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Clinical and Diagnostic Laboratory Immunology, March 2005, p. 447-452, Vol. 12, No. 3
1071-412X/05/$08.00+0 doi:10.1128/CDLI.12.3.447-452.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Medical Microbiology and Immunology,1 Göteborg University Vaccine Institute,2 Department of Obstetrics and Gynecology,3 Department of Infectious Diseases, Sahlgrenska University Hospital, Göteborg University, Göteborg, Sweden4
Received 30 September 2004/ Returned for modification 18 October 2004/ Accepted 13 December 2004
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It is generally believed that direct application of an antigen at the target mucosa is the most efficient way of inducing a protective mucosal immune response (4-6, 8, 22). We and others have shown that vaginal administration is superior to both oral vaccination and rectal vaccination in generating strong IgA and IgG antibody responses in cervical and vaginal secretions of humans (15, 27). According to recent studies, the nasal route can also be considered for the induction of antibodies in the female genital tract (2, 12, 17). However, information concerning the kinetics of immune responses in the human genital tract after mucosal vaccination is scarce.
The aim of the present study was to examine the kinetics of the local and systemic immune responses in healthy fertile women given three vaginal vaccinations with an inactivated cholera vaccine containing recombinant cholera toxin B subunit (CTB). CTB is one of the best-characterized mucosal antigens with regard to both safety and immunogenicity in humans (2, 7, 10, 27). CTB-specific IgA and IgG antibodies in cervical secretions along with antitoxin antibodies in serum were determined, and the immune responses after one, two, and three vaginal doses of CTB were compared. The kinetics of the immune responses were also monitored for responding volunteers for up to 12 months after the last vaccination.
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None of the volunteers had previously been vaccinated against cholera or had traveled to areas where cholera or enterotoxigenic Escherichia coli is endemic during the 5 years preceding the study. All women were regularly menstruating, and none used oral contraceptives.
Each volunteer was vaginally immunized with three doses of a licensed, inactivated B-subunit-whole-cell (B-WC) cholera vaccine (Dukoral; SBL Vaccin AB, Stockholm, Sweden) administered at 2-week intervals. The vaccine contained 1 mg of recombinant CTB and 1011 inactivated cholera vibrios per dose (7). Each dose (3 ml) of the vaccine was mixed with 650 mg of a biologically inert polysaccharide (Eldexomer, batch 020; Perstorp Pharma, Perstorp, Sweden). The freshly made vaccine-gel mixture was deposited in the upper fornix of the vagina, and the women remained in a horizontal position for 10 min after each vaccination (12, 27). The first immunization was initiated on day 10 of the menstrual cycle (i.e., 10 days after the last menstrual bleeding had started). The second immunization was given on day 24 of the menstrual cycle, and the third immunization was given on day 10 of the following menstrual cycle.
Nine volunteers participated in the first part of the study, in which the immune responses after one, two, and three vaginal immunizations with CTB were evaluated.
Cervical secretions and serum were collected immediately before immunization (day 0, i.e., day 10 of the menstrual cycle), 14 days after the first immunization (day 14, i.e., day 14 of the menstrual cycle), 14 days after the second immunization (day 28, i.e., day 24 of the following menstrual cycle), and 8 to 10 days after the third immunization (days 36 to 38, i.e., days 18 to 20 of the following menstrual cycle).
In the second part of the study, the kinetics of the immune responses in seven of the nine volunteers who had participated in the first part of the study were monitored. The remaining volunteers were withdrawn from the study; one woman was a nonresponder, and the other woman did not give her consent to continue. In addition, three women were recruited and vaginally immunized three times with CTB according to the above-mentioned schedule. Thus, 10 volunteers who responded with significant IgA antitoxin titer increases in cervical secretions after vaginal vaccination participated in the second part of the study. In this part of the study, cervical and serum specimens were obtained 1, 2, 3, 6, and 12 months after the third vaccination. Samples of cervical secretions were obtained with a syringe (Aspiglaire; Biotechnologies International, Aigle, France), and the volume was recorded. Before determination of the antibodies, the cervical samples were diluted 1:10 with phosphate-buffered saline and treated with bromelain (Sigma Chemical Company, St. Louis, Mo.) to solubilize the mucus (27). Bromelain-treated specimens were stored at 70°C until analyzed. Serum specimens were stored in aliquots at 20°C until analyzed.
