Acinetobacter baumannii is a gram-negative bacterium, that is widespread in hospitals and causes soft tissue infections such as pneumonia and meningitis, or sepsis in immunocompromised patients. The clinical A. baumannii infection treatment has become challenging due to rapid emergence of multidrug-resistant strains [1]. In 2017, the World Health Organization named A. baumannii as a top priority pathogen in critical need of new therapeutic approaches [2]. Additionally, the COVID-19 pandemic has increased the rate of coinfections or secondary infections caused by multidrug-resistant A. baumannii that are directly associated with poor clinical outcomes, prolonged hospitalization, and higher mortality risk [3].
Vaccination is a promising strategy for preventing A. baumannii infections. Currently, there are no vaccine candidates that have stepped into clinical trials. The development of vaccines is complicated by a thick polysaccharide capsule that protects the bacterium and shields most outer membrane proteins from immune recognition [4]. Nonetheless, some proteins, such as Blp1, can penetrate the capsule, enabling the pathogen to adhere to host epithelium cells and form biofilms. Previous studies using active and passive immunizations have shown that surface-exposed C-terminal 711 amino acid length fragment of Blp1 is a suitable vaccine candidate and protects mice from A. baumannii infection. Conveniently, within this fragment, a stretch of 163 amino acids is conserved and therefore fit as a universal antigen to target the most prevalent A. baumannii clinical strains [5].
The yeast surface display technology has potential in prototyping of oral vaccines against various pathogens. The display of antigens on the yeast surface occurs through cell wall proteins, such as α-agglutinin. Saccharomyces cerevisiae cell wall protein α-agglutinin consists of two subunits Aga1 and Aga2. The antigen is expressed as a fusion with Aga2 subunit, linked by two disulfide bonds to cell wall-anchored Aga1 subunit. Importantly, S. cerevisiae has GRAS (Generally Recognized As Safe) status, which allows its use in food and drug production. Finally, it is worth noting that the immunostimulatory properties of yeast cell wall major component β-glucan allow yeast cells to serve not only as a vaccine carrier but also as an immune adjuvant [6].
The aim of this study was to immobilize the C-terminal fragment of A. baumannii Blp1 protein on the surface of S. cerevisiae. Fusion protein composed of conserved 163 amino acid fragment of the target protein and Aga2 subunit was displayed on yeast cell surface. Antigen expression and display was confirmed by western blot analysis and visualized by immunofluorescence microscopy. Flow cytometry analysis indicated that 48.5% of singlets displayed the conserved fragment of A. baumannii. These initial results provide compelling evidence that the yeast surface display technology has bright prospects in development of oral vaccine candidate against A. baumannii.