Background/objectives: Acinetobacter baumannii is an opportunistic nosocomial pathogen characterized by its multidrug-resistant (MDR) phenotype, increasing patient mortality and healthcare costs as a result. Previously, we constructed an immunoinformatics-based Acinetobacter Multi-Epitope Vaccine (AMEV2) candidate and demonstrated robust protection against this MDR pathogen. In this study, we delineate the mechanisms of AMEV2-mediated protective immunity.
Methods: In vivo passive immunization with AMEV2 antisera and in vitro opsonophagocytic killing assays (OPKAs) were used to assess the critical role of antibody-mediated protection induced by AMEV2 vaccination.
Results: The passive transfer of AMEV2 immune sera to naïve mice afforded 67% protection in a pulmonary challenge mouse model. Although AMEV2 sera reacts with bacterial antigens, it is not bactericidal on its own and does not enhance the complement-mediated direct killing of A. baumannii. However, OPKAs demonstrate AMEV2 sera enhancement of the killing of A. baumanniiin the presence of primary bone marrow-derived macrophages. This killing occurs via complement and Fc gamma receptor-mediated phagocytosis. A highly immunogenic AMEV2 component peptide, pTonB, elicits pTonB-specific antibodies and protection in vivo. The depletion of pTonB antibodies from AMEV2 immune sera by pTonB absorption significantly reduced the opsonophagocytic killing of A. baumannii in vitro.
Conclusions: The data presented here demonstrate the importance of humoral immunity and its protective mechanisms against A. baumannii. These findings further expand the in vivo evaluation of in silico-designed vaccines as a viable alternative to combat the current global MDR pathogen health crisis.
