An endogenous caspase-11 ligand elicits interleukin-1 release from living dendritic cells

Abstract

Dendritic cells (DCs) use pattern recognition receptors to detect microorganisms and activate protective immunity. These cells and receptors are thought to operate in an all-or-nothing manner, existing in an immunologically active or inactive state. Here, we report that encounters with microbial products and self-encoded oxidized phospholipids (oxPAPC) induce an enhanced DC activation state, which we call "hyperactive." Hyperactive DCs induce potent adaptive immune responses and are elicited by caspase-11, an enzyme that binds oxPAPC and bacterial lipopolysaccharide (LPS). oxPAPC and LPS bind caspase-11 via distinct domains and elicit different inflammasome-dependent activities. Both lipids induce caspase-11-dependent interleukin-1 release, but only LPS induces pyroptosis. The cells and receptors of the innate immune system can therefore achieve different activation states, which may permit context-dependent responses to infection.

Fig. 1. oxPAPC does not induce TLR4 signaling

(A) MΦs or DCs were treated with LPS or oxPAPC for the indicated time points. TLR4 dimerization and endocytosis was measured by flow cytometry. Line graph represents means and standard deviations of four replicates. (B) MΦs or DCs were treated with LPS or oxPAPC. Cytokine production was analyzed 18 hours later. Means and standard deviations of four replicates are shown. (C) Myddosome formation in iMΦs was assessed at the indicated time points after treatment with LPS or oxPAPC by co-immunoprecipitation (IP) of IRAK4 with MyD88 followed by western analysis of the proteins indicated. (D) Whole cell lysates (WCL) were collected and DCs were monitored for STAT-1 phosphorylation and viperin expression after treatment with LPS oxPAPC. (C and D) One experiment representative of three is shown.

Fig. 2. oxPAPC induces the activation of the NLRP3 inflammasome in DCs

(A) DCs primed with LPS, followed by ATP or oxPAPC treatment. Cell culture supernatant from DCs subjected to indicated treatments were collected, and processed IL-1β (p17) production was assessed. One experiment representative of three is shown. (B) DCs were treated with LPS alone, 10, 50, or 120μM oxPAPC or were primed with LPS for 3 hours and then treated with oxPAPC. For this experiment, commercially available oxPAPC and an oxPAPC enriched in PEIPC were used. 18 hours after LPS administration, secreted (left panel) and cell associated (right panel) IL-1β were measured by ELISA. Means and standard deviations of four replicates are shown. (C) DCs of the genotypes indicated were treated with LPS alone, oxPAPC alone or were primed with LPS for 3 hours and then treated with oxPAPC. 18 hours after LPS administration, IL-1β and secretion was measured by ELISA. Means and standard deviations of four replicates are shown. (D) MΦs and DCs were treated with Pam3CSK (P3C) alone, oxPAPC alone, or were primed with Pam3CSK for 3 hours and then treated with oxPAPC, DOTAP alone, and LPS or oxPAPC encapsulated in DOTAP. 18 hours after P3C administration, IL-1β was measured by ELISA. Means and standard deviations of four replicates are shown. (E) DCs (left panel) or MΦs (right panel) were primed with LPS for three hours and treated with ATP. At indicated time points, IL-1β was measured by ELISA and cell death was measured by PI permeabilization assay. Means and standard deviations of four replicates are shown.

Fig. 3. oxPAPC promotes noncanonical inflammasome activation

(A) WT DC and caspase-11 KO DC were treated with LPS alone, oxPAPC alone or were primed with LPS for 3 hours and then treated with oxPAPC. 18 hours after LPS administration, IL-1β secretion was measured by ELISA. Means and standard deviations of four replicates are shown. (B) DCs were left untreated or primed with LPS and then stimulated with ATP or oxPAPC. Specks containing ASC (green) and caspase-1 (Casp1, red) were analyzed 18 hours after LPS stimulation. Nuclei are shown in blue. Panels are representative of four independent experiments. Scale bar: 10 μm. (C) S100 fractions of nontreated (nt) or P3C-primed (P3C) MΦs were incubated with biotin-LPS (Bio-LPS), biotin-oxPAPC (Bio-oxPAPC) or biotin-MDP (Bio-MDP). Endogenous proteins associated with biotinylated-ligands were captured by streptavidin beads and revealed by western analysis. Shown is a representative blot out of three independent experiments. (D) SPR analysis of the interactions between the proteins and lipids indicated. (E) Gel filtration analysis of the size of caspase-11 complexes before and after exposure to oxPAPC. Complex size was monitored by A280 or western analysis, as indicated. Shown is a representative blot out of three independent experiments. (F) Bone marrow cells were infected with the pMSCV2.2-IRES-GFP vector (empty), the pMSCV2.2-IRES-GFP vector encoding WT caspase-11 (WT caspase-11) or the same vector containing a catalytic mutant caspase-11 (C254A). DCs were primed or not with LPS and then stimulated with oxPAPC, or transfected with LPS-containing FuGENE (Trans. LPS). 18 hours after LPS priming, supernatant were collected and IL-1β was measured by ELISA. Cell viability was assessed by measuring LDH release. Means and standard deviations of four replicates are shown.

Fig. 4. oxPAPC prevents DC death and potentiates adaptive immune responses

(A and B) DCs were treated with LPS alone, ATP alone, oxPAPC alone or FuGENE complexed LPS [Fugene (LPS)], or were primed for three hours with LPS and then treated with the indicated stimuli. Cell death was measured by LDH release (A) or IL-1β secretion was measured by ELISA (B). Means and standard deviations of four replicates are shown. (C and D) DCs were pretreated with LPS for 3 hours and then activated with ATP or oxPAPC. 18 hours later, cells were stained for ASC (green), nuclei (blue) Zombie Dye (red) (C) or active mitochondria (red). Scale bar: 10 μm. (D). Panels are representative of three independent experiments. (E) CD4+ T-cells were isolated from the draining lymph nodes 40 days after immunization with OVA + LPS in IFA (LPS), OVA + LPS + oxPAPC in IFA (LPS+oxPAPC) or OVA + oxPAPC in IFA (oxPAPC) of WT, caspase-1/-11 dKO or caspase-11 KO mice. CD4+ T-cells were restimulated or not with OVA in the presence of DCs. IFNγ (left panel) and IL-17 (right panel) secretion was measured 5 days later by ELISA. Bar graphs represent means and standard errors of two experiments with five animals per group. *P < 0.05; **P < 0.01; ***P < 0.005.