Beyond Senolytics: immunotherapy targeting senescent cells

Taken from:  Cellular senescence: immunosurveillance and future immunotherapy

"Immunotherapeutic strategies already in development for combating cancer may one day be repurposed for targeting senescent cells for the alleviation of age-related diseases (see Fig. 3). Discussed in detail elsewhere (Burton and Krizhanovsky, 2014; Childs et al., 2015; Muñoz-Espín and Serrano, 2014; van Deursen, 2014), such age-related diseases include diabetes, Alzheimer’s, pulmonary fibrosis, cardiovascular disease and osteoporosis. In addition, identifying further molecular changes associated with senescent cells, especially cell-type specific alterations, would be advantageous for developing therapeutic approaches for targeting senescent cells. However, since senescent cells can also be beneficial, the elimination of acute senescent cells could be problematic. Therefore, the identification of therapeutic targets specific to chronic senescent which are absent in acute senescent cells would be highly desirable.

A recent study identified the expression of CD26/dipeptidyl peptidase 4 (DPP4) on the membrane surface of senescent fibroblasts which have undergone replicative senescence (Kim et al., 2017). This study assessed the potential use of DPP4 as a membrane target for promoting antibody-dependent cell-mediated cytotoxicity (ADCC) against senescent cells. ADCC is a natural mechanism of cell-mediated immune defence involving the activation NK cells by antibodies which has been adapted to promote NK cell-mediated ADCC in cancer immunotherapy (Wang et al., 2015). Kim et al. demonstrated that targeting DPP4 via an ADCC assay in vitro preferentially sensitised senescent, but not proliferating cells to cytotoxicity by NK cells. These finding highlight a potential immunotherapeutic strategy for targeting senescent cells.

As stated previously, one of the mechanisms by which NK cells specifically recognise and kill senescent cells is via the surface expression of NKG2D ligands (Sagiv et al., 2016). Since many tumour cells also express NKG2D ligands, such ligands have been suggested to be a useful target for immunotherapeutic approaches in cancer (Spear et al., 2013), and so could be adapted for senescent cell clearance. For example, the use of engineered immune cells such as chimeric antigen receptor (CAR) T cells to target specific molecules on cancer cells has great potential as an anti-cancer therapy (Yu et al., 2017). As such, it may be possible to target senescent cells by engineering T cells to express a NKG2D CAR which recognise NKG2D ligands on the surface of senescent cells.

An adaption of cancer vaccines could also be considered for boosting immune clearance of senescent cells. Although a universal biomarker of cell senescence has not been identified, the exposure of senescent cell membranes to immune cells may evoke an immune response to antigens not yet identified. In one approach, senescence vaccines would involve the isolation of senescence specific antigens (SSAs) which are then exposed to dendritic cells, professional antigen presenting cells. In response to SSA uptake, dendritic cells process and express these antigens on their cell surface which can then be recognised by T cells. T cell interaction with these antigens promotes T cell activation, differentiation and ultimately killing of target cells.

As discussed previously, the senescence program can also be activated in immune cells, which likely contributes towards impaired immune function, contributing to senescent cell accumulation. In addition to impaired immune cell function, the persistence of senescent immune cells would likely take up valuable immunological space required for expansion of functionally competent immune cells. T cells for example undergo rapid expansion in response to antigenic stimuli followed by cell-mediated apoptosis (Chou and Effros, 2013), a process that may be prevented if senescent T cells are present. As such, another strategy for promoting the elimination of non-immune senescent cells may be the removal of senescent immune cells with the anticipation that such an approach would enhance immune function.

One possibility could be to filter the blood of the unwanted senescent immune cells, maybe through the use of antibodies linked to magnetic nanoparticles/beads (Chen et al., 2017; Rebo et al., 2010). Another strategy may be the use of senolytic drugs to specifically kill senescent immune cells or senescent hematopoietic stem cells (HSCs) which give rise to immune cells. Senolytic targeting of senescent HSCs in mice leading to rejuvenation of aged tissue stem cells has already provided promising results (Chang et al., 2015). Rather than eliminating senescent immune cells, it may one day be possible to rejuvenate senescent immune cells, reversing their detrimental phenotype to improve immune function. For example, inhibition of p38 signalling in CD8+ T cells which exhibit features of cell senescence, increased proliferation, telomerase activity, mitochondria biogenesis and fitness (Henson et al., 2014). Small molecules based on resveratrol have also been reported to rescue cells from cell senescence (Latorre et al., 2017).

