In vitro interaction between Acinetobacter baumannii and human peripheral blood mononuclear cells.

Acinetobacter baumannii (A. baumannii) is recognized as an ESKAPE pathogen and a major cause of nosocomial infections. Despite advances in understanding the pathogenesis of A. baumannii and host immune responses, the direct interactions between A. baumannii and immune cells, as well as their influence on bacterial behavior, remain poorly defined. To address this gap, the present study established an in vitro model by co-culturing A. baumannii with human peripheral blood mononuclear cells (PBMCs) to investigate these interactions. A. baumannii isolated from patients’ pneumonia samples was co-cultured with PBMCs and their cell-free supernatants. After 24 h, the proliferation and viability of PBMC, the CFU count of bacteria, the PBMC cytokine profile, immune evasion gene expression (bap and ompA), biofilm formation, and bacterial motility were investigated. Co-culture with A. baumannii has significantly reduced the proliferation and viability of PBMCs compared with PBMCs alone. No significant change in bacterial CFU was observed in the co-culture group and cell-free supernatant. Cytokine analysis revealed a marked increase in TNF-α, whereas the production of IL-6 and IL-10 decreased in the co-cultured groups. Gene expression analysis revealed significant downregulation of bap in interaction with PBMC, whereas ompA expression remained unchanged. Co-culturing bacteria with PBMC resulted in an increase in biofilm formation; however, this difference was not statistically significant. Furthermore, there was no significant difference in surface-associated motility and twitching motility in the co-culture and bacterial alone groups. Overall, this is the first study to show direct interaction of A. baumannii and human PBMCs. The findings indicate that A. baumannii impairs the proliferation and viability of immune cells, while simultaneously resisting immune-mediated clearance. Interestingly, the downregulation of the biofilm-associated bap gene and the inflammatory microenvironment did not lead to significant changes in bacterial growth, biofilm formation, or motility. These findings demonstrate the robust resistance of A. baumannii to human immune defenses, underscoring the challenges in controlling and treating infections caused by this pathogen.