Historically, the major difference between the adaptive and innate immune systems has been considered the ability of the adaptive immune system to retain memory for specific infectious agents, whilst the innate immune system has been considered neither capable of memory nor specificity {Roitt:1992ti}. This memory function of the adaptive immune system is critical to immunity in vertebrates, often offering lifelong protection against reinfection. Recently however, cells of the innate immune system such as monocytes/macrophages have been discovered capable of long-term functional memory following a single immunostimulatory event, resulting in the dramatically enhanced production of pro-inflammatory molecules in response to a second, very distant, immunostimulatory event. This has been termed “trained innate immunity” or “innate immune memory” (Netea, Quintin, & van der Meer, 2011). In animal models of dysfunctional or absent adaptive immunity, trained innate immune cells can protect against lethal infection with various pathogens. Trained innate immunity is considered by many in the field of immunology to represent a dramatic paradigm shift in our understanding of innate immunity, with important implications for immunotherapeutics (Netea, Latz, Mills, & O'Neill, 2015). Trained innate immunity is distinct from priming. Priming is a well described phenomenon during certain infections or other stimulations in which a secondary stimulation results in an exaggerated innate immune response (Condliffe, Kitchen, & Chilvers, 1998; Netea, 2013). Classically, priming is mediated by cytokines, in particular IFN-y released in response to infection, pre-activating innate immune cells {Schroder:2006jh, Perry:2014hx}. Often, pre-exposure of the cell to IFN-y or another cytokine is necessary to fully activate the cell {Schroder:2006jh}. Typically therefore, primed cells produce little, if any, cytokines ({Ransohoff:2009jy, Wynne:2009cy}, whereas trained cells do. Priming effects also usually rapidly decline upon resolution of the primary infection or stimulation. Whilst priming is short-lived and is often dependent on the co-existence of one stimuli followed by another, training involves long-term changes in innate immune cell function that persist well beyond the primary immunostimulatory event. Thus, following certain infections or stimuli, the functional state of innate immune cells may not return to baseline – in which case the innate immune response is said to have been trained. As a result, the mechanisms underlying innate immune training are distinct from innate immune priming. One of the mechanisms underlying priming is the mobilisation of pre-formed receptors in the cytoplasm to the plasma membrane as a result of a stimulus, resulting in short-term term changes in receptor number or affinity (Condliffe et al., 1998). In contrast, training is underwritten by stable epigenetic changes in a number of immune and metabolic pathways (Netea et al., 2015). This long term memory effect can persist for months, and possibly years, and is not exclusive or specific to the type of primary immune stimulating event (Kleinnijenhuis et al., 2014). Intrestingly, recent discoveries hint at the possibility that innate immune cells may also retain long-term memory for non-infectious challenges, such as stress (Ramirez, Shea, McKim, F, & Sheridan, 2015).
Any long-term training of innate immune cells in response to infectious and non-infectious challenges could have important implications for neuropsychiatric and neurodegenerative disorders. It is well known that infectious and non-infectious immunostimulatory events are powerful risk factors for the development and progression of such diseases.(Berk et al., 2013; Brown & Derkits, 2010; De Chiara et al., 2012; Perry, Cunningham, & Holmes, 2007). This could be due to priming or training (is this correct?) Whilst the contribution of priming of innate immune cells to the pathogenesis of neuropsychiatric and neurodegenerative diseases has been described and reviewed in detail (Perry & Holmes, 2014), to date there has been no discussion of I WOULD SAY LESS IS KNOWN ABOUT, OR SIMILARtrained innate immunity in the behavioural and neuroscience literature. As a result of immunostimulatory events, activated innate immune cells, such as monocytes, macrophages and microglia, can release a host of pro-inflammatory and neurotoxic molecules, driving and exacerbating neuropsychiatric and neurodegenerative disorders (Heneka, Kummer, & Latz, 2014; Jones & Thomsen, 2013). The contribution that innate immune training may make to these disorders may be more significant and complex than priming, given the long-term and heterogeneous nature of training. Here, we focus on recent evidence for innate immune training and contemplate why it may have important implications for neuropsychiatric and neurodegenerative disorders. We focus primarily on monocytes, macrophages and microglia, though there is also extensive evidence for trained immunity in NK cells (Netea et al., 2015), which have themselves been recently hypothesised to play a role in neuropsychiatric and neurodegenerative disorders {Poli:2013cq}.