The ability of immune cells to migrate and kill target cells can become compromised with age, suggesting that therapeutically boosting these responses may be beneficial for promoting senescent cell clearance. One approach that can be adapted within immune cells is the engineering of cells that recognise specific components of the senescent secretome. As proof of principle, Qudrat et al. generated cells with a chimeric IL-6 receptor (IL6Rchi) that generates a Ca2+ signal in response to IL-6, a component of the senescent secretome (Qudrat et al., 2017). When IL6Rchi was expressed with an engineered Ca2+-activated RhoA (CaRQ), it enabled directed migration to IL-6 in cells which normally have no such function. Furthermore, the expression of vesicular stomatitis virus glycoprotein G (VSVG) and herpes simplex virus type 1 thymidine kinase (TK) in these IL-6 seeking cells allowed cell-cell fusion to occur with target cells and consequently cell death upon administration of ganciclovir, an anti-viral drug."

Killing 'zombie' cells to improve health in old age

 

Dominick Burton, Research Fellow, Aston University

This article was originally published on The Conversation. Read the original article.


 Imagine a world where you could take just a single pill for the treatment or prevention of several age-related diseases. Although still in the realms of science fiction, accumulating scientific data now suggests that despite their biological differences a variety of these diseases share a common cause: senescent cells. This has led scientists to find drugs that can destroy these cells.

When cells become damaged, they either self-destruct (apoptosis) or they lose their ability to grow and remain stuck within the body. These are the non-growing senescent cells that no longer carry out their tasks properly. They spew out chemicals that cause damage to cells nearby, sometimes turning them into “zombies” – hence why they are sometimes referred to as “zombie cells”. Eventually, the damage builds up so much that the function of bodily organs and tissues, such as skin and muscle, becomes impaired. At this point, we identify the changes as disease.

Depending on where these senescent cells gather within the body will determine which disease will develop. Senescent cells have now been shown to be linked to several diseases, including cardiovascular disease, type 2 diabetes, osteoarthritis and cancer.

In 2011 and in 2016, researchers at the Mayo Clinic in the US showed, through the use of genetically engineered (transgenic) mice, that the removal of senescent cells reduced cancer formation, delayed ageing and protected the mice against age-related diseases. The mice also lived 25% longer, on average. A similar result in humans would mean an increase in life expectancy from 80 years to 100 years. It was proof-of-principle studies like these that laid the groundwork and inspired other researchers to build on these findings.

           
  I’ll live how much longer? Kirill Kurashov/Shutterstock.com
         

Killing a few to save the many

It is not known how many senescent cells need to be present to cause damage to the body, but the harmful effects of the chemicals they release can spread quickly. A few zombie cells may have a huge impact. Drugs for specifically killing senescent cells in order to extinguish their destructive force have recently been revealed and tested on mice. The collective term for these drugs is “senolytics”.

In 2016, two research groups independently published findings on the discovery of two new senolytic drugs which target proteins responsible for protecting senescent cells from cell death. Research lead by scientists from the University of Arkansas, US, showed that the drug ABT-263 (Navitoclax) could selectively kill senescent cells in mice, making aged tissues young again. And scientists from the Weizmann Institute of Science in Israel used the drug ABT-737 to kill senescent cells in the lungs and skin of mice.

There has also been a lot of interest in the role of senescent cells in pulmonary diseases caused by damage to the lungs. Among the risk factors, smoking is known to speed up lung ageing and disease, partly by attacking healthy cells with toxic chemicals from cigarette smoke which can result in cells becoming senescent.

In late 2016, Japanese scientists showed that the removal of senescent cells using genetically engineered mice greatly restored lung function in old mice. A more recent study, lead by scientists at the Mayo Clinic in the US, showed that idiopathic pulmonary fibrosis (scarring of the lungs) was linked to an increase in the number of senescent cells and the damaging effects of the chemicals they release. The killing of senescent cells using genetically engineered mice again greatly improved lung function. In the same study, this group also reported the possible use of a combination of drugs, dasatinib and quercetin, to destroy senescent cells.

