Mice were sequentially treated with P. acnes and LPS, and their serum IL-18 levels and liver injuries were determined by ELISA and ALT/AST measurement, respectively. Active caspase-1 in LPS-stimulated Kupffer cells was determined by Western blotting.
Macrophage-ablated mice lacked P. acnes-induced hepatic granuloma formation and LPS-induced serum IL-18 elevation and liver injury. Myd88−/− Kupffer cells, but not Trif−/− cells, exhibited normal caspase-1 activation upon TLR4 engagement in vitro. Myd88−/− mice failed to develop hepatic granulomas after P. acnes treatment and liver injury induced by LPS challenge. In contrast, Trif−/− mice normally formed the hepatic granulomas, but could not release IL-18 or develop the liver injury. Nalp3−/− mice showed the same phenotypes of Trif−/− mice.
Propionibacterium acnes treatment MyD88-dependently induced hepatic granuloma formation. Subsequent LPS TRIF-dependently activated caspase-1 via Nalp3 inflammasome and induced IL-18 release, eventually leading to the liver injury.