Innate immunity and neuropsychiatric & neurodegenerative disorders
It is worth first briefly reviewing the relationship between inflammation, innate immunity, and neuropsychiatric and neurodegenerative disorders. There is a substantial body of evidence indicating that neuroinflammation contributes to the development and trajectories of major depressive disorder (MDD), schizophrenia, Alzheimer’s disease (AD) and Parkinson’s disease (Heppner, Ransohoff, & Becher, 2015; S. Najjar, Pearlman, Alper, Najjar, & Devinsky, 2013). In the context of MDD, for example, multiple causal factors, all capable of inducing inflammation, have been proposed, including some direct links as general infections, periodontal disease and atopic disorders (is this a type of infection?), and more indirect links including stress, diet, sedentary lifestyle, obesity, smoking, changes in gut permeability and disturbed sleep (Berk et al., 2013). Some of these factors are also associated with an increased risk for the development and progression of other neuropsychiatric disorders, such as schizophrenia, as well as neurodegenerative disorders (Benros et al., 2011; Sastre, Richardson, Gentleman, & Brooks, 2011). These factors can induce neuroinflammation as a result of the direct activation of immune cells in the central nervous system (CNS) and resulting release of pro-inflammatory molecules, the migration of activated peripheral immune cells into the CNS where they release pro-inflammatory molecules, or the passage of pro-inflammatory molecules produced by activated peripheral immune cells into the CNS. Irrespective of the origin, these pro-inflammatory molecules can directly exert negative effects on neuronal cells. Innate immune cells such as monocytes, macrophages and microglia, either in the CNS or in the periphery, are important sources of pro-inflammatory molecules (Beumer et al., 2012; Perry, Nicoll, & Holmes, 2010), and microglia, the resident innate immune cells of the CNS, are the major mediators of neuroinflammation (Streit, Mrak, & Griffin, 2004). As a result all of these innate immune cells have been implicated in the pathogenesis of neuopsychiatric and neurodegenerative disorders. In the context of AD for example, amyloid-β accumulation in the CNS extracellular space and neuronal injury can induce the M1 pro-inflammatory activation of microglia (Mandrekar & Landreth, 2010; Tang & Le, 2016). The presence of M1 activated microglia is a hallmark of AD, and contribute to disease progression through their release of pro-inflammatory neurotoxic cytokines such as IL-1B, IL-6, TNF-a and INF-y, as well as reactive reactive oxygen species (ROS), reactive nitrogen species (NOS), which capable of causing disruptions in neurogenesis, neuronal excitability, synaptic transmission, synaptic plasticity and neuronal survival (Lull & Block, 2010). In rodent models of neurodegenerative disorders, systemic infections with gram negative bacteria such as Escherichia coli and Salmonella typhimurium can further contribute to disease progression through systemic inflammatory response induced microglial activation and neuroinflammation (Perry et al., 2007). In humans, systemic infections are known to exacerbate cognitive decline and disease progression in AD (Holmes et al., 2003; Starkstein, Jorge, Mizrahi, & Robinson, 2006). M1 activated microglia are also a hallmark of disease in animal models of depression, and are also a feature of other neuropsychiatric disorders (Yirmiya, Rimmerman, & Reshef, 2015). Further, activated peripheral monocytes and macrophages are important drivers of systemic inflammation in animal models of several psychiatric disorders, and can, additionally, infiltrate the CNS and induce an inflammatory response and exacerbate disease (Zheng, Zhang, Wang, & Hao, 2015). A number of studies in humans have shown increased levels of immune activation in blood monocytes from patients with depression and depressive symptoms (Alcocer-Gómez et al., 2014; Carvalho et al., 2014). As a result, blocking innate immune pathways, both centrally and peripherally, has been shown to prevent disease onset and progression in various animal models of neuropsychiatric and neurodegenerative disorders (Soczynska et al., 2012; Zheng et al., 2015). In models of stress or infection induced depression for example, minocycline prevents microglial activation and mitigates neuroinflammation induced decreases in neurogenesis and depressive symptomatology. Blocking microglial activation has also been shown to slow disease progression in animal models of neurodegenerative diseases, both as a result of the native disease process and in response to events that result in systemic inflammation induced neuroinflammation, such as peripheral infection.