A study published earlier this month from the University of Arkansas, extended their previous findings on the drug ABT-263 to pulmonary fibrosis. They found that ABT-263 treatment reduced the problems caused by senescent cells and reversed the disease in mice.

There’s money in senolytics

In light of these accumulating and highly promising findings, a number of start-up biotechnology companies have been created to exploit the health benefits of targeting senescent cells.  Probably the most well funded is Unity Biotechnology in the US which raised US$116m for research and development.

It will likely be several years before we see senolytic drugs being tested on humans. If you can’t wait that long, exercise may be the answer. A study published in March 2016 by the Mayo Clinic showed that exercise prevented the accumulation of senescent cells caused by a high-fat diet in mice. So if the regular health benefits of exercise were not enough to get you off the sofa, maybe the anti-ageing benefits will be.

Original Article: The Conversation

Senescent cells communicate via intercellular protein transfer

Abstract

Mammalian cells mostly rely on extracellular molecules to transfer signals to other cells. However, in stress conditions, more robust mechanisms might be necessary to facilitate cell–cell communications. Cellular senescence, a stress response associated with permanent exit from the cell cycle and the development of an immunogenic phenotype, limits both tumorigenesis and tissue damage. Paradoxically, the long-term presence of senescent cells can promote tissue damage and aging within their microenvironment. Soluble factors secreted from senescent cells mediate some of these cell-nonautonomous effects. However, it is unknown whether senescent cells impact neighboring cells by other mechanisms. Here we show that senescent cells directly transfer proteins to neighboring cells and that this process facilitates immune surveillance of senescent cells by natural killer (NK) cells. We found that transfer of proteins to NK and T cells is increased in the murine preneoplastic pancreas, a site where senescent cells are present in vivo. Proteomic analysis and functional studies of the transferred proteins revealed that the transfer is strictly dependent on cell–cell contact and CDC42-regulated actin polymerization and is mediated at least partially by cytoplasmic bridges. These findings reveal a novel mode of intercellular communication by which senescent cells regulate their immune surveillance and might impact tumorigenesis and tissue aging.

http://genesdev.cshlp.org/content/early/2015/04/03/gad.259341.115.abstract

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The senescent phenotype and promiscuous gene expression

Senescent cells are often associated with changes in gene expression that appear to occur independent of the regulated gene expression linked to aspects of the senescent phenotype such as cell cycle arrest, the secretory response and apoptosis resistance. This phenomenon has been termed promiscuous gene expression (pGE) (Burton and Krizhanovsky, 2014) and can be more specifically defined as gene expression that is uncoupled from tissue or developmental regulation. 

pGE can be observed in microarray analysis by comparing the gene expression profiles of different senescent cell types and lines. Zhang et al (2003) has demonstrated that the up-regulation of genes in senescent fibroblasts was associated with gene clustering (150 of the 376 gene up-regulated), whereas the down-regulation of genes (313) was not. 48.1% of the up-regulated genes were designated as membrane-associated proteins, 10.5% related to apoptosis and 15.8% to transport, whereas 17.9% of the down-regulated genes are involved in cell cycle regulation. Gene expression changes in senescent human mammary epithelial cells (HMECs) were shown to be drastically different than that of the fibroblasts, despite both undergoing senescence induced by telomere attrition. Only five genes up-regulated and seven genes down-regulated in HMECs showed similar regulation in fibroblasts. However, like senescent fibroblasts, HMECs also demonstrated gene clustering associated with up-regulated genes only. Zhang et al postulated at the time, that if senescence is a response to DNA damage, then the observed differences in gene expression between senescent fibroblasts and HMECs imply that the effects of DNA damage must vary according to cell type and line. This study also suggested that processes occurring during senescence may lead to localized alteration in chromatin and the consequent up-regulation of groups of genes within “opened” domains. 

