Rational engineering of Omp34 loop-based immunogen enhances protection against Colistin-resistant Acinetobacter baumannii

The surface antigens are promising immunogenic targets for vaccine development and immunotherapies. Here, the loop3 to loop7 (L3-L7) region of the Omp34 from Acinetobacter baumannii was evaluated. A DNA fragment encoding the L3-L7 region was amplified from the Omp34 gene. The structures of ΔL5/5Gly-L3-L7 and ΔL5/Acr-L3-L7 were achieved through the modification of the loop 5 sequence in the L3-L7 of Omp34. Optimization of Omp34 loop epitopes via Acr sequence linkage was performed. Omp34, L3-L7 region, ΔL5/5Gly-L3-L7 and ΔL5/Acr-L3-L7 were recombinantly expressed. Mice immunized with the purified antigens, were infected with the clinical isolate of A. baumannii 58ST. Active immunizations with ΔL5/Acr-L3-L7, L3-L7, Omp34 and ΔL5/5Gly-L3-L7 resulted in 75%, 50%, 37.5% and 37.5% survival rates, respectively. Passive immunization with sera from these antigens in the same order results in survival rates of 62.5%, 37.5%, 25% and 25%, respectively. Anti-ΔL5/Acr-L3-L7 sera mediated the greatest opsonophagocytic killing activity. In addition, active and passive immunizations with ΔL5/Acr-L3-L7 resulted in the highest bacterial clearance in the lung, spleen, and liver. These findings suggest that the L3-L7 region of Omp34 is an appropriate scaffold and antigen against A. baumannii , in which replacing loop 5 with the Acr sequence could enhance its antigenicity and protectivity.