T cell mediated suppression of neurotropic coronavirus replication in neural precursor cells

Neuro-axonal injury in COVID-19: the role of systemic inflammation and SARS-CoV-2 specific immune response

Background:

In coronavirus disease-2019 (COVID-19) patients, there is increasing evidence of neuronal injury by the means of elevated serum neurofilament light chain (sNfL) levels. However, the role of systemic inflammation and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific immune response with regard to neuronal injury has not yet been investigated.

Methods:

In a prospective cohort study, we recruited patients with mild–moderate (n = 39) and severe (n = 14) COVID-19 and measured sNfL levels, cytokine concentrations, SARS-CoV-2-specific antibodies including neutralizing antibody titers, and cell-mediated immune responses at enrollment and at 28(±7) days. We explored the association of neuro-axonal injury as by the means of sNfL measurements with disease severity, cytokine levels, and virus-specific immune responses.

Results:

sNfL levels, as an indicator for neuronal injury, were higher at enrollment and increased during follow-up in severely ill patients, whereas during mild–moderate COVID-19, sNfL levels remained unchanged. Severe COVID-19 was associated with increased concentrations of cytokines assessed [interleukin (IL)-6, IL-8, interleukin-1 beta (IL-1β), and tumor necrosis factor-alpha (TNF-α)], higher anti-spike IgG and anti-nucleocapsid IgG concentrations, and increased neutralizing antibody titers compared with mild–moderate disease. Patients with more severe disease had higher counts of defined SARS-CoV-2-specific T cells. Increases in sNfL concentrations from baseline to day 28(±7) positively correlated with anti-spike protein IgG antibody levels and with titers of neutralizing antibodies.

Conclusion:

Severe COVID-19 is associated with increased serum concentration of cytokines and subsequent neuronal injury as reflected by increased levels of sNfL. Patients with more severe disease developed higher neutralizing antibody titers and higher counts of SARS-CoV-2-specific T cells during the course of COVID-19 disease. Mounting a pronounced virus-specific humoral and cell-mediated immune response upon SARS-CoV-2 infection did not protect from neuro-axonal damage as by the means of sNfL levels.

Assessing the Expression of Major Histocompatibility Complex Class I on Primary Murine Hippocampal Neurons by Flow Cytometry

Increasing evidence supports the hypothesis that neuro-immune interactions impact nervous system function in both homeostatic and pathologic conditions. A well-studied function of major histocompatibility complex class I (MHCI) is the presentation of cell-derived peptides to the adaptive immune system, particularly in response to infection. More recently it has been shown that the expression of MHCI molecules on neurons can modulate activity-dependent changes in the synaptic connectivity during normal development and neurologic disorders. The importance of these functions to the brain health supports the need for a sensitive assay that readily detects MHCI expression on neurons. Here we describe a method for primary culture of murine hippocampal neurons and then assessment of MHCI expression by flow cytometric analysis. Murine hippocampus is microdissected from prenatal mouse pups at the embryonic day 18. Tissue is dissociated into a single cell suspension using enzymatic and mechanical techniques, then cultured in a serum-free media that limits growth of non-neuronal cells. After 7 days in vitro, MHCI expression is stimulated by treating cultured cells pharmacologically with beta interferon. MHCI molecules are labeled in situ with a fluorescently tagged antibody, then cells are non-enzymatically dissociated into a single cell suspension. To confirm the neuronal identity, cells are fixed with paraformaldehyde, permeabilized, and labeled with a fluorescently tagged antibody that recognizes neuronal nuclear antigen NeuN. MHCI expression is then quantified on neurons by flow cytometric analysis. Neuronal cultures can easily be manipulated by either genetic modifications or pharmacologic interventions to test specific hypotheses. With slight modifications, these methods can be used to culture other neuronal populations or to assess expression of other proteins of interest.

The microbiota protects from viral-induced neurologic damage through microglia-intrinsic TLR signaling

Symbiotic microbes impact the function and development of the central nervous system (CNS); however, little is known about the contribution of the microbiota during viral-induced neurologic damage. We identify that commensals aid in host defense following infection with a neurotropic virus through enhancing microglia function. Germfree mice or animals that receive antibiotics are unable to control viral replication within the brain leading to increased paralysis. Microglia derived from germfree or antibiotic-treated animals cannot stimulate viral-specific immunity and microglia depletion leads to worsened demyelination. Oral administration of toll-like receptor (TLR) ligands to virally infected germfree mice limits neurologic damage. Homeostatic activation of microglia is dependent on intrinsic signaling through TLR4, as disruption of TLR4 within microglia, but not the entire CNS (excluding microglia), leads to increased viral-induced clinical disease. This work demonstrates that gut immune-stimulatory products can influence microglia function to prevent CNS damage following viral infection.

