Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model

Abstract

Pertussis is a highly contagious respiratory illness caused by the bacterial pathogen Bordetella pertussis. Pertussis rates in the United States have been rising and reached a 50-y high of 42,000 cases in 2012. Although pertussis resurgence is not completely understood, we hypothesize that current acellular pertussis (aP) vaccines fail to prevent colonization and transmission. To test our hypothesis, infant baboons were vaccinated at 2, 4, and 6 mo of age with aP or whole-cell pertussis (wP) vaccines and challenged with B. pertussis at 7 mo. Infection was followed by quantifying colonization in nasopharyngeal washes and monitoring leukocytosis and symptoms. Baboons vaccinated with aP were protected from severe pertussis-associated symptoms but not from colonization, did not clear the infection faster than naïve animals, and readily transmitted B. pertussis to unvaccinated contacts. Vaccination with wP induced a more rapid clearance compared with naïve and aP-vaccinated animals. By comparison, previously infected animals were not colonized upon secondary infection. Although all vaccinated and previously infected animals had robust serum antibody responses, we found key differences in T-cell immunity. Previously infected animals and wP-vaccinated animals possess strong B. pertussis-specific T helper 17 (Th17) memory and Th1 memory, whereas aP vaccination induced a Th1/Th2 response instead. The observation that aP, which induces an immune response mismatched to that induced by natural infection, fails to prevent colonization or transmission provides a plausible explanation for the resurgence of pertussis and suggests that optimal control of pertussis will require the development of improved vaccines.

Keywords: IL-17; T-cell memory; adaptive immunity; animal models; whooping cough.

Fig. 1.

The effect of vaccination or convalescence on colonization and leukocytosis. Naïve animals, aP-vaccinated animals, wP-vaccinated animals, and previously infected [convalescent conv.)] animals were directly challenged with B. pertussis (n = 3–4 per group). (A) Colonization was monitored by quantifying B. pertussis cfu per mL in biweekly nasopharyngeal washes with a limit of detection of 10 cfu per mL. For each animal the time to clearance is defined as the first day that no B. pertussis cfu were recovered from nasopharyngeal washes. (B) The mean time to clearance is shown for each group (n = 3 per group). Because no B. pertussis organisms were recovered from the conv. animals, the mean time to clearance was defined as the first day of sampling (day 2, indicated by the dashed line). *P < 0.05 vs. Naïve, †P < 0.05 vs. aP, ‡P < 0.05 vs. wP. (C) The mean circulating white blood cell counts before and after challenge are shown for each group of animals (n = 3–4 per group). **P < 0.01 vs. preinfection from same group.

Fig. 2.

aP does not protect against colonization following natural transmission. A naïve animal was directly challenged. After 24 h, a naïve animal and two aP-vaccinated animals were placed in the same cage as the directly challenged animal and followed for colonization as in Fig. 1.

Fig. 3.

Infected aP vaccinees can transmit pertussis to naïve contacts. Two animals vaccinated with aP were housed in separate cages, and each was directly challenged. Twenty four hours after challenge, an unchallenged naïve animal was placed in each cage. All animals were followed for colonization as in Fig. 1. One cage pairing is shown with turquoise lines with circles, and the other is shown with maroon lines with squares. Solid lines with closed symbols indicate the aP-vaccinated, directly challenged animals, and open symbols with dashed lines are used for the unchallenged, naïve contacts.

Fig. 4.

Vaccination and previous infection induce robust serum antibody responses. Antibody responses to the four vaccine antigens—PRN, FIM, FHA, and PT—and to heat-killed B. pertussis (B. p.) were measured by ELISA. Preimmune sera were collected from vaccinated animals before immunization and from conv. animals before initial infection (n = 3–4 per group). Because Infanrix does not contain FIM, four Daptacel-vaccinated animals were included in the anti-FIM ELISA. Prechallenge sera were collected from all animals 1 wk before challenge. 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. ***P < 0.001, **P < 0.01, *P < 0.05 vs. Convalescent. †P < 0.001 vs. wP.

Fig. 5.

Helper T-cell responses induced by vaccination and infection. PBMC collected from naïve, aP-vaccinated, wP-vaccinated, and conv. animals 1 wk before infection were incubated overnight with either medium alone or medium containing heat-killed B. pertussis (n = 3–4 per group). For each growth condition, nonadherent cells were collected and either left unseparated (total nonadherent cells) or separated using anti-CD4 and anti-CD95 magnetic particles. Total nonadherent, CD4−, CD4+, and CD95−CD4+ cells were then cultured under the same conditions as before (with medium alone or stimulated with heat-killed B. pertussis). After 36 h, supernatants were collected and analyzed for IL-17 (A), IFN-γ (B), and IL-5 (C). Cytokine secretion in response to B. pertussis stimulation is presented for total nonadherent cells (Left) and separated cells (Right). ***P < 0.001, **P < 0.01, *P < 0.05 vs. same fraction from naïve animals.