Activated innate immune cells, therefore, contribute significantly to the pathogenesis of neuropsychiatric and neurodegenerative disorders through the production of pro-inflammatory neurotoxic molecules. An exagerated production of these molecules, or a lower threshold for their production, as a consequence of trained innate immunity therefore may be a potential factor involved in the development and progression of these disorders and is worth investigating further. Reconsider this previous paragraph –what’s the main point? How can exampkes flow from each other?
Trained innate immunity
The innate immune system and training
The innate immune system recognizes pathogens through receptors called pathogen recognition receptors (PRRs), which are present on the cell surfaces, in the endosome or in the cytoplasm of innate immune cells (Akira, Uematsu, & Takeuchi, 2006). PRRs recognise conserved microbial molecules including lipids, proteins, nucleic acids and carbohydrates. These microbial molecules are known as pathogen-associated molecular patterns (PAMPs) (Akira et al., 2006). A diverse range of PRR types exist, including Toll-like receptors (TLRs), C-type lectin receptors, nucleotide-binding oligomerisation domain-like receptors (NLRs), and retinoic acid-inducible gene I (RIG-I)-helicases (Akira et al., 2006). A specific PRR may be capable of recognising a single or multiple different PAMPs. TLR-4, for example, recognizes lipopolysaccharide (LPS), an endotoxin present in the cell wall of multiple different gram negative bacteria (Lien et al., 2000). Often microorganisms will have multiple PAMPs, which will together stimulate a suite of PRRs. Herpes simplex virus (HSV) glycoproteins activate TLR2 on the surface of macrophages, whilst HSV activates TLR9 in the endosome {KurtJones:2004fp, Krug:2004cx}, for example .
Typically, stimulation of a PRR by a PAMP leads to activation of the cell and the release of various pro-inflammatory cytokines, chemokines, ROS and RNS (Akira et al., 2006). The exact response may vary dependent on the type of PAMP-PRR interaction. However, many different PAMP-PRR interactions activate the same or similar downstream signaling pathways. Activation of a PRR by a PAMP can sometimes lead to an altered cellular immune response upon subsequent reactivation of the same PRR or the activation of a different PRR that feeds into similar signaling pathways. This has been known about for some time in the context of innate immune tolerance (Biswas & Lopez-Collazo, 2009; Martin, 2014). Innate immune tolerance was first described in relation to the immunoparalysis that occurs after Gram-negative sepsis in critically ill patients (Biswas & Lopez-Collazo, 2009). Here, cells of the innate immune system fail to mount an appropriate pro-inflammatory response to re-stimulation by LPS or Gram-negative bacteria following Gram-negative sepsis. Innate immune cells thus, under certain contexts, appear to retain an inhibitory memory for high dose LPS stimulation. Recently howeverwhy however? isn’t this an example?, innate immune cells have been shown to be capable of retaining long term memory for certain PAMPs or pathogens that results in an exaggerated immune response upon subsequent re-stimulation with similar or dissimilar PAMPs or pathogens. This heterogenous pro-inflammatory memory effect, trained innate immunity, appears to be prolonged, lasting sometimes for months, and possibly even years (Netea et al., 2015). Whether tolerance or training occurs is dependent on a number of factors, including the amount as well as the nature of the PAMP or pathogen. For example, whilst high dose LPS results in decreased production of pro-inflammatory molecules upon re-exposure to LPS, low dose LPS exposure results in increase production of pro-inflammatory molecules upon re-exposure to LPS (Ifrim et al., 2014).
