The Bordetella pertussis protein Pertactin: role in immunity and immune evasion

Pertussis is a highly contagious infectious disease of the respiratory tract which is caused by Bordetella pertussis. Before widespread introduction of vaccination against pertussis, almost every child contracted pertussis. The disease is most severe in neonates and children under the age of 1. Introduction of mass vaccination reduced the number of pertussis cases significantly and resulted in a 10-fold decrease in pertussis mortality in the Netherlands in the late 1950s. Despite vaccination, pertussis remained endemic. A major epidemic occurred in the Netherlands in 1996. Since the 1996 outbreak, the Netherlands now has an epidemic pertussis cycle with peaks every 2 to 3 years during the last decade. Also in several other European countries, as well as Canada, The United States and Australia, a re-emergence of pertussis was observed. Several explanations were proposed for the sudden increase of pertussis in the Netherlands including waning immunity in adolescents and adults, increased reporting, improved diagnosis of the disease and the adaptation of the B. pertussis population. In the Netherlands, pathogen adaptation has probably played an important role in the resurgence of pertussis. Vaccine adapted strains showed polymorphisms in two proteins implicated in protective immunity: P.69 Prn and Ptx. The role of P.69 Prn in protective immunity has been well documented in both humans and in animal models. Furthermore, P.69 Prn is present in several of the currently most used ACVs. Until recently, relatively little was known about the antibody response to P.69 Prn, the location of epitopes on P.69 Prn and the role of variation in Prn on immune evasion. Variation in the variable region1 was described to affect the Ab response to this region, but no direct evidence or a feasible mechanism for immune evasion was described. The goal of this thesis was to identify the location of (protective) epitopes to which human Abs are directed, and to investigate the role of variation in P.69 Prn and the implications for the anti-P.69 Prn Ab response. Our results clearly indicate that Prn has evolved several ways to escape antibody and possibly phage binding. We showed that the N-terminus of P.69 Prn harbors important epitopes. Furthermore, our data suggests that the variable region1 has evolved to hide those important epitopes from immune recognition and possibly phage binding. In addition, Prn has several other flexible (and variable) loops that are employed to hide adjacent epitopes from the immune system (epitope masking). The N- and C-termini of P.69 Prn interact physically, which results in the masking of a part of Prn. Since the C-terminus has a linear structure that folds towards the N-terminus, it is likely that the Ab response to this region is less efficient then an Ab response directed to the conformational epitopes located in parts of the molecule that are covered by the C-terminus (conformational masking). The loops that are exposed, e.g. region1, region2, and several other loops, were shown to be highly variable (antigenic variation). We showed that deletion of the variable regions does not improve protection against infection. Apparently deletion of the decoy regions does not automatically skew the Ab response towards the conserved protective epitopes. However, we have shown that it is possible to employ synthetic peptides to induce a protective response towards conserved and protective epitopes. The results presented in this thesis have shed new light on the mechanisms employed by bacteria, and B. pertussis in particular, to evade immune recognition. These results will facilitate the development of new, possibly synthetic, but mainly more effective vaccines