The Coagulation and Immune Systems Are Directly Linked through the Activation of Interleukin-1α by Thrombin

Abstract

Ancient organisms have a combined coagulation and immune system, and although links between inflammation and hemostasis exist in mammals, they are indirect and slower to act. Here we investigated direct links between mammalian immune and coagulation systems by examining cytokine proproteins for potential thrombin protease consensus sites. We found that interleukin (IL)-1α is directly activated by thrombin. Thrombin cleaved pro-IL-1α at a site perfectly conserved across disparate species, indicating functional importance. Surface pro-IL-1α on macrophages and activated platelets was cleaved and activated by thrombin, while tissue factor, a potent thrombin activator, colocalized with pro-IL-1α in the epidermis. Mice bearing a mutation in the IL-1α thrombin cleavage site (R114Q) exhibited defects in efficient wound healing and rapid thrombopoiesis after acute platelet loss. Thrombin-cleaved IL-1α was detected in humans during sepsis, pointing to the relevance of this pathway for normal physiology and the pathogenesis of inflammatory and thrombotic diseases.

Graphical abstract

Figure 1

IL-1α Is Activated by Direct Thrombin Cleavage (A) Protein alignment showing conservation of a (K)PRS motif in diverse species. (B and C) Western blots for IL-1α showing cleavage of recombinant p33 to an ∼18kDa form by thrombin (B), and inhibition of cleavage after mutating Arg¹¹² to His (C). (D) N-terminal sequencing of thrombin-cleaved human p33 IL-1α detected one sequence (italic underlined) corresponding to processing between Arg¹¹² and Ser¹¹³. (E–G) IL-1-dependent IL-6 production by HeLa cells incubated with calpain cleaved (E) or thrombin cleaved (F) p33 IL-1α, ± a neutralizing IL-1α pAb (+αAb), or increasing concentrations of recombinant p17 or p18 (G). (H and I) Cleavage and activation of p33 IL-1α during clotting of platelet-rich plasma (PRP) as shown with a cleaved IL-1α-specific ELISA (H) or IL-1-dependent IL-6 production by HeLa cells (I), ± a calpain inhibitor (+Ci), or an IL-1α pAb (+αAb). (J) Mutation of Arg¹¹² to His prevents p33 activation during clotting of PRP. (K) Pictogram showing cleavage sites within p33 IL-1α. Data represent mean ± SEM; n = 3 (E–G, I), n = 5 (H), n = 2 (J); p = ^∗≤0.05, ^∗∗≤0.01, ^∗∗∗≤0.001; NS = not significant. See also Figure S1.

Figure 2

Key Cell Types Contain p33 IL-1α that Can Be Cleaved by Thrombin (A and B) Flow cytometry plots and mean fluorescence intensity (MFI) for cell surface IL-1α in murine J2 macrophages treated ± LPS (A), or LPS followed by thrombin (+Tmb), ± a thrombin inhibitor (PPACK) (B). (C and D) Cleaved IL-1α-specific ELISA (C) and IL-1 activity assay (D) showing release of active p18 from the surface of LPS-treated J2 macrophages into the conditioned media by thrombin. (E and F) Human skin sections stained (brown) for IL-1α (E) or tissue factor (F). (G) Western blot for IL-1α in human keratinocyte necrotic lysates, ± a calpain inhibitor (+Ci), ± thrombin. (H) Cleaved IL-1α-specific ELISA showing thrombin cleavage of p33 IL-1α within human keratinocyte conditioned media. (I) Western blot for IL-1α showing p33 within human platelets. (J) Cleaved IL-1α-specific ELISA showing thrombin cleavage of p33 IL-1α in human platelet lysates or recombinant p33. (K) Flow cytometry for CD41 and IL-1α on resting, and collagen or collagen-related peptide (CRP) treated human platelets. Data represent mean ± SEM; n = 3 (C, D, J), n = 2 (H), n = 5 (K); p = ^∗≤0.05, ^∗∗≤0.01, ^∗∗∗≤0.001. Scale bars represent 100μm. See also Figure S1.