Given the diverse array of inflammatory factors that have been identified as etiological factors in neuropsychiatric disorders, including some in early life that are remote from the time of onset of disease in adulthood, trained innate immunity is potentially highly relevant to our understanding of neuropsychiatric and potentially also neurodegenerative disorders. That this memory effect is heterogeneous and long lasting has important implications as it may set the stage for a pro-inflammatory cycle in response to a diverse array of temporally distant immune stimulating events. Table 1 lists some of the main features of trained innate immunity, as well as potential consequences for neuropsychiatric and neurodegenerative disorders.
Training of monocytes and macrophages
Given that the adaptive immune system is highly specific and the long held view that the innate immune system displays no memory for pathogens, we would not expect an enhanced immune response upon re-infection at a distant time with dissimilar organisms. Nor would we expect an enhanced immune response upon re-stimulation in the absence of a functional adaptive immune system. Several studies performed decades ago demonstrated just this though. Mice immunized with Bacillus Calmette–Guérin (BCG) displayed enhanced immunity and protection against lethal infection with non-tuberculous organisms such as Candida albicans and Schistosoma mansoni (TRIBOULEY). T lymphocyte depletion failed to dampen this protective effect, which was subsequently found to be mediated by activated tissue macrophages (van 't Wout, Poell, & van Furth, 1992). Mice previously infected with the fungus Candida albicans also showed enhanced immune responses and increased survival in response to infection with the gram positive bacterium Staphylococcus aureus (Bistoni et al., 1986), an effect which was again found to be dependent on macrophage responses (Bistoni et al., 1988).
The importance of these findings and the implications for innate immune memory were largely ignored, however, until two recent studies replicated and extend these findings to human innate immune cells. Kleinnijenhuis et al (2012) isolated monocytes from adult humans before and two weeks after BCG vaccination, and then stimulated them ex-vivo with BCG, Staphylococcus aureus, Candida albicans, or LPS (Kleinnijenhuis et al., 2012). Post BCG vaccination, monocytes showed dramatically enhanced production (up to 7 fold) of various pro-inflammatory cytokines, including IFN-γ, TNF-α and IL-1β upon homogenous and heterogenous stimulation, relative to monocytes pre BCG vaccination. This up-regulation in pro-inflammatory function was still present 3 months after BCG vaccination, and even one year later (Kleinnijenhuis et al., 2014), and was accompanied by increased expression of various activation markers and PRRs, including CD11b, CD14, and TLR4. Primary stimulation of NOD2, a PRR for mycobacteria, with the NOD2-specific ligand MDP, mimicked the training effects of BCG. Training was prevented when inhibiting NOD2 during primary stimulation, as well as its downstream signaling pathway, Rip2 kinase ref here. Such training effects were also seen in neonatal human monocytes following BCG vaccination (up to 14 fold increases of`?) (Jensen et al., 2015).
Similar training effects were observed following stimulation of purified adult human monocytes in-vitro with either Candida albicans or Β-Glucan (a component of fungal cell walls) for 24 hours (Quintin et al., 2012). Upon a second stimulation with Candida albicans, bacteria, or various PAMPs/PRR ligands up to two weeks later, pre-stimulated monocytes showed significant increases (of up to 10-fold) in TNF-α and IL-6 production relative to naive monocytes. The size of training effects was dependent on the doses of both the primary and secondary stimulation. Again, specifically inhibiting a PRR for Β-Glucan and its downstream signaling pathways during primary stimulation, in this case dectin-1 and Raf-1, prevented training. These training effects were accompanied by dramatic and long lasting changes in gene expression. Genes for multiple PRRs unrelated to Candida/β-glucan were up-regulated, including several TLRs, which might partly explain the heterogenous effect of training. Other up-regulated genes included chemokines and nitric oxide synthases, as well as several histone methyltransferases, suggesting a potential role for epigenetic remodelling in underwriting training.
