Cell injury/death leads to Increased pro-inflammatory mediators

Cell death, including both necrosis and apoptosis can lead toward inflammation. Faouzi and colleagues showed that apoptosis can induce hepatic inflammation equally as necrosis (Faouzi et al., 2001). Some studies indicate that phagocytes can produce inflammatory cytokines upon ingestion of apoptotic bodies (Uchimura et al., 1997).

When cells undergo necrosis they lose the integrity of their plasma membrane and release their intracellular contents, into the extracellular space. The same process can occur when apoptotic cells aren't cleared fast enough and their membrane becomes permeable to macromolecules, which presents secondary necrosis (Majno et al., 1995). There is evidence that the immune system has evolved the capacity to detect the release of intracellular molecules which stimulates the generation of adaptive immune responses to dying cells.

Intracellular content of dying cells that triggers immune response when excreted contains molecules named danger associated molecular patterns (DAMPs). DAMPs include for example HMGB-1, IL-1α, uric acid, DNA fragments, mitochondrial content, and ATP (Eigenbrod et al., 2008; Kono et al., 2010a; Sauter et al., 2000). DAMPs can be molecules that have non-inflammatory functions in living cells (such as HMGB-1, ATP) and acquire immunomodulatory properties when released (Rock and Kono, 2008), or alarmins, molecules that have cytokine-like properties (such as IL-1α, IL-6), which are stored in cells and released after cell lysis and contribute to the inflammatory response (Oppenheim and Yang, 2005; Vanden Berghe et al., 2006).

One of the most investigated DAMPs is HMGB-1 (Lotze et al., 2005). HMGB-1 is a nuclear protein that binds to chromatin and has a role in bending DNA and regulating gene transcription (Landsman et al., 1993). HMGB-1 is released by both necrotic and apoptotic cells (Scaffidi et al., 2002; Bell et al., 2006), but also apoptotic cells activate macrophages that engulf them to secrete HMGB-1 (Qin et al., 2006). This protein induces inflammation, dendritic cells maturation, migration, and T-cell activation (Scaffidi et al., 2002; Messmer et al., 2004; Rovere –Querini et al., 2004; Dumitriu et al., 2005; Yang et al., 2007).

HMGB-1 is a stimulus for tumour necrosis factor (TNF) synthesis and release, but it also significantly activates the synthesis of IL-1 α, IL-1 β, IL-1RA, IL-6, IL-8, MIP-1 a, and MIP-1 (Andersson et al., 2000). It was shown that HMGB-1 released from late apoptotic cells remains bound to nucleosomes and that HMGB1-nucleosome complexes activate antigen-presenting cells (APC) and induce secretion of cytokines by macrophages and expression of co-stimulatory molecules in DCs (Urbonaviciute et al., 2008).

HMGB-1 is not the only pro-inflammatory DAMP released from dying cells. Other DAMPs, S100A8/A9 and S100A12 proteins induce pro-inflammatory cytokine production by macrophages (Hofmann et al., 1999; Yang et al., 2001; Viemann et al., 2004; Ehlerman et al., 2006; Pouliot et al., 2008).

The adjuvant activity of cells was reduced by enzymatic depletion of uric acid, indicating that it is a major DAMP, at least in some cells (Shi et al., 2003). Uric acid is a mediator released from necrotic or apoptotic cells that has immunostimulatory properties in vivo (Gordon et al., 1985; Shi et al., 2003). 

Insufficient autophagy of deteriorated mitochondria could lead to massive release of DAMPs such as mtDNA and possibly other mitochondrial proteins (Oka et al., 2012).

Receptors on host cells sense when DAMPs are released and that triggers the inflammatory process. These receptors are pattern-recognition receptors (PRRs) (Chen and Nunez, 2010). PRRs represent proteins by which cells recognize microbial entities, but also some of the host's own molecules and direct an immune response (Piccinini et al., 2010). PRRs can be broadly divided in five subfamilies: Toll-like receptors (TLRs), RIG-1-like receptors (RLRs), NOD like receptors (NLRs), AIM2-like receptors (ALRs) and C-type lectin receptors (CLRs) (Takeuchi and Akira, 2010; Wang et al., 2014). For example, HMGB-1 was reported to stimulate TLR2 and TLR4 (Park et al. 2004) and receptor for advanced glycation end products (RAGE) (Dumitriu et al., 2005), while NLRP3 has been involved in the inflammatory response to mono-sodium urate (MSU) (Martinon et al., 2006). Cellular nucleic acids can stimulate TLR7 and TLR9 on B cells to promote antibody responses (Green and Marshak-Rothstein, 2011; Leadbetter et al., 2002).

