Escaping Death: How Cancer Cells and Infected Cells Resist Cell-Mediated Cytotoxicity

Abstract

Cytotoxic lymphocytes are critical in our immune defence against cancer and infection. Cytotoxic T lymphocytes and Natural Killer cells can directly lyse malignant or infected cells in at least two ways: granule-mediated cytotoxicity, involving perforin and granzyme B, or death receptor-mediated cytotoxicity, involving the death receptor ligands, tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas ligand (FasL). In either case, a multi-step pathway is triggered to facilitate lysis, relying on active pro-death processes and signalling within the target cell. Because of this reliance on an active response from the target cell, each mechanism of cell-mediated killing can be manipulated by malignant and infected cells to evade cytolytic death. Here, we review the mechanisms of cell-mediated cytotoxicity and examine how cells may evade these cytolytic processes. This includes resistance to perforin through degradation or reduced pore formation, resistance to granzyme B through inhibition or autophagy, and resistance to death receptors through inhibition of downstream signalling or changes in protein expression. We also consider the importance of tumour necrosis factor (TNF)-induced cytotoxicity and resistance mechanisms against this pathway. Altogether, it is clear that target cells are not passive bystanders to cell-mediated cytotoxicity and resistance mechanisms can significantly constrain immune cell-mediated killing. Understanding these processes of immune evasion may lead to novel ideas for medical intervention.

Keywords: cancer; cell-mediated cytotoxicity; cytolytic T cells; immune synapse; lymphocytes; natural killer cells; resistance; viral infection.

Figure 1

Mechanisms of lymphocyte cytotoxicity. Following activation, cytotoxic effector cells can kill through granule-mediated cytotoxicity, death receptor-mediated cytotoxicity, or TNF-mediated cytotoxicity. (A) During granule-mediated cytotoxicity, perforin and granzymes are released from lytic granules into the synaptic cleft. Perforin forms pores in the target cell membrane. At high concentrations of perforin, osmotic flux through pores leads to cell swelling and necrotic cell death. Perforin can facilitate the uptake of granzyme B through direct diffusion or endocytosis. Granzyme B directly cleaves caspase 3 to induce apoptosis or triggers the mitochondrial apoptotic pathway via Bid cleavage into tBid. tBid recruits Bax/Bak leading to mitochondrial outer membrane permeabilization (MOMP) and apoptosis. Granzyme B may also degrade Mcl-1 releasing Bim to activate MOMP. Granzyme B can also cleave ICAD contributing to DNA damage, α-tubulin leading to cytoskeletal degradation, or gasdermin E, which forms pores in the cell membrane to induce pyroptosis. (B) Ligation of death receptors (Fas/DR4/DR5) by FasL or TRAIL triggers assembly of the death-inducing signalling complex (DISC) composed of FADD and pro-caspase 8/10. Caspase 8/10 induces apoptosis via direct caspase 3 cleavage or the mitochondrial apoptotic pathway via Bid cleavage. (C) Ligation of TNFR1 by TNF triggers the assembly of complex I (TRADD, RIPK1, TRAF2, cIAP1/2). LUBAC ubiquitinates complex I components leading to pro-survival signalling via NF-κB and MAPK pathways. In the absence of ubiquitination, RIPK1 dissociates and forms complex II with FADD and pro-caspase 8/10. Cleavage of pro-caspase 8/10 triggers apoptosis by the same pathways as FasL/TRAIL. In the presence of insufficient pro-caspase 8, RIPK1 can also recruit RIPK3, which activates MLKL to trigger necroptosis.

Figure 2

Resistance to granule-mediated cytotoxicity. Activation of a cytotoxic effector cell at an immune synapse with a target cell leads to the polarisation and secretion of lytic granules containing perforin and granzyme B. Under normal circumstances, in the absence of any resistance mechanisms, the secreted perforin will form pores in the target cell membrane and allow entry of granzyme B. This process will initiate cell death through both direct cell lysis and the activation of apoptotic pathways. Target cells can employ multiple mechanisms to evade cytotoxicity. (A) Increased plasma membrane lipid order to reduce perforin binding. (B) Externalisation of phosphatidylserine to induce perforin aggregation rather than pore formation. (C) Reduced cell stiffness to prevent efficient perforin pore formation. (D) Expression of Serpin B9 to directly inhibit Granzyme B activity. (E) Autophagy of Granzyme B to prevent activation of apoptotic pathways. (F) Secretion of Cathepsin B to degrade perforin. (G) Reduced gasdermin B and E expression or IpaH7.8-mediated ubiquitination and degradation of gasdermin B can reduce pyroptosis or lysis of shigella, respectively.

Figure 3

Inhibiting death receptor-mediated killing. Death receptor-mediated killing is a critical method of lymphocyte cytotoxicity. However, target cells have developed multiple mechanisms to inhibit the effectiveness of these processes. (A) Expression of decoy receptors, including membrane bound decoy receptors 1 and 2 that lack functional death domains, thereby preventing signalling while sequestering TRAIL. Decoy receptor 2 also inhibits death receptor 5, preventing death receptor 4 recruitment and DISC formation. Decoy receptor 3 is soluble and binds to FasL preventing it acting upon target cell Fas. (B) Autophagy inhibits FADD-dependent caspase-8 activation. (C) Decreased expression of death receptors, such as Fas, DR4 and DR5 inhibits apoptotic pathways. (D) Increased expression of cFLIP sequesters pro-caspase 8 into heterodimers to prevent its cleavage to caspase 8 and the subsequent activation of apoptotic pathways.