For some PAMPs, the dose can determine whether training or tolerance is induced, whereas for others, different doses appear to consistently or only induce training. For example, moderate to high doses of a variety of PAMPs and PRR ligands, such as LPS (100 μg/ml), Pam3CSK4 (100 μg/ml), flagellin (10 μg/ml) and Poly(I:C) (100 μg/ml) (which all engage TLRs) induced heterogeneous tolerance in human monocytes (Ifrim et al., 2014). Lower doses (0.1 pg/ml to 1 μg/ml) however, induced heterogeneous training, with up to 3 fold increases in TNF-α protein. In contrast, low to moderate doses of Β-Glucan (1 μg/ml), Tri-DAP (10 μg/ml) and MDP (10 μg/ml) (which engage NODs), all induced training (up to 5 fold increases in TNF-α). Is there an explanation for this? Add- If none, can you speculate?
Training of microglia
Fewer studies have investigated the long-term up-regulation of microglial function as a result of a single immunostimulatory challenge, and none in fact within the context of training. Nevertheless, there are suggestions that microglia are capable of being trained. Hippocampal microglia isolated from adult mice that had been injected once intraperitonealy with Salmonella typhimurium and then given an intrahippocampal injection of LPS four weeks later showed increased immunoreactivity for the activation markers CD11c and MHCII relative to microglia from LPS exposed only mice (Püntener, Booth, Perry, & Teeling, 2012). Salmonella typhimurium injection alone did not result in an increase in CD11c and MHCII immunoreactivity, suggesting that the increase seen following LPS injection was not simply due to an increase in the frequency of microglia. Neonatal rats injected with Escherichia coli subcutaneously and then given a peripheral injection of LPS in adulthood had increased microglial CD11b gene expression, as well as faster and more prolonged increases in microglial production of IL-1β protein, without there being any differences in microglial frequency (Bilbo, 2005; Bland, Beckley, Watkins, Maier, & Bilbo, 2010). These changes were accompanied by decreased neurogenesis in the hippocampus and memory impairments, which were prevented when Caspase-1 (a protease that prevents the synthesis of IL-1β and IL-18 {Fantuzzi:1999wi}) was administered prior to LPS challenge in adulthood. Although suggestive of training, it is possible however that interactions between the adaptive and the innate immune system were responsible for the observed effects in these studies. To specifically rule out interactions with other cell types, live microglia would have to be isolated and tested in-vitro. Although animals with severe combined immunodeficiency could be used to rule out interactions with the adaptive immune system, this would still leave the possibility that other cell types such as astrocytes and neurons, which are known to have immunomodulatory properties, are responsible for any effects. Microglia isolated from adult rats that had been infected neonatally with Escherichia coli showed increased production of IL-1β mRNA on a per cell basis when exposed to low dose LPS in-vitro (Williamson, Sholar, Mistry, Smith, & Bilbo, 2011). Again these changes were accompanied by memory impairments, which were prevented by administering minocycline prior to adult LPS challenge (which also prevented increases in IL-1β mRNA). Further, microglia isolated from the fetuses of maternal sheep injected with LPS, and then maintained in-vitro for 3 weeks, had significantly higher production of IL-1β (> 4 fold) in response to LPS in-vitro (Cao et al., 2015). After 3 weeks of in-vivo LPS exposure and isolation, microglia, LPS/LPS microglia, heme oxygenase (decycling) 1 (HMOX1), which is thought to play an anti-inflammatory role (Ye et al., 2014), strongly down-regulated in LPS/LPS microglia. The other 5? If not, remove mention to six, leave only HMOX
Training of innate immune cells by non-microbial stimuli
