The adenylate cyclase toxin as a target for antibody therapeutics and vaccination against whooping cough
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Whooping cough, also known as pertussis, is caused by the bacterium Bordetella pertussis. Since widespread vaccination with heat-killed whole cell vaccines (wP) in the 1950s, the number of cases dropped dramatically. However, there has been a consistent resurgence in the past two decades, coinciding with the switch from wP vaccines to acellular vaccines (aP). US CDC estimates 16 million cases and 195,000 deaths worldwide per year. Accumulating evidence show that aP vaccines provide short protection against the symptoms but not against subclinical infection and transmission of the disease. Changing the adjuvant to induce more protective immunity or inclusion of additional protective antigens are some of the strategies to improve the efficacy of aP vaccines. Adenylate cyclase toxin (ACT) is a 177 kDa protein produced by B. pertussis and related species. ACT mainly targets leukocytes through [alpha] [subscript M] [beta] ₂ integrin and translocates its N-terminal cyclase domain into the cytosol, generating supraphysiological level of cAMP. Studies have shown that ACT-deficient stains are less pathogenic and passive immunization with polyclonal antibodies or active immunization with ACT protected mice against bacterial challenges. However, ACT is not included in any of the current aP vaccines, due to a lack of understanding of its protective epitopes and its poor solubility and stability. We aimed to identify potent neutralizing antibodies (nAbs) as therapeutic candidates, and map their epitopes to guide the design of vaccine antigen overcoming the solubility and stability issues. Two nAbs, M2B10 and M1H5, were discovered from antibody phage display libraries from ACT-immunized mice. They bind non-overlapping conformational epitopes in the C-terminal RTX domain of ACT. Our data suggest their mode of action is interrupting the interaction between the toxin and its cellular receptor. On the other hand, individual domains of ACT were expressed in E. coli and purified. The catalytic and RTX domains retained antigenicity but are biophysically superior to full-length ACT. We further showed that the RTX domain elicited similar level of neutralizing antibody response to ACT in mice. These antibodies, together with those neutralizing other major B. pertussis toxins, may constitute a therapy for severe pertussis infection, and the epitopes provide the basis for structure-based antigen design for superior stability and immunogenicity.