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Infection and Immunity, September 2003, p. 5402-5406, Vol. 71, No. 9
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.9.5402-5406.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
J. R. Schreiber,1,2,3* and N. S. Greenspan1*
Departments of Pathology,1 Pediatrics, Case Western Reserve University,2 Divisions of Infectious Diseases, Allergy, Immunology, and Rheumatology, Rainbow Babies and Children's Hospital, Cleveland Ohio 441063
Received 28 March 2003/ Returned for modification 1 May 2003/ Accepted 27 May 2003
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We previously described low 23F-specific antibody titers after immunization with the 23F-CRM197 vaccine in CBA/J mice, despite high CRM197-specific antibody titers and the presence of CRM197-reactive T cells (3). Immunization of CBA/J mice with 6B-CRM197 or 19F-CRM197 conjugate vaccines resulted in high levels of both PS- and CRM197-specific antibodies. Although immunization with 23F-CRM197 yielded responses to different sets of CRM197-derived peptides than did immunization with the 6B-CRM197 and 19F-CRM197 conjugates, the overall levels of T-cell proliferation in response to any of the different conjugates were comparable. Thus, we concluded that the apparent qualitative alterations in the T-cell response to 23F-CRM197 in CBA/J mice were unlikely to be responsible for the poor immunogenicity of serotype 23F PS (3).
In the present study, we asked if the weak PS-specific antibody responses of CBA/J mice following immunization with 23F-CRM197 is influenced by genetic differences between distinct inbred strains of mice. Therefore, we compared responses to 23F-CRM197 conjugate vaccine among CBA/J, BALB/c, and (CBA/J x BALB/c)F1 (CCBAF1) mice. Both serum titers of 23F-specific antibody and numbers of 23F-specific antibody-secreting cells (ASC) after immunization were assessed.
Mice of the three genetic backgrounds (CBA/J, BALB/c, and CCBAF1) were immunized intraperitoneally with either 19F-CRM197 or 23F-CRM197 and boosted 2 weeks later. Eight-week-old pathogen-free female mice were immunized with 10 µg (PS content) of 19F-CRM197 or 23F-CRM197 (kindly supplied by Ron Eby, Wyeth-Lederle Vaccines, West Henrietta, N.Y.). The PS/protein mass ratios of the vaccines were as follows: 19F-CRM197, 0.66; 23F-CRM197, 0.52 (R. Eby, personal communication). CBA/J and BALB/c mice were obtained from The Jackson Laboratory (Bar Harbor, Maine). CCBAF1 mice were obtained by in-house breeding. Ten micrograms of PnPS was chosen as the immunizing dose because previous experiments had demonstrated that this is the minimal amount of antigen that is able to yield maximal or near-maximal antibody titers in CBA/J mice (3). Mice immunized with sterile phosphate-buffered saline served as negative controls. Sera were screened for anti-PnPS and anti-CRM197 antibodies via an enzyme-linked immunosorbent assay (ELISA) in which 96-well plates were coated with PnPS or CRM197 as previously described (3). Quantitation of antigen-specific antibody levels was based on standard curves generated with distinct murine monoclonal antibodies specific for, respectively, 19F PS (59-1; immunoglobulin G1 [IgG1]), 23F PS (53-2; IgG1), or CRM197 (E7-10; IgG1). The monoclonal antibodies were all generously provided by Phil Fernsten, Wyeth-Lederle Vaccines.
Immunization of CBA/J mice with 23F-CRM197 resulted in low-level anti-23F antibody titers. In contrast, immunization of BALB/c and CCBAF1 mice with 23F-CRM197 resulted in 23F-specific antibody titers significantly greater than those of CBA/J mice (P < 0.004) (Fig. 1A). CBA/J and BALB/c mice also produced significantly lower titers of CRM197-specific antibodies (Fig. 1C).
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FIG. 1. 23F- and CRM197-specific responses of BALB/c, CBA/J, and CCBAF1 mice after two immunizations with 23F-CRM197. (A) Total 23F-specific serum immunoglobulin chain titers (micrograms per milliliter) detected by PnPS solid-phase ELISA. (B) Frequency of 23F-specific ASC determined by PnPS solid-phase ELISpot assay. The data shown are a compilation of two experiments with the mean ± the standard error of the mean of a total of 24 mice per group. *, P < 0.004. (C)Total CRM197-specific serum immunoglobulin chain titers (micrograms per milliliter) detected by CRM197 solid-phase ELISA. (D) Frequency of CRM197-specific ASC determined by PnPS solid-phase ELISpot assay. The data shown are a compilation of two experiments with the mean ± the standard error of the mean of a total of 24 mice per group. *, P < 0.003 for comparisons of selected groups with CCBAF1.