Promoting remyelination through cell transplantation therapies in a model of viral-induced neurodegenerative disease

Multiple sclerosis (MS) is a central nervous system (CNS) disease characterized by chronic neuroinflammation, demyelination, and axonal damage. Infiltration of activated lymphocytes and myeloid cells are thought to be primarily responsible for white matter damage and axonopathy. Several United States Food and Drug Administration-approved therapies exist that impede activated lymphocytes from entering the CNS thereby limiting new lesion formation in patients with relapse-remitting forms of MS. However, a significant challenge within the field of MS research is to develop effective and sustained therapies that allow for axonal protection and remyelination. In recent years, there has been increasing evidence that some kinds of stem cells and their derivatives seem to be able to mute neuroinflammation as well as promote remyelination and axonal integrity. Intracranial infection of mice with the neurotropic JHM strain of mouse hepatitis virus (JHMV) results in immune-mediated demyelination and axonopathy, making this an excellent model to interrogate the therapeutic potential of stem cell derivatives in evoking remyelination. This review provides a succinct overview of our recent findings using intraspinal injection of mouse CNS neural progenitor cells and human neural precursors into JHMV-infected mice. JHMV-infected mice receiving these cells display extensive remyelination associated with axonal sparing. In addition, we discuss possible mechanisms associated with sustained clinical recovery. Developmental Dynamics 248:43-52, 2019. © 2018 Wiley Periodicals, Inc.

Neural precursor cells derived from induced pluripotent stem cells exhibit reduced susceptibility to infection with a neurotropic coronavirus

The present study examines the susceptibility of mouse induced pluripotent stem cell-derived neural precursor cells (iPSC-NPCs) to infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV). Similar to NPCs derived from striatum of day 1 postnatal GFP-transgenic mice (GFP-NPCs), iPSC-derived NPCs (iPSC-NPCs) are able to differentiate into terminal neural cell types and express MHC class I and II in response to IFN-γ treatment. However, in contrast to postnatally-derived NPCs, iPSC-NPCs express low levels of carcinoembryonic antigen-cell adhesion molecule 1a (CEACAM1a), the surface receptor for JHMV, and are less susceptible to infection and virus-induced cytopathic effects. The relevance of this in terms of therapeutic application of NPCs resistant to viral infection is discussed.

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The neurotropic virus JHMV infects and kills mouse post-natal neural progenitor cells (NPCs).

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This study examines if JHMV infects mouse inducible pluripotent stem cell-derived NPCs.

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iPSC-NPCs are less susceptible to infection with JHMV and subsequent lytic effects.

MicroRNA 155 and viral-induced neuroinflammation

MicroRNA (miRNA) regulation of gene expression is becoming an increasingly recognized mechanism by which host immune responses are governed following microbial infection. miRNAs are short, non-coding RNAs that repress translation of target genes, and have been implicated in a number of activities that modulate host immune responses, including the regulation of immune cell proliferation, survival, expansion, differentiation, migration, polarization, and effector function. This review highlights several examples in which mammalian-encoded miR-155 influences immune responses following viral infection of the CNS.

MicroRNA-155 enhances T cell trafficking and antiviral effector function in a model of coronavirus-induced neurologic disease

BACKGROUND

MicroRNAs (miRNAs) are noncoding RNAs that modulate cellular gene expression, primarily at the post-transcriptional level. We sought to examine the functional role of miR-155 in a model of viral-induced neuroinflammation.

METHODS

Acute encephalomyelitis and immune-mediated demyelination were induced by intracranial injection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into C57BL/6 miR-155 (+/+) wildtype (WT) mice or miR-155 (-/-) mice. Morbidity and mortality, viral load and immune cell accumulation in the CNS, and spinal cord demyelination were assessed at defined points post-infection. T cells harvested from infected mice were used to examine cytolytic activity, cytokine activity, and expression of certain chemokine receptors. To determine the impact of miR-155 on trafficking, T cells from infected WT or miR-155 (-/-) mice were adoptively transferred into RAG1 (-/-) mice, and T cell accumulation into the CNS was assessed using flow cytometry. Statistical significance was determined using the Mantel-Cox log-rank test or Student's T tests.

