Pertussis is a highly contagious respiratory infection caused by Bordetella pertussis bacterium. The mainstay of treatment is macrolide antibiotics that reduce transmissibility, shorten the duration of symptoms and decrease mortality in infants. Recently, the macrolide resistance of B. pertussis has been reported globally but is especially widespread in mainland China. In this review, we aim to summarise the current understanding of the epidemiology, resistance mechanisms and clinical implications of B. pertussis macrolide resistance. Since the first appearance of macrolide-resistant B. pertussis in Arizona, USA, in 1994, only sporadic cases have been reported outside China. In certain parts of China, on the other hand, up to 70–100% of the recent clinical isolates have been found to be macrolide resistant. Reasons for macrolide resistance being centred upon China during the last decade can only be speculated on, but the dominant B. pertussis lineage is different between China and most of the high-income countries. It seems evident that efforts to increase awareness, guide molecular epidemiological surveillance and carry out systematic screening of B. pertussis positive samples for macrolide resistance should be implemented globally. In addition, practices to improve the clinical care of infants with pertussis caused by resistant strains should be studied vigorously.

Pertussis toxin (PT) is considered the main virulence factor causing whooping cough or pertussis. The protein is widely studied and its composition was revealed and sequenced already during the 1980s. The human immune system creates a good response against PT when measured in quantity. However, the serum anti-PT antibodies wane rapidly, and only a small amount of these antibodies are found a few years after vaccination/infection. Therefore, multiple approaches to study the functionality (quality) of these antibodies, e.g., avidity, neutralizing capacity, and epitope specificity, have been investigated. In addition, the long-term B cell memory (Bmem) to PT is crucial for good protection throughout life. In this review, we summarize the findings from functional PT antibody and Bmem studies. These results are discussed in line with the quantity of serum anti-PT antibodies. PT neutralizing antibodies and anti-PT antibodies with proper avidity are crucial for good protection against the disease, and certain epitopes have been identified to have multiple functions in the protection. Although PT-specific Bmem responses are detectable at least five years after vaccination, long-term surveillance is lacking. Variation of the natural boosting of circulating Bordetella pertussis in communities is an important confounding factor in these memory studies.

Serological diagnosis of pertussis is mainly based on anti-pertussis toxin (PT) IgG antibodies. Since PT is included in all acellular vaccines (ACV), serological assays do not differentiate antibodies induced by ACVs and infection. Adenylate cyclase toxin (ACT) is not included in the ACVs, which makes it a promising candidate for pertussis serology with the specific aim of separating infection- and ACV-induced antibodies. A multiplex lateral flow test with PT and ACT antigens was developed to measure serum antibodies from pertussis-seropositive patients (n = 46), healthy controls (n = 102), and subjects who received a booster dose of ACV containing PT, filamentous hemagglutinin, and pertactin (n = 67) with paired sera collected before and one month after the vaccination. If the diagnosis was solely based on anti-PT antibodies, 98.5–44.8% specificity (before and after vaccination, respectively) and 78.2% sensitivity were achieved, whereas if ACT was used in combination with PT, the sensitivity of the assay increased to 91.3% without compromising specificity. No increase in the level of anti-ACT antibodies was found after vaccination. This exploratory study indicates that the use of ACT for serology would be beneficial in combination with a lower quantitative cutoff for anti-PT antibodies, and particularly in children and adolescents who frequently receive booster vaccinations.

Bordetella pertussis continues to cause whooping cough globally even in countries with high immunisation coverage. Booster vaccinations with acellular pertussis vaccines are thus used in children, adolescents, and adults. T cell immunity is crucial for orchestrating the immune response after vaccination. However, T cell assays can be expensive and difficult to implement in large clinical trials. In this study, a whole blood (WB) stimulation assay was developed to identify secreted T cell associated cytokines in different age groups after acellular pertussis booster vaccination. Longitudinal WB samples were collected from a small set of subjects (n = 38) aged 7-70 years participating in a larger ongoing clinical trial. For assay development, samples were diluted and incubated with purified inactivated pertussis toxin (PT), filamentous haemagglutinin (FHA), inactivated B. pertussis lysate, and complete medium (M) as stimulating conditions, with anti-CD28 and anti-CD49d as co-stimulants. Different timepoints around the vaccination (D0, D7, D14, D28), WB dilution factor (1:2, 1:4) and incubation time (24 h, 48 h, 72 h) were compared. Responses to 15 cytokines were tested with Luminex/multiplex immunoassay. The optimized assay consisted of WB incubation with M, PT, and FHA (including the two co-stimulants). After 48 h incubation, supernatants were collected for measurement of seven selected T cell associated cytokines (IL-2, IL-5, IL-10, IL-13, IL-17 A, IL-17F, and IFN-y) from samples before and 28 days after vaccination. PT stimulation showed a trend for upregulation of IL-2, IL-13, and IL-17 A/F for adult subjects, whereas the responses of all cytokines were downregulated for the paediatric subjects. Furthermore, PT and FHA-stimulated WB showed diverse cytokine producing profiles. The developed WB-based cytokine assay was shown to be less costly, easy to perform, and functional in differently aged individuals. Further, it requires only a small amount of fresh blood, which is beneficial especially for studies including infants. Our results support the use of this assay for other immunological studies in the future.

