Inflammation (Latin, īnflammō, “I ignite, set alight”) is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants.

Acute activation of innate immune response in the brain, or neuroinflammation, protects this vital organ from a range of external pathogens and promotes healing after traumatic brain injury. However, chronic neuroinflammation leading to the activation of immune cells like microglia and astrocytes cause damage to the nervous tissue, and it is causally linked to a range of neurodegenerative diseases such as Alzheimer's diseases (AD), Multiple Sclerosis (MS), Parkinson's disease (PD), and many others. While neuroinflammation is a key target for a range of neuropathological diseases, there is a lack of effective countermeasures to tackle it, and existing experimental therapies require fairly invasive intracerebral and intrathecal delivery due to difficulty associated with the therapeutic crossover between the blood-brain barrier, making such treatments impractical to treat neuroinflammation long-term. Sharma et al. present the development of an optimal neurotherapeutic using a Nanoligomer Discovery Engine, by screening downregulation of several pro-inflammatory cytokines (e.g., Interleukin-1β or IL-1β, tumor necrosis factor-alpha or TNF-α, TNF receptor 1 or TNFR1, Interleukin 6 or IL-6), inflammasomes (e.g., NLRP1), key transcription factors (e.g., nuclear factor kappa-B or NF-κβ) and their combinations, as upstream regulators and canonical pathway targets, to identify and validate the best-in-class treatment. Using our high-throughput drug discovery, target validation, and lead molecule identification via a bioinformatics and artificial intelligence-based ranking method to design sequence-specific peptide molecules to up- or downregulate gene expression of the targeted gene at will, they used the discovery engine to perturb and identify most effective upstream regulators and canonical pathways for therapeutic intervention to reverse neuroinflammation. The lead neurotherapeutic was a combination of Nanoligomers targeted to NF-κβ (SB.201.17D.8_NF-κβ1) and TNFR1 (SB.201.18D.6_TNFR1), which were identified using in vitro cell-based screening in donor-derived human astrocytes and further validated in vivo using a mouse model of lipopolysaccharide (LPS)-induced neuroinflammation. The combination treatment SB_NI_111 was delivered without any special formulation using a simple intraperitoneal injection of low dose (5 mg/kg) and was found to significantly suppress the expression of LPS-induced neuroinflammation in mouse hippocampus. These results point to the broader applicability of this approach towards the development of therapies for chronic neuroinflammation-linked neurodegenerative diseases, sleep countermeasures, and others, and the potential for further investigation of the lead neurotherapeutic molecule as reversible gene therapy 1).

Inflammatory response plays a vital role in the pathological mechanism of intracerebral hemorrhage. It has been recently reported that neutrophil to lymphocyte ratio (NLR) could represent a novel composite inflammatory marker for predicting the prognosis of intracranial hemorrhage (ICH).

The inflammatory response in the cerebral cortex serves an important role in the progression of secondary injury following traumatic brain injury (TBI).

The primary physical effect of the inflammatory response is for blood circulation to increase around the infected area. In particular, the blood vessels around the site of inflammation dilate, permitting increased blood flow to the area. Gaps appear in the cell walls surrounding the infected area, allowing the larger cells of the blood, i.e. the immune cells, to pass. As a result of the increased blood flow, the immune presence is strengthened. All of the different types of cells that constitute the immune system congregate at the site of inflammation, along with a large supply of proteins, which fuel the immune response. There is an increase in body heat, which can itself have an anti-biotic effect, swinging the balance of chemical reactions in favour of the host. The main symptoms of the inflammatory response are as follows.

The tissues in the area are red and warm, as a result of the large amount of blood reaching the site.

The tissues in the area are swollen, again due to the increased amount of blood and proteins that are present.

The area is painful, due the expansion of tissues, causing mechanical pressure on nerve cells, and also due to the presence of pain mediators.

Once the inflammatory process has begun, it continues until the infection that caused it has been eradicated. Phagocytes continue to consume and destroy bacteria, the acquired immune system binds and disposes of harmful toxins. Pus is produced, pus being the debris that is left over from the battle between the invader and the immune system. The colour of the pus depends on the organism causing the infection.

Chronic inflammation has earlier been detected in ruptured intracranial aneurysms. A study detected both dental bacterial DNA and bacterial-driven inflammation in ruptured intracranial aneurysm walls.