Shelton et al (1999) also demonstrated that senescence-mediated gene expression between different cell lineages varies greatly. BJ fibroblasts, HUVECs and retinal pigment epithelial cells (RPE340) that underwent replicative senescence demonstrated substantial variation in gene expression. A genomic comparison of three different senescent fibroblasts strains also demonstrated significant differences in gene expression, but also shared trends were apparent. If indeed pGE is uncoupled from tissue or developmental regulation, then stochastic processes that alter chromatin structure could be at play and the different response between cell types and cell strains could reflect differences in cell-specific chromatin architecture important for cell-specific gene expression. Elevated levels of oxidative stress, a feature of senescent cells could be one such stochastic process. 

Bahar et al demonstrated that although gene expression levels varied among cardiomyocytes taken from hearts of young mice, the heterogeneity is elevated with age (Bahar et al. 2006). This increased stochastic gene expression with age was suggested to be the result of genomic damage, as mouse embryonic fibroblasts treated with hydrogen peroxide in culture resulted in significant cell-cell variation in gene expression in conjunction with these cells showing morphological signs of cellular senescence (Bahar et al. 2006). 

So how could DNA damage induced by oxidative stress result in stochastic changes in gene expression? When cells sustain DNA damage, chromatin undergoes remodeling to facilitate DNA repair (Price and D’Andrea, 2013, House et al. 2014). This remodeling or “opening” of tightly packed DNA could allow transcription factors access to previously inaccessible genes. Therefore, persistent DNA damage and consequently continuous chromatin remodeling may facilitate pGE. While the induction of DNA damage is likely a stochastic process, the sites of DNA damage may not be completely random, as certain areas of the genome may be more or less prone to genomic insults (Ma et al. 2012). The clustering phenomenon reported by Zhang et al may be the result of these DNA damage prone sites (Zhang et al. 2003). If this were indeed the case, while there may be substantial differences in gene expression at a cell-cell comparison, an overall comparison between cell cultures would likely demonstrate consistent gene alterations resulting from an average expression of all cells within a culture. 

In addition to oxidative stress, a number of other possible mechanisms may exist for generating pGE. Senescent fibroblasts are known to undergo methylation changes (Cruickshanks et al. 2013) and these alterations may lead to epigenetic alterations that promote stochastic changes in gene expression. Alternatively, it has been suggested that DNA damage may modulate gene expression by altering the binding capacity of transcription factors (Rose et al. 2012). 

Interestingly, the reprogramming of fibroblasts into induced pluripotent stem cells (iPSCs) via the addition of OCT4, SOX2, KLF4 and MYC (OSKM) requires a long stochastic phase of gene activation associated with changes in histone modifications at somatic genes and activation of DNA repair and RNA processing (Buganim et al. 2013). This stochastic gene expression may be the result of “promiscuous binding” by OCT4, SOX2 and KLF4, where they occupy accessible chromatin and bind to promoters of genes that are active or repressed (Buganim et al. 2013). It is possible that pGE in senescent cells partly mimics stochastic gene activation associated with cellular reprogramming. However, whether pGE in senescent cells is associated with factors that can undergo “promiscuous binding” has yet to be determined. 

Whether pGE plays a functional role in cell senescence has yet to be determined. However, it can be speculated that pGE may function to generate an array of tissue-restricted proteins that can subsequently be processed into peptides by autophagic proteases for presentation on MHC molecules (Dengjel et al. 2005). Similar to the presentation of tumour-associated antigens (Reuschenbach et al. 2009), senescent cells may also present antigens that can be recognized by immune cells, thereby becoming antigen-presenting cells (APCs). Although the up-regulation of MHC molecules on senescent cells have yet to be fully evaluated, the up-regulation of MHC class I but not MHC class II in response to DNA damage in fibroblasts has been reported (Tang et al. 2014). It remains to be determined whether pGE is a component of immunogenic conversion.

Taken from: Cellular Senescence: From Growth Arrest to Immunogenic Conversion  

Atypical senescent states: Experimental induction of cyclin-dependent kinase inhibitors (e.g. p16, p21)

For many researchers, irreversible cell cycle arrest is the canonical trait of senescent cells.   Such growth arrest can be induced experimentally by the up-regulation or over-expression of cyclin dependent kinase inhibitors (CDKi).  Thus valuable models are, at least potentially, available in which to study the physiological effect of growth arrest distinct from the DDR or any other upstream response.   Unfortunately there has been little characterization of the phenotype of cells rendered ‘senescent’ by this means.