Figure 3

Generation of a Mouse Model in which IL-1α Cannot Be Activated by Thrombin (A) IL-1-dependent IL-6 production by murine fibroblasts incubated with increasing concentrations of recombinant mouse p17 or p33 IL-1α. (B) Western blot for IL-1α showing thrombin cleavage of recombinant mouse wild-type p33 to a ∼18kDa form, but less cleavage of Arg¹¹⁴His p33. (C–E) IL-1-dependent IL-6 production by murine fibroblasts incubated with increasing concentrations of recombinant mouse p17 or p18 (C), or wild-type (D) and Arg¹¹⁴His mutant (E) p33 incubated with clotting platelet-rich plasma (PRP), ± a calpain inhibitor (+Ci) and/or an IL-1α pAb (+αAb). (F) Western blot for IL-1α showing no thrombin cleavage of an Arg¹¹⁴Gln mutant p33. (G and H) IL-1-dependent IL-6 production by murine fibroblasts incubated with wild-type or mutant p33 incubated with clotting platelet-rich plasma (G) or active calpain (H). (I) Western blot for IL-1α showing equivalent expression of exogenous wild-type or mutant p33 in HeLa cells. (J) Cleaved IL-1α-specific ELISA reporting the level of calpain or thrombin processing of p33 IL-1α derived from LPS-treated control or IL-1α thrombin mutant (IL-1α TM) mBMDMs. (K) Flow cytometry for surface IL-1α on LPS-treated control or IL-1α TM mBMDMs incubated ±thrombin (+Tmb) and ± a thrombin inhibitor (PPACK). Data represent mean ± SEM; n = 3 (A, E, and J), n = 2 (C and H), n = 5 (G); p = ^∗≤0.05, ^∗∗≤0.01, ^∗∗∗≤0.001; NS = not significant. See also Figures S2 and S3, and Table S1.

Figure 4

p18 IL-1α Drives Rapid Thrombopoiesis and Wound Healing, and Is Generated during Sepsis in Humans (A and B) Platelet count by flow cytometry in mice under basal conditions (A), or over time after anti-CD42b-mediated platelet depletion (B). (C) Flow cytometry for the number of CD41⁺ and CD61⁺ MKs in the bone marrow of control and IL-1α TM mice under basal conditions. (D) Cleaved IL-1α-specific ELISA reporting the level of cleaved IL-1α within the serum of control mice treated ±dabigatran (+Dabi) and IL-1α TM mice over time after anti-CD42b-mediated platelet depletion. (E–G) Representative images showing gross healing of 4 mm excisional skin wounds (E), quantitation of wound area (F) over time, and rate of closure (G) in mice as indicated. (H–K) Representative images and quantitation of Ly6G+ neutrophils (H and I) or Mac3+ macrophages (J and K) recruited to the granulation tissue underlying wounds at the times indicated. (L) ELISA data showing release of cleaved IL-1α or IL-1β from wounded skin. (M) Sandwich ELISA data showing reactivity of the p18-specific ELISA to p17, p18, or p33 IL-1α. (N and O) p18-specific ELISA data reporting level of p18 in plasma from control individuals or patients with sepsis-associated ARDS (N). Red circles indicate +VE microbiology in bronchoalveolar lavage fluid. (O) Cleaved IL-1α-specific ELISA reporting the level of cleaved IL-1α within the serum of control and IL-1α TM mice during LPS-induced endotoxemia. Data represent mean ± SEM; n = ≥4 (A, B, and D), n = 3 (C), n = ≥5 (F and G), n = ≥10 wounds (I and K), n = 20 wounds (L), n = 2 (M); p = ^∗≤0.05, ^∗∗≤0.01, ^∗∗∗≤0.001; NS = not significant. See also Figure S4 and Table S2, S3.