Cooperation and constraint: the complex association of caspase-8 and cFLIPDownload PDF Download PDF EditorialOpen accessPublished: 27 July 2026Betsaida Bibo-Verdugo1 &Guy S. Salvesen ORCID: orcid.org/0000-0002-7933-67322 Cell Death & Differentiation (2026) Cite this articleSubjectsCell death and immune responseProteolysisCaspases exert their function by cleaving proteins, primarily self-cleavage as part of their maturation in cell death signaling complexes, and cleavage of other caspases to engage downstream cell death signaling pathways. Caspase-8 has a unique place in cell death decisions because it can both promote apoptosis and prevent necroptosis (Fig. 1), thereby engaging opposing pro-and anti-inflammatory pathways [1]. Mechanistically-speaking, these opposing outcomes depend on interaction with the adaptor protein FADD (Fas-Associated Death Domain protein), dimerization state and self-cleavage. Downstream from recognition of death ligands by their cognate receptors, including Fas and TNFR1 (Tumor Necrosis Factor Receptor 1), latent caspase-8 is activated by dimerization via recruitment to the oligomeric platform known as the death inducing signaling complex (DISC). Once activated, a caspase-8 homodimer can cleave itself at position Asp387 in mouse, or Asp374 or Asp384 in human. This processing event splits the catalytic domain into two subunits, resulting in a more active and stable conformation that is crucial for its proapoptotic function in the context of homodimerization [2]. Adjacent to the Casp8 genetic locus is the gene encoding the pseudocaspase cFLIP (Cellular FLICE-like Inhibitory Protein), Cflar. A variant of this protein (cFLIPL) contains the same protein fold as caspase-8 but lacks the catalytic residues. Increased concentration of cFLIPL favor the formation of caspase-8/cFLIPL heterodimers, which inhibit caspase-8 proapoptotic function but can promote its anti-necroptotic function [3]. When caspase-8 partners with cFLIPL, intersubunit linker proteolysis is not required. In vitro, the heterodimer is an attenuated enzyme [4]. Thus, cFLIPL can activate caspase-8 by dimerization to generate a heterodimer with different substrate specificity than the homodimeric caspase-8, which has an apoptotic role.Fig. 1Full size imageCell death pathways influenced by the dimerization state of caspase-8.Elucidating the consequences of caspase-8 influence on death decisions heavily relies on genetic mouse models. Mice lacking caspase-8 or expressing inactive caspase-8 do not survive embryonic development. Although apoptosis is compromised, necroptosis is unrestrained [5,6,7]. Mutations that prevent necroptosis (deletion of Mlkl or Ripk3) have a rescuing effect on the lethal phenotype [8, 9]. On the other hand, mutations that prevent full caspase-8 activation cause apoptosis insufficiency and produce lymphoproliferative disease (LPR) in mice and autoimmune lymphoproliferative syndrome (ALPS) in humans [10]. Although self-processing of caspase-8 is considered important for apoptosis, self-processing-deficient caspase-8 (D384A) mice do not develop LPR.A recent paper by Newton and colleagues addresses this somewhat confusing conundrum using mice with specific mutations in Casp8 and Cflar bred together [11]. In a Casp8 WT background, Cflar loss-induced lethality is rescued by deletion of Mlkl. However, while embryos resulting from a cross of mice bearing an inactivation of the caspase-8 self-cleavage site (D384A) with Mlkl-/- mice are viable, concomitant loss of Cflar leads to perinatal lethality, atrophy of small intestine and thymus, and upregulation of cytokine and chemokine transcripts. The culmination of this work indicates that cleavage-deficient caspase-8 D387A can form active heterodimers with cFLIPL to promote apoptosis, whereas caspase-8 D387A homodimers cannot sustain this function. This model is supported by a previous work where ablation of caspase-8 homodimers was achieved by mutations in the pro-domain [12]. Importantly, this outcome depends on restraining the necroptotic pathway, highlighting the fact that dominance of cell death paradigms depends on the dynamic competition of cell death signaling proteins. Caspase-8 D387A homodimers could not cleave caspase-3 to trigger extrinsic apoptosis, supporting prior biochemical findings that the first cleavage in the DISC must be a self-cleavage to generate a conformation of caspase-8 competent for downstream signaling. Over the years, the supposed function of cFLIPL has changed from apoptosis inhibitory protein to modulator of caspase-8 specificity, culminating in the latest publication that cleverly uses mouse genetics to reveal the mechanism that this pseudocaspase utilizes to modulate cell death signaling pathways.Notwithstanding the attractive answers to how caspase-8 activity is controlled in mice to signal distinct cell death outcomes we note that humans contain a caspase-8 paralog in the genomic region between Casp8 and Cflar. Caspase-10 is absent in mice, and although it can also be activated by homodimerization or heterodimerization the resulting enzymes can promote apoptosis but cannot block necroptosis in vitro [13]. While the core apoptotic machinery is functionally conserved across mammals [14], central components of the necroptotic pathway have been deleted or show inactivating mutations in several mammalian lineages [15]. From an evolutionary perspective, it appears that the apoptotic pathway is indispensable and that genes regulating necroptosis are under extreme selection, resulting in their loss in some mammal orders and we look forward to future investigations of this interesting phenomenon.ReferencesTummers B, Green DR. Caspase-8: regulating life and death. Immunol Rev. 2017;277:76–89.Article PubMed PubMed Central Google Scholar Boatright KM, Renatus M, Scott FL, Sperandio S, Shin H, Pedersen IM, et al. A unified model for apical caspase activation. Mol Cell. 2003;11:529–41.Article PubMed Google Scholar Oberst A, Pop C, Tremblay AG, Blais V, Denault J-B, Salvesen GS, et al. Inducible Dimerization and inducible cleavage reveal a requirement for both processes in caspase-8 activation. J Biol Chem. 2010;285:16632–42.Article PubMed PubMed Central Google Scholar Pop C, Oberst A, Drag M, Van Raam Bram J, Riedl Stefan J, Green Douglas R, et al. 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Front Immunol. 2021;12:747737. https://doi.org/10.3389/fimmu.2021.747737.Article PubMed PubMed Central Google Scholar Download referencesAuthor informationAuthors and AffiliationsCentro de Investigaciones Biológicas del Noroeste S.C., La Paz, MexicoBetsaida Bibo-VerdugoSanford Burnham Prebys, La Jolla, CA, USAGuy S. SalvesenAuthorsBetsaida Bibo-VerdugoView author publicationsSearch author on:PubMed Google ScholarGuy S. SalvesenView author publicationsSearch author on:PubMed Google ScholarContributionsBBV and GS contributed equally to the conceptualization of the original draft. Both authors jointly worked on writing, reviewing and editing the article.Corresponding authorsCorrespondence to Betsaida Bibo-Verdugo or Guy S. 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