A whole-cell pertussis vaccine engineered to elicit reduced reactogenicity protects baboons against pertussis challenge

Abstract

Whole-cell pertussis (wP) vaccines introduced in the 1940s led to a dramatic reduction of pertussis incidence and are still widely used in low- and middle-income countries (LMICs) worldwide. The reactogenicity of wP vaccines resulted in reduced public acceptance, which drove the development and introduction of acellular pertussis (aP) vaccines in high-income countries in the 1990s. Increased incidence of pertussis disease has been observed in high-income countries following the introduction of aP vaccines despite near universal rates of pediatric vaccination. These increases are attributed to the reduced protection against colonization, carriage, and transmission as well as reduced duration of immunity conferred by aP vaccines relative to the wP vaccines they replaced. A reduced reactogenicity whole-cell pertussis (RRwP) vaccine was recently developed with the goal of achieving the same protection as conferred by wP vaccination but with an improved safety profile, which may benefit countries in which wP vaccines are still in routine use. In this study, we tested the RRwP vaccine in a baboon model of pertussis infection. We found that the RRwP vaccine induced comparable cellular and humoral immune responses and comparable protection following challenge relative to the wP vaccine, while significantly reducing injection-site reactogenicity.IMPORTANCEThe World Health Organization (WHO) recommended in 2015 that countries administering wP vaccines in their national vaccine programs should continue to do so, and that switching to aP vaccines for primary infant immunization should only be considered if periodic booster vaccinations and/or maternal immunization could be assured and sustained in their national immunization schedules (WHO, Vaccine 34:1423-1425, 2016, https://doi.org/10.1016/j.vaccine.2015.10.136). Due to the considerably higher cost of aP vaccines and the larger number of doses required, most LMICs continue to use wP vaccines. The development and introduction of a wP vaccine that induces fewer adverse events without sacrificing protection would significantly benefit countries in which wP vaccines are still in routine use. The results of this study indicate this desirable goal may be achievable.

Keywords: baboon model; pertussis; whole cell vaccines.

Fig 1

Difference in systemic body temperature post-vaccination. The area under the curve (AUC) of the body temperature recorded by the dataloggers was calculated for the same 18-h period during the 3 days before each vaccination as described in Materials and Methods, and the average pre-vaccination body temperature AUC was calculated and subtracted from the body temperature AUC of the same animal on day 1 post-vaccination recorded for the same 18-h period. The 18-h interval between 3:00 PM and 9:00 AM was chosen to avoid the impact of sedation on body temperature. Data are presented as mean ± standard deviation (SD). A repeated measures two-way ANOVA did not detect any significant differences between groups.

Fig 2

Injection site reactions measured on day 1 post-injection of high-dose RRwP and high-dose wP vaccines. Four baboons each were vaccinated with high-dose wP or high-dose RRwP vaccines prepared as described in Materials and Methods. On day 1 post-vaccination, the injection sites of all eight animals were evaluated for redness, induration, swelling, and temperature by a veterinarian blinded to treatment groups. Relative clinical scores were assigned by the veterinarian. The non-injected arm served as a negative control for each animal. The injection site temperature is reported as a ratio of the injected arm injection site temperature relative to the control arm mock injection site temperature. In addition to the evaluation of the injection site, the core body temperature was measured and reported as a ratio relative to the temperature of each animal measured on day 0 just before vaccination. Comparisons between groups were made using the nonparametric two-tailed unpaired Mann–Whitney U test. Data are presented as mean ± standard deviation (SD). Statistical difference in site-reactions between high-dose vaccines was observed for redness.

Fig 3

T-cell responses of aP, wP, and RRwP-vaccinated baboons. PBMCs were isolated from whole blood collected from vaccinated baboons following the third vaccination and from age-matched unvaccinated baboons. The cells were stimulated with heat-killed B. pertussis, and supernatants were collected 72 h later and were assessed for (A) IFNγ, (B) IL5, and (C) IL17a protein levels using ELISA as described in Materials and Methods. Comparisons between groups were made using the nonparametric two-tailed unpaired Mann–Whitney U test. Data are presented as mean ± standard deviation (SD).

Fig 4

Pertussis-specific serum IgG responses 2 months after aP, wP, or RRwP vaccination. Two months following the third vaccination, (A) anti-HKBp and (B) anti-PT IgG titers were measured with serum collected from the vaccinated animals using ELISA. Comparisons between groups were made using the nonparametric two-tailed unpaired Mann–Whitney U test. Data are presented as mean ± standard deviation (SD).

Fig 5

Bacterial colonization and white blood cell count in vaccinated and unvaccinated baboons following B. pertussis challenge. Vaccinated and unvaccinated baboons (n = four per group) were inoculated with B. pertussis strain D420 as described in Materials and Methods. Blood and nasopharyngeal washes were collected twice weekly following inoculation. Nasopharyngeal washes were diluted and plated on Regan–Lowe plates, and the CFUs/mL of nasopharyngeal wash were enumerated following incubation. The numbers of circulating white blood cells/µL of whole blood were determined by complete blood cell differentiation as described in Materials and Methods.

Fig 6

Induction of IFNγ- and IL17-producing T cells in vaccinated and unvaccinated baboons following challenge. PBMCs isolated from animals on day 7, 14, and 21 post-challenge were subject to stimulation, staining, and flow cytometry analysis as described in Materials and Methods. Percentage of IFNγ- or IL17a-producing CD4 T cells were characterized.

Fig 7

Pertussis-specific serum IgG responses to aP, wP, and RRwP vaccination pre- and post-challenge. Pre-challenge and on days 7 and 28 post-challenge, (A) anti-HKBp and (B) anti-PT IgG titers were measured with serum collected from the animals by ELISA. Data are presented as mean ± standard deviation (SD).