Antibody-mediated targeting of human microglial leukocyte Ig-like receptor B4 attenuates amyloid pathology in a mouse model.

Microglia help limit the progression of Alzheimer's disease (AD) by constraining amyloid-ß (Aß) pathology, effected through a balance of activating and inhibitory intracellular signals delivered by distinct cell surface receptors. Human leukocyte Ig-like receptor B4 (LILRB4) is an inhibitory receptor of the immunoglobulin (Ig) superfamily that is expressed on myeloid cells and recognizes apolipoprotein E (ApoE) among other ligands. Here, we find that LILRB4 is highly expressed in the microglia of patients with AD. Using mice that accumulate Aß and carry a transgene encompassing a portion of the LILR region that includes LILRB4, we corroborated abundant LILRB4 expression in microglia wrapping around Aß plaques. Systemic treatment of these mice with an anti-human LILRB4 monoclonal antibody (mAb) reduced Aß load, mitigated some Aß-related behavioral abnormalities, enhanced microglia activity, and attenuated expression of interferon-induced genes. In vitro binding experiments established that human LILRB4 binds both human and mouse ApoE and that anti-human LILRB4 mAb blocks such interaction. In silico modeling, biochemical, and mutagenesis analyses identified a loop between the two extracellular Ig domains of LILRB4 required for interaction with mouse ApoE and further indicated that anti-LILRB4 mAb may block LILRB4-mApoE by directly binding this loop. Thus, targeting LILRB4 may be a potential therapeutic avenue for AD.

Inducible nitric oxide synthase deficiency promotes murine-ß-coronavirus induced demyelination.

BACKGROUND: Multiple sclerosis (MS) is characterized by neuroinflammation and demyelination orchestrated by activated neuroglial cells, CNS infiltrating leukocytes, and their reciprocal interactions through inflammatory signals. An inflammatory stimulus triggers inducible nitric oxide synthase (NOS2), a pro-inflammatory marker of microglia/macrophages (MG/Mφ) to catalyze sustained nitric oxide production. NOS2 during neuroinflammation, has been associated with MS disease pathology; however, studies dissecting its role in demyelination are limited. We studied the role of NOS2 in a recombinant ß-coronavirus-MHV-RSA59 induced neuroinflammation, an experimental animal model mimicking the pathological hallmarks of MS: neuroinflammatory demyelination and axonal degeneration. OBJECTIVE: Understanding the role of NOS2 in murine-ß-coronavirus-MHV-RSA59 demyelination. METHODS: Brain and spinal cords from mock and RSA59 infected 4-5-week-old MHV-free C57BL/6 mice (WT) and NOS2-/- mice were harvested at different disease phases post infection (p.i.) (day 5/6-acute, day 9/10-acute-adaptive and day 30-chronic phase) and compared for pathological outcomes. RESULTS: NOS2 was upregulated at the acute phase of RSA59-induced disease in WT mice and its deficiency resulted in severe disease and reduced survival at the acute-adaptive transition phase. Low survival in NOS2-/- mice was attributed to (i) high neuroinflammation resulting from increased accumulation of macrophages and neutrophils and (ii) Iba1 + phagocytic MG/Mφ mediated-early demyelination as observed at this phase. The phagocytic phenotype of CNS MG/Mφ was confirmed by significantly higher mRNA transcripts of phagocyte markers-CD206, TREM2, and Arg1 and double immunolabelling of Iba1 with MBP and PLP. Further, NOS2 deficiency led to exacerbated demyelination at the chronic phase as well. CONCLUSION: Taken together the results imply that the immune system failed to control the disease progression in the absence of NOS2. Thus, our observations highlight a protective role of NOS2 in murine-ß-coronavirus induced demyelination.

Astrocytic APOE4 removal confers cerebrovascular protection despite increased cerebral amyloid angiopathy.