Blagosklonny and co-workers (Korotchkina et al. 2009) used an isopropyl-thio-galactosidase (IPTG)-inducible p21 expression construct to induce a senescence-like state in an HT1080-derived cell line (HT-p21-9).   Characterisation of the phenotype of these cells does not appear to have been attempted beyond observing irreversible growth arrest and the presence of increased SA-β-Gal activity.  Given that HT1080 is a highly tumorigenic fibrosarcoma carrying an activated N-ras oncogene (Benedict et al. 1984), it probably represents a poor genetic background in which to assess whether markers of immunogenic conversion or resistance to cell death can be induced by CDKi overexpression alone.  However, the basic principle of using such a construct for that purpose is sound.

Tokarsky-Amiel et al (2013) showed that overexpression of p14^ARF in the epidermis of the skin of mice (using a tetracyclin-inducible construct) resulted in mass apoptosis and cell cycle arrest.  As measured by SA-β-Gal activity, the p14^ARF transgene drove senescence in up to 8% of the surviving cells in the epithelium by a p53-dependent mechanism (demonstrated by ablation of p53 through co-expression of a specific shRNA directed against it).  These senescent cells were viable within the epidermis for several weeks consistent with lack of clearance.  Unfortunately, minimal analysis of their phenotype was conducted (beyond assessment of the message levels for the senescence-associated genes Pai-1 and Dcr2).  Thus, the immune state of the p14^ARF-senescent cells is currently unclear and the picture is complicated by the fact that senescent rodent cells do not display a senescent secretome under some conditions.  However, given that alopecia and follical stem cell dysfunction were observed in the animals, it is clear that cells rendered ‘senescent’ in this manner can exert phenotypic effects.  Thus, there is some evidence that cell cycle arrest alone may be sufficient to cause problems in highly mitotic tissues such as the epidermis, but large amounts of work remain to be done.  

CDKi overexpression systems clearly have the potential to be valuable tools.  However the extent to which these are physiologically reflective can legitimately be challenged.  This can be understood in two ways (i) the mechanism by which the growth arrest is induced has not been reported in vivo and (ii) cells do not become senescent en mass but gradually as a result of tissue turnover throughout life.  Thus, findings made with these systems could be considered ‘artefactual’

By way of addressing these concerns, it is worth remembering that for many years replicative senescence was dismissed as a ‘tissue culture artefact’ because senescent cells had not been observed in vivo (evidence for their existence in tissue remained severely limited until the late 1990s).  By the same token, elevation of CDKi alone in cells in vivo is not impossible.  Absence of evidence is never evidence of absence.   Similarly, many over-expression systems model systems can be said to be non-physiological.  However, valuable data is routinely gathered using them and in this instance could allow researchers to gage the maximum physiological impact that irreversible growth arrest can have on tissue function.  Thus, if these limits are recognized, such models are potentially utile, especially when combined with detailed analysis of phenotypes known to exist in other ‘senescent cells’ (e.g. apoptosis resistance, immune ligand presentation and the secretory response) 

Taken from: Cellular Senescence: From Growth Arrest to Immunogenic Conversion

Atypical senescent states: Endoplasmic Reticulum stress induced senescence

Endoplasmic reticulum (ER) stress may also promote a senescent-like response.  The accumulation of unfolded proteins in the ER triggers a stress-signaling pathway that can result in cell cycle arrest mediated by p27 (Han et al. 2013) and the p53/47 isoform (Bourougaa et al. 2010).  Furthermore, ER stress has also been shown to induce an inflammatory response via NFkB activation (Garg et al. 2012) and induce cytokines such as MCP-1, IL-6 and IL-8 (Schroder, 2008), which are capable of attracting and activating immune cells (Sagiv and Krizhanovsky, 2013). ER stress has also been shown to promote cell survival, another feature of cell senescence (Raciti et al. 2012).  Interestingly, a senescent state via activation of ER stress-dependent p21 signaling has been reported in proximal tubular epithelial cells, triggered by receptors for advanced glycation end-products (RAGE) (Liu et al. 2014).  Although, ER stress-induced senescence has the potential induce an immunogenic phenotype in the absence of DNA damage, a full evaluation of the phenotype is required to determine if this is so.