Upon cellular damage or stress, host cells release a variety of molecules. Some of these released molecules can activate PRRs and initiate an inflammatory/immune response, much like PAMPs. These endogenous PRR-stimulating non-microbial molecules are known as danger associated molecular patterns (DAMPs) (Seong & Matzinger, 2004). Examples of DAMPs include high mobility group box-1 (HMGB1), S-100 proteins, heat-shock proteins (HSPs), hyaluronan, surfactant protein, IFN-α, uric acid, fibronectin, beta defensin, and cardiolipin, amongst others (Seong & Matzinger, 2004). Many of these DAMPs are oxidized versions of proteins and other molecules present on apoptotic host cells or in cellular debris (Matt, Sharif, Martins, & Knapp, 2014). These molecules are oxidized as a result of cellular damage/death induced ROS generation, resulting in hydrophobic regions being exposed, and so available be recognised by PRRs as DAMPs (Seong & Matzinger, 2004). Many of these endogenous oxidized molecules and hydrophobic regions share molecular identity with microbial PAMPs (Matt et al., 2014; Seong & Matzinger, 2004). Thus, we might expect DAMPs to be also capable of inducing training. Exposure of human monocytes to oxidised low density lipoprotein (LDL) (1-10ug/ml) for 24 hours, but not LDL, for example results in increased protein production (up to 5 fold) of TNF-α, IL-6, IL-8, upon re-stimulation 7 days later with various TLR-4 (e.g. LPS) and TLR-2 agonists (e.g. Pam3Cys) (Bekkering et al., 2014). Blocking these TLR receptors or some of their downstream pathways (extracellular regulated kinase and phosphoinositide 3 kinase ) inhibited the training effects.
The study above is the only study that we are aware that has directly investigated whether DAMPs can induce training. There are however a few studies that provide indirect evidence for DAMP induced training. Sterile traumatic brain injury (TBI) releases a suite of DAMPs (Manson, Thiemermann, & Brohi, 2012), and might therefore be a systemic non-infectious immunostimulatory event capable of inducing microglial training. In humans, TBI is associated with microglial activation on PET decades later (Ramlackhansingh et al., 2011), and is a risk factor for depression (Jorge et al., 2004). Indeed, microglia isolated from mice that had been injected peripherally with LPS one month after TBI showed increased IL-1β and TNF-α mRNA expression relative to microglia from LPS exposed only mice (Fenn, Gensel, Huang, & Popovich, 2014). This heightened microglial activity was accompanied by depressive-like behaviors. Although suggestive, this is not direct evidence for DAMP induced microglial training however, as TBI is a complex multifaceted phenomenon and the microglia were not stimulated ex-vivo, thus raising the possibility that interactions with other cell types may have been responsible for the up-regulation of microglial pro-inflammatory function. Microglia isolated from mice 24 days after repeated social defeat stress (RSD) however had higher IL-1β, IL-6, and TNF-α mRNA expression and IL1-β protein production in response to ex-vivo LPS stimulation relative to non stressed controls (Ramirez et al., 2015). This upregulation in microglial pro-inflammatory function was accompanied by increase social avoidance behavior in RSD mice. Although, whether specifically stress induced release of DAMPs resulted in microglial training was not investigated?. Psychological and/or physical stress resultscheck verb tenses in the release of a variety of DAMPs however both peripherally and centrally, including heat shock protein 72, uric acid and high mobility group box-1 (Fleshner, Campisi, Amiri, & Diamond, 2004; Maslanik et al., 2013; Weber, Frank, Tracey, Watkins, & Maier, 2015). Stressor exposure also releases microbial associated molecular patterns (MAMPs) from the gut microbiota into the blood and/or extracellular environment (Fleshner, 2013). These DAMPs are all capable of priming innate immune cells to pathogenic challenges (Edwards, 2005), suggesting that they might also be capable of inducing training. Given the strong association between stress, inflammation and neuropsychiatric illnesses, investigating whether stress induced release of DAMPs may train microglia over the medium to long-term in a homogeneous and heterogenous manner is an intriguing avenue of future study.
