Activated protein C (APC) is effective in treating septic patients who have already experienced multiorgan failure. The mechanisms responsible for the efficacy remain unclear. Macrophages were activated with endotoxin and gamma interferon, the supernatant harvested and subjected to incubation with activated protein C. The toxicity of the supernatant toward endothelium was blocked by APC incubation. APC incubation generated 3 new peptides which sequence determination demonstrated were derived from histones including H3 and H4. Histones incubated with endothelium were cytotoxic. APC cleaved and eliminated this cytotoxicity. Histones were identified in the circulation of mice challenged with endotoxin, baboons challenged with E. coli and some septic human patients. In mice, blocking the histone toxicity with a monoclonal antibody effectively reduced endotoxin-mediated death. Histone injection resulted in death with pathologies resembling those of sepsis including fibrin deposition and neutrophil infiltration. APC could also prevent death in a histone-mediated model. The histones elicit an inflammatory response, suggesting some recognition by the immune system. Using 293 cells expressing individual toll receptor reporter constructs, we found that the histones activated toll 2 and toll 4. Mice with either of these tolls deleted were resistant to histone-mediated death. This data, coupled with earlier studies demonstrating that histones can kill bacteria, suggest that histones play a role in innate immunity but are also toxic. The results suggest that the inflammatory response in trauma may be triggered by the cellular destruction that releases histones and further suggest that histone antagonists might be an effective mechanism to block progressive organ failure.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.