Does Infection-Induced Immune Activation Contribute to Dementia?

The central nervous system (CNS) is protected by a complex blood-brain barrier system; however, a broad diversity of virus, bacteria, fungi, and protozoa can gain access and cause illness. As pathogens replicate, they release molecules that can be recognized by innate immune cells. These molecules are pathogen-associated molecular patterns (PAMP) and they are identified by pattern-recognition receptors (PRR) expressed on antigen-presenting cells. Examples of PRR include toll-like receptors (TLR), receptors for advanced glycation endproducts (RAGE), nucleotide binding oligomerisation domain (NOD)-like receptors (NLR), c-type lectin receptors (CLR), RIG-I-like receptors (RLR), and intra-cytosolic DNA sensors. The reciprocal action between PAMP and PRR triggers the release of inflammatory mediators that regulate the elimination of invasive pathogens. Damage-associated molecular patterns (DAMP) are endogenous constituents released from damaged cells that also have the ability to activate the innate immune response. An increase of RAGE expression levels on neurons, astrocytes, microglia, and endothelial cells could be responsible for the accumulation of αβ-amyloid in dementia and related to the chronic inflammatory state that is found in neurodegenerative disorders.

The central nervous system (CNS) is protected by a complex barrier system; however, a broad diversity of virus, bacteria, fungi, and protozoa can gain access and cause illness [1][2]. The blood-brain barrier (BBB) is an important structure consisting of microvascular endothelial cells, astrocytes, and pericytes. This barrier controls the passage of molecules into and out of the brain to preserve the microenvironment of the brain [3]. Pathogenic bacteria can cross the BBB through different mechanisms, including: transcellular traversal (Escherichia coli, Streptococcus pneumoniae, Neisseria meningitidis, Candida sp., and Cryptococcus neoformans); paracellular traversal (Trypanosoma spp. and Borrelia spp.); and Trojan-horse (Listeria monocytogenes and Mycobacterium tuberculosis) [2][3]. Transcellular traversal occurs when the pathogen crosses the BBB without any traces of intracellular tight-junction rupture. In paracellular traversal, the microorganism penetrates the BBB with or without evidence of tight-junction rupture.
In the Trojan-horse mechanism the pathogen crosses endothelial cells by transmigrating in infected phagocytes [2]. Several virus species can directly infect endothelial cells to cross the BBB into the CNS [4][5][6].  Replication of the pathogen occurs concomitantly with the release of their component molecules. These components are all referred to as pathogen-associated molecular patterns (PAMP). PAMP are identified by pattern-recognition receptors (PRR), which are important mechanisms of the innate immune system [7][8]. The host immune response occurs when pathogens are identified by antigen-presenting cells. This identification occurs via the binding of PAMP to PRR on antigen-presenting cells. PPR examples include toll-like receptors (TLR), receptors for advanced glycation endproducts (RAGE), nucleotide binding oligomerisation domain (NOD)-like receptors (NLR), c-type lectin receptors (CLR), RIG-I-like receptors (RLR), and intra-cytosolic DNA sensors [8][9]. The reciprocal action between PAMP and PRR triggers the release of inflammatory mediators that regulate the response necessary to eliminate the invasive pathogens [10]. Innate immune cells are also activated by endogenous constituents that are released from damaged cells, such as high mobility group box-1 protein (HMGB-1], nucleic acids, amyloid β, cytochrome c, uric acid crystals, adenosine 5′-triphosphate (ATP), members of S100 protein family, histones, advanced glycation end products (AGE), and heat shock protein (HSP). These molecules are called damage-associated molecular patterns (DAMP) [11][12]. Thus, damage to CNS during infection commonly involves pathogenic mechanisms and the innate immune host response [13][14].

Recognition of Pathogen Infection by Innate Immune Sensors TLR
The innate immune system identifies pathogens via numerous receptor families. Toll-like receptors (TLR) recognize molecular motifs that are expressed by pathogens or endogenous ligands released from damaged cells [15]. The TLR receptors can be separated into two categories: one group of receptors is expressed on the cell membrane for ligand recognition (TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10) and the other group is localised in intracellular endosomal space for recognition of pathogen nucleic acids (TLR3, TLR7, TLR8, and TLR9) [16], Figure 1.

NOD1 and NOD2 Signalling
NLR are members of a family of cytosolic sensors that are responsible for the detection of different kinds of pathogens and the detection of DAMPs [16]. The NLR intracellular proteins have C-terminal leucine-rich repeat (LRR) domains and central NACHT nucleotide-binding domains (NBD) [22]. NOD1 (CARD4) and NOD2 (CARD15) activate numerous transcription factors to trigger the release of pro-inflammatory mediators [23). Another group of NLR members activates the formation of a multiprotein complex known as the inflammasome. The inflammasome activates procaspase-1 and maturation of the IL-1 cytokines [17].
NOD1 and NOD2 recognize compounds from Grampositive and Gram-negative bacteria and recruit the receptor-interacting protein-2 (RIP2), cellular inhibitors of apoptosis (cIAP2), and X-linked inhibitor of apoptosis protein (XIAP). This cascade provides a link to the TAK1/TAB1/TAB2/TAB3 complex and IKK complex. These complexes phosphorylate IKB, resulting in the release and nuclear translocation of the transcription factor, NF-κB and MAPK, to trigger the transcription of interleukin (IL)-1 cytokines [17,24].

Cytosolic DNA Sensors
Intracellular DNA from virus and bacteria is important to initiate an efficient host immune response. The signaling adaptor protein Stimulator of Interferon Genes (STING) is a necessary innate immune pathway for cytosolic DNA sensing that drives type I IFN production [28]. STING is activated by cyclic dinucleotides generated by cyclic GMP-AMP synthase (cGAS). STING activation leads to activation of the serine/threonine-protein kinase tankbinding kinase 1 (TBK1). TBK1 phosphorylates interferon regulatory factor 3 (IRF3), thereby inducing type I IFN gene transcription. TBK1 also plays an important role in the regulation of the immune response expression of pro-inflammatory cytokines [29][30].

