A promising approach to control carbapenem-resistant Acinetobacter baumannii: Investigating immune pathways through rFilF immunization.

Acinetobacter baumannii is recognized as a critical priority for drug development due to the growing threat posed by antibiotic-resistant strains to public health. In response, research into non-antibiotic therapies, including passive immunization, has emerged as a promising alternative. This study investigates the immune responses induced by a fimbrial adhesin (FilF) from A. baumannii, used here as an immunogen. Recombinant FilF (rFilF) was formulated with Alhydrogel® adjuvant and administered intramuscularly in a three-dose schedule on days 0, 14 and 28 (n = 4). A control group (n = 4) received adjuvant diluted in 1 x PBS. Serum samples were collected weekly, and the spleens were harvested 14 days after the final immunization. Anti-rFilF IgG levels were evaluated by ELISA using pooled serum samples corresponding to each time point. To evaluate the functional activity of the immune response, anti-rFilF serum was incubated with A. baumannii ATCC® 19606™ and a carbapenem-resistant clinical strain. The opsonization ability of anti-rFilF antibodies to enhance macrophage phagocytosis was assessed using RAW264.7 cell cultures. Treatment with anti-rFilF serum significantly reduced bacterial viability in both strains, associated with increased macrophage phagocytic activity. By flow cytometry analysis we observe increased bacterial reactive oxygen species (ROS) levels and alterations in bacterial membrane integrity following treatment with anti-rFilF serum, suggesting potential mechanisms of bacterial clearance. Additionally, cytokine profiling of splenocyte cultures from immunized mice indicated a balanced cytokine response, with upregulation of IL-1β, IL-4, and IL-10, alongside the downregulation of IFN-γ. These findings indicate that rFilF immunization induces functional antibodies with therapeutic potential against A. baumannii infection.