Regulated necrosis and innate immunity
Unlike apoptosis, which is considered a less immunogenic variant of PCD,
all forms of regulated necrosis are highly inflammatory
(Pasparakis & Vandenabeele, 2015;
Vanden Berghe et al., 2015). This type of
PCD involves the massive release of intracellular molecules which act as
danger associated molecular patterns (DAMPs), activating the immune
system and amplifying the inflammatory response
(D’Arcy, 2019;
Linkermann et al., 2014). The same
process is triggered during innate immune response to microbes, inducing
the release of pathogen associated molecular patterns (PAMPs) from
infected cells, leading to immune cell recruitment and activation
(Kaczmarek, Vandenabeele, & Krysko,
2013). This concept was introduced by certain studies that demonstrated
that unlike apoptotic cells, necrotic cells are able to release specific
molecules that activate immune cells like macrophages and antigen
presenting cells. For example the research of Basu and colleges in 2000
who demonstrate that necrotic but no apoptotic E.G7 cells release HSPs
such as hsp90, hsp70 and gp96 which can activate macrophages and
dendritic cells through the stimulation of the NF-κB pathway. Although
in this work they do not induce an specific necroptotic process (just a
mimic or a regular necrosis), this was a first approach to the idea that
during oncosis and non-apoptotic cell death the release of these
intracellular factors is a crucial internal signaling to activate the
immune response (Basu, Binder, Suto,
Anderson, & Srivastava, 2000). This has been experimentally
corroborated in the context of inflammatory necroptosis. For example, in
spite of their pleiotropic role in apoptosis-necrosis transition,
inflammosome activation and TNF signaling, the kinases RIPK1 and RIPK3
are actively involved in antiviral and antibacterial response in a
necroptotic-dependent way (reviewed in
(Humphries et al., 2015;
Silke et al., 2015)). It has been
demonstrated in vitro and in vivo that certain viral
entities like cytomegalovirus or herpes simplex virus regulate the
necroptotic response in murine infected cells through the interaction
between viral encoded proteins and the RIP homotypic interaction motif
(RHIM) domains of RIPK1 and RIPK3 (Huang
et al., 2015; Upton, Kaiser, & Mocarski,
2008, 2010). This has also been observed
in human infected cells (Omoto et al.,
2015). However, necroptosis can be also manipulated as a microbial
strategy to impair immune function. In line with this, intracellular
bacteria such as Salmonella enterica are also strong inducers of
RIPK3-dependent necroptosis in infected macrophages
(Robinson et al., 2012). The same
mechanism is used by other pathogenic entities, which induce necroptosis
in immune cells to interfere with its defensive function and amplify the
inflammatory loop (Weinlich, Oberst,
Beere, & Green, 2017). In spite of that, necroptosis is still an
efficient mechanism to eliminate and expose intracellular pathogens.
Some bacteria have even evolved to suppress necroptosis as an infective
mechanism. That is the case of Porphyromonas gingivalis , which is
able to cleave RIPK1 in human infected endothelial cells via a
lysine-specific (Kgp) protease (Madrigal,
Barth, Papadopoulos, & Genco, 2012). The key role of RIP kinases and
necroptosis in the maintenance of an immunocompetent status has been
confirmed in vivo , in RIPK3-knockout mice which are more
susceptible to viral and bacterial infections
(Jorgensen, Rayamajhi, & Miao, 2017;
Kaiser, Upton, & Mocarski, 2013).
Moreover, the role of RIP kinases in innate immunity is beyond
necroptosis induction. RIPK1 and RIPK3 are key regulators of cell final
destination during an infection or immune challenge, but they can also
participate in signal transduction for the downstream activation of
NF-κB acting as adaptor proteins in the signaling cascade of nucleic
acid sensing proteins like the DNA-dependent activator of IFN regulatory
factors (DAI) or Toll-like receptors such as TLR3 and TLR4
(Humphries et al., 2015).
Not only necroptosis but other forms of regulated necrosis are also
implicated in particular and interesting mechanisms of innate immunity.
One of the clearest examples is the form of PCD called
Pyroptosis/Pyronecrosis, which receives this denomination due to its
final and distinctive molecular event: the releasing of pyrogenic
cytokines like IL-1β and IL-1α (Κεππ ετ
αλ., 2010). This special form of PCD occurs in different cell types but
primary in immune cells, mainly macrophages, as a direct response to
pathogens leading to the Caspase 1-dependent processing and release of
IL-1β, a potent inflammation mediator and IL18, which induces
Interferon-γ (IFN-γ) production in TH1 cells, NK cells and cytotoxic T
lymphocytes (CTLs), and also promotes TH2 cell development
(Dinarello, 2009;
B. Liu et al., 2004;
Miao et al., 2011). However, besides of
its function in cytokine maturation and release, pyroptosis itself seems
to be an efficient mechanism to eliminate and release intracellular
bacteria from infected macrophages, exposing them to neutrophil-mediated
clearance trough a molecular mechanism that relies on Caspase 1
activity. This has been demonstrated in vivo for Salmonella
typhimurium, Legionlla pneumophila and Burkholderia
thailandensis in Casp1 knockout mice
(Miao et al., 2011). This form of
programed necrosis has been recently recognized by its role in the
immune response against bacterial infections of the gastrointestinal
tract and even in the control of transformed cells in the context of
gastrointestinal cancer (Zhou & Fang,
2019).
On the other hand, NETosis is another important example of how regulated
necrosis actively mediates microbe killing and immune enhancing. This
cell death modality is known to be induced in specialized immune cells
like neutrophils, leading to the release of the so called
“extracellular traps”, complex structures composed of chromatin
fragments and histones which opsonize and enclose circling pathogens,
limiting their spreading within the body and facilitating their
clearance by phagocytocis (Allam et al.,
2014; Andrade & Darrah, 2013). These
extracellular traps also contain antimicrobial proteins such as, MPO,
NE, Proteinase 3 (PR3), Cathepsin G, Lysozyme and α-defensins
representing a non-phagocytic mode of neutralize and degrade invading
pathogens (Branzk & Papayannopoulos,
2013; Urban et al., 2009) and are able
to stimulate the production of Interferon-α (IFN-α), a strong antiviral
cytokine (Garcia-Romo et al., 2011). In
the past 10 years, this form of PCD has been extensively studied to
elucidate the sequence of its molecular events and its role in immune
response, demonstrating that is actually one of the most specialized
immune mechanism, which can be activated depending on microbe size and
location, and as a strategy to control pathogens which are able to
escape from neutrophil phagocytosis (reviewed in
(Burgener & Schroder, 2020)). The
effectiveness of this antimicrobial defense is evident in some pathogens
such as Group A Streptococcus and Staphylococcus aureus ,
that produce nucleases and DNA binding proteins to impair NET formation
and block NETosis as an active evasion mechanism
(Storisteanu et al., 2017). In the same
way, it has been reported that chronic granulomatouse disease (CGD)
patients are more susceptible to Aspergillus nidulans growth, due
to their deficiency in NADPH oxidase function and consequently in NET
formation (Bianchi et al., 2009). Invasive
aspergillosis is a leading cause of death in CGD patients, suggesting
the importance of NETosis in pathogen control and immunocompetent
status.