BACKGROUND: Alzheimer Disease (AD) and cerebral amyloid angiopathy (CAA) are both characterized by amyloid-ß (Aß) accumulation in the brain, although Aß deposits mostly in the brain parenchyma in AD and in the cerebrovasculature in CAA. The presence of CAA can exacerbate clinical outcomes of AD patients by promoting spontaneous intracerebral hemorrhage and ischemia leading to CAA-associated cognitive decline. Genetically, AD and CAA share the ε4 allele of the apolipoprotein E (APOE) gene as the strongest genetic risk factor. Although tremendous efforts have focused on uncovering the role of APOE4 on parenchymal plaque pathogenesis in AD, mechanistic studies investigating the role of APOE4 on CAA are still lacking. Here, we addressed whether abolishing APOE4 generated by astrocytes, the major producers of APOE, is sufficient to ameliorate CAA and CAA-associated vessel damage. METHODS: We generated transgenic mice that deposited both CAA and plaques in which APOE4 expression can be selectively suppressed in astrocytes. At 2-months-of-age, a timepoint preceding CAA and plaque formation, APOE4 was removed from astrocytes of 5XFAD APOE4 knock-in mice. Mice were assessed at 10-months-of-age for Aß plaque and CAA pathology, gliosis, and vascular integrity. RESULTS: Reducing the levels of APOE4 in astrocytes shifted the deposition of fibrillar Aß from the brain parenchyma to the cerebrovasculature. However, despite increased CAA, astrocytic APOE4 removal reduced overall Aß-mediated gliosis and also led to increased cerebrovascular integrity and function in vessels containing CAA. CONCLUSION: In a mouse model of CAA, the reduction of  APOE4 derived specifically from astrocytes, despite increased fibrillar Aß deposition in the vasculature, is sufficient to reduce Aß-mediated gliosis and cerebrovascular dysfunction.

CD40L protects against mouse hepatitis virus-induced neuroinflammatory demyelination.

Neurotropic mouse hepatitis virus (MHV-A59/RSA59) infection in mice induces acute neuroinflammation due to direct neural cell dystrophy, which proceeds with demyelination with or without axonal loss, the pathological hallmarks of human neurological disease, Multiple sclerosis (MS). Recent studies in the RSA59-induced neuroinflammation model of MS showed a protective role of CNS-infiltrating CD4+ T cells compared to their pathogenic role in the autoimmune model. The current study further investigated the molecular nexus between CD4+ T cell-expressed CD40Ligand and microglia/macrophage-expressed CD40 using CD40L-/- mice. Results demonstrate CD40L expression in the CNS is modulated upon RSA59 infection. We show evidence that CD40L-/- mice are more susceptible to RSA59 induced disease due to reduced microglia/macrophage activation and significantly dampened effector CD4+ T recruitment to the CNS on day 10 p.i. Additionally, CD40L-/- mice exhibited severe demyelination mediated by phagocytic microglia/macrophages, axonal loss, and persistent poliomyelitis during chronic infection, indicating CD40-CD40L as host-protective against RSA59-induced demyelination. This suggests a novel target in designing prophylaxis for virus-induced demyelination and axonal degeneration, in contrast to immunosuppression which holds only for autoimmune mechanisms of inflammatory demyelination.

DJ-1-Nrf2 axis is activated upon murine ß-coronavirus infection in the CNS.

Coronaviruses have emerged as alarming pathogens owing to their inherent ability of genetic variation and cross-species transmission. Coronavirus infection burdens the endoplasmic reticulum (ER.), causes reactive oxygen species production and induces host stress responses, including unfolded protein response (UPR) and antioxidant system. In this study, we have employed a neurotropic murine ß-coronavirus (M-CoV) infection in the Central Nervous System (CNS) of experimental mice model to study the role of host stress responses mediated by interplay of DJ-1 and XBP1. DJ-1 is an antioxidant molecule with established functions in neurodegeneration. However, its regulation in virus-induced cellular stress response is less explored. Our study showed that M-CoV infection activated the glial cells and induced antioxidant and UPR genes during the acute stage when the viral titer peaks. As the virus particles decreased and acute neuroinflammation diminished at day ten p.i., a significant up-regulation in UPR responsive XBP1, antioxidant DJ-1, and downstream signaling molecules, including Nrf2, was recorded in the brain tissues. Additionally, preliminary in silico analysis of the binding between the DJ-1 promoter and a positively charged groove of XBP1 is also investigated, thus hinting at a mechanism behind the upregulation of DJ-1 during MHV-infection. The current study thus attempts to elucidate a novel interplay between the antioxidant system and UPR in the outcome of coronavirus infection.

Spike Glycoprotein Is Central to Coronavirus Pathogenesis-Parallel Between m-CoV and SARS-CoV-2.