Pathogenic inflammation contributes to aneurysm formation by mediating the destruction of the endothelium and the extracellular matrix and promoting pathogenic proliferation of smooth muscle cells. In mouse models, tolerance-inducing regulatory T cell (Treg) cells could significantly reduce the incidence and severity of aneurysms. Hence, it should be investigated why in human intracranial aneurysm (IA) patients, Treg cells failed to provide protection against aneurysm formation. In this study, the frequency and function of Treg cells in IA patients were examined. The frequency of Foxp3+ Treg cells was significantly lower in IA patients than in healthy controls. This downregulation was only specific to the Treg subset of CD4+ T cells, as the frequency of total CD4+ T cell was increased in IA patients. Subsequently, we found that the expressions of Treg-associated molecules, including Foxp3, CTLA-4, TGF-β, and IL-10, were significantly lower in Foxp3+ Treg cells from IA patients than in Foxp3+ Treg cells from healthy controls. In both healthy controls and IA patients, Foxp3+ Treg cells were distinguished into a more potent Tim-3+ subset and a less potent Tim-3- subset. The Tim-3+ subset of Foxp3+ Treg cells was significantly reduced in IA patients. Signaling via IL-2, IL-7, IL-15 and IL-21 was shown to promote Tim-3 upregulation in CD4+ and CD8+ T cells. Interestingly, we found that Tim-3 could be upregulated in Treg cells via the same mechanism, but compared to the Treg cells from healthy controls, the Treg cells from IA patients presented defects in Tim-3 upregulation upon cytokine stimulation. Together, our results demonstrated that Foxp3+ Treg cells in IA patients presented reduced function, which was associated with a defect in Tim-3 upregulation 2).

Neuroinflammation is inflammation of the nervous tissue. It may be initiated in response to a variety of cues, including infection, traumatic brain injury, toxic metabolites, or autoimmunity.

see Inflammatory response.

In the central nervous system, including the brain and spinal cord, microglia are the resident innate immune cells that are activated in response to these cues.

The CNS is typically an immunologically privileged site because peripheral immune cells are generally blocked by the blood brain barrier (BBB), a specialized structure composed of astrocytes and endothelial cells.

However, circulating peripheral immune cells may surpass a compromised BBB and encounter neurons and glial cells expressing major histocompatibility complex molecules, perpetuating the immune response.

Although the response is initiated to protect the central nervous system from the infectious agent, the effect may be toxic and widespread inflammation as well as further migration of leukocytes through the blood brain barrier.

The onset of aneurysmal subarachnoid hemorrhage (aSAH) elicits activation of the inflammatory cascade, and ongoing neuroinflammation is suspected to contribute to secondary complications, such as vasospasm and delayed cerebral ischemia.

In a review, of Watson et al. analyze the extent literature regarding the relationship between neuroinflammation and cognitive dysfunction after aSAH. Pro-inflammatory cytokines appear to play a role in maintaining normal cognitive function in adults unaffected by aSAH. However, in the setting of aSAH, elevated cytokine levels may correlate with worse neuropsychological outcomes. This seemingly dichotomous relationship between neuroinflammation and cognition suggests that the action of cytokines varies, depending on their physiologic environment. Experimental therapies which suppress the immune response to aSAH appear to have a beneficial effect on cognitive outcomes. However, further studies are necessary to determine the utility of inflammatory mediators as biomarkers of neurocognitive outcomes, as well as their role in the management of aSAH 3).

Neuroinflammation is a crucial factor contributing to neurological injuries after intracerebral hemorrhage (ICH).

Neuroinflammation has been increasingly implicated as a pathological mechanism in dementia and demonstration that it is a key event accelerating cognitive or functional decline would inform novel therapeutic approaches, and may aid diagnosis. Much research has therefore been done to develop technology capable of imaging neuroinflammation in vivo.

The majority of the studies used positron emission tomography (PET) imaging of the TSPO microglial marker and found increased neuroinflammation in at least one neuroanatomical region in dementia patients, most usually Alzheimer's disease, relative to controls, but the published evidence to date does not indicate whether the regional distribution of neuroinflammation differs between dementia types or even whether it is reproducible within a single dementia type between individuals. It is less clear that neuroinflammation is increased relative to controls in mild cognitive impairment than it is for dementia, and therefore it is unclear whether neuroinflammation is part of the pathogenesis in early stages of dementia. Despite its great potential, a review of Stefania et al. demonstrates that imaging of neuroinflammation has not thus far clearly established brain inflammation as an early pathological event. Further studies are required, including those of different dementia subtypes at early stages, and newer, more sensitive, PET imaging probes need to be developed 4).