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-alkaline phosphatase and 5-bromo-4-chloro-3-indolylphosphate (BCIP)-nitroblue tetrazolium in nitroblue tetrazolium buffer (Sigma). The spots were enumerated with an ImmunoSpot series 1.0 analyzer (Optimas, Bothell, Wash.). Spots could not be detected in unimmunized mice or in mice immunized once in our assay system (data not shown). The frequencies of both 23F- and CRM197-specific ASC were substantially decreased (P < 0.004 and P < 0.03, respectively) in CBA/J mice in comparison to the 23F- and CRM197-specific ASC frequencies of BALB/c and CCBAF1 mice (Fig. 1B and D), consistent with the serum antibody measurements. In contrast to these results, immunization of CBA/J, BALB/c, and CCBAF1 mice with 19F-CRM197 yielded roughly comparable serum antibody titers and ASC with specificities for both 19F PS and CRM197 (Fig. 2A to D). Thus, CBA/J mice have the ability to respond, similarly to BALB/c mice, following immunization with CRM197. The low numbers of 23F-specific ASC in CBA/J mice and the presence of significantly higher numbers in BALB/c and CCBAF1 mice suggest a genetic influence over the antibody response to 23F, such that a stronger response is expressed in a dominant fashion.
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FIG. 2. 19F- and CRM197-specific responses of BALB/c, CBA/J, and CCBAF1 mice after two immunizations with 19F-CRM197. (A) Total 19F-specific serum immunoglobulin chain titers (micrograms per milliliter) detected by PnPS solid-phase ELISA. (B) Frequency of 19F-specific ASC determined by PnPS solid-phase ELISpot assay. The data shown are a compilation of two experiments with the mean ± the standard error of the mean of a total of 16 mice per group. (C) Total CRM197-specific serum immunoglobulin chain titers (micrograms per milliliter) detected by CRM197 solid-phase ELISA. (D) Frequency of CRM197-specific ASC determined by CRM197 solid-phase ELISpot assay. The data shown are a compilation of two experiments with the mean ± the standard error of the mean of a total of 16 mice per group.
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FIG. 3. PS- and CRM197- specific antibody titers (micrograms per milliliter) of CBA/J mice primed with either 19F-CRM197 or 23F-CRM197 and boosted with either conjugate. Total serum immunoglobulin chain titers (micrograms per milliliter) of 19F-specific (A), 23F-specific (B), and CRM197-specific (C) antibodies in CBA/J mice detected by PnPS (A and B) and CRM197 (C and D) solid-phase ELISAs. 19F-specific (A) and CRM197-specific (D) IgG1 levels detected in the same sera are also shown. The mean titers (micrograms per milliliter) ± the standard error of the mean of groups of five mice are shown.
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In order to determine if the differences in 23F PS- or CRM197-specific immunogenicity between mouse strains after immunization with 23F-CRM197 might involve differences in T-cell responsiveness, we compared both proliferative responses of lymph node cells to whole CRM197 and the specificities of T-cell proliferative responses for CRM197-derived peptides. While the sets of peptides for the CBA/J and BALB/c cells were not identical (although overlapping), the overall proliferative responses to whole CRM197 were comparable (data not shown). These results, in conjunction with the results from sequential immunization with 23F-CRM197 and 19F-CRM197, suggest that the lack of response to 23F PS (and CRM197) by CBA/J mice is not primarily attributable to the T-cell compartment.
The most likely explanation for these findings is a genetically influenced paucity of 23F-specific B-lymphocyte precursors. If in mice, as Zhou et al. (9) found in humans, antibody responses to 23F PS involve a restricted number of light- or heavy-chain variable-region genes, polymorphisms in the relevant variable-region genes might influence the production of 23F-specific precursors. How a relative deficiency of 23F PS-specific precursors in CBA/J mice would account for the relatively poor CRM197-specific antibody responses following immunization of these mice with 23F-CRM197 remains unclear. Perhaps PS-specific B cells represent a significant proportion of the antigen-presenting cell pool for carrier-specific T cells. Although we cannot absolutely rule out the possibility that conjugation of 23F PS to CRM197 generates a uniquely nonstimulatory or inhibitory epitope(s), the ability of 23F-CRM197 to prime effectively for an antibody response to CRM197 upon a challenge with 19F-CRM197 (Fig. 3C and D) argues against these possibilities. Furthermore, we have previously shown that simultaneous immunization with both 23F- and 19F-CRM197 conjugates does not preclude a strong response to 19F PS, suggesting that CRM197-specific T-cell responses were not inhibited by the 23F-CRM197 conjugate (3).
In conclusion, CBA/J mice were poorly responsive after immunization with Pn serotype 23F-CRM197, with antibody titers and 23F-specific ASC frequencies markedly lower than those of BALB/c mice. These results suggest that genetic differences between CBA/J and BALB/c mice contribute to the quantitative differences in the 23F PS-specific antibody responses elicited by 23F-CRM197. Further experiments are necessary to identify murine loci at which allelic variation influences the magnitude of the antibody response to protein-conjugated Pn capsular PS.
This study was supported by National Institutes of Health grants AI41657 (N.S.G.), AI27862 (J.R.S.), and AI32596 (J.R.S.) and a grant from Wyeth Lederle Vaccines.
Present address: Department of Microbiology, University of Pennsylvania, Philadelphia, PA 19104. ![]()
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