NLRC5-dependent activation of the inflammasome

The nucleotide-binding domain (NBD) leucine rich repeat (LRR) containing proteins, NLRs, are intracellular sensors of PAMPs and DAMPs. A subgroup of NLRs can form inflammasome complexes, which facilitate the maturation of pro-caspase-1 to caspase-1, leading to IL-1β and IL-18 cleavage and secretion. NLRC5 is predominantly expressed in hematopoetic cells and has not been studied for inflammasome function. RNAi-mediated knockdown of NLRC5 nearly eliminated caspase-1, IL-1β and IL-18 processing in response to bacterial infection, PAMPs and DAMPs. This was confirmed in primary human monocytic cells. NLRC5 together with procaspase-1, pro-IL-1β and the inflammasome adaptor, ASC, reconstituted inflammasome activity which showed cooperativity with NLPR3. The range of pathogens that activate NLRC5 inflammasome overlaps with those that activate NLRP3. Furthermore, NLRC5 biochemically associates with NLRP3 in an NBD-dependent but LRR-inhibitory fashion. These results invoke a model where NLRC5 interacts with NLRP3 to cooperatively activate the inflammasome.