died prior to antibody administration, and the remainder displayed signs of plague indistinguishable from those in untreated control animals, suggesting that even when symptoms of plague are apparent, antibody therapy can delay death. Mice were treated with MAb 7.3 at -4, +24, +48, or +60 h relative to aerosol infection (Fig. 1B). Protection was seen in groups that received antibody 24 and 48 h after challenge. All mice treated at +60 h died, but a statistically significant delay in the TTD was observed, compared with that in untreated animals (Fig. 1B).
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Passive transfer of LcrV-specific polyclonal antiserum protected mice against plague, and the protective epitopes were assigned to region 168 to 275 (21). Similarly, we have mapped MAb 7.3 binding to a conformational epitope between amino acids 135 to 275 of LcrV (13). Therefore, this central region of LcrV appears to be a good target for plague-protective antibodies. To date, epitope mapping studies have not been conducted with F1-04-A-G1.
LcrV has a key role in type III secretion (TTS) by Yersinia spp., a process that allows the injection of a set of effector proteins directly into the cytosol of eukaryotic target cells upon intimate contact (4, 14, 28, 29). The effector proteins (termed "Yops") have a range of functions that promote the killing of phagocytic host cells. Protective polyclonal antisera inhibited Yersinia TTS in HeLA cell cytotoxicity experiments, and LcrV was detected at the bacterial surface prior to contact with eukaryotic cells by confocal microscopy analysis (26). A similar study showed that MAb 7.3, but not other nonprotective Mabs, protected J774 macrophage-like cells against Yersinia-mediated killing (37). Antiserum raised against the LcrV homologue of Pseudomonas aeruginosa (PcrV) protected mice in a lung infection model, antiserum inhibited TTS-mediated cytotoxicity of J774 cells (9, 31), and anti-PcrV F(ab')2 fragment provided therapeutic protection in a model of disease (32). However, other studies did not show a correlation between protective LcrV-specific polyclonal antiserum in cytotoxicity assays (8). LcrV is also reported to have immunomodulatory properties (20, 22, 34, 38), so it remains a possibility that antibodies inhibit both TTS as well as the anti-inflammatory properties of Y. pestis, by blocking the interaction of secreted LcrV with an unidentified eukaryotic receptor.
F1 is expressed optimally at 37°C, is thought to inhibit phagocytosis through the formation of a capsule-like structure on the bacterial surface, and is an effective plague vaccine (2, 6, 12, 36). A recent report showed that an isogenic F1 plague mutant has impaired resistance to phagocytosis by J774 cells (6). Also, a virulence plasmid-cured strain, deficient for TTS, was less resistant to phagocytosis, and an additive effect was seen with the double mutant (F1-negative, plasmid-cured strain). It was proposed that the TTS system and F1 capsule synthesis contribute in different ways to maintain the extracellular lifestyle of Y. pestis (6). The fact that we have targeted both the TTS system and the F1 capsule might explain the high level of protection achieved with MAbs 7.3 and F1-04-A-G1.
A number of strategies can be used to generate clinically useful antibodies (3). For example, the specificity of animal antibodies can be transferred to a human antibody framework, a process termed "humanization" (35, 44), or animal antibodies can be chemically treated to improve their therapeutic properties (18). Alternatively, antibodies can be generated from naïve human single-chain antibody libraries (5, 16, 24) or from immunized transgenic animals that express a human antibody repertoire (23). Our findings have highlighted the benefits of combining antibodies specific for LcrV and F1. The next challenge will be to identify further targets for antibody intervention and to generate antibodies that are suitable for clinical use as a fast-acting pretreatment or postexposure therapy for plague.
| ACKNOWLEDGMENTS |
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Present address: Amersham Biosciences, Amersham, Buckinghamshire HP7 9LL, United Kingdom. ![]()
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