Determination of total immunoglobulin and specific antibodies.
The total IgA and IgG antibody contents in cervical specimens were determined by a modified enzyme-linked immunosorbent assay (ELISA) (2, 26). Specimens with low IgA and/or IgG concentrations (
40 µg ml1) were excluded from further analyses, since our previous studies showed that antibody titers in such specimens were unreliable.
IgA and IgG antibodies to cholera toxin in cervical secretions and serum were determined by the GM1 ELISA method as previously described (25, 27). The CTB-specific IgA and IgG antibody activities (in units per microgram) in cervical secretions were determined by dividing the ELISA immunoglobulin titer (in units per milliliter) by the total immunoglobulin concentration (in micrograms per milliliter) in the specimens to compensate for variations in immunoglobulin contents in specimens collected on different days. Responders were defined as having a greater than twofold increase in specific antibody activities between pre- and postvaccination specimens (11). In serum, a twofold or greater increase in endpoint titers between pre- and postvaccination specimens was used to signify seroconversion at a P value of
0.05 (10, 11).
Statistical methods.
The frequencies and magnitudes of antibody responses after one, two, and three vaginal vaccinations were compared for statistical significance by using Fisher's exact test and Student's t test (paired), respectively. Two-tailed significance tests were used, and P values of
0.05 were considered statistically significant.
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A single vaccination failed to induce any CTB-specific IgA antibody responses in cervical secretions (Fig. 1A), and an increase in the ratios of IgG antitoxin titers to total IgG concentrations was found only in one of nine individuals (Fig. 1B). Two doses of CTB induced significant increases in IgA antitoxin titers in seven of nine volunteers, and four volunteers developed IgG antitoxin responses. A 20-fold increase in CTB-specific IgA titers was induced in the entire group of vaccinees, and the IgG responses were increased 7.1-fold; the magnitudes of the titer increases in the responders were 50-fold for IgA antitoxin and 47-fold for IgG antitoxin. The postvaccination levels of IgA as well as IgG antitoxin were significantly higher after two vaccinations than after a single dose of vaccine (P = 0.0078 and P = 0.0336, respectively) (Fig. 1). A third dose of CTB did not significantly increase the antitoxin responses in cervical secretions, although the frequency of IgG responses was slightly higher than that after the second vaccination. No differences in the postvaccination levels of IgA and IgG antitoxins were found after three versus two vaccinations (Fig. 1).
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FIG. 1. Geometric mean ± standard error of the ratios of IgA (A) and IgG (B) antitoxin titers to total immunoglobulin concentrations in cervical secretions from nine healthy women before and after one, two, and three vaginal immunizations with recombinant CTB. The values have been multiplied by 100. The numbers of responding volunteers are indicated above the bars. Immunizations are indicated by arrows.
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TABLE 1. Antitoxin antibody responses in serum from healthy women after one, two, and three vaginal administrations of recombinant CTB
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TABLE 2. Kinetics of frequencies of IgA and IgG antitoxin responses in cervical secretions and serum from healthy women after three vaginal doses of recombinant CTB
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FIG. 2. Kinetics of geometric mean ± standard error of the fold increases in the ratios of IgA (A) and IgG (B) antitoxin titers to total immunoglobulin concentrations in cervical secretions from 10 healthy women after three vaginal immunizations with recombinant CTB.
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The endocervix appears to be the predominant immunological organ in the female genital tract (18), although no organized inductive sites containing membranous epithelial cells have been found in the genital mucosa. Immunohistochemical examinations of tissue sections or dispersed cells have indicated that the human endocervix contains larger numbers of immunoglobulin-secreting cells than do the ectocervix, fallopian tubes, and vagina (3, 18). Almost all IgA-producing cells contain joining chains and the secretory component (SC), a marker of synthesis of polymeric IgA. Further, the single-layer epithelium of the endocervix as well as the fallopian tubes, uterus, and ectocervix express SC, which is necessary for the transportation of locally produced polymeric IgA into genital secretions. In contrast, the multilayer epithelium of the vagina does not stain for SC (18; J. Mestecky, R. P. Edwards, P. A. Crowley-Nowick, A. M. Pitts, and W. H. Kutteh, unpublished data). For the determination of local immune responses in the female genital tract after various routes of mucosal vaccination, cervical secretions and/or vaginal fluids have been used. Vaginal vaccination has been shown to be the route of choice for stimulating both IgA and IgG antibody responses in cervical secretions (12, 16, 27), while nasal vaccination seems to be superior in inducing IgA responses in vaginal fluids (12). These findings indicate that there is compartmentalization within the genital tract and that the induction of specific antibodies in cervical secretions is regulated in a manner different from that in vaginal secretions (12).