RESULTS

Compared to WT mice, JHMV-infected miR-155 (-/-) mice developed exacerbated disease concomitant with increased morbidity/mortality and an inability to control viral replication within the CNS. In corroboration with increased susceptibility to disease, miR-155 (-/-) mice had diminished CD8(+) T cell responses in terms of numbers, cytolytic activity, IFN-γ secretion, and homing to the CNS that corresponded with reduced expression of the chemokine receptor CXCR3. Both IFN-γ secretion and trafficking were impaired in miR-155 (-/-) , virus-specific CD4(+) T cells; however, expression of the chemokine homing receptors analyzed on CD4(+) cells was not affected. Except for very early during infection, there were not significant differences in macrophage infiltration into the CNS between WT and miR-155 (-/-) JHMV-infected mice, and the severity of demyelination was similar at 14 days p.i. between WT and miR-155 (-/-) JHMV-infected mice.

CONCLUSIONS

These findings support a novel role for miR-155 in host defense in a model of viral-induced encephalomyelitis. Specifically, miR-155 enhances antiviral T cell responses including cytokine secretion, cytolytic activity, and homing to the CNS in response to viral infection. Further, miR-155 can play either a host-protective or host-damaging role during neuroinflammation depending on the disease trigger.

Promoting remyelination: utilizing a viral model of demyelination to assess cell-based therapies

Multiple sclerosis (MS) is a chronic inflammatory disease of the CNS. While a broad range of therapeutics effectively reduce the incidence of focal white matter inflammation and plaque formation for patients with relapse-remitting forms of MS, a challenge within the field is to develop therapies that allow for axonal protection and remyelination. In the last decade, growing interest has focused on utilizing neural precursor cells (NPCs) to promote remyelination. To understand how NPCs function in chronic demyelinating environments, several excellent pre-clinical mouse models have been developed. One well accepted model is infection of susceptible mice with neurotropic variants of mouse hepatitis virus (MHV) that undergo chronic demyelination exhibiting clinical and histopathologic similarities to MS patients. Combined with the possibility that an environmental agent such as a virus could trigger MS, the MHV model of demyelination presents a relevant mouse model to assess the therapeutic potential of NPCs transplanted into an environment in which inflammatory-mediated demyelination is established.

Activating receptor NKG2D targets RAE-1-expressing allogeneic neural precursor cells in a viral model of multiple sclerosis

Transplantation of major histocompatibility complex-mismatched mouse neural precursor cells (NPCs) into mice persistently infected with the neurotropic JHM strain of mouse hepatitis virus (JHMV) results in rapid rejection that is mediated, in part, by T cells. However, the contribution of the innate immune response to allograft rejection in a model of viral-induced neurological disease has not been well defined. Herein, we demonstrate that the natural killer (NK) cell-expressing-activating receptor NKG2D participates in transplanted allogeneic NPC rejection in mice persistently infected with JHMV. Cultured NPCs derived from C57BL/6 (H-2(b) ) mice express the NKG2D ligand retinoic acid early precursor transcript (RAE)-1 but expression was dramatically reduced upon differentiation into either glia or neurons. RAE-1(+) NPCs were susceptible to NK cell-mediated killing whereas RAE-1(-) cells were resistant to lysis. Transplantation of C57BL/6-derived NPCs into JHMV-infected BALB/c (H-2(d) ) mice resulted in infiltration of NKG2D(+) CD49b(+) NK cells and treatment with blocking antibody specific for NKG2D increased survival of allogeneic NPCs. Furthermore, transplantation of differentiated RAE-1(-) allogeneic NPCs into JHMV-infected BALB/c mice resulted in enhanced survival, highlighting a role for the NKG2D/RAE-1 signaling axis in allograft rejection. We also demonstrate that transplantation of allogeneic NPCs into JHMV-infected mice resulted in infection of the transplanted cells suggesting that these cells may be targets for infection. Viral infection of cultured cells increased RAE-1 expression, resulting in enhanced NK cell-mediated killing through NKG2D recognition. Collectively, these results show that in a viral-induced demyelination model, NK cells contribute to rejection of allogeneic NPCs through an NKG2D signaling pathway.