Mechanisms underlying training
Several studies have specifically investigated whether training is underwritten by epigenetic remodeling. Human monocyte training as a result of BCG or Candida/β-glucan exposure is results in an increase in H3K4me3, a histone modification reported to be associated with the regulation of immune-related genes, at the promoters of target genes including TNF-α, IL-6, IL-18 (Kleinnijenhuis et al., 2012; Quintin et al., 2012). Consequently, blocking histone methylation, specifically by inhibition of histone methyltransferases using 5′-deoxy-5′ (methylthio) adenosine (MTA), prevented training was this shown? How?. Training of monocytes by non-microbial stimuli, such as oxidized LDL, is also dependentbecause effect was abolished when methylation was blocked? If not, say accompanied or similar on the enrichment of H3K4me3 at the promoters of various immune related genes, with MTA again preventing training (Bekkering et al., 2014). β-glucan training in human monocytes has also been associated with epigenetic remodeling of various metabolic pathways (Cheng et al., 2014; Saeed et al., 2014). β-glucan trained monocytes display reduced oxygen consumption and increased glucose consumption, consistent with a switch from oxidative metabolism to glycolysis. This metabolic shift is characteristic of activated monocytes and macrophages, and, as result, disrupting these changes in cellular metabolism inhibits training. Further, in mice, LPS induced macrophage training results in phosphorylation of the stress-response transcription factor ATF7 (Yoshida et al., 2015). ATF7 normally suppresses a group of genes encoding factors involved in innate immunity in macrophages by recruiting the histone H3K9 dimethyltransferase G9a. Training leads to the release of ATF7 from chromatin and a decrease in repressive histone H3K9me2 marks, and thus increased expression of immune related genes. Interestingly, ATF7 is also phosphorylated as a result of social isolation stress and various other stressors (Maekawa et al., 2009).
Whilst to our knowledge there have been no studies investigating whether epigenetic remodeling is responsible for training effects in microglia, there is evidence that epigenetic remodeling does underly tolerance in microglia. In response to a high dose in-vivo LPS exposure, isolated mouse microglia displayed reduced pro-inflammatory cytokine production upon a secondary LPS stimulation in-vitro (Schaafsma et al., 2015). These effects were mediated by epigenetic remodeling, and associated with a reduction in the levels of H3K4me3 at the promoters of various immune related genes. Interestingly, whilst pro-inflammatory cytokine production was diminished, phagocytic activity and nitric oxide production were enhanced. This study is interesting therefore, not only because it demonstrates that microglia are capable of functional epigenetic remodeling, but also because it shows that the dichotomy between innate immune tolerance vs training is potentially simplistic. Whilst some pro-inflammatory and neurotoxic innate immune cell functions may be down-regulated, others may be simultaneously up-regulated. Since pro-inflammatory cytokines, ROS and RNS all have the potential to be neurotoxic, it will be important to delineate the extent to which production of these different molecules are affected as a result of training
Discussion
There remain a lot of unanswered questions regarding the details of trained innate immunity. We have listed some of these in table 2. Most important, given the association between activated innate immunity, neuroinflammation and disease, is whether trained innate immunity contributes to the onset and progression of neuropsychiatric and neurodegenerative disorders. A key unknown is also the duration for which training is capable of lasting. That training is human monocytes has been detected up to a year post exposure suggests that training effects can be long term. This is an important observation. Early-life immunostimulatory events such as stress and infection are risk factors for the development of neuropsychiatric disorders, and possibly neurodegenerative disorders, in adulthood (Knuesel et al., 2014). These events are associated with long-term changes in the inflammatory environment, that last into adulthood. One way in which these events are thought to contribute to the development of neuropsychiatric illnesses is through the induction or a pro-inflammatory phenotype, leading to neuroinflammation, and the disruption of key processes involved in CNS development throughout childhood and adolescence, especially following a second, distant, immunostimulatory event (Knuesel et al., 2014). These