Atypical senescent states: Metabolic stress-induced senescence

Metabolic stress, defined here as a combination of aerobic glycolysis and mitochondria dysfunction can potentially trigger a senescent state.  All organisms that use aerobic glycolysis form reactive acyclic α-oxoaldehydes (e.g. methylglyoxal and glyoxal) spontaneously from triosephosphates and by a wide variety of other routes (Thornalley, 2009).  These dicarbonyl compounds are highly reactive and damage proteins through non-enzymatic modification producing a wide variety of covalent adducts (AGEs).  Elevated levels of methylglyoxal and glyoxal are known to be cytotoxic and although the mechanism of action remains imprecisely defined, it can be blocked by ROS scavengers, suggesting that oxidative stress mediates at least some of the deleterious effects (Shangari and O’Brian, 2004).

Cytosolic and mitochondrial protection from dicarbonly damage is primarily mediated through the action of the glyoxalase system that consists of two enzymes, glyoxalase I and II.  However, in cultures of WI38 fibroblasts a significant reduction in the activity of glyoxalase-I occurs with serial passage (Ahmed et al. 2010).  Treatment of cultures of ASF2 human adult dermal fibroblasts with micro or millimolar concentrations of glyoxal or methylglyoxal renders them senescent within 72 hours.  This was defined by the presence of typical senescent morphology, irreversible growth arrest and increased SA-β-Gal activity (Sejersen & Rattan, 2009).  Further studies (Larsen et al. 2012) extended these observations to immortalized human mesenchymal stem cells (MSCs) and demonstrated that treatment with physiologically reflective (Han et al. 2007) concentrations of glyoxal for 72 hours led to senescence without significant cell death (although massive cell death occurred at higher glyoxal concentrations).  Elevated levels of SA-β-Gal, p16 and DNA damage (as measured by COMET) accompanied the growth arrest.  Interestingly, a profound reduction in the ability of these senescent MSCs to differentiate into functional osteoblasts (as determined by alkaline phosphatase and mineralization assays) was also observed.   Given the imbalances in glucose metabolism that accompany mammalian ageing (and diabetes), the authors proposed that this type of metabolic stress might underlie age-related changes in bone function.   Unfortunately, no markers of immunogenic conversion have yet been measured in this system and whilst the presence of DNA damage could indicate the likelihood of a secretory response, this cannot be assumed.  Thus, the propensity of senescence human MSCs to be cleared by the immune system remains unknown and is of considerable physiological significance.

Taken from: Cellular Senescence: From Growth Arrest to Immunogenic Conversion

Immune Ligand Expression in Senescent Cells

In addition to secreting soluble factors for the attraction of immune cells, senescent cells can also become immunogenic through the up-regulation of ligands that can specifically be recognized by immune cells.  While research into the recognition and interaction of immune cells with senescent cells is at its infancy, a number of studies have reported the up-regulation of the Natural Killer Group 2D (NKG2D) ligands in senescent cells that can be recognized by receptors on Natural Killer (NK) cells and CD8+ T-cells.  Since NKG2D ligands are not widely expressed on healthy cells, this would allow for specific recognition, interaction and elimination of senescent cells by immune cells.  As with the senescent secretome, this response is likely not exclusive to cell senescence as the same mechanism functions in immunosurveillance of tumour cells (López-Soto et al. 2014).  The human NKG2D ligands primarily consist of MICA, MICB, ULBP1, ULBP2, ULBP3, ULBP4, ULBP5 and ULBP6.  The transcriptional up-regulation of MICA and ULBP2 during cell senescence have been reported in senescent activated hepatic stellate cells, replicative senescent fibroblasts and HUVECs, etoposide-induced senescent fibroblasts, fusion-induced senescent fibroblasts and chemotherapy-induced senescent multiple myeloma cells (Krizhanovsky, et al. 2008, Kim et al. 2008 Chuprin et al. 2013, Soriani et al. 2014, Lackner et al, 2014).  In addition to MICA and ULBP2, microarray analysis of replicative senescent fibroblasts demonstrated an increase in the expression of ULBP1 (2.75 fold) compared to growing cells, in addition to the up-regulation of HLA-E (2 fold) (Lackner et al. 2014).  HLA-E is a non-classical MHC class I molecule that plays a role in cell recognition by NK cells. However, replicative senescent vascular smooth muscle cells do not appear to up-regulate MICA, ULBP2 or ULBP1, at least not greater than 2 fold as assessed by microarray analysis (Burton et al. 2009).  Therefore, it should not be assumed that all senescent cell types up regulate NKG2D ligands and this should be evaluated in underexplored senescent cell types. Mechanisms involved in the interaction of senescent cells with T-cells is less understood, but it appears that major histocompatibility complex class II (MHCII) expression is required for killing of pre-malignant senescent hepatocytes by T-cells (Kang et al. 2011).  Mice with liver specific MHCII deficiency resulted in impaired immunosurveillance of senescent cells.

