Microglia Reduce Herpes Simplex Virus 1 Lethality of Mice with Decreased T Cell and Interferon Responses in Brains

Microglia in Neurodegenerative Diseases

Neurodegenerative diseases are manifested by a progressive death of neural cells, resulting in the deterioration of central nervous system (CNS) functions, ultimately leading to specific behavioural and cognitive symptoms associated with affected brain regions. Several neurodegenerative disorders are caused by genetic variants or mutations, although the majority of cases are sporadic and linked to various environmental risk factors, with yet an unknown aetiology. Neuroglial changes are fundamental and often lead to the pathophysiology of neurodegenerative diseases. In particular, microglial cells, which are essential for maintaining CNS health, become compromised in their physiological functions with the exposure to environmental risk factors, genetic variants or mutations, as well as disease pathology. In this chapter, we cover the contribution of neuroglia, especially microglia, to several neurodegenerative diseases, including Nasu-Hakola disease, Parkinson's disease, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease, infectious disease-associated neurodegeneration, and metal-precipitated neurodegeneration. Future research perspectives for the field pertaining to the therapeutic targeting of microglia across these disease conditions are also discussed.

The anti-viral immune response of the adult host robustly modulates neural stem cell activity in spatial, temporal, and sex-specific manners

Viruses induce a wide range of neurological sequelae through the dysfunction and death of infected cells and persistent inflammation in the brain. Neural stem cells (NSCs) are often disturbed during viral infections. Although some viruses directly infect and kill NSCs, the antiviral immune response may also indirectly affect NSCs. To better understand how NSCs are influenced by a productive immune response, where the virus is successfully resolved and the host survives, we used the CD46+ mouse model of neuron-restricted measles virus (MeV) infection. As NSCs are spared from direct infection in this model, they serve as bystanders to the antiviral immune response initiated by selective infection of mature neurons. MeV-infected mice showed distinct regional and temporal changes in NSCs in the primary neurogenic niches of the brain, the hippocampus and subventricular zone (SVZ). Hippocampal NSCs increased throughout the infection (7 and 60 days post-infection; dpi), while mature neurons transiently declined at 7 dpi and then rebounded to basal levels by 60 dpi. In the SVZ, NSC numbers were unchanged, but mature neurons declined even after the infection was controlled at 60 dpi. Further analyses demonstrated sex, temporal, and region-specific changes in NSC proliferation and neurogenesis throughout the infection. A relatively long-term increase in NSC proliferation and neurogenesis was observed in the hippocampus; however, neurogenesis was reduced in the SVZ. This decline in SVZ neurogenesis was associated with increased immature neurons in the olfactory bulb in female, but not male mice, suggesting potential migration of newly-made neurons out of the female SVZ. These sex differences in SVZ neurogenesis were accompanied by higher infiltration of B cells and greater expression of interferon-gamma and interleukin-6 in female mice. Learning, memory, and olfaction tests revealed no overt behavioral changes after the acute infection subsided. These results indicate that antiviral immunity modulates NSC activity in adult mice without inducing gross behavioral deficits among those tested, suggestive of mechanisms to restore neurons and maintain adaptive behavior, but also revealing the potential for robust NSC disruption in subclinical infections.

Ablation of microglia following infection of the central nervous system with a neurotropic murine coronavirus infection leads to increased demyelination and impaired remyelination

Intracranial inoculation of susceptible mice with a glial-tropic strain of mouse hepatitis virus (JHMV), a murine coronavirus, results in an acute encephalomyelitis followed by viral persistence in white matter tracts accompanied by chronic neuroinflammation and demyelination. Microglia are the resident immune cell of the central nervous system (CNS) and are considered important in regulating events associated with neuroinflammation as well as influencing both white matter damage and remyelination. To better understand mechanisms by which microglia contribute to these immune-mediated events, JHMV-infected mice with established demyelination were treated with the small molecular inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, to deplete microglia. Treatment with PLX5622 did not affect viral replication within the CNS yet the severity of demyelination was increased and remyelination impaired compared to control mice. Gene expression analysis revealed that targeting microglia resulted in altered expression of genes associated with immune cell activation and phagocytosis of myelin debris. These findings indicate that microglia are not critical in viral surveillance in persistently JHMV-infected mice yet restrict white matter damage and remyelination, in part, by influencing phagocytosis of myelin debris.

