The use of transfection technology as a Babesia bovis vaccine development tool
Carlos E. Suarez, Animal Disease Research Unit, ARS-USDA.
Babesia bovis is an intraerythrocytic pathogen responsible for one of the most prevalentand costly tick borne diseases of cattle worldwide. The parasite is transmitted by
Rhiphicephallus (Boophilus) ticks, and have the ability to evade the immune system of
the vertebrate hosts, causing persistent disease. Acute babesiosis can be prevented quite
effectively by combining tick control and vaccination with living attenuated organisms.
However these methods of control have numerous limitations and improved approaches
are needed. A new research toolbox that includes B. bovis full genome sequencing
combined with the improved ability to genetically modify the organism is enhancing ourunderstanding of their biology and allows the use of recently developed Babesia
transfection methods for functional gene characterizations and for vaccine development.
However, significant knowledge gaps on the role of key parasite molecules involved incell invasion, adhesion, asexual and sexual reproduction, tick transmission, and evasionof the immune system, remain. Overall, a better understanding of the biology of theseorganisms will positively contribute toward the goal of developing improved
immunological and pharmacological interventions against B. bovis. While developing
Babesia subunit vaccines might still be an achievable goal, transfection tools will allow
for the production of improved genetically modified live vaccines to be developed under
the framework of the current effective live vaccines. For instance, an important limitationof current vaccines is the lack of a simple method to discriminate between vaccinated andnaturally infected animals. One approach toward addressing this limitation is the use ofmarker vaccines using recombinant approaches. In addition, because attenuated parasites
also establish persistent infection, expression of exogenous genes by stably transfected,attenuated B. bovis provides an opportunity to explore the possibility of combining
antigens from multiple infectious agents and parasites into a single live vaccine. The
ability to persistently infect the host makes this approach particularly promising when
immune protection requires frequent boosting, for example, with other hemoparasites andticks. However, to serve as efficient live vectors for vaccination, recombinant attenuatedB. bovis vaccines would be required to establish mild acute infection leading topersistence, express exogenous antigens throughout infection, and remain geneticallystable. A Babesia bovis line stably transfected with the gfp-bsd gene was shown to beable to establish acute and persistent infections with a pattern that was undistinguishableof its parental wild type strain. Furthermore, exogenous transgenes can be expressed andremain stable throughout acute and persistent infection in calves, which remained
protected of clinical disease upon challenge with a virulent strain. New transfection
strategies are been currently developed using bidirectional promoters that provide theability to express more than a single transfected gene, thus increasing the potential oftransfected attenuated parasites as vaccine delivery systems. Overall, the use of genome
data in conjunction with novel transfection and genomic related approaches will beinstrumental for closing our current knowledge gaps toward a better understanding of thebiology of these organisms and for formulating new vaccines and other therapeuticinterventions. Together with a better understanding of the protective immune responses,
these new experimental approaches will likely be the keys leading to the developing of
improved methods of control of this devastating cattle disease.