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.

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.

The pandemic caused by SARS-CoV-2 has caused widespread infection and significant mortality across the globe. Combined virology perspective of SARS-CoV-2 with a deep-rooted understanding of pathophysiological and immunological processes underlying the clinical manifestations of COVID-19 is of prime importance. The characteristic symptom of COVID-19 is respiratory distress with diffused alveolar damage, but emerging evidence suggests COVID-19 might also have neurologic consequences. Dysregulated homeostasis in the lungs has proven to be fatal, but one cannot ignore that the inability to breathe might be due to defects in the respiratory control center of the brainstem. While the mechanism of pulmonary distress has been documented in the literature, awareness of neurological features and their pathophysiology is still in the nascent state. This review makes references to the neuro-immune axis and neuro-invasive potential of SARS-CoV and SARS-CoV2, as well as the prototypic H-CoV strains in human brains. Simultaneously, considerable discussion on relevant experimental evidence of mild to severe neurological manifestations of fellow neurotropic murine-β-CoVs (m-CoVs) in the mouse model will help understand the underpinning mechanisms of Neuro-COVID. In this review, we have highlighted the neuroimmunopathological processes in murine CoVs. While MHV infection in mice and SARS-CoV-2 infection in humans share numerous parallels, there are critical differences in viral recognition and viral entry. These similarities are highlighted in this review, while differences have also been emphasized. Though CoV-2 Spike does not favorably interact with murine ACE2 receptor, modification of murine SARS-CoV2 binding domain or development of transgenic ACE-2 knock-in mice might help in mediating consequential infection and understanding human CoV2 pathogenesis in murine models. While a global animal model that can replicate all aspects of the human disease remains elusive, prior insights and further experiments with fellow m-β-CoV-induced cause-effect experimental models and current human COVID-19 patients data may help to mitigate the SARS-CoV-2-induced multifactorial multi-organ failure

Objective Monkeypox virus (MPXV) disease has been declared a public health emergency by the World Health Organization, creating an urgent need for neurologists to be able to recognize, diagnosis, and treat MPXV‐associated neurologic disease. Methods Three cases of MPXV‐associated central nervous system (CNS) disease occurring during the 2022 outbreak, and their associated imaging findings are presented, with 2 cases previously published in a limited capacity in a public health bulletin. Results Three previously healthy immunocompetent gay men in their 30s developed a febrile illness followed by progressive neurologic symptoms with presence of a vesiculopustular rash. MPXV nucleic acid was detected by polymerase chain reaction (PCR) from skin lesions of 2 patients, with the third patient having indeterminate testing but an epidemiologic link to a confirmed MPXV disease case. Cerebrospinal fluid demonstrated a lymphocytic pleocytosis, elevated protein, and negative MPXV‐specific PCR. In 2 patients, magnetic resonance imaging of the brain and spine demonstrated partially enhancing, longitudinally extensive central spinal cord lesions with multifocal subcortical, basal ganglia, thalamic, cerebellar, and/or brainstem lesions. The third patient had thalamic and basal ganglia lesions. All patients received 14 days of tecovirimat, and 2 patients also received multiple forms of immunotherapy, including intravenous immunoglobulin, pulsed high‐dose steroids, plasmapheresis, and/or rituximab. Good neurologic recovery was observed in all cases. Interpretation MPXV can be associated with CNS disease. It is unclear whether this is from a parainfectious immune‐mediated injury or direct CNS viral invasion. ANN NEUROL 2023;93:893–905

CD4+ T cells play critical roles in mediating adaptive immunity and developing protective host immunity by eliminating infectious viruses. CD40 Ligand expressed on CD4+ T cells known to interact with its cognate receptor CD40 expressed on monocyte/macrophage/microglia to induce several immune functions during inflammation and host immunity. Our study aimed to delineate a potential CD4+ T cell-microglia nexus at the cellular and molecular level using CD4−/− and CD40L−/− mice. Results showed that the absence of CD40L renders mice highly susceptible to murine-coronavirus-MHV-A59/RSA59 infection due to reduced microglia/macrophage activation and significantly dampened effector CD4+ T recruitment to the CNS at the acute-adaptive bridging phase of neuroinflammation. CD40L deficiency significantly impaired the priming, expansion, and activation of lymphoid cells in the deep cervical lymph nodes. Additionally, both CD4−/− and CD40L−/− mice presented with severe chronic phase demyelination and axonal pathology in the brain and spinal cord which was correlated with prolonged virus persistence. Results from this study provide exciting information about a critical role of CD40L in mediating RSA59 induced activation of CD4+ T cells in the CLN and their consequent infiltration into CNS in addition to affecting the numbers and activation of monocyte/macrophage/microglia during the acute-adaptive transition phase. CD40-CD40L interaction in linking the innate-acute and chronic-adaptive immune responses may suggest a novel target in designing prophylaxis for virus-induced demyelination and axonal degeneration, in contrast to autoimmune suppression, which holds only for autoimmune mechanisms of inflammatory demyelination. This work was supported by a Department of Biotechnology, India, research grant (BT/PR 20922/MED/122/37/2016). We thank the Council of Scientific and Industrial Research (CSIR) India for providing fellowships to F.S. the Ministry of Human Resource Development (MHRD), India for providing fellowship to D.C; and the University Grants Commission (UGC), India, for providing fellowship to M.K.

