Differing caspase‐1 activation states in monocyte versus macrophage models of IL‐1β processing and release

The release of IL‐1β as an active, mature cytokine requires proteolytic processing by caspase‐1, which is recruited to signaling complexes that facilitate its autocatalytic proteolysis and activation. Caspase‐1 processing has been characterized in human monocyte and murine macrophage model systems, and comparative analyses indicate significant mechanistic differences in caspase‐1 activation by these cell types. In this study, we used an in vitro processing assay to compare caspase‐1 activation in THP‐1 human monocytes vs. Bac1.2F5 murine macrophages. These in vitro caspase‐1 and IL‐1β processing reactions indicated a higher rate of constitutive caspase‐1 activation in lysates from THP‐1 vs. Bac1 cells. Transfer of small amounts of THP‐1 lysate to Bac1 lysate rapidly increased in vitro procaspase‐1 and proIL‐1β processing in the latter preparation. The transferable activation factor(s) was heat‐labile, ≥10 kDa, and unaffected by immunodepletion of procaspase‐1 from the THP‐1 lysate. Thi transactivating effect of THP‐1 lysate on processing in Bac1 lysates could be mimicked by addition of purified recombinant human caspase‐1. The constitutive caspase‐1 and IL‐1β processing reactions in THP‐1 lysates were insensitive to pharmacological blockade by the tyrphostin, AG126, and the phospholipase A2 inhibitor bromoenol lactone (BEL); contrarily, the same processing reactions were inhibited in lysates from Bac1 cells pretreated with either AG126 or BEL. These observations indicate significant biochemical differences in the assembly and regulation of caspase‐1 signaling complexes within human monocyte and murine macrophage models of inflammatory activation. These differences need to be considered when comparing or pharmacologically manipulating IL‐1β processing and release in various model systems.