Nontypeable haemophilus influenzae (NTHi) leads to caspase-1-dependent upregulation of interleukin 1-beta (IL-1β) in respiratory cells and human lung tissue – A role of the “inflammasome” in respiratory tract infections

The role of the inflammasome in pulmonary inflammation due to respiratory infections is still not well understood. As the inflammasome is a heterogeneous group of proteins, we focused on one obligatory component, caspase-1. We investigated if inhibition leads to a decrease of IL-1β after infection with NTHi and if it induces other proinflammatory cytokines like tumor necrosis factor-alpha (TNF-α) and CXCL2. Murine alveolar macrophages (RAW 264.7) and human lung tissue were stimulated in-vitro with NTHi 10 6 cfu/ml for 24-48h. A caspase-1 inhibitor (CI) was added 8h after infection. Supernatant and cells were collected for ELISA, PCR and Western Blot analysis. Cell and tissue culture experiments showed a significant induction of IL-1β-production after NTHi- in vitro -infection (RAW: Med 24h and 48h <15.6±0pg/ml vs. NTHi 24h 408±64pg/ml and NTHi 48h 717±72pg/ml, both n=6, p<0.01). Caspase-1 was activated 60min after infection. Inhibition of caspase-1 significantly decreases IL-1β levels after 24h (NTHi 24h 408±64pg/ml vs. NTHi+CI 24h 174±12pg/ml, n=6, p<0.01) and 48h of NTHi infection (NTHi 48h 717±72pg/ml vs. NTHi 48h 432±49pg/ml, n=6, p<0.01). PCR data confirmed the inhibitory effect. We did not observe significant changes in TNF-α and CXCL2 release after caspase-1 inhibition what means that these cytokines are not affected by inhibiting caspase-1. These results indicate that caspase-1-mediated IL-1β-upregulation is an important mechanism of NTHi-induced inflammation in pulmonary tissues and might be a central mediator in the pathogenesis of respiratory tract infections.