Innate immune mechanisms promote human response to Acinetobacter baumannii infection.

Acinetobacter baumannii is an opportunistic Gram-negative bacterium representing one of the leading causes of ventilator-associated pneumonia. The development of pneumonia results from a complex interplay between pathogens and pulmonary innate mucosal immunity. Therefore, the knowledge of the host immune responses is pivotal for the development of effective therapeutics to treat A. baumannii infections. Previous studies were conducted using cell lines and animal models, but a comprehensive understanding of the interaction between A. baumannii and primary human immune cells is still lacking. To bridge this gap, we investigated the response of primary monocytes, macrophages, and dendritic cells to the A. baumannii-type strain and an epidemic clinical isolate. We found that all immune cells trigger different responses when interacting with A. baumannii. In particular, macrophages and monocytes mediate bacterial clearance, whereas monocytes and dendritic cells activate a late response through the production of cytokines, chemokines, and the expression of co-stimulatory molecules. The epidemic strain induces lower expression of interleukin-10 and CD80 compared with the type strain, potentially constituting two immune evasion strategies.