Intrathecal complement activation by the classical pathway in tick-borne encephalitis

An autochthonous case of severe tick-borne encephalitis virus associated meningoencephalitis in France: Is there a place for polyvalent intravenous immunoglobulins?

Tick-borne encephalitis virus (TBEV) is a common cause of viral encephalitis in parts of Central and Eastern Europe, with a recent resurgence of cases and geographical expansion. Active immunization results in a high rate of seroconversion and is the most effective measure to reduce the incidence of tick-borne encephalitis (TBE). In France, an endemic country, vaccination is recommended only for travelers staying in rural or forested areas in endemic regions. Polyvalent intravenous immunoglobulin (IVIG) is sometimes used as rescue treatment of viral encephalitis. However, few cases of TBEV meningoencephalitis treated with polyvalent IVIG have been described. We report here a case of autochthonous TBEV meningoencephalitis in a French patient with cranial nerve involvement that was treated with IVIG and discuss the possible mechanisms of action.

The activation of complement C5a-C5aR1 axis in astrocytes facilitates the neuropathogenesis due to EV-A71 infection by upregulating CXCL1

Enterovirus A71 (EV-A71) is a common small RNA virus that is highly neuroinvasive. Emerging evidence indicates that the complement fragment C5a and its receptor C5aR1 are important drivers of neuroinflammation. However, the potential role of the C5a-C5aR1 axis in EV-A71 encephalitis remains largely elusive. Our previous studies revealed that EV-A71 can infect astrocytes and result in complement activation in vivo. Here, we investigated how complement factors interact with astrocytes to promote a severe inflammatory response upon EV-A71 infection. Our data revealed that EV-A71 infected mainly astrocytes and caused astrocyte activation in the mouse brain, which was further verified in patients with EV-A71 infection and U87-MG cells. Notably, EV-A71 infection led to activation of the C5a-C5aR1 axis in U87-MG cells, and knockdown (siC5aR1) or blockade (PMX53) of C5aR1 significantly suppressed EV-A71-induced astrocyte activation and proinflammatory cytokine (e.g., CXCL1) production. Next, the activation of the C5a-C5aR1 axis in mouse astrocytes was confirmed. Compared with C5aR1 knockout mice, wild-type mice presented more severe symptoms and lower survival rates after EV-A71 infection. C5aR1 deficiency or blockade significantly reduced EV-A71-induced pathological damage and proinflammatory cytokine production in the mouse brain. Importantly, an increased level of soluble C5a was strongly correlated with the severity of symptoms in patients with EV-A71 infection. By using confocal microscopy, primary astrocytes, and human specimens, we observed that the increase in CXCL1 levels resulted mainly from astrocytes. Neutralizing CXCL1 significantly alleviated the neuropathological changes caused by EV-A71 infection, and the production of CXCL1 in astrocytes was regulated by p38 MAPK signaling. Taken together, our findings indicate that the activation of the C5a-C5aR1 axis in astrocytes facilitates the neuropathological changes resulting from EV-A71 infection, emphasizing the potential role of p38 MAPK-mediated CXCL1 production in these alterations.

IMPORTANCE

Enterovirus A71 (EV-A71) is a common small RNA virus with highly neuroinvasive tendencies. Our previous studies took the view that EV-A71 could infect astrocytes and result in complement activation in vivo. We investigated how complement interacts with astrocytes to promote a severe inflammatory response upon EV-A71 infection in the study. As expected, our data demonstrate that EV-A71 triggers robust activation of the C5a-C5aR1 axis in astrocytes and that knockout or blockade of C5aR1 in animals exposed to lethal doses of EV-A71 significantly enhances survival by diminishing the production of the chemokines CXCL1 and IL-6. In addition, neutralizing CXCL1 significantly alleviates the neuropathogenesis caused by EV-A71 infection. Thus, inhibiting the C5a-C5aR1 axis has emerged as a potential therapeutic strategy to mitigate neural damage caused by EV-A71 infection.

Long-term Elevation of Complement Factors in Cerebrospinal Fluid of Patients With Borna Disease Virus 1 Encephalitis

BACKGROUND

Borna disease virus 1 (BoDV-1) causes rare but severe zoonotic infections in humans, presenting as encephalitis. The case-fatality risk is very high and no effective countermeasures have been established so far. An immunopathology is presumed, while data on immune responses in humans are limited. Evidence of a role of the complement system in various neurological disorders and in viral infections of the central nervous system is increasing and specific inhibitors are available as therapeutic options.

METHODS

In this study, we investigated factors of the complement system in the cerebrospinal fluid (CSF) of patients with BoDV-1 infections (n = 17) in comparison to noninflammatory control CSF samples (n = 11), using a bead-based multiplex assay. In addition, immunohistochemistry was performed using postmortem brain tissue samples.

RESULTS

We found an intrathecal elevation of complement factors of all complement pathways and an active cascade during human BoDV-1 infections. The increase of certain complement factors such as C1q was persistent, and C3 complement deposits were detected in postmortem brain sections. Intrathecal complement levels were negatively correlated with survival.

CONCLUSIONS

Further investigations are warranted to clarify whether targeting the complement cascade by specific inhibitors might be beneficial for patients suffering from severe BoDV-1 encephalitis.

The complement system in neurodegenerative and inflammatory diseases of the central nervous system

Neurodegenerative and neuroinflammatory diseases, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis, affect millions of people globally. As aging is a major risk factor for neurodegenerative diseases, the continuous increase in the elderly population across Western societies is also associated with a rising prevalence of these debilitating conditions. The complement system, a crucial component of the innate immune response, has gained increasing attention for its multifaceted involvement in the normal development of the central nervous system (CNS) and the brain but also as a pathogenic driver in several neuroinflammatory disease states. Although complement is generally understood as a liver-derived and blood or interstitial fluid operative system protecting against bloodborne pathogens or threats, recent research, particularly on the role of complement in the healthy and diseased CNS, has demonstrated the importance of locally produced and activated complement components. Here, we provide a succinct overview over the known beneficial and pathological roles of complement in the CNS with focus on local sources of complement, including a discussion on the potential importance of the recently discovered intracellularly active complement system for CNS biology and on infection-triggered neurodegeneration.

Tick-Borne Encephalitis (TBE): From Tick to Pathology

Tick-borne encephalitis (TBE) is a viral arthropod infection, endemic to large parts of Europe and