Extensive apoptosis of leukocytes during sepsis and endotoxic shock constitutes an important mechanism linked to the excessive mortality associated with these disorders. Caspase inhibitors confer protection from endotoxin-induced lymphocyte apoptosis and improve survival, but it is not clear which caspases mediate lipopolysaccharide (LPS)-induced lymphocyte apoptosis and mortality. We report here that the apoptotic executioner caspase-7 was activated in the splenocytes of LPS-injected mice, suggesting a role for caspase-7 in lymphocyte apoptosis. Indeed, caspase-7-deficient mice were resistant to LPS-induced lymphocyte apoptosis and were markedly protected from LPS-induced lethality independently of the excessive production of serum cytokines. These results reveal for the first time a nonredundant role for caspase-7 in vivo and identify caspase-7 inhibition as a component of the mechanism by which caspase inhibitors protect from endotoxin-induced mortality. (Blood. 2009;113:2742-2745)
IntroductionSepsis is the most common cause of mortality in patients treated in the intensive care setting, with more than 210 000 sepsis-related deaths occurring annually in the United States. 1 Extensive apoptotic death of leukocytes is commonly observed in patients who died of sepsis 2,3 and was suggested to contribute significantly to immune suppression and lethality. [4][5][6][7][8] In this regard, synthetic caspase inhibitors and overexpression of the antiapoptotic protein Bcl-2 were shown to diminish lymphocyte apoptosis and improve survival in experimental sepsis models. [8][9][10][11][12] However, it is currently incompletely understood which caspases promote lymphocyte apoptosis and contribute to lethality.Together with caspase-3, the executioner caspase-7 performs central roles in the execution phase of apoptosis by cleaving a large set of substrates, ultimately resulting in the morphologic and biochemical hallmarks of apoptosis such as DNA fragmentation. [13][14][15][16] Caspase-3/-7 double-deficient mice were recently shown to exhibit embryonic lethality, whereas mice singly deficient in either caspase are born at normal Mendelian ratios and display no gross abnormalities when maintained on a C57BL/6 genetic background. 17 At this stage, the precise roles of caspase-7 in the adult animals remain to be elucidated.In this study, we show that caspase-7 was activated in splenocytes of lipopolysaccharide (LPS)-treated mice and that caspase-7 Ϫ/Ϫ mice were protected from LPS-induced splenocyte apoptosis. As a result, caspase-7 deficiency improved survival during endotoxemia without affecting cytokine levels.
Methods
MiceCaspase-1 Ϫ/Ϫ , caspase-3 Ϫ/Ϫ , and caspase-7 Ϫ/Ϫ mice were backcrossed to C57BL/6 background for 10 generations and have been described previously. 17,18 Mice were originally purchased from The Jackson Laboratory (Bar Harbor, ME) and housed in a pathogen-free facility. The animal studies were conducted under protocols approved by St Jude Children's Research Hospital Committee on Use and Care of Animals.
LPS-...