Alternative inflammasome activation enables IL-1β release from living cells

Highlights

• Alternative NLRP3 inflammasome activation comprises a distinct signaling entity.

• Genetic and phenotypic criteria distinguish alternative and classical NLRP3 signaling.

• This includes absence of pyroptosis and GSDMD-independent IL-1β secretion.

Classical modes of NLRP3 activation entail a priming step that enables its activation (signal 1) and a potassium efflux-dependent activation signal (signal 2) that triggers pyroptosome formation and pyroptosis, a lytic cell death necessary for IL-1β release. Opposing to that, human monocytes engage an alternative NLRP3 inflammasome pathway in response to LPS that proceeds in the absence of signal 2 activation and enables IL-1β secretion without pyroptosis. This specifically relies on Caspase-8 to propagate signaling to NLRP3, leading to inflammasome activation in absence of pyroptosome formation. Here, we summarize the current knowledge about alternative inflammasome activation, discuss potential extensions of this signaling entity beyond LPS-dependent activation, speculate about its role in tissue homeostasis and sterile inflammation and highlight the implications of pyroptosis-independent IL-1β secretion.

Introduction

The pleotropic cytokine IL-1β serves instrumental functions in initiation and orchestration of innate and adaptive immune responses. Its receptor IL-1R shares the intracellular TIR (Toll IL-1 receptor) signaling-domain with the well-characterized PRR-family of TLRs, thereby enabling TLR-like transcriptional responses in cells not expressing TLRs. This homology in signaling explains why the biological effects of IL-1β closely resemble activation of the innate immune system. IL-1β is a cytokine of numerous functions, exerting diverse effects on multiple cells types to culminate in broadly inflammatory events. Systemic consequences of IL-1β signaling include fever, vasodilatation, hypotension and acute phase response. Locally, IL-1 signaling enables mesenchymal and endothelial cells to express adhesion molecules facilitating recruitment of lymphocytes and myeloid cells. Consequently, the infiltrating immune cells are activated and triggered to further amplify inflammation. Besides its crucial roles in the induction and orchestration of innate immune responses, IL-1β has been implicated in adaptive responses by influencing Th1 and Th17-mediated immune activation. Aberrant IL-1β signaling has been associated with rare heritable monogenetic auto-inflammatory diseases like Cryopyrin-Associated Periodic Syndromes (CAPS) or Familial Mediterranean Fever (FMF), yet also with common multifactorial diseases, such as Type 2 diabetes or gout [1, 2, 3].

IL-1β belongs to the family of IL-1 cytokines, of which several members, including IL-1β and IL-18, are synthesized as inactive precursors within the cytosol, where they await proteolytic cleavage to gain bioactivity. This maturation step is most commonly executed by the cysteine protease Caspase-1, whose activity is controlled by a cytosolic activation platform called the inflammasome (Figure 1). At the same time, other enzymes, such as Caspase-8 or neutrophil proteases have been described to mature IL-1β. Inflammasome signaling entails a sensor molecule (e.g. NLRP1, NLRP3, NLRC4 or AIM2) that seeds the assembly of the adapter molecule ASC into helically structured filaments forming a high-molecular weight complex that is called the pyroptosome [4]. In turn, Caspase-1 becomes recruited to the pyroptosome to also form helical structures, which enable its proximity-induced proteolytical auto-activation. Once Caspase-1 has become matured into the active p10²:p20² hetero-tetramer it is licensed to cleave the cytokine-precursors pro-IL-1β/pro-IL-18. Besides cytokine maturation, induction of a lytic type of cell death called pyroptosis constitutes a hallmark of inflammasome activation. This programmed cell death is mediated by Caspase-1-dependent cleavage of the pyroptosis-effector molecule GSDMD (Gasdermin-D). Caspase-1 maturation of GSDMD abolishes the intra-molecular auto-inhibition of the C-terminal domain [5••, 6••, 7••]. Subsequently, phospholipid binding specificities of the matured N-terminal fragment of GSDMD enable its translocation to the inner leaflet of the plasma membrane, where it forms round, pore-like structures of approximately 15 nm in diameter [8, 9, 10•, 11]. Consequently, breakdown of ion gradients over the plasma membrane leads to water influx driven by oncotic pressure, inducing the characteristic cell swelling and rupture of the plasma membrane. Although soluble cytosolic proteins are released during pyroptotic lysis, it has been shown that so called pore-induced intracellular traps (PITs) maintain larger fragments of the pyroptosed cell (including previously phagocytosed bacteria) within the cellular debris [12]. The previously noted correlation of pyroptosis and IL-1β release [13, 14] has been genetically supported by the defect in IL-1β secretion but not in maturation in murine macrophages deficient for GSDMD [5••, 6••, 7••]. To this end, the picture emerges that IL-1β secretion critically depends on GSDMD in the course of classical inflammasome pathways, either directly through the nascent pore or in the course of plasma membrane disintegration.