Immunological Cytotoxic Factors


The host immune system has evolved a variety of strategies to control pathogens and tumour development. The beneficial outcome is the death of the offending cell (transformed or pathogen‐infected) and consequently the demise of the invading pathogen. However, if uncontrolled, these mechanisms will also contribute to host immunopathology. Many of the mechanisms used by the immune system to eliminate intracellular and extracellular pathogens and transformed cells include not only death mediators associated with cytotoxic lymphocytes, but also soluble proteins such as the complement system and the defensins, which are designed to eliminate extracellular pathogens. Recent evidences suggest that the major cytotoxic mediators of host cell‐mediated immunity may possess noncytotoxic functions such as induction of proinflammatory cytokines or inactivation of proteins involved in the pathogen life cycle.

Key Concepts:

  • The host immune system can kill transformed and infected cells by apoptosis.

  • The pleiotropism of host immune system‐induced cell death is a consequence of evolutionary pressure of tumour/pathogens.

  • Host cell‐mediated immunity use cytotoxic granule exocytosis, death receptors and the release of soluble cytokines to combat cancer and infection.

  • Death receptors are involved in the regulation of cell homoeostasis and prevention of autoimmune disorders.

  • Granule exocytosis possesses noncytotoxic functions like regulation or proinflammatory processes, cardiovascular diseases or inactivation of viruses.

  • Cytotoxic function of granule exocytosis critically depends on perforin.

  • Interferons regulate cell cycle and apoptosis susceptibiltiy of pathogen‐infected or cancer cells.

  • Complement system induces lysis of pathogen or tumour cells and helps to clear apoptotic cells.

  • Antimicrobial peptides eliminate extracellular and intracellular pathogens and induce apoptosis in some tumour cells.

  • Selectivity of antimicrobial peptides for pathogens is based on the lipid composition of membranes.

Keywords: apoptosis; cytotoxic cell granule‐mediated apoptosis; death receptor‐mediated apoptosis; interferon; perforin; granzymes; granulysin; Fas; FasL; TRAIL; TNF; complement membrane attack complex; bactericidal proteins; defensins

Figure 1.

Proapoptotic and antiapoptotic signalling through DR family. Proapoptotic function: ligation of the TNF receptor (TNFR), death receptor 3 (DR3), Fas (CD95) or death receptors 4 and 5 (DR5/DR4) by their respective ligands TNF, TL1A, FasL (CD95L) or TRAIL results in receptor trimerisation. Consequently, the oligomerised death domains of the receptor bind cognate domains of TNFR‐associated death domain (TRADD) and Fas‐associated death domain (FADD). The DED of FADD in turn binds a related domain in the propeptide region of caspase‐8, facilitating transactivation of the enzyme. The active enzyme is able to trigger two different apoptotic pathways depending on the cell type. In type I cells, active caspase‐8 cleaves procaspase‐3, which is fully activated by autocleaving. Active caspase‐3 is able to degrade and/or activate several proteins inducing cell death by apoptosis. In type II cells caspase‐8 cleaves the Bcl‐2 family member Bid generating the active truncated form of the protein (tBid), which translocates to the mitochondria where it activates Bak and Bax. Active Bak/Bax induces cytochrome c release, which in the presence of dATP form a complex with Apaf‐1 and recruit and activates caspase‐9 generating a structure known as apoptosome. Active caspase‐9 induces caspase‐3 activation and apoptosis. In addition, other proapoptotic proteins such as AIF, endonuclease G (EndoG) or the serineprotease HtrA2 are released from the mitochondria by this pathway. Antiapoptotic function: In the case of DR3, DR, DR5 and TNFR, TRADD also binds RIP, which then interacts with TNFR‐associated factor 2 (TRAF2). These later interactions activate the NFκB offering antiapoptotic regulation (survival) of the DRs. DcRs with either a truncated or an absent death domain (DD) act as a ‘sink’ for TRAIL minimising activation of DR4 and DR5. FLIP (FLICE inhibitor protein) is able to block proapoptotic signals by blocking procaspase‐8 recruitment by FADD. Finally, it has been described that in the presence of FLIP a complex known as necrosome is formed which induces necrotic cell death.

Figure 2.