Despite extensive vaccinations, pertussis remains endemic and epidemic in multiple countries. The persistence of cases can be partly attributed to the significant individual variation in vaccine responses. This study evaluated the association of baseline cytokines (before booster vaccination) on antibody concentrations to Tdap-vaccine antigens. Healthy Finnish children (7-10y, n = 36), adolescents (11-15y, n = 37), young adults (20-34y, n = 25), and older adults (60-70y, n = 23) received a Tdap3-IPV booster. Serum antibodies against pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (Prn), fimbriae 2/3, diphtheria toxoid (DT), and tetanus toxoid (TT), as well as PT neutralizing antibodies were measured before, one month, and one year after the booster. Baseline serum concentrations of IFN-γ, IL-2, IL-5, IL-10, IL-13, IL-17 A and IL-17F were determined. The proportion of detectable and undetectable baseline cytokines varied between age groups 58.3 % of children had a higher proportion of detectable IL-5, IL-10, IL-13, and IL-17F compared to adolescents (IL-5, 37.8 %; IL-10, 48.6 %; IL-13, 48.6 %; IL-17F, 37.7 %), young adults (IL-5, 36.0 %; IL-10, 28.0 %; IL-13, 36.0 %; IL-17F, 44.0 %), and older adults (IL-5, 26.1 %; IL-10, 21.7 %; IL-13, 39.1 %; IL-17F, 30.4 %). IFN-γ had a lower detectability in children (44.4 %) and young (40.0 %) and older adults (39.1 %) in contrast to adolescents (62.2 %). IL-2 was undetectable in all age groups while the proportion of detectable IL-17 A decreased with age. A mixed model showed that undetectable baseline levels of IFN-γ, IL-2, IL-10, and IL-17 A were associated with higher antibody concentrations in children before and after vaccination, particularly against PT. Positive associations were observed in adolescents for anti-TT concentrations and young adults for anti-FHA IgA concentrations. These findings indicate a possible role of existing cytokines in pertussis booster antibody concentrations in children and warrant further studies in different populations. However, the results should be interpreted with caution as the number of subjects is limited.

Despite widespread use of whole-cell (wP) and acellular (aP) pertussis vaccines, outbreaks persist in many countries. Polymorphisms in HLA class II molecules, which present antigens to CD4⁺ T cells, may play a vital role in vaccine responses. Emerging evidence suggests that HLA-DR/DQ variants can significantly influence wP vaccine responses. This study investigated the possible association of HLA class II haplotypes with antibody concentrations after aP booster vaccination. Healthy Finnish children (7-10y, n = 37), adolescents (11-15y, n = 37), young adults (20-34y, n = 25), and older adults (60-70y, n = 25) received a Tdap3-IPV booster. Serum antibodies against pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (PRN), diphtheria toxoid (DT), and tetanus toxoid (TT), as well as PT-neutralizing antibodies (PTNA), were measured before, one month, and one year after the booster. Participants were HLA-typed with a stepwise HLA-DR/DQ screening system using PCR followed by allele-specific probe hybridization. The frequency of HLA haplotypes was compared to the control cohort from the Finnish Pediatric Diabetes Register, which was constructed from parental haplotypes not passed down to diabetic children. The HLA DR-DQ haplotype frequencies in our cohort did not differ from the control group. Two associations survived FDR correction: (DR7)-DQA1*02:01-DQB1*02 with lower anti-PT IgG and (DR15)-DQB1*06:02 with a time-dependent anti-FHA IgG response with higher pre-booster concentrations. Nominally, (DR8)-DQB1*04 and (DR7)-DQA1*02:01-DQB1*02 carriers had lower PTNA; (DR9)-DQA1*03-DQB1*03:03, (DR15)-DQB1*06:02, and DRB1*04:01-DQA1*03-DQB1*03:02 carriers had higher anti-PT and anti-Prn concentrations; and (DR13)-DQB1*06:03 carriers had consistently lower anti-DT across all timepoints. The present study highlights a potential role of certain HLA haplotypes in modulating immune responses to aP vaccination. These findings emphasize the importance of further research to clarify how HLA class II haplotypes influence aP vaccine responses in various populations.