The present study shows that two vaginal vaccinations were required for the induction of strong IgA and IgG antitoxin responses in the human genital tract. None of the vaccinees responded with IgA antitoxin titer increases in cervical secretions after a single vaccination, and only one of nine vaccines developed a significant IgG antitoxin response. These results agree with the notion that one nasal dose of CTB was less efficient than two doses in stimulating IgA and IgG antibody responses in nasal and vaginal secretions (23). Two oral doses of B-WC cholera vaccine have also been reported to be optimal for the induction of an effective local IgA immune response in the intestine (26). Our results also showed that two vaginal doses of CTB were as effective as three doses in generating local IgA as well as IgG antibody responses in cervical secretions. In contrast, Kozlowski et al. (15) reported higher levels of CTB-specific antibodies in cervical and vaginal secretions after three compared to two vaginal vaccinations. However, the number of vaccinees was small, and there was a large distribution of CTB-specific antibody titers around the mean value in their study. According to our data, immunity at the systemic level does not directly reflect the local antibody responses in the genital tract. CTB-specific IgA and IgG antibody responses in serum were found already after a single vaginal vaccination, and the magnitudes of the responses in serum after two and three vaginal vaccinations were lower than those in cervical secretions.
Several studies have evaluated the kinetics of local immune responses to CTB in various secretions after various routes of mucosal vaccination. In volunteers given two oral doses of B-WC cholera vaccine, peak levels of IgA antitoxin antibodies in intestinal, nasal, and vaginal fluids appeared 2 weeks after the second vaccination (13, 23). After nasal administration of CTB, maximal increases in specific IgA antibodies in nasal and vaginal secretions were found 1 week after the second vaccination, whereas IgG antibodies showed a delayed onset, with peak responses appearing after 6 weeks (23). Our analyses of the kinetics of antibody responses in cervical secretions after vaginal administration of B-WC cholera vaccine indicated that IgA as well as IgG antitoxin antibodies showed peak responses 8 to 10 days after the third vaccination. Kozlowski et al. (15) also reported similar findings of higher levels of antitoxin antibodies in cervical secretions 2 weeks compared to 4 weeks after vaginal vaccination.
Immune responses at mucosal surfaces are generally regarded to be short-lived. However, analyses of fecal antibody responses in Swedish volunteers have shown that increases in IgA antitoxin and antibacterial antibodies can be demonstrated in 50 and 43% of initially responding subjects 6 months after oral immunization with B-WC cholera vaccine (14). Nasal administration of CTB has also been shown to result in persisting IgA and IgG antibody responses in nasal as well as vaginal fluids for at least 6 months after vaccination (2). Prior to this study, the duration of local IgA and IgG antibody responses in genital secretions after vaginal vaccination had been monitored for only 2 months (17). The relative ease in collecting cervical specimens made it possible for us to monitor local mucosal immune responses in the genital tract for a longer period. After 3 months, significant IgA antitoxin responses could be demonstrated in eight of nine initially responding volunteers, and in five vaccinees, the antitoxin levels were still elevated at 12 months of follow-up. The IgG antibody titer increases in cervical secretions displayed a similar pattern, with persisting responses being found in half of the initially responding volunteers 12 months after vaccination. The effective induction and long-term persistence of locally produced antibodies in cervical secretions suggest that CTB and probably also other antigens linked to CTB may be used for effective mucosal immunization of the genital tract against specific pathogens. These findings may be of relevance for the development of vaccines against human immunodeficiency virus infection as well as other STDs.
We gratefully acknowledge Camilla Johansson and Kerstin Andersson for excellent technical assistance and Perstorp Pharma for providing Eldexomer.
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