The role of signaling crosstalk of microglia in hippocampus on progression of ageing and Alzheimer's disease

Based on single-cell sequencing of the hippocampi of 5× familiar Alzheimer's disease (5× FAD) and wild type mice at 2-, 12-, and 24-month of age, we found an increased percentage of microglia in aging and Alzheimer's disease (AD) mice. Blood brain barrier injury may also have contributed to this increase. Immune regulation by microglia plays a major role in the progression of aging and AD, according to the functions of 41 intersecting differentially expressed genes in microglia. Signaling crosstalk between C-C motif chemokine ligand (CCL) and major histocompatibility complex-1 bridges intercellular communication in the hippocampus during aging and AD. The amyloid precursor protein (APP) and colony stimulating factor (CSF) signals drive 5× FAD to deviate from aging track to AD occurrence among intercellular communication in hippocampus. Microglia are involved in the progression of aging and AD can be divided into 10 functional types. The strength of the interaction among microglial subtypes weakened with aging, and the CCL and CSF signaling pathways were the fundamental bridge of communication among microglial subtypes.

Temporal tracking of microglial and monocyte single-cell transcriptomics in lethal flavivirus infection

As the resident parenchymal myeloid population in the central nervous system (CNS), microglia are strategically positioned to respond to neurotropic virus invasion and have been implicated in promoting both disease resolution and progression in the acute and post-infectious phase of virus encephalitis. In a mouse model of West Nile virus encephalitis (WNE), infection of the CNS results in recruitment of large numbers of peripheral immune cells into the brain, the majority being nitric oxide (NO)-producing Ly6C^hi inflammatory monocyte-derived cells (MCs). In this model, these cells enhance immunopathology and mortality. However, the contribution of microglia to this response is currently undefined. Here we used a combination of experimental tools, including single-cell RNA sequencing (scRNA-seq), microglia and MC depletion reagents, high-dimensional spectral cytometry and computational algorithms to dissect the differential contribution of microglia and MCs to the anti-viral immune response in severe neuroinflammation seen in WNE. Intriguingly, analysis of scRNA-seq data revealed 6 unique microglia and 3 unique MC clusters that were predominantly timepoint-specific, demonstrating substantial transcriptional adaptation with disease progression over the course of WNE. While microglia and MC adopted unique gene expression profiles, gene ontology enrichment analysis, coupled with microglia and MC depletion studies, demonstrated a role for both of these cells in the trafficking of peripheral immune cells into the CNS, T cell responses and viral clearance. Over the course of infection, microglia transitioned from a homeostatic to an anti-viral and then into an immune cell-recruiting phenotype. Conversely, MC adopted antigen-presenting, immune cell-recruiting and NO-producing phenotypes, which all had anti-viral function. Overall, this study defines for the first time the single-cell transcriptomic responses of microglia and MCs over the course of WNE, demonstrating both protective and pathological roles of these cells that could potentially be targeted for differential therapeutic intervention to dampen immune-mediated pathology, while maintaining viral clearance functions.

Asiatic Acid Attenuates Inflammation Induced by Salmonella via Upregulating LncRNA TVX1 in Microglia

Salmonella typhimurium (S.T) induces damage to the central nervous system; however, the role of Asiatic acid (AA) in this is still unknown. Microglia play a role as macrophages to recognize the invaded pathogenic microbes in the brain. The aim of this study was to investigate the protective effect and mechanism of AA on the central nervous system through an in vitro model of S.T infection in microglia. We pre-treated microglia with AA before S.T infection and explored the anti-infection mechanism of AA by sequencing, quantitative reverse transcription PCR (RT-qPCR), and Western blotting. Long non-coding RNA (lncRNA) sequencing demonstrated that inflammation is a major factor in S.T infection of microglia. RT-qPCR data demonstrated that AA inhibited S.T-induced increases in the mRNA levels of the pro-inflammatory factors interleukin (IL)-1β, IL-6, and IL-18. Western blotting demonstrated that AA inhibited S.T-induced activation of the nuclear factor (NF)-κB pathway and activation of the NLR family, pyrin domain-containing 3 (NLRP3) inflammasome. Expression of the lncRNA TVX1 in microglia was decreased by S.T infection and increased by pretreatment with AA. Inhibition of TVX1 expression reversed the anti-inflammatory effect of AA, and overexpression of TVX1 in microglia suppressed S.T-induced inflammation. In conclusion, AA attenuated S.T-induced microglial inflammation by upregulating the expression of the lncRNA TVX1.

