Comparison of Three Whole-Cell Pertussis Vaccines in the Baboon Model of Pertussis

Abstract

Pertussis is a highly contagious respiratory illness caused by the bacterial pathogen Bordetella pertussis. Pertussis rates in the United States have escalated since the 1990s and reached a 50-year high of 48,000 cases in 2012. While this pertussis resurgence is not completely understood, we previously showed that the current acellular pertussis vaccines do not prevent colonization or transmission following challenge. In contrast, a whole-cell pertussis vaccine accelerated the rate of clearance compared to rates in unvaccinated animals and animals treated with the acellular vaccine. In order to understand if these results are generalizable, we used our baboon model to compare immunity from whole-cell vaccines from three different manufacturers that are approved outside the United States. We found that, compared to clearance rates with no vaccine and with an acellular pertussis vaccine, immunization with any of the three whole-cell vaccines significantly accelerated the clearance of B. pertussis following challenge. Whole-cell vaccination also significantly reduced the total nasopharyngeal B. pertussis burden, suggesting that these vaccines reduce the opportunity for pertussis transmission. Meanwhile, there was no difference in either the duration or in B. pertussis burden between unvaccinated and acellular-pertussis-vaccinated animals, while previously infected animals were not colonized following reinfection. We also determined that transcription of the gene encoding interleukin-17 (IL-17) was increased in whole-cell-vaccinated and previously infected animals but not in acellular-pertussis-vaccinated animals following challenge. Together with our previous findings, these data are consistent with a role for Th17 responses in the clearance of B. pertussis infection.

FIG 1

Vaccination and previous infection induce robust serum antibody responses. Serum samples were collected from unvaccinated animals (n = 4), DTaP-vaccinated animals (n = 3), convalescent animals (n = 3), and animals vaccinated with DTwP (Triple Antigen, DT-COQ, or Quinvaxem; n = 2 per group) 1 week prior to challenge. Antibody responses to the four vaccine antigens, PT (A), FHA (B), PRN (C), FIM (D), and to heat-killed B. pertussis (E) were measured by enzyme-linked immunosorbent assay (ELISA). International units (IU) or relative units (RU) in each sample were determined by comparing the responses to the WHO international standard pertussis antiserum on each plate. For each group, the mean response is presented with error bars representing the standard error (SE).

FIG 2

Vaccination and prior infection prevent leukocytosis. Unvaccinated animals (n = 4), DTaP-vaccinated animals (n = 3), convalescent animals (n = 3), and animals vaccinated with DTwP (Triple Antigen, DT-COQ, or Quinvaxem; n = 2 per group) were directly challenged with B. pertussis (n = 2 to 4 per group). The mean circulating white blood cell counts before and after challenge are shown for each group of animals. *, P < 0.05 versus preinfection from the same group. For each group, the mean count is presented at every time point with error bars representing the SE.

FIG 3

Effects of vaccination and previous infection on colonization. Unvaccinated animals (n = 4), DTaP-vaccinated animals (n = 3), convalescent animals (n = 3), and animals vaccinated with DTwP (Triple Antigen, DT-COQ, or Quinvaxem; n = 2 per group) were directly challenged with B. pertussis. (A) Colonization was monitored by quantifying B. pertussis CFU per milliliter in biweekly nasopharyngeal washes with a limit of detection of 10 CFU/ml. The mean colonization is presented for each group and time point with error bars representing the SE. (B) For each animal, the time to clearance is defined as the first day that no B. pertussis CFU were recovered from nasopharyngeal washes. The mean time to clearance is shown for each group (same sizes as above except n = 3 for the unvaccinated group since one unvaccinated animal was not followed to clearance). Since no B. pertussis organisms were recovered from the convalescent animals, the mean time to clearance was defined as the first day of sampling (day 2, indicated by the dashed line). *, P < 0.01 versus naive.

FIG 4

Whole-cell pertussis vaccines reduce total nasopharyngeal B. pertussis burden. The total nasopharyngeal burden over the course of the infection was estimated by calculating the area under curve of the serial nasopharyngeal wash counts for each animal. The data are shown for each unvaccinated animal (n = 3), DTaP-vaccinated animal (n = 3), convalescent animal (n = 3), and each animal vaccinated with DTwP (Triple Antigen, DT-COQ, or Quinvaxem; n = 2 per group), with a bar representing the mean value for each group. *, P < 0.05 versus naive; **, P < 0.01 versus naive.

FIG 5

Cytokine responses during infection. Blood was collected into PAXgene tubes to stabilize RNA at the indicated time points before and during the infection. RNA was converted to cDNA, and the transcript levels of the genes encoding IL-17A (A), IFN-γ (B), and IL-4 (C) were analyzed by real-time PCR. For each cytokine gene, the data are presented relative to ICOS transcript levels and are normalized to preinfection values. For each group, the mean response is presented with error bars representing the SE. (D) The ICOS transcript was similarly analyzed relative to the housekeeping gene RPLP0. For all graphs, n = 3 for unvaccinated and convalescent (Conv.), n = 2 for DTaP, n = 6 for DTwP. *, P ≤ 0.01 for convalescent versus unvaccinated and DTwP versus unvaccinated.