Characterization of ospA genetic diversity in the west coast

Background/Question/Methods

Lyme borreliosis is a global tick-borne disease caused by pathogenic species within Borrelia burgdorferi senso lato (s.l.). The three most common etiological agents of Lyme disease are: B. burgdorfei senso stricto (Bbss) in the U.S. and, B. afzelii and B. garinii in Europe. In the western U.S., Bbss is vectored by the tick Ixodes pacificus. Bbss expresses outer surface protein A, ospA, that plays a role in pathogen colonization of the tick vector and is a major target for Lyme vaccines. Yet, our understanding of ospA genetic diversity is limited, particularly in the western U.S. Genetic variability of the ospA gene could have important implications for the success of ospA targeted vaccines. The goal of this study is to characterize the genetic diversity of the ospA locus in Bbss infected I. pacificus in the western U.S. to better understand the broad applicability of an ospA targeted vaccine in the U.S. Questing nymphal I. pacificus ticks were collected from 14 field sites in northern California by drag sampling. Tick samples were extracted for DNA and screened for Bbss infection by 5S-23S IGS targeted nested PCR. Then, ticks identified as positive for Bbss were further tested to amplify the ospA gene.

Results/Conclusions

We conducted a phylogenetic analysis of our ospA sequences as well as published sequences from ticks in Europe and the U.S. Sequencing of Bbss infected I. pacificus ticks from the western U.S. found some limited genetic variability of the ospA locus. All ospA sequences that we examined clustered together with other Bbss from North America, but there was some distinction between North American and European samples. In addition, we found Borrelia bissettiae, a potential Bbsl pathogen, forms a distinct lineage from the Bbss clade. European Lyme disease species, B. afzelii and B. garinii, comprise another clade based on ospA sequences. The efficacy of ospA targeted Lyme vaccines will need to consider genetic variability of the ospA gene across bacterial species and geographic regions and may need to accommodate differences exhibited by different Bbsl species. This study sheds light on the genetic diversity of a gene that is a target of vaccine development and stresses the importance of studying geographic genetic variability of important vaccine targets, such as Bbss ospA, to more effectively protect against Lyme disease.