Mutations in the sphB1 Gene of Bordetella pertussis and Their Association with the Introduction of Acellular Vaccines in Finland, 1991-2020
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Whooping cough is a highly infectious respiratory disease caused by Bordetella pertussis bacterium. The disease is dangerous for unvaccinated newborns. In adults, the symptoms of the disease may remain mild. Despite high vaccination rate, B. pertussis has caused epidemics in multiple industrialized countries. Several genomic changes have been identified in the pathogen after the introduction of acellular vaccines (aPv). The mutations are associated with the vaccine-induced selection pressure on B. pertussis.
In Finland, large-scale vaccination against whooping cough started in 1952 with a whole-cell vaccine. This led to a significant reduction in cases. In 2005, an acellular vaccine was introduced in Finland. The vaccine contained two antigens/virulence factors of the pathogen until 2009, when it was replaced by a new vaccine containing three components of the pathogen.
So far the genomic changes identified in B. pertussis are mainly in genes encoding the virulence factors included in the acellular vaccines. The globally increased number of cases is associated with decrease in vaccine immunity, which is partially caused by these genomic changes.
SphB1 is a protease in B. pertussis that cleaves two virulence factors – one of which is included in the acellular vaccines – from the cell surface of the pathogen. Two mutations have been identified in the gene encoding SphB1. The aim of this study was to evaluate the prevalence of each allele in Finland between 1991 and 2020 and their associations with the introduction of aPv in this country. 300 randomly selected clinical isolates were included in the study.
The first part of the experiment was cultivation of the bacteria on a charcoal plate. After the cultivation, the DNA was extracted, and a PCR run was performed using specific primer sets. The size of the product was confirmed with agarose gel electrophoresis. In the third part of the experiment, the PCR products were enzymatically purified and sent for the Sanger sequencing. Finally, the sequencing results were analyzed against the gene of an international reference B. pertussis strain. The impact of the mutations on the protein function was studied using 3D modeling software.
The results show that the prevalence of the wildtype sphB1-1 decreased from 95.5% to 0% between 1991 and 2020. The prevalence of variant genotype sphB1-2 (G358A, Val120Ile) increased from 1.9% to 73.7%, while variant genotype sphB1-3 (G463A, Ala155Thr) increased from 2.6% to 26.3%. After the introduction of the acellular vaccine, the prevalence of the wildtype allele started to decrease rapidly.
Based on the 3D modeling, both mutations are in the active site of the protein. They might affect the kinetics of ligand binding or cleavage. This study indicates that the introduction of the acellular vaccine has possibly influenced the structural changes in the sphB1 gene.