Analysis of the intrathecal humoral immune response in Brown Norway (BN) rats, infected with the murine coronavirus JHM

Neurological Complications Associated with the Blood-Brain Barrier Damage Induced by the Inflammatory Response During SARS-CoV-2 Infection

The main discussion above of the novel pathogenic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has focused substantially on the immediate risks and impact on the respiratory system; however, the effects induced to the central nervous system are currently unknown. Some authors have suggested that SARS-CoV-2 infection can dramatically affect brain function and exacerbate neurodegenerative diseases in patients, but the mechanisms have not been entirely described. In this review, we gather information from past and actual studies on coronaviruses that informed neurological dysfunction and brain damage. Then, we analyzed and described the possible mechanisms causative of brain injury after SARS-CoV-2 infection. We proposed that potential routes of SARS-CoV-2 neuro-invasion are determinant factors in the process. We considered that the hematogenous route of infection can directly affect the brain microvascular endothelium cells that integrate the blood-brain barrier and be fundamental in initiation of brain damage. Additionally, activation of the inflammatory response against the infection represents a critical step on injury induction of the brain tissue. Consequently, the virus’ ability to infect brain cells and induce the inflammatory response can promote or increase the risk to acquire central nervous system diseases. Here, we contribute to the understanding of the neurological conditions found in patients with SARS-CoV-2 infection and its association with the blood-brain barrier integrity.

Coronaviruses: a challenge of today and a call for extended human postmortem brain analyses

While there is abounding literature on virus-induced pathology in general and coronavirus in particular, recent evidence accumulates showing distinct and deleterious brain affection. As the respiratory tract connects to the brain without protection of the blood-brain barrier, SARS-CoV-2 might in the early invasive phase attack the cardiorespiratory centres located in the medulla/pons areas, giving rise to disturbances of respiration and cardiac problems. Furthermore, brainstem regions are at risk to lose their functional integrity. Therefore, long-term neurological as well as psychiatric symptomatology and eventual respective disorders cannot be excluded as evidenced from influenza-A triggered post-encephalitic Parkinsonism and HIV-1 triggered AIDS-dementia complex. From the available evidences for coronavirus-induced brain pathology, this review concludes a number of unmet needs for further research strategies like human postmortem brain analyses. SARS-CoV-2 mirroring experimental animal brain studies, characterization of time-dependent and region-dependent spreading behaviours of coronaviruses, enlightening of pathological mechanisms after coronavirus infection using long-term animal models and clinical observations of patients having had COVID-19 infection are calling to develop both protective strategies and drug discoveries to avoid early and late coronavirus-induced functional brain disturbances, symptoms and eventually disorders. To fight SARS-CoV-2, it is an urgent need to enforce clinical, molecular biological, neurochemical and genetic research including brain-related studies on a worldwide harmonized basis.

Viruses and multiple sclerosis

Multiple sclerosis (MS) is a chronic demyelinating disorder of unknown etiology, possibly caused by a virus or virus-triggered immunopathology. The virus might reactivate after years of latency and lyse oligodendrocytes, as in progressive multifocal leukoencephalopathy, or initiate immunopathological demyelination, as in animals infected with Theiler's murine encephalomyelitis virus or coronaviruses. The argument for a viral cause of MS is supported by epidemiological analyses and studies of MS in identical twins, indicating that disease is acquired. However, the most important evidence is the presence of bands of oligoclonal IgG (OCBs) in MS brain and CSF that persist throughout the lifetime of the patient. OCBs are found almost exclusively in infectious CNS disorders, and antigenic targets of OCBs represent the agent that causes disease. Here, the authors review past attempts to identify an infectious agent in MS brain cells and discuss the promise of using recombinant antibodies generated from clonally expanded plasma cells in brain and CSF to identify disease-relevant antigens. They show how this strategy has been used successfully to analyze antigen specificity in subacute sclerosing panencephalitis, a chronic encephalitis caused by measles virus, and in neuromyelitis optica, a chronic autoimmune demyelinating disease produced by antibodies directed against the aquaporin-4 water channel.