RAGE Receptor
The RAGE receptor is a central signalling molecule of the innate immune system. This receptor is involved in the inflammatory host response and is able to bind a variety of ligands with different properties and origin [31]. RAGE is an important receptor that recognizes AGE, members of S100 protein family, amyloid β and amyloid fibrils, HMGB-1, and β-integrin macrophage 1 antigen (Mac-1) [31][32]. Activated RAGE receptors trigger the Rasextracellular signal-regulated kinase 1/2 (ERK1/2), Cdc42/Rac, stress-activated protein kinase/c-Jun-NH2terminal kinase (SAPK/JNK), and p38 mitogen-activated protein (MAP) kinase pathways. These cascades result in the activation of the transcription factor NF-κB [33][34]. NF-κB then triggers gene transcription and the release of TNF-α, IL-1β, IL-6, M-CSF, and S100B. Secreted S100B and chronic RAGE activation contribute to microglial activation and increased oxidative stress. By these mechanisms, up-regulation NF-κB [35] leads to neurite degeneration and neuronal apoptosis that results in memory impairment [33], Figure 1.

Infectious Disease and Dementia
Chronic activation of microglia and astrocytes potentially cause injury to the BBB and neuronal impairment through the release of cytokines, chemokines, reactive oxygen species, DAMPs, and pro-inflammatory mediators. The exacerbated host immune response leads to perturbations in neuronal function and extensive brain degeneration. An important characteristic of dementia is its association with dysfunctions of cognitive memory such as calculation, space orientation, and speech impairment [36]. Systemic inflammation and neuroinflammation are emerging as important targets in cognitive decline in the aged and susceptible brain [37]. Sepsis-associated delirium is a clinical indication that there is CNS involvement in sepsis [38]. Severe sepsis can result in significant brain damage that is discernible when the patients present with delirium. A longer duration of delirium in the hospital is normally associated with worse global cognition and executive function scores of the patients [39]. The pathophysiology of sepsis-associated delirium involves hypoxemia, glucose dysregulation, systemic inflammatory host response, BBB disruption, abnormal neurotransmitter composition, impaired astrocyte function, and neuronal degeneration [38,40]. Sepsis patients who present elevated levels of TNF-α, IL-1β, and IL-6 in the CSF may face long-term cognitive impairment [41][42]. In another study, septic patients presented higher expression of HMGB1 mRNA and protein in peripheral blood, and the levels of HMGB1 gradually decreased to normal after treatment [43]. Therefore, the endogenous proneurogenic response to sepsis could be triggered by, or involve signals in, pathway activation by amyloid β peptide, HMGB-1, and RAGE/NF-κB [44]. These potential proneurogenic response pathways also contribute to production of inflammatory mediators and neurotoxic compounds in the brain [44] .
In an experimental sepsis model of cecal ligation and puncture (CLP) HMGB-1, IL-1β, and NADPH oxidase levels are responsible for long-term cognitive impairment [45][46]. These findings are interesting because these proinflammatory mediators are agonists of the RAGE receptor and trigger exacerbated pro-inflammatory cascades.
In another sepsis model using lipopolysaccharide (a constituent of the cell wall of Gramnegative bacteria that activates TLR-4), induction of neuroinflammation and increased intracellular accumulation of amyloid precursor protein and amyloid β peptide was observed in transgenic mice [47][48]. Similarly, in the CLP sepsis model, brains from sepsissurvivor animals presented an increase in amyloid β content, a decrease in synaptophysin levels, and cognitive impairment [48].
Despite advances in medical care, severe bacterial meningitis often causes neurological deficits, including dementia, and infection is also associated with long-term cognitve impairment, principally in childhood [49][50]. Bacterial meningitis is accompanied by inflammatory mechanisms that can cause cell injury and release of DAMPs. An important DAMP in this process is HMGB1, Aging and Disease • Volume 6, Number 5, October 2016 346 a nuclear protein that has been detected in the CSF of paediatric patients with bacterial meningitis [51]. When released from cells, HMGB1 is recognized by TLR2, TLR4, and RAGE receptors [52][53]. Pro-inflammatory cytokines produced in the first hours post-infection include TNF-α, IL-1β, and IL-6 [54]. In one meningitis study, animals presented depressive-like-behavior and increased cytokines at ten days after infection. In this period the animals were free of infection, but the brain continued producing TNF-α [55].The increase of these cytokines has been demonstrated in both patients and in rodent meningitis models. The chronic production of cytokines could be related to long-term cognitive impairment [54,[56][57]. In another meningitis study, rodents receiving adjuvant treatment with antioxidant or immunomodulators had a decrese of TNF-α and a concurrent improvment in memory [58][59][60]. In clinical sepsis-associated delirium, patients with infectious meningitis, particularly mycotic or bacterial meningitis, present high levels of serum amyloid β, when compared to healthy controls [61]. Our hypothesis is that long-term cognitive impairment in sepsis-associated delirium or meningitis is associated with acumulation of amyloid peptide in the brain.

Conclusion
During infection, the severe inflammatory response may lead to neuronal damage and could potentially be detrimental for patients. Many stimuli including PAMPs, DAMPs and pro-inflammatory mediators are responsible for the activation of microglia cells that produce cytokines, chemokines, and reactive oxygen and nitrogen species to eliminate the invading pathogen. We propose that inhibition of the activity of HMGB-1 could prevent longterm cognitive impairment in patients with bacterial meningitis or sepsis.