| Mast cells have been shown to play a critical role in bacterial infections. However, the fate of these cells after encountering bacteria has not yet been studied. Human mast cells (HMC-1 5C6 and cord blood derived mast cells, CBMC) were co-incubated with Pseudomonas aeruginosa clinical isolate (cystic fibrosis) strain 8821 between 3 and 24 hours and then analysed for apoptosis by a variety of methods. Logarithmic, but not stationary, P. aeruginosa can induce apoptosis in mast cells, indicating that bacterial growth phase is important in apoptotic induction. Apoptosis involves the caspase cascade (activation of caspase-3), the intrinsic pathway (loss of mitochondrial membrane potential, release of cytochrome c and AIF from the mitochondria, and an increase in Bcl-xs to Bcl-xL ratio), and may also involve the extrinsic pathway (FLIP degradation). Fas, Exotoxin S, ceramide, and calpain were not involved in P. aeruginosa-induced apoptosis in mast cells. However, calpain inhibition itself induced apoptosis in HMC-1 5C6 (but not in CBMC). Calpain inhibitor-induced apoptosis involves calpain-associated molecules (calpastatin, fodrin), the caspase cascade (activation of caspase-3), the intrinsic apoptotic pathway (an increase in Bcl-xs to Bcl-xL ratio, caspase-9 activation), and possibly the extrinsic pathway (FLIP degradation). Calpain inhibition also resulted in a down-regulation of P. aeruginosa-induced production of TNF, and IL-6 by CBMC, but not of IL-1beta. |