Proposed model for perforin‐mediated intracellular granzymes delivery. Perforin pore model: Perforin (perf) is able to form pores in the membrane of target cells and granzymes (gzms) would enter the cytosol via those pores. Granzyme endocytosis model: granzymes would be endocyted via specific receptors such as mannose‐6‐phosphate receptor (M6PR) or by membrane charge‐dependent interactions and pinocytosis. At the same time, perforin insertion in the cell membrane would induce a cell repairment response that would induce perforin endocytosis. It is not clear if perforin and granzymes would be endocytosed in the same endosome or if granzyme endosomes fuse with perforin endosomes during intracellular traffic. Finally, perforin would disrupt the endosome allowing granzymes to release in the cytosol where they would induce cell death.

Figure 3.

Antitumoural and antibacterial action mechanism of granulysin. (a) Two mechanisms of apoptosis induced by granulysin have been described depending on the presence or absence of perforin. The first studies showed that granulysin‐induced plasma membrane‐associated sphingomyelinase (neutral SMase) activation, degradation of sphingomyelin and generation of ceramide, a lipid responsible of the activation of some proapoptotic pathways. However, ceramide generation was only detected after 8–10 h and granulysin was able to induce apoptosis faster. Later on, it was found that granulysin induced a fast caspase‐independent apoptosis pathway mediated by the release of AIF from the mitochondria. This release was Bcl‐2‐dependent and mediated by the opening of the mitochondrial transition pore by a Ca2+ increase in the cytosol. Cytosolic Ca2+ increase is a consequence of granulysin‐induced disruption of plasma membrane. This mechanism is already evident after 3–4 h. Recently, it has been described that perforin‐delivered intracellular granulysin induces caspase‐7 dependent cell death though activating the ER stress response. (b) Although granulysin is able to directly lyse pathogens, it needs perforin to reach intracellular bacteria.

Figure 4.

Proapoptotic pathways activated by granzyme B. Intracellular granzyme B is able to induce cell death by at least three different pathways. Granzyme B cleaves and activates the effector caspase‐3 and ‐7, which are mandatory for phosphatidylserine translocation and ROS production. On the other side, Bid/Bak/Bax pathway is activated and is crucial for cytochrome c (cyt c) release from the mitochondria and apoptosome formation. Both pathways independently contribute to depolarisation of the mitochondria. Finally, granzyme B induces cell death without apoptotic phenotype by a caspase‐ and Bid‐independent pathway. Intracellular granzyme B is also able to interfere with virus replication by cleaving different proteins involved in virus life cycle without affecting host cell survival. Extracellular granzyme B is able to cleave proteins from the extracellular matrix (ECM) such as filamin, vitronectin or laminin, inducing cell detachment and death by anoikis. Cells die because they need integrin‐mediated prosurvival signalling, which is disrupted when cells are not attached to ECM. Finally, it has been described that extracellular granzyme B contributes to skin ageing and atherosclerosis‐related pathology.

Figure 5.

Mechanism of action of granzyme A. It has been described that intracellular granzyme A is able to induce mitochondrial depolarisation and ROS production by cleaving a protein of the complex I known as NDUFS3. ROS generation is crucial for PS translocation. In addition, ROS production would induce DNA damage and the subsequent activation of DNA‐repairing mechanisms, among them, the translocation of the SET complex from the ER to the nucleus. In the nucleus, granzyme A would degrade some proteins of that complex such as SET, pp32 and Ape1 releasing from the nuclease NM23H1 that would induce DNA damage and cell death. This mechanism has been tested with purified proteins and is not clear. In addition, granzyme A is able to regulate the production of proinflammatory cytokines in macrophages, likely by activating the inflammasome platforms. Extracellular granzyme A is able to induce ECM degradation and cell detachment.

Figure 6.

Mechanism of action of other granzymes. Purified granzymes C, F, H, K and M are able to induce cell death in the presence of perforin by activating diverse intracellular pathways. In addition, it has been shown that both granzymes K and M regulates the production of proinflammatory cytokines in macrophages. It is not known whether, like granzymes A and B, other granzymes are able act extracellularly.



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Martinez-Lostao, Luis, Anel, Alberto, Regner, Matthias, Froelich, Christopher J, and Pardo, Julian(Oct 2013) Immunological Cytotoxic Factors. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000928.pub3]