Alcohol induced neuroinflammation.

SARS-CoV-2-mediated neuroinflammation

Microglia-related neuroinflammation is associated with a variety of neurodegenerative diseases. Flavonoids have demonstrated different pharmacological effects, such as antioxidation, neuroprotection and anti-inflammation However, the effect of flavonoid 6-methoxyflavone (6-MeOF) on microglia-mediated neuroinflammation remain unknown.

A study aimed to study the antineuroinflammatory effects of 6-MeOF in lipopolysaccharide- (LPS-) induced microglia in vitro and in vivo.

Pretreatment of BV2 microglia cells with 6-MeOF for 1 h then stimulated with LPS (100 ng/ml) for 24 h. The expression levels of pro-inflammatory factors, NO and reactive oxygen species (ROS) were performed by the enzyme-linked immunosorbent assay (ELISA), Griess assay and flow cytometry. Western blotting was used to assess MAPK, NF-κB signal transducer and antioxidant enzymes-related proteins. Analysis of ROS and microglial morphology was confirmed in the zebrafish and mice brain, respectively.

The results demonstrated that 6-MeOF dose-dependently prevent cell death and decreased the levels of pro-inflammatory mediators in LPS-stimulated BV2 microglia cells. Phosphorylated NF-κB/IκB and TLR4/MyD88/p38 MAPK/JNK proteins after exposure to 6-MeOF was suppressed in LPS-activated BV-2 microglial cells. 6-MeOF also presented antioxidant activity by reduction of NO, ROS, iNOS and COX-2 and the induction of the level of HO-1 and NQO1 expressions in LPS-activated BV2 microglial cells. Furthermore, we demonstrated that 6-MeOF inhibited LPS-induced NO generation in an experimental zebrafish model and prevent the LPS-induced microgliosis in the prefrontal cortex and substantia nigra of mice.

These results explored that 6-MeOF possesses potential as anti-inflammatory and anti-oxidant agents against microglia-associated neuroinflammatory disorders 5).

Sharma S, Borski C, Hanson J, Garcia MA, Link CD, Hoeffer C, Chatterjee A, Nagpal P. Identifying an Optimal Neuroinflammation Treatment Using a Nanoligomer Discovery Engine. ACS Chem Neurosci. 2022 Nov 21. doi: 10.1021/acschemneuro.2c00365. Epub ahead of print. PMID: 36410860.
Zhang HF, Liang GB, Zhao MG, Zhao GF, Luo YH. Patients with intracranial aneurysms presented defects in regulatory T cells, which were associated with impairment in Tim-3 upregulation. Int Immunopharmacol. 2018 Sep 19;64:350-355. doi: 10.1016/j.intimp.2018.09.020. [Epub ahead of print] PubMed PMID: 30243071.
Watson E, Ding D, Khattar NK, Everhart DE, James RF. Neurocognitive outcomes after aneurysmal subarachnoid hemorrhage: Identifying inflammatory biomarkers. J Neurol Sci. 2018 Jun 25;394:84-93. doi: 10.1016/j.jns.2018.06.021. [Epub ahead of print] Review. PubMed PMID: 30240942.
Stefaniak J, O'Brien J. Imaging of neuroinflammation in dementia: a review. J Neurol Neurosurg Psychiatry. 2016 Jan;87(1):21-8. doi: 10.1136/jnnp-2015-311336. Epub 2015 Sep 18. Review. PubMed PMID: 26384512.
Chen WF, Shih YH, Liu HC, Cheng CI, Chang CI, Chen CY, Lin IP, Lin MY, Lee CH. 6-methoxyflavone suppresses neuroinflammation in lipopolysaccharide- stimulated microglia through the inhibition of TLR4/MyD88/p38 MAPK/NF-κB dependent pathways and the activation of HO-1/NQO-1 signaling. Phytomedicine. 2022 Mar 1;99:154025. doi: 10.1016/j.phymed.2022.154025. Epub ahead of print. PMID: 35272244.
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