Caspase-7 deficiency protects from endotoxin-induced lymphocyte apoptosis and improves survival

Abstract

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.

Figure 1

Caspase-7 deficiency protects against LPS-induced splenocyte apoptosis and lethality. (A,B) Wild-type (WT) and caspase-7^−/− mice (Casp7^−/−; both n = 4) were injected intraperitoneally with 20 mg kg⁻¹ LPS for 24 hours before spleens were collected and sections were stained with H&E (A) or TUNEL (B). (C) The number of TUNEL-positive cells in 5 high-power fields (hpf, ×400) from the white pulp of the spleen of each animal was quantified. Results represent mean plus or minus SD for each genotype. Data were analyzed by the Student t test. (D) Wild-type and caspase-7^−/− mice (n = 2) were either sham-operated or injected intraperitoneally with 20 mg kg⁻¹ LPS for 24 hours before spleens were collected and extracts were probed with an antibody against active caspase-7 (listed as Casp7; p19 denotes the large catalytic subunit) and Grb2. (E) Spleen extracts of wild-type mice that were either sham-operated or injected intraperitoneally with 20 mg kg⁻¹ LPS for 24 hours were probed with an antibody against caspase-1 (p45, procaspase-1; p20, large catalytic subunit) and active caspase-3 (p19 and p17 denote the large catalytic subunit). (F) Caspase-7^−/− (Casp7^−/−; n = 9) and wild-type mice (WT; n = 8) were injected intraperitoneally with 20 mg kg⁻¹ LPS, and their survival was monitored. Data were analyzed with the Kaplan-Meier test. Results are representative of 5 independent experiments. (G) Wild-type (WT; n = 8), caspase-1^−/− (Casp1^−/−; n = 8), caspase-3^−/− (Casp3^−/−; n = 6), and caspase-7^−/− (Casp7^−/−; n = 9) mice were injected intraperitoneally with 30 mg kg⁻¹ LPS and their survival was monitored. Data were compared with wild-type mice and analyzed with the Kaplan-Meier test. Results are representative of 2 independent experiments.