The TNF-α of mast cells induces pro-inflammatory responses during infection with Acinetobacter baumannii

Mast cells serve important roles as sentinels against bacterial infection by secreting mediators stored in granules. Much of their effectiveness depends upon recruiting and/or modulating other immune cells. The location of mast cells implies that they recognize pathogens invading tissues or mucosal tissues. Acinetobacter baumannii is a gram-negative bacterium that is considered an emerging nosocomial pathogen and causes a wide range of infections associated with high morbidity and mortality. To date, the interaction of A. baumannii with mast cells remains unclear. In this study, we demonstrated an interaction between human LAD2 mast cells and A. baumannii in vitro. When LAD2 cells were co-cultured with live A. baumannii or Pseudomonas aeruginosa PAO1 in vitro for 4h, TNF-α and IL-8 were produced in the culture supernatant. These inflammatory cytokines were not detected in the supernatant after the cells were treated with live bacteria without serum. Gene expression analysis showed that TNF-α and IL-8 mRNA expression increased in A. baumannii- and P. aeruginosa-infected LAD2 cells. Scanning electron microscopy showed that A. baumannii was tightly attached to the surface of LAD2 cells and suggested that A. baumannii may bind to FcγRII (CD32) on LAD2 cells. TNF-α in the culture supernatant from A. baumannii-infected LAD2 cells, showed that PMN activation and migration increased in Boyden chamber assays. These results suggest that mast cells recognize and initiate immune responses toward A. baumannii by releasing the preformed mediator TNF-α to activate effector neutrophils.