At the mechanistic level, little is currently known about the regulation of NKG2D ligand expression in senescent cells.  Nonetheless, some extrapolation from others models is possible.  For example, MICA and MICB have been reported to be regulated by endogenous miRNAs in tumours and as a result of infection with cytomegalovirus (Stern-Ginossar et al. 2008).  Since miRNAs appear to play a role in regulating cellular senescence (Feliciano et al. 2011, Liu et al. 2012 Benhamad et al. 2012) and their expression is altered in response to DNA damage (Dolezalova et al. 2012, Wang and Taniguchi, 2013), it is possible that changes in miRNA expression also regulate the expression of immune ligands in senescent cells. 

Soriani et al demonstrated that the up-regulation of MICA in senescent multiple myeloma cells was dependent upon the DDR (Soriani et al. 2014).   In other systems, NKG2D ligands have also been shown to be up-regulated in response to DNA damage and Ras activation via ATM and ATR (Gasser et al. 2005, Cerboni et al. 2014).  Inhibition of the ATM or ATR pathways prevented the up-regulation of immune ligands. 

It is also possible that the up-regulation of immune ligands on senescent cells is mediated via the secretory response.  In addition to activating and attracting immune cells, the senescent secretome may serve to up-regulate immune ligands in an autocrine or paracrine manner.  It has been shown for example, that TNFα can up-regulate MICA on human endothelial cells and that the addition of exogenous MICA seems to induce senescence in HUVECs (Lin et al. 2011), but the extent to which this occurs under more physiologically reflective situations remains unclear. 

Immune ligands can also be up-regulated in response to various other forms of cell stress such as heat shock, metabolic stress and endoplasmic reticulum (ER) stress (Cerwenka, 2009, Valés-Gómez et al. 2008).  Thus, as with the secretory response, mechanisms exists that can up-regulate immune ligands independent of DNA damage.  Given that this is an important aspect of senescent cell clearance and the number of cell types in which the up-regulation of immune ligands has been shown is limited, a more detailed study of this aspect of immunogenic conversion seems warranted.

While senescent cells are likely eliminated by the immune system during normal physiological processes, it has been speculated that the accumulation of senescent cells with age could be due to inefficient elimination by an ageing immune system (Burton, 2009).  In fact, immune cells may themselves undergo cellular senescence, a process that requires further investigations (Effros et al. 2005, Rajagopalan et al. 2012). As such, induction of cell senescence in immune cells may represent one aspect of immunosenescence, the gradual deterioration of the immune system, which consequently leads to impaired immunosurveillance of non-immune senescent cells.  It can be speculated that impaired immunosurveillance may result from altered expression of surface receptors on immune cells that impair recognition and interaction with target senescent cells (and cancer cells).  In addition, it is possible that aged or senescent immune cells do not respond as efficiently to chemoattractants secreted by senescent cells.  In order to understand the mechanisms associated with age-related changes resulting in impaired immunosurveillance of senescent cells, we must first fully understand the normal processes governing immune clearance of senescent cells.  However, evaluating the hypothesis that aged or senescent immune cells display a reduced capacity to target senescent cells and the physiological impact of this decline can still be assessed.  If this were indeed found to be the case, the rejuvenation of an ageing immune system would represent an attractive approach for promoting health span.

Taken from: Cellular Senescence: From Growth Arrest to Immunogenic Conversion