Non-canonical roles of caspase-4 and caspase-5 in heme driven- IL-1β release and cell death

Excessive release of heme from red blood cells is a key pathophysiological feature of several disease states including bacterial sepsis, malaria, and sickle cell disease. This hemolysis results in an increased level of free heme that has been implicated in the inflammatory activation of monocytes, macrophages, and endothelium. Here, we show that extracellular heme engages the human inflammatory caspases, caspase-1, caspase-4, and caspase-5, resulting in the release of IL-1β. Heme-induced IL-1β release was further increased in macrophages from patients with sickle cell disease. In human primary macrophages, heme activated caspase-1 in an inflammasome-dependent manner, but heme-induced activation of caspase-4 and caspase-5 was independent of canonical inflammasomes. Furthermore, we show that both caspase-4, and caspase-5 are essential for heme-induced IL-1β release, while caspase-4 is the primary contributor to heme-induced cell death. Together, we have identified that extracellular heme acts as a damage associated molecular pattern (DAMP) that can engage canonical and non-canonical inflammasome activation as a key mediator of inflammation in macrophages. Footnotes Funding for the project includes NIH/NIDDK T32DK060445 (BEB, BAR), NIH/NIDDK F32DK121479 (BEB), NIH/NIGMS R01GM121389 (LBH), NIH/NHLBI R01HL114567 (JDB, GMV), NIH/NHLBI R01-HL136415 (JMF), and CPRIT-RP180672, NIHCA125123 and NIHRR024574 (Cytometry and Cell Sorting Core at Baylor College of Medicine) Key Points Heme induces oligomerization of caspase-1, caspase-4, and caspase-5. Heme-induced IL-1β release requires both caspase-4 and caspase-5. Caspase-4 alone contributes to heme-induced cell death.