Controlled Human Infection With Bordetella pertussis Induces Asymptomatic, Immunizing Colonization

Abstract

Background: Bordetella pertussis is among the leading causes of vaccine-preventable deaths and morbidity globally. Human asymptomatic carriage as a reservoir for community transmission of infections might be a target of future vaccine strategies, but has not been demonstrated. Our objective was to demonstrate that asymptomatic nasopharyngeal carriage of Bordetella pertussis is inducible in humans and to define the microbiological and immunological features of presymptomatic infection.

Methods: Healthy subjects aged 18-45 years with an antipertussis toxin immunoglobin G (IgG) concentration of <20 international units/ml were inoculated intranasally with nonattenuated, wild-type Bordetella pertussis strain B1917. Safety, colonization, and shedding were monitored over 17 days in an inpatient facility. Colonization was assessed by culture and quantitative polymerase chain reaction. Azithromycin was administered from Day 14. The inoculum dose was escalated, aiming to colonize at least 70% of participants. Immunological responses were measured.

Results: There were 34 participants challenged, in groups of 4 or 5. The dose was gradually escalated from 103 colony-forming units (0% colonized) to 105 colony-forming units (80% colonized). Minor symptoms were reported in a minority of participants. Azithromycin eradicated colonization in 48 hours in 88% of colonized individuals. Antipertussis toxin IgG seroconversion occurred in 9 out of 19 colonized participants and in none of the participants who were not colonized. Nasal wash was a more sensitive method to detect colonization than pernasal swabs. No shedding of Bordetella pertussis was detected in systematically collected environmental samples.

Conclusions: Bordetella pertussis colonization can be deliberately induced and leads to a systemic immune response without causing pertussis symptoms.

Clinical trials registration: NCT03751514.

Keywords: Bordetella pertussis; carriage; human challenge; mmune response.

Figure 1.

Subject flowchart. Abbreviations: B. pertussis, Bordetella pertussis; cfu, colony-forming units; IgG, immunoglobin G; IU, international units; PT, pertussis toxin.

Figure 2.

Colonization density in nasal wash samples of colonized subjects (n = 19) over time. A, Culture results in total cfu, measured by dilutional plating. B, Quantative polymerase chain reaction results, expressed as Ct value. Day 0 was the day of inoculation. Results are presented as box plots with medians and 25% and 75% interquartiles, and the whiskers represent the minimum and maximum values. Abbreviations: cfu, colony-forming units; Ct, cycle threshold.

Figure 3.

Antigen-specific serum IgG concentration after Bp exposure, comparing dose groups. A, Anti-PT. B, Anti-PRN. C, Anti-FHA. D, Anti-FIM 2/3. Infected with: inoculum dose 10³ cfu (n = 5), inoculum dose 10⁴ cfu (n = 9), inoculum dose 5x10⁴ cfu (n = 5), inoculum dose 10⁵ cfu (n = 15). Day 0 was the day of inoculation. Results are presented as scatter plots with median values. *Significance between time points (P < .05), using the Wilcoxon test. Abbreviations: AU, arbitrary units; Bp, Bordetella pertussis; cfu, colony-forming units; FHA, filamentous hemaglutinin; FIM 2/3, fimbriae 2/3; IgG, immunoglobin G; IU, international units; PRN, pertactin; PT, pertussis toxin.

Figure 4.

Serum IgG concentration against Bp-specific antigens after a challenge with 10⁵ cfu of Bp. IgG concentration of n = 15 subjects exposed to 10⁵ cfu Bp. A, Anti-PT. B, Anti –PRN. C, Anti-FHA. D, Anti-FIM 2/3. The black lines indicate colonized cases and the dashed lines indicate noncolonized cases. Day 0 was the day of inoculation. Abbreviations: AU, arbitrary units; Bp, Bordetella pertussis; cfu, colony-forming units; FHA, filamentous hemaglutinin; FIM 2/3, fimbriae 2/3; IgG, immunoglobin G; IU, international units; PRN, pertactin; PT, pertussis toxin.

Figure 5.

IgG- and IgA-secreting plasma B-cell responses to Bp challenge. Numbers of plasma B cells secreting IgG and IgA are specific for (A–B) PT, (C–D) FHA, (E–F) PRN, and (G–H) FIM 2/3 by ELISPOT. PBS (I–J) and TT (K–L) were used as a background control and negative control antigen, respectively. Results for volunteers who were uncolonized (○; n = 9) and colonized (∎; n = 16) are shown as box plots representing the median, with 25% and 75% interquartile ranges, and whiskers representing minimum and maximum values. *Significance between time points (P < .05), using a Kruskel-Wallis test with Dunn’s correction. ^#Significance between noncolonized and colonized responses (P < .05), using a Kruskel-Wallis test with Dunn’s correction. Abbreviations: Bp, Bordetella pertussis; FHA, filamentous hemaglutinin; FIM 2/3, fimbriae 2/3; Ig, immunoglobin; PBMC, peripheral blood mononuclear cells; PBS, phosphate buffered saline; PRN, pertactin; PT, pertussis toxin; TT, tetanus toxoid.