TLRs are placed either at the cell surface (TLR1, TLR2, TLR4, TLR5, and TLR6) or in the endolysosomal compartment (TLR3, TLR7, and TLR9) (Barton and Kagan, 2009). Upon binding with the ligand, they undergo a conformational change and initiate a signalling cascade via signal adaptor molecules: myeloid differentiation primary response gene 88 (MyD88), MyD88 adaptor-like protein (MAL, also known as TIR-domain-containing adaptor protein; TIRAP), TIR domain-containing adaptor protein inducing interferon-β (TRIF), and TRIF-related adaptor molecule (TRAM). MyD88 was essential for the inflammatory response to injected dead cells (Chen et al., 2007).

All TLRs, except TLR3, associate with MyD88, and this stimulates a kinase cascade resulting in the activation of mitogen activated protein kinases (MAPKs), c-Jun N-terminal kinases, p38, and extracellular signal–regulated kinases, and nuclear factor NF-kB (Akira and Takeda, 2004; Lee and Kim, 2007). NF-kB is an important transcription factor for IL -1β and NLRP3 (Wang et al., 2004; Bauernfeind et al., 2009).

NF-kB is a central mediator of pro-inflammatory gene induction and functions in both types of immune cells. NF-kB pathway is responsible for transcriptional induction of pro-inflammatory cytokines, chemokines and additional inflammatory mediators, such as NLRP3, pro-IL-1β and pro-IL-18 (Sun et al., 2013; Ghosh and Karin, 2002; Hayden and Ghosh, 2013).

Macrophages must first be ‘primed’ with a stimulus that induces the synthesis of pro-IL -1β and also upregulates the expression of NLRP3 (Bauernfeid et al., 2009; Franchi et al., 2009). The stimuli that can prime macrophages include TLR agonists and cytokines like TNF. When macrophages producing pro-IL -1β are stimulated with ATP or irritant particles, inactive pro-caspase 1 assembles into a molecular complex called the inflammasome and is cleaved into active form (Stutz et al., 2009; Schroder and Tschopp, 2010). Inflammasomes consist of caspase 1, apoptosis-associated speck-like protein containing CARD (ASC) and an NLRP (Schroder and Tschopp, 2010). The catalytically active caspase 1 then cleaves pro-IL-1β to its mature and active form (Stutz et al., 2009).  Macrophages lacking any of the inflammasome components don't make mature IL-1 when stimulated in culture with sterile particles (Hornung et al., 2008; Halle et al., 2008). NF-κB signaling pathway is also involved in the regulation of inflammasome (Guo et al., 2015).

Sometimes substantial sterile inflammatory response can be seen in caspase 1-deficient mice (eg. Chen et al., 2007). This contrasts with the much more marked reduction of these responses that is consistently observed in IL-1β -deficient mice. These data imply that there must be a caspase 1-independent pathway for generating mature IL-1β in vivo (Dinarello, 2009).

In the sterile inflammatory response to cell death, the contribution of TNF appears to be more modest than IL-1 (Rock et al., 2011). A possible explanation might be that the IL-1 is being released from the dying cells themselves (Eigenbrod et al., 2008).

After engulfment of apoptotic bodies, Kupffer cells in liver express TNF, TNF-related apoptosis-inducing ligand (TRAIL), and Fas ligand (FasL) (Canbay et al. 2003), which can induce apoptosis in hepatocytes and further aggravate liver inflammation. Engulfment of apoptotic bodies by macrophages also induces FasL expression (Kiener et al., 1997), which is known to exert a pro-inflammatory activity (Chen et al., 1998).

Human (Andersson et al., 2000; Scaffidi et al., 2002; Bell et al., 2006; Clarke et al., 2010)

Mouse (Faouzi et al., 2001; Chen et al., 2007)