Chronic progressive neuroinflammatory disease multiple sclerosis (MS) is characterized by loss of neuronal functions resulting from demyelination with or without axonal degeneration. Infiltration of T lymphocytes and activation of microglia and their interplay are the major pathophysiological events leading to neurodegeneration in MS. Our studies in Mouse Hepatitis Virus (MHV) induced neuroinflammatory model demonstrated a protective role of CNS infiltrating CD4+ T cells. In the absence of CD4+ T cells, microglial activation fails to resolve, and mice are more susceptible to acute poliomyelitis and chronic demyelination with axonal bulbar vacuolation. Our studies also revealed that CD40L (expressed on CD4+ T cells) upregulates upon MHV infection but is downregulated in CD4−/− mice CNS. This led to a further delineation of the CD4-microglia nexus at the molecular level using CD40L−/− mice. Results showed that the absence of CD40L renders mice highly susceptible to MHV infection due to reduced microglia/macrophage activation and significantly dampened effector CD4+ T recruitment to the CNS at the acute-adaptive bridging phase (day 7–10 p.i.) of inflammation. Moreover, CD40L−/− mice exhibited severe demyelination, axonal loss, and persistent poliomyelitis at the chronic phase of infection, highlighting the protective role of CD40-CD40L in MHV induced neuroinflammatory demyelination. Together, these studies highlight that migration of peripheral T cells and their interaction with microglia via CD40-CD40L is essential to eliminate the virus and provide long-term neuroprotection. These findings can lead to designing potential therapeutic interventions against MS, targeting CD40-CD40L interaction.

Meningoencephalomyelitis emanates under the umbrella relating inflammatory changes of the Central Nervous System (CNS). Meningitis denotes inflammation in the meningeal layers, encephalitis is an acute diffuse inflammation of the brain, and inflammation in the spinal cord is denoted as myelitis. These can be interrelated or independent of each other depending on the etiology. The entire mechanism of meningoencephalomyelitis is governed by an acute innate inflammatory branch followed by a chronic progressive, adaptive branch of immunity with clinical signs like hyperthermia, weight loss, hypoxia, leukocytosis. This book chapter will focus on viral-induced meningitis, encephalitis, and myelitis. Thirty years of experience working with a murine-β-coronavirus (m-CoV); Mouse hepatitis virus (MHV)-A59 induced experimental model system provided us a thorough understanding of neuroglial cell-mediated acute neuroinflammation, denoted by the accumulation of leukocyte-common-antigen (LCA) positive or CD45+ leukocytes in perivascular infiltrates referred to as perivascular cuff formation and microglial nodules in the brain parenchyma, which mimics specific pathology of human neurological disease multiple sclerosis (MS). Additionally, in this chapter, we summarized the role of CNS resident microglial activation and its interaction with peripheral migratory T cells in mounting neuropathogenesis and host immunity in different families of neurotrophic encephalomyelitis viruses that cause CNS inflammation.

Interferon-induced protein with tetratricopeptide repeats 2, Ifit2, is critical in restricting neurotropic murine-β-coronavirus, RSA59 infection. RSA59 intracranial injection of Ifit2-deficient (-/-) compared to wild-type (WT) mice results in impaired acute microglial activation, reduced CX3CR1 expression, limited migration of peripheral lymphocytes into the brain, and impaired virus control followed by severe morbidity and mortality. While the protective role of Ifit2 is established for acute viral encephalitis, less is known about its influence during the chronic demyelinating phase of RSA59 infection. To understand this, RSA59 infected Ifit2 -/- and Ifit2 +/+ (WT) were observed for neuropathological outcomes at day 5 (acute phase) and 30 post-infection (chronic phase). Our study demonstrates that Ifit2 deficiency causes extensive RSA59 spread throughout the spinal cord gray and white matter, associated with impaired CD4 + T and CD8 + T cell infiltration. Further, the cervical lymph nodes of RSA59 infected Ifit2 -/- mice showed reduced activation of CD4 + T cells and impaired IFNγ expression during acute encephalomyelitis. Interestingly, BBB integrity was better preserved in Ifit2 -/- mice, as evidenced by tight junction protein Claudin-5 and adapter protein ZO-1 expression surrounding the meninges and blood vessels and decreased Texas red dye uptake, which may be responsible for reduced leukocyte infiltration. In contrast to sparse myelin loss in WT mice, the chronic disease phase in Ifit2 -/- mice was associated with severe demyelination and persistent viral load, even at low inoculation doses. Overall, our study highlights that Ifit2 provides antiviral functions by promoting acute neuroinflammation and thereby aiding virus control and limiting severe chronic demyelination. IMPORTANCE Interferons execute their function by inducing specific genes collectively termed as interferon-stimulated genes (ISGs), among which interferon-induced protein with tetratricopeptide repeats 2, Ifit2, is known for restricting neurotropic viral replication and spread. However, little is known about its role in viral spread to the spinal cord and its associated myelin pathology. Toward this, our study using a neurotropic murine β-coronavirus and Ifit2-deficient mice demonstrates that Ifit2 deficiency causes extensive viral spread throughout the gray and white matter of the spinal cord accompanied by impaired microglial activation and T cell infiltration. Furthermore, infected Ifit2-deficient mice showed impaired activation of T cells in the cervical lymph node and relatively intact blood–brain barrier integrity. Overall, Ifit2 plays a crucial role in mounting host immunity against neurotropic murine coronavirus in the acute phase while preventing mice from developing viral-induced severe chronic neuroinflammatory demyelination, the characteristic feature of human neurological disease multiple sclerosis (MS).