Role of Microglia in Herpesvirus-Related Neuroinflammation and Neurodegeneration

Neuroinflammation is defined as an inflammatory state within the central nervous system (CNS). Microglia conprise the resident tissue macrophages of the neuronal tissue. Upon viral infection of the CNS, microglia become activated and start to produce inflammatory mediators important for clearance of the virus, but an excessive neuroinflammation can harm nearby neuronal cells. Herpesviruses express several molecular mechanisms, which can modulate apoptosis of infected neurons, astrocytes and microglia but also divert immune response initiated by the infected cells. In this review we also describe the link between virus-related neuroinflammation, and development of neurodegenerative diseases.

PLX5622 Reduces Disease Severity in Lethal CNS Infection by Off-Target Inhibition of Peripheral Inflammatory Monocyte Production

PLX5622 is a CSF-1R inhibitor and microglia-depleting reagent, widely used to investigate the biology of this central nervous system (CNS)-resident myeloid population, but the indirect or off-target effects of this agent remain largely unexplored. In a murine model of severe neuroinflammation induced by West Nile virus encephalitis (WNE), we showed PLX5622 efficiently depleted both microglia and a sub-population of border-associated macrophages in the CNS. However, PLX5622 also significantly depleted mature Ly6C^hi monocytes in the bone marrow (BM), inhibiting their proliferation and lethal recruitment into the infected brain, reducing neuroinflammation and clinical disease scores. Notably, in addition, BM dendritic cell subsets, plasmacytoid DC and classical DC, were depleted differentially in infected and uninfected mice. Confirming its protective effect in WNE, cessation of PLX5622 treatment exacerbated disease scores and was associated with robust repopulation of microglia, rebound BM monopoiesis and markedly increased inflammatory monocyte infiltration into the CNS. Monoclonal anti-CSF-1R antibody blockade late in WNE also impeded BM monocyte proliferation and recruitment to the brain, suggesting that the protective effect of PLX5622 is via the inhibition of CSF-1R, rather than other kinase targets. Importantly, BrdU incorporation in PLX5622-treated mice, suggest remaining microglia proliferate independently of CSF-1 in WNE. Our study uncovers significantly broader effects of PLX5622 on the myeloid lineage beyond microglia depletion, advising caution in the interpretation of PLX5622 data as microglia-specific. However, this work also strikingly demonstrates the unexpected therapeutic potential of this molecule in CNS viral infection, as well as other monocyte-mediated diseases.

Absence of the lectin-like domain of thrombomodulin reduces HSV-1 lethality of mice with increased microglia responses

Background

Herpes simplex virus 1 (HSV-1) can induce fatal encephalitis. Cellular factors regulate the host immunity to affect the severity of HSV-1 encephalitis. Recent reports focus on the significance of thrombomodulin (TM), especially the domain 1, lectin-like domain (TM-LeD), which modulates the immune responses to bacterial infections and toxins and various diseases in murine models. Few studies have investigated the importance of TM-LeD in viral infections, which are also regulated by the host immunity.

Methods

In vivo studies comparing wild-type and TM-LeD knockout mice were performed to determine the role of TM-LeD on HSV-1 lethality. In vitro studies using brain microglia cultured from mice or a human microglia cell line to investigate whether and how TM-LeD affects microglia to reduce HSV-1 replication in brain neurons cultured from mice or in a human neuronal cell line.

Results

Absence of TM-LeD decreased the mortality, tissue viral loads, and brain neuron apoptosis of HSV-1-infected mice with increases in the number, proliferation, and phagocytic activity of brain microglia. Moreover, TM-LeD deficiency enhanced the phagocytic activity of brain microglia cultured from mice or of a human microglia cell line. Co-culture of mouse primary brain microglia and neurons or human microglia and neuronal cell lines revealed that TM-LeD deficiency augmented the capacity of microglia to reduce HSV-1 replication in neurons.

Conclusions

Overall, TM-LeD suppresses microglia responses to enhance HSV-1 infection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02426-w.