T-cell immune response assessment as a complement to serology and intranasal protection assays in determining the protective immunity induced by acellular pertussis vaccines in mice

Abstract

The relative value of antibodies and/or T-cell immune responses to Bordetella pertussis antigens in the immunity induced by acellular pertussis (aP) vaccines is still an open issue, probably due to the incomplete knowledge on the mechanisms of protective immunity to pertussis. The relevance of T-cell immune responses in protection from pertussis has been demonstrated in murine and human models of infection; thus, in this study, the ability of different vaccine preparations of three component (pertussis toxin, filamentous hemagglutinin, and pertactin) aP vaccines to induce T-cell responses was investigated in mice. All vaccine preparations examined passed the immunogenicity control test, based on antibody titer assessment, according to European Pharmacopoeia standards, and protected mice from B. pertussis intranasal challenge, but not all preparations were able to prime T cells to pertussis toxin, the specific B. pertussis antigen. In particular, one vaccine preparation was unable to induce proliferation and gamma interferon (IFN-gamma) production while the other two gave borderline results. The evaluation of T-cell responses to pertussis toxin antigen may provide information on the protective immunity induced by aP vaccines in animal models. Considering the critical role of the axis interleukin-12-IFN-gamma for protection from pertussis, our results suggest that testing the induction of a key protective cytokine such as IFN-gamma could be an additional tool for the evaluation of the immune response induced by aP vaccines.

FIG. 1.

Kinetics of B. pertussis clearance from lungs of intranasally challenged unvaccinated mice (NT) or mice vaccinated with aP vaccine preparations. Groups of CD1 (A) or BALB/c (B) mice were immunized i.p. at 0 and 3 weeks with one-half a human dose. Two weeks after completion of vaccination, mice were challenged with B. pertussis 18323 and CFU counts were performed on individual lung homogenates on the indicated days. Results are means ± SE (error bars) of viable B. pertussis counts from four mice per group at each time point. The results are representative of three experiments performed. The limit of detection of the assay was approximately log₁₀ 0.5 CFU per lung.

FIG. 2.

Lymphoproliferative response from immune CD1 (A) or BALB/c mice (B) was assessed at 15 (black bars) and 30 (grey bars) days after completion of the immunization schedule. Spleen cells (2 × 10⁶/ml) were stimulated in vitro with the indicated antigens, as detailed in Materials and Methods. DNA synthesis was measured after 5 days by counting [³H]thymidine incorporation (counts per minute, 10³). Results are mean values of SI obtained from three mice per group assessed individually and are representative of three experiments performed. The counts per minute (mean ± SE) of unstimulated cultures of spleen cells from the mice under study was 0.2 ± 0.06. The threshold of positive proliferative response is shown (SI = 4).