Immunoprotective potential of in silico predicted Acinetobacter baumannii outer membrane nuclease, NucAb

• An outer membrane nuclease, NucAb, reported as vaccine candidate for the first time.
• Reverse vaccinology approach: In silico prediction validated in mouse pneumonia model.
• Highly conserved NucAb can provide broad protection against Acinetobacter strains.
• NucAb immunization decreased bacterial load, cytokines and inflammation in mice lungs.
• Immunization reduced disease severity, delayed death and improved survival.

Acinetobacter baumannii is an emerging multi-drug resistant pathogen causing significant mortality in hospitalized ICU patients which demands developing new methods for prevention and treatment. A. baumannii 19606 proteome was analysed in silico through the online tool Vaxign for finding potential vaccine candidates. The selected nuclease (NucAb) was predicted to possess all the attributes of a promising vaccine candidate like outer membrane localization, high adhesin probability (0.53), one transmembrane helix only, non-homology to human proteins and presence of B-cell and T-cell epitopes binding with high affinity (percentile rank ≤ 1) to HLA alleles prevalent at high frequency in North Indian populations. nucAb gene was highly prevalent (100%) among the clinical isolates (40/40) and conserved (>98%) among NCBI sequenced Acinetobacter strains. It was cloned in pET28a, purified and its immunoprotective potential was validated in murine pneumonia model. Immunization of BALB/c mice with recombinant NucAb (25 μg) elicited high antibody titre (1–5 × 105) which reduced bacterial load by 5 log cycles in lungs of mice challenged with optimized lethal dose (108 CFU). Lung histopathology revealed marked suppression of inflammation. Pro-inflammatory cytokines (TNF-α and IL-6) levels were reduced significantly and anti-inflammatory (IL-10) cytokine increased in lungs and serum leading to decreased severity and slow progression of disease. Though active immunization showed low survival rate (20%), passive immunization improved the survival (40%). This is the first study reporting an outer membrane nuclease as a vaccine candidate in Gram negative bacterium, A. baumannii through reverse vaccinology approach.