Polymorphisms in the Bordetella pertussis virulence factor SphB1 and altered antigen expression of clinical isolates before and after acellular pertussis vaccination in Finland

Background Bordetella pertussis has adapted significantly following the transition from whole-cell vaccines (WCV) to acellular vaccines (ACV). While changes in vaccine antigens are well-documented, the evolution of processing factors like sphB1, a protease essential for the maturation of filamentous hemagglutinin (FHA) and adenylate cyclase toxin (ACT), remains unexplored. This study was aimed to characterize sphB1 genotypes among circulating isolates collected in Finland during 1991–2020 and their association with different antigen expressions . Material and methods Altogether, 273 selected clinical isolates were analysed. The sphB1 gene was genotyped using PCR and Sanger sequencing. Results were correlated with fimbrial (Fim) serotypes, ptxP and prn genotypes, and expression levels of ACT, FHA, pertussis toxin (PT), and pertactin (PRN). Results Prior to ACV introduction in 2005, 95.5% of isolates carried wildtype sphB1-1 . However, frequencies of variant genotypes sphB1-2 (c.358G > A, p. (Val120Ile)) and sphB1-3 (c.463G > A, p. (Ala155Thr)) increased with time reaching 100% during 2016–2020. Both variants emerged in 2003 within the ptxP3 lineage. Isolates carrying sphB1 variant genotypes were associated with significantly higher FHA (1.7-fold), ACT (1.6-fold) and PT (1,2-fold) (all p values <0.001) expression levels than isolates carrying the wildtype allele. Furthermore, sphB1-3 genotype was exclusively associated with Fim3 serotype, whereas sphB1-2 was distributed across both Fim2 and Fim3 serotypes. Conclusions The association between sphB1 mutations and elevated FHA/ACT expressions suggests that B. pertussis adaptation involves not only antigenic shifts but also optimized processing of key virulence factors. These lineage-specific combinations may play a critical role in pertussis resurgence.