Necrotic cells trigger a sterile inflammatory response through the Nlrp3 inflammasome

Abstract

Dying cells are capable of activating the innate immune system and inducing a sterile inflammatory response. Here, we show that necrotic cells are sensed by the Nlrp3 inflammasome resulting in the subsequent release of the proinflammatory cytokine IL-1beta. Necrotic cells produced by pressure disruption, hypoxic injury, or complement-mediated damage were capable of activating the Nlrp3 inflammasome. Nlrp3 inflammasome activation was triggered in part through ATP produced by mitochondria released from damaged cells. Neutrophilic influx into the peritoneum in response to necrotic cells in vivo was also markedly diminished in the absence of Nlrp3. Nlrp3-deficiency moreover protected animals against mortality, renal dysfunction, and neutrophil influx in an in vivo renal ischemic acute tubular necrosis model. These findings suggest that the inhibition of Nlrp3 inflammasome activity can diminish the acute inflammation and damage associated with tissue injury.

Fig. 1.

Inflammation induced in vivo by pressure-disrupted necrotic cells is dependent on the Nlrp3 inflammasome. (A–C) Neutrophil influx into the peritoneum of WT, IL-1R-, caspase-1-, ASC-, and Nlrp3-deficient mice 16 h after i.p. challenge with pressure-disrupted B16 cells (B16). Control WT mice were challenged i.p. with PBS. *, P = 0.0017; **, P = 0.0021; ***, P = 0.0159; ****, P = 0.0240.

Fig. 2.

Cellular injury induced by pressure-disruption is sensed by the Nlrp3 inflammasome. (A) LPS-primed WT Mϕ were stimulated with either UV-treated, freeze-thawed, or pressure-disrupted B16 cells at a ratio of 10 necrotic cells per Mϕ; culture supernatants were collected 12 h later, and IL-1β release was measured by ELISA. (B) Neutrophil influx into the peritoneum of WT and caspase-1-deficient mice 16 h after i.p. challenge with either 1 × 10⁷ pressure-disrupted B16 cells (B16) or freeze-thawed B16 cells. *, P = 0.0041. (C and D) LPS-primed WT, Nlrp3-, caspase-1-, or ASC-deficient Mϕ were stimulated with pressure-disrupted B16 cells and supernatants collected 12 h later or at the indicated time. IL-1β release was measured by ELISA. (E) Lysates from LPS-primed WT, Nlrp3-, or ASC-deficient Mϕ stimulated with pressure-disrupted B16 cell for 12 h were immunoblotted with antibodies against the p10 subunit of caspase-1, IL-1β, and GAPDH. (A, C, and D) Determinations were performed in triplicate and expressed as the mean ± SEM. Results are representative of two (A) and three (C and D) separate experiments.

Fig. 3.

Complement- and hypoxia-induced cellular injury is sensed by the Nlrp3 inflammasome. (A) LPS-primed WT, caspase-1-, ASC-, Nlrp3-, or Nlrc4-deficient Mϕ were stimulated with complement-lysed splenocytes at a ratio of 25 splenocytes per Mϕ; supernatants were collected 12 h later, and IL-1β release assessed by ELISA. (B–D) Splenocytes or the hybridoma cell line 6F10 were either left untreated or opsonized with anti-MHC class II Aβ(b) IgG2a antibody followed by incubation with human serum (serum) or heat-inactivated human serum (HI-serum). LPS-primed WT or Nlrp3−/− Mϕ were stimulated with complement-damaged cells at a ratio of 25 splenocytes per Mϕ, 20 6F10 cells per Mϕ, or as indicated; supernatants were collected 12 h later, and IL-1β release assessed by ELISA. *, P = 0.0003. (E) LPS-primed WT or Nlrp3−/− Mϕ were stimulated with B16 cells grown under normoxic (control) or hypoxic conditions at a ratio of five cells per Mϕ; supernatants were collected 12 h later, and IL-1β release assessed by ELISA. **, P = 0.0013. (F) WT Mϕ were primed with either biglycan (8 μg/mL) or hyaluronic acid (25 μg/mL) for 12 h followed by stimulation with pressure-disrupted B16 cell at a ratio of 10 necrotic cells per Mϕ; supernatants were collected 12 h later, and IL-1β release measured by ELISA. (A–F) Determinations were performed in triplicate and expressed as the mean ± SEM. Results are representative of two (C–F) and three (A and B) separate experiments.

Fig. 4.

Mitochondria released from necrotic cells stimulate Nlrp3 inflammasome activation. (A) LPS-primed WT Mϕ were stimulated with pressure-disrupted B16 cells or nuclei, plasma membrane, cytosol, and mitochondrial fractions derived from B16 cells at a ratio of five cell equivalents per Mϕ; supernatants were collected 12 h later, and IL-1β release was measured by ELISA. (B) LPS-primed WT, caspase-1-, ASC-, Nlrp3-, or Nlrc4-deficient Mϕ were left untreated or stimulated with mitochondria (100 μg/mL); supernatants were collected 12 h later, and IL-1β release was assessed by ELISA. (C) Lysates from LPS-primed WT, Nlrp3-, or ASC-deficient Mϕ stimulated with mitochondria for 12 h were immunoblotted with antibodies against the p10 subunit of caspase-1 and GAPDH. (D) Neutrophil influx into the peritoneum of WT and Nlrp3-deficient mice 16 h after i.p. challenge with mitochondria isolated from 1 × 10⁷ B16 cells. Control WT mice were challenged i.p. with homogenization buffer. *, P = 0.0317. (E) Mitochondria were treated with rotenone (10 μM), myxothiazol (10 μM), apyrase (1 U) for 20 min or heat-treated at 65 °C for 20 min, washed, and ATP content quantified. (F) LPS-primed WT Mϕ were stimulated with mitochondria (100 μg/mL) that had been treated as described; supernatants were collected 12 h later, and IL-1β release was measured by ELISA. (G) LPS-primed WT or P2X7R-deficient Mϕ were stimulated with ATP (5 mM), silica (50 μg/cm²), pressure-disrupted B16 cells, or mitochondria; supernatants were collected 12 h later, and IL-1β release was measured by ELISA. **, P = 0.0041; ***, P = 0.0009. Determinations were performed in triplicate and expressed as the mean ± SEM. Results are representative of two (A, E, and F) and three (B and G) separate experiments.

Fig. 5.

Nlrp3-deficiency protects animals against mortality, renal dysfunction, and impairs an inflammatory response during renal ischemic acute tubular necrosis. (A) Relative Nlrp3 mRNA levels were measured in kidneys of WT mice 1 day after nonlethal renal I/R or sham operation. (B) Tubular necrosis score of WT mice (n = 8) and Nlrp3-deficient mice (n = 8) after nonlethal renal I/R injury using PAS-D-stained renal tissue sections. (C) Survival of Nlrp3-deficient mice (n = 14) compared to WT mice (n = 13) after lethal renal ischemia. (D and E) Renal dysfunction of Nlrp3-deficient mice (n = 8) compared to WT mice (n = 8) as reflected by increased levels of urea (D) and creatinine (E) in plasma after nonlethal renal acute tubular necrosis. (F and G) Neutrophil influx in kidneys from WT and Nlrp3-deficient mice 1 day after renal I/R or sham operation as assessed by immunohistochemistry (F) and counted in at least 10 randomly selected high-power fields in the outer medulla (G). Original magnification of pictures, ×400. (H and I) Total KC and IL-1β levels in kidneys from Nlrp3-deficient and WT mice subjected for 1 day to renal I/R injury. *, P < 0.05.