second immunostimulatory events, that result in heightened inflammatory responses, can be completely unrelated in nature to the primary immunostimulatory event, for example, prenatal polyI:C (which activates TLR-3 (Alexopoulou, Holt, Medzhitov, & Flavell, 2001)) followed by peripubertal stress (Giovanoli et al., 2013) or TBI followed by LPS (Fenn et al., 2014)??. However, the mechanism by which such early life events can lead to persistent changes in the neuroinflammatory environment is currently unknown. Trained innate immunity, particularly in microglia, may offer an explanation. Adult microglia arise almost exclusively from a founding population of bone marrow-derived primitive macrophages is this so? during development and are maintained throughout life {Ginhoux:2010fg}. Thus, microglia have the potential to be not replaced following training events, such as after infection or traumatic events in early life for example. Such events early in life may lead to microglia in adolescence and adulthood having a lower threshold for activation and/or the enhanced production of pro-inflammatory and neurotoxic molecules, which are known to cause impairments in neuroplasticity and neurogenesis, upon secondary immunostimulatory events. That secondary immunostimulatory events that are completely unrelated in nature to primary events in early life can result in heightened inflammatory responses cannot be easily explained by adaptive immunity or classical understandings of innate immunity. However, that trained innate immunity is heterogenous in nature, likely because different PAMPs and DAMPs can activate the same PRRs or downstream signaling pathways and result in the epigenetic reprogramming of identical pathways, offers a potential mechanistic explanation.
Furthermore, there are a number of different adult risk factors for the development of neuropsychiatric disordersre-write, particularly for MDD {Berk:2013dj}. Yet, even after accounting for genetic factors, it is not clear why some individuals with these risk factors develop inflammation whilst others don’t, and why some but not others go on to develop MDD. Trained innate immunity might account for some of the variation in how individuals respond to these risk factors. It is possible that the presence of one inflammatory risk factor may train the innate immune system to react more vigorously in response to a second to a inflammatory risk factor, even if distant in time and not temporally co-existent.
Once cells of the innate immune system such as microglia are trained, this could lead to a vicious cycle in which (neuro)inflammation begets (neuro)inflammation. (Neuro)inflammation can result in cell damage and death, and thus the release of numerous DAMPs. As alluded to, DAMPs may themselves be capable of inducing training. Thus, as a result of infection or stress induced (neuro)inflammation and cell injury/death for example, the release of DAMPs may train the innate immune system towards a pro-inflammatory phenotype, even if the initial (neuro)inflammatory insult did not. Upon secondary challenge, at a distant time, the enhanced release of pro-inflammatory and neurotoxic molecules can then result in greater (neuro)inflammation, cell death, and DAMP release, and so on and so on (Fig.1). Indeed, this may explain how prior episodes of depression for example appear to sensitize immune responses to subsequent depressive episodes, with levels of pro-inflammatory cytokines and chemokines increasing with sequential depressive episodes (Celik et al., 2010; Maes, Mihaylova, Kubera, & Ringel, 2012). This is obviously a hypothesis which awaits confirmation, and we recognize that the opposite is also just as plausible and that the release of DAMPs in response to (neuro)inflammation could theoretically result in tolerance. Whether tolerance or training occurs may well be dependent on the nature, doses, duration and frequency of immunostimulation.
Conclusion
We have presented some of the recent evidence for trained innate immunity. Many questions, including the contribution of trained innate immunity to disease onset and progression in neuropsychiatric and neurodegenerative disorders remain to be addressed. The discovery of trained innate immunity has opened up a new dimension in fields of immunotherapeutics such as vaccine development. It is likely that understanding the features and mechanisms of trained innate immunity in greater detail will also have an impact on the development of treatments for a range of disorders in which activation of the innate immune system is implicated, including autoimmune diseases, airway diseases, neuropsychiatric diseases, and neurodegenerative disorders.