Background: Coronaviruses (CoVs) are single-stranded, polyadenylated, enveloped RNA of positive polarity with a unique potential to alter host tropism. This has been exceptionally demonstrated by the emergence of deadly virus outbreaks of the past: Severe Acute Respiratory Syndrome (SARS-CoV) in 2003 and Middle East Respiratory Syndrome (MERS-CoV) in 2012. Summary: The 2019 outbreak by the new cross-species transmission of SARS-CoV-2 has put the world on alert. CoV infection is triggered by receptor recognition, membrane fusion, and successive viral entry mediated by the surface Spike (S) glycoprotein. S protein is one of the major antigenic determinants and the target for neutralizing antibodies. It is a valuable target in antiviral therapies because of its central role in cell-cell fusion, viral antigen spread, and host immune responses leading to immunopathogenesis. The receptor-binding domain of S protein has received greater attention as it initiates host attachment and contains major antigenic determinants. However, investigating the therapeutic potential of fusion peptide as a part of the fusion core complex assembled by the heptad repeats 1 and 2 (HR1 and HR2) is also warranted. Along with receptor attachment and entry, fusion mechanisms should also be explored for designing inhibitors as a therapeutic intervention. Key message: In this article, we review the S protein function and its role in mediating membrane fusion, spread, tropism, and its associated pathogenesis with notable therapeutic strategies focusing on results obtained from studies on a murine ß-Coronavirus (m-CoV) and its associated disease process.

CD4 Deficiency Causes Poliomyelitis and Axonal Blebbing in Murine Coronavirus-Induced Neuroinflammation.

Mouse hepatitis virus (MHV) is a murine betacoronavirus (m-CoV) that causes a wide range of diseases in mice and rats, including hepatitis, enteritis, respiratory diseases, and encephalomyelitis in the central nervous system (CNS). MHV infection in mice provides an efficient cause-effect experimental model to understand the mechanisms of direct virus-induced neural-cell damage leading to demyelination and axonal loss, which are pathological features of multiple sclerosis (MS), the most common disabling neurological disease in young adults. Infiltration of T lymphocytes, activation of microglia, and their interplay are the primary pathophysiological events leading to disruption of the myelin sheath in MS. However, there is emerging evidence supporting gray matter involvement and degeneration in MS. The investigation of T cell function in the pathogenesis of deep gray matter damage is necessary. Here, we employed RSA59 (an isogenic recombinant strain of MHV-A59)-induced experimental neuroinflammation model to compare the disease in CD4-/- mice with that in CD4+/+ mice at days 5, 10, 15, and 30 postinfection (p.i.). Viral titer estimation, nucleocapsid gene amplification, and viral antinucleocapsid staining confirmed enhanced replication of the virions in the absence of functional CD4+ T cells in the brain. Histopathological analyses showed elevated susceptibility of CD4-/- mice to axonal degeneration in the CNS, with augmented progression of acute poliomyelitis and dorsal root ganglionic inflammation rarely observed in CD4+/+ mice. Depletion of CD4+ T cells showed unique pathological bulbar vacuolation in the brain parenchyma of infected mice with persistent CD11b+ microglia/macrophages in the inflamed regions on day 30 p.i. In summary, the current study suggests that CD4+ T cells are critical for controlling acute-stage poliomyelitis (gray matter inflammation), chronic axonal degeneration, and inflammatory demyelination due to loss of protective antiviral host immunity.IMPORTANCE The current trend in CNS disease biology is to attempt to understand the neural-cell-immune interaction to investigate the underlying mechanism of neuroinflammation, rather than focusing on peripheral immune activation. Most studies in MS are targeted toward understanding the involvement of CNS white matter. However, the importance of gray matter damage has become critical in understanding the long-term progressive neurological disorder. Our study highlights the importance of CD4+ T cells in safeguarding neurons against axonal blebbing and poliomyelitis from murine betacoronavirus-induced neuroinflammation. Current knowledge of the mechanisms that lead to gray matter damage in MS is limited, because the most widely used animal model, experimental autoimmune encephalomyelitis (EAE), does not present this aspect of the disease. Our results, therefore, add to the existing limited knowledge in the field. We also show that the microglia, though important for the initiation of neuroinflammation, cannot establish a protective host immune response without the help of CD4+ T cells.