Viruses and Multiple Sclerosis

Multiple sclerosis (MS) is a chronic demyelinating disorder of unknown etiology, possibly caused by a virus or virus-triggered immunopathology. The virus might reactivate after years of latency and lyse oligodendrocytes, as in progressive multifocal leukoencephalopathy, or initiate immunopathological demyelination, as in animals infected with Theiler’s murine encephalomyelitis virus or coronaviruses. The argument for a viral cause of MS is supported by epidemiological analyses and studies of MS in identical twins, indicating that disease is acquired. However, the most important evidence is the presence of bands of oligoclonal IgG (OCBs) in MS brain and CSF that persist throughout the lifetime of the patient. OCBs are found almost exclusively in infectious CNS disorders, and antigenic targets of OCBs represent the agent that causes disease. Here, the authors review past attempts to identify an infectious agent in MS brain cells and discuss the promise of using recombinant antibodies generated from clonally expanded plasma cells in brain and CSF to identify disease-relevant antigens. They show how this strategy has been used successfully to analyze antigen specificity in subacute sclerosing panencephalitis, a chronic encephalitis caused by measles virus, and in neuromyelitis optica, a chronic autoimmune demyelinating disease produced by antibodies directed against the aquaporin-4 water channel.

The B cell response in multiple sclerosis

Multiple sclerosis (MS) plaques and CSF contain increased amounts of intrathecally synthesized IgG, manifest as oligoclonal bands (OCBs) after protein electrophoresis. OCBs are not unique to MS and are also produced in infectious diseases of the CNS, in which the oligoclonal IgG has been shown to be antibody directed against the disease-causing agent. Thus, analysis of antibody specificity may identify the causative agent/antigen in MS. This review discusses recent studies that have analyzed the phenotypes of B cells in MS which infiltrate the CNS and the molecular features of their antigen-binding regions. Together with histologic studies showing the presence of ectopic lymphoid follicles in the meninges of some MS patients, this data supports the notion of a targeted and compartmentalized humoral response in MS.

CNS viral infection diverts homing of antibody-secreting cells from lymphoid organs to the CNS

Neurotropic coronavirus infection of mice results in acute encephalomyelitis followed by viral persistence. Whereas cellular immunity controls acute infection, humoral immunity regulates central nervous system (CNS) persistence. Maintenance of serum Ab was correlated with tissue distribution of virus-specific Ab-secreting cells (ASC). Although virus-specific ASC declined in cervical lymph node and spleen after infectious virus clearance, virus-specific serum Ab was sustained at steady levels, with a delay in neutralizing Ab. Virus-specific ASC within the CNS peaked rapidly 1 wk after control of infectious virus and were retained throughout chronic infection, consistent with intrathecal Ab synthesis. Surprisingly, frequencies of ASC in the BM remained low and only increased gradually. Nevertheless, virus-specific ASC induced by peripheral infection localized to both spleen and BM. The data suggest that CNS infection provides strong stimuli to recruit ASC into the inflamed tissue through sustained up-regulation of the CXCR3 ligands CXCL9 and CXCL10. Irrespective of Ag deprivation, CNS retention of ASC coincided with elevated BAFF expression and ongoing differentiation of class II+ to class II-CD138+CD19+ plasmablasts. These results confirm the CNS as a major ASC-supporting environment, even after resolution of viral infection and in the absence of chronic ongoing inflammation.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

Recruitment kinetics and composition of antibody-secreting cells within the central nervous system following viral encephalomyelitis

Infection by the neurotropic JHM strain of mouse hepatitis virus produces an acute demyelinating encephalomyelitis. While cellular immunity initially eliminates infectious virus, CNS viral persistence is predominantly controlled by humoral immunity. To better understand the distinct phases of immune control within the CNS, the kinetics of humoral immune responses were determined in infected mice. Early during clearance of the JHM strain of mouse hepatitis virus, only few virus-specific Ab-secreting cells (ASC) were detected in the periphery or CNS, although mature B cells and ASC without viral specificity were recruited into the CNS concomitant with T cells. Serum antiviral Ab and CNS virus-specific ASC became prominent only during final elimination of infectious virus. Virus-specific ASC peaked in lymphoid organs before the CNS, suggesting peripheral B cell priming and maturation. Following elimination of infectious virus, virus-specific ASC continued to increase within the CNS and then remained stable during persistence, in contrast to declining T cell numbers. These data comprise three novel findings. Rapid recruitment of B cells in the absence of specific Ab secretion supports a potential Ab-independent effector function involving lysis of virus-infected cells. Delayed recruitment relative to viral clearance and subsequent maintenance of a stable CNS ASC population demonstrate differential regulation of T and B lymphocytes within the infected CNS. This supports a critical role of humoral immunity in regulating viral CNS persistence. Lastly, altered antiviral ASC specificities following clearance of infectious virus suggest ongoing recruitment of peripheral memory cells and/or local B cell differentiation.

B-cell-mediated lysis of cells infected with the neurotropic JHM strain of mouse hepatitis virus

Cells expressing the spike (S) glycoprotein of the neurotropic JHM strain (JHMV) of mouse hepatitis virus (MHV) are susceptible to lysis by B cells derived from naïve mice, including B cells from perforin-deficient mice. Cytolysis requires interaction of the virus receptor and the viral S glycoprotein, is independent of other viral-induced components, and is not a unique property of B cells. Neutralizing anti-S-protein monoclonal antibodies (mAb) and a mAb specific for the viral receptor inhibit lysis. However, cells infected with an MHV strain unable to induce cell-cell fusion are resistant to lysis and lysis of JHMV-infected cells is inhibited by an anti-S-protein nonneutralizing mAb which prevents S-protein-mediated cell fusion. These data suggest that B cells may function as antibody-independent innate immune response during JHMV infection in vivo.

Specific deposition of passively transferred monoclonal antibodies against herpes simplex virus type 1 in rat brain infected with the virus

The kinetics of human monoclonal antibody (anti-gB) to herpes simplex virus type 1 (HSV-1) were investigated after intravenous injection of anti-gB into an HSV-1 encephalitis animal model. Immunohistochemical study revealed specific deposition of passively transferred anti-gB in the hippocampus and thalamus of the infected rat brain, and it bound to the same neurons in which HSV-1 antigen was positively stained. To examine the macroscopic distribution of anti-gB in the infected brain, we undertook an 125I-labeled anti-gB injection study, and the same distribution of 125I-labeled anti-gB deposition was observed by brain semimicroautoradiography as in the immunohistochemical study. These results suggest that anti-gB easily permeates the capillary wall and is deposited in the inflammatory site where HSV-1-specific antigen is detectable. The use of radioisotope-labeled anti-gB injection and external brain imaging could lead to a noninvasive diagnostic tool for the early detection of HSV-1 antigen in cases of suspected HSV-1 encephalitis.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

Coronavirus induced primary demyelination: indications for the involvement of a humoral immune response

Coronavirus MHV-JHM infection of rodents can result in demyelinating encephalomyelitis. We analysed histological changes induced by coronavirus MHV-JHM infection in Lewis rats. Besides an acute disease (AE), chronic panencephalitis (CPE) and subacute demyelinating encephalomyelitis (SDE) were induced. These disease types were differentiated by the incubation period, the localization of lesions, the type of tissue damage and distribution of virus antigen. In AE and CPE, virus antigen was detected in neurons, astrocytes and oligodendrocytes, whereas in SDE neurons lacked virus antigen. Viral nucleocapsid protein (N) was present in the cytoplasm and the spike protein (S) was displayed on the surface of infected neural cells. However, expression of S protein relative to N protein was severely impaired in SDE lesions. Quantitative analysis of infiltrating inflammatory cells revealed that the number of macrophages and T cells were similar in lesions of AE, CPE and SDE. In contrast to that, SDE lesions contained a significantly higher number of IgG + B cells and plasma cells. In addition active demyelinating SDE lesions displayed an enhanced IgG content and deposits of complement C9. These results indicate that virus induced primary demyelination could be a consequence of antibody mediated cytotoxicity. Furthermore, a reduction in the number of cells producing spike protein in the chronic forms of the disease indicates down-regulation of this protein, possibly mediated by anti-S antibodies.

Population dynamics of lymphocyte subsets in the central nervous system of rats with different susceptibility to coronavirus-induced demyelinating encephalitis

The inflammatory response in the central nervous system (CNS) of rats with differing susceptibility to demyelinating encephalitis induced by coronavirus MHV4 was characterized. Topographical maps showing the arrangement of infiltrating lymphocyte subsets in virus-infected tissue were developed by digital-image processing of immunohistologically stained CNS sections. The kinetics of the inflammatory process was evaluated by flow-cytometry on lymphocytes isolated from the CNS. Cumulative data obtained with these two techniques demonstrated the following features. In susceptible Lewis (LE) rats, viral antigens were disseminated throughout the CNS, including spinal cord. Onset as well as recovery from neurological disease was associated with a steep rise of infiltrating CD8+ T cells, which localized in close contact to virus-infected cells. Accompanying convalescence was a slight increase in B(OX33+) cells in the CNS and the accumulation of immunoglobulin-containing cells in the centre of virus-infected areas. In clinically resistant Brown Norway (BN) rats, virus-infected cells were mainly restricted to small periventricular foci and the extent of lymphocyte infiltration was never as high as that found at any time during the course of infection in LE rats. There were striking differences in the CD8+ T-cell population compared to LE rats. Cells of this phenotype were identified in virus-affected areas of BN rats only early after infection, and their infiltration profile revealed much lower quantities than in the CNS of susceptible LE rats. Although the population dynamics of B(OX33+) lymphocytes were comparable in BN and LE rats, as determined by flow-cytometry, less immunoglobulin-containing B cells were detected in virus-infected areas of BN rats.

The pathogenic role of virus-specific antibody-secreting cells in the central nervous system of rats with different susceptibility to coronavirus-induced demyelinating encephalitis

The humoral immune response in the central nervous system (CNS) of susceptible Lewis (LE) rats and resistant Brown Norway (BN) rats was analysed after intracerebral infection with the murine coronavirus JHM (MHV4). The subclinical course of the infection in BN rats was characterized by an early rise of neutralizing antibodies in the cerebrospinal fluid (CSF) 7 days post-infection. At this time in LE rats, neutralizing antibodies were not detectable in the CSF and the animals developed neurological signs of infection. Subsequently, LE rats recovered from disease. This process was accompanied by increasing titres of virus-neutralizing antibodies. Within the CNS parenchyma of both rat strains, equivalent numbers of IgM-secreting cells were detected. However, in BN rats, virus-specific IgG secreting cells appeared earlier and in higher numbers. Moreover, based on the size of zones of antibody secreted by single cells in the Spot-ELISA assay, it appeared that cells from BN rats secreted IgG antibody of higher affinity. These data suggest that early maturation of antiviral antibody responses in the resistant BN rat probably restricts the spread of viral infection to small foci within the CNS, resulting in a subclinical level of primary demyelination. In contrast, the absence of neutralizing antibodies in the susceptible LE rats favours spread of the virus throughout the CNS, resulting finally in severe neurological disease.

Identification of the spinal cord as a major site of persistence during chronic infection with a murine coronavirus

After intranasal inoculation, mouse hepatitis virus (MHV) gains entry into the central nervous system (CNS) via the olfactory and trigeminal nerves. Under the appropriate conditions, some mice develop clinically apparent demyelinating encephalomyelitis several weeks later, with virus always present in the spinal cord. To determine the pathway by which virus reaches the cord, brains and spinal cords of infected, asymptomatic mice were analyzed by in situ hybridization. Viral RNA was always detected in the anterior part of the upper spinal cord. A similar analysis of mice with the recent onset of hindlimb weakness showed that viral RNA was detected in the same location. The results suggest that MHV is transported to the spinal cord via well-defined neuroanatomic pathways and that viral amplification with resultant clinical disease occurs from this site of persistence in the anterior spinal cord. This process of viral amplification may involve the generation of viral variants as has been described for MHV-infected rats. No major changes in viral RNA or protein could be detected when MHV isolated from mice with hindlimb paralysis was analyzed. The data suggest that the generation of viral variants is not important in the pathogenesis of the late onset of neurological disease induced by MHV in mice.

Astrocytes as antigen presenting cells for primary and secondary T cell responses: effect of astrocyte infection by murine hepatitis virus

CD4+ T cell lines specific for murine hepatitis virus (MHV) - JHM or myelin basic protein (MBP) proliferated when cultured together with MHC class I and II positive syngeneic rat astrocytes and either inactivated virus or MBP as antigen. The magnitude of the T cell proliferative response was comparable to that seen when thymocytes were used as a source of antigen presenting cells (APC). In contrast, MHC class I and II positive astrocytes were unable to significantly stimulate the proliferation of highly purified populations of naive CD4+ and CD8+ T cells in an allogeneic mixed lymphocyte reaction (MLR). Both T cell populations proliferated when mixed with allogeneic lymph node cells. Infection of the astrocytes with a variant of MHV-JHM (PI-AS22D) did not alter this cells incapacity to stimulate the naive CD4+ and CD8+ T cells to proliferate.

Comparative analysis of virus-specific antibodies and immunoglobulins in serum and cerebrospinal fluid of subacute measles virus-induced encephalomyelitis (SAME) in rats and subacute sclerosing panencephalitis (SSPE)

The intrathecal humoral immune response was analysed in patients with subacute sclerosing panencephalitis (SSPE) and Lewis rats with subacute measles virus (MV)-induced encephalomyelitis (SAME). SSPE patients as well as SAME rats revealed oligoclonal, intrathecal antibody synthesis with MV specificity. SAME rats synthesized MV-specific antibodies intracerebrally to a higher extent than SSPE patients. Although a restricted isoelectric pattern of MV-specific antibodies was detected in the cerebrospinal fluid (CSF) of SSPE patients as well as of SAME rats, the heterogeneity within clusters of immunoglobulin bands was higher in the rat specimens. Increase in the blood-brain barrier permeability for albumin was exclusively detected in SAME rats but not in SSPE patients. These data suggest that the rat model offers excellent opportunities to study the initial humoral events in MV-induced encephalitides.

Borrelia burgdorferi--specific and autoreactive T-cell lines from cerebrospinal fluid in Lyme radiculomyelitis

In 3 patients with Lyme radiculomyelitis, cellular immune reactions of cerebrospinal fluid (CSF) lymphocytes were analyzed. Phenotypic analysis of CSF cells demonstrated that the majority were T cells (CD3+) of the helper/inducer subset (CD4+). These T cells were directly expanded from the CSF by limiting dilution. A total of 505 T-cell lines were tested for Borrelia burgdorferi (Bb)-specific proliferation and also partly tested for reactivity to a panel of central and peripheral nervous system antigens. Proliferative assays revealed 33 of them to be Bb specific, 16 to be specific for myelin basic protein, 16 to be specific for peripheral myelin, 1 to be specific for cardiolipin, and 2 to be specific for galactocerebrosides. The antigen-specific proliferation was restricted by autologous human leukocyte antigen (HLA) class II molecules. The majority of CSF-derived T-cell lines stained positively for CD3, CD4, and HLA class II antigens and negatively for CD8 (cytotoxic/suppressor subset). One T-cell line provided help for the production of specific IgG by autologous B cells and secreted gamma-interferon upon stimulation with Bb antigen in the presence of autologous antigen-presenting cells. These data show that in patients with severe neurological manifestations of late Lyme disease, not only Bb-specific T-cell lines but also T cells reactive to central or peripheral nervous system autoantigens can be found.

Persistent intrathecal secretion of oligoclonal, Borrelia burgdorferi-specific IgG in chronic meningoradiculomyelitis

In the cerebrospinal fluid IgG of five patients with lymphomeningoradiculitis (Bannwarth's syndrome) and radiculomyelitis studied by immunoblot technique an oligoclonal pattern was found. Most of these oligoclonal bands were specific for Borrelia burgdorferi. In patients suffering from chronic meningoradiculomyelitis, repeated CSF examination by this technique showed persistent secretion of identical IgG bands. Thus, the specific humoral immune response and the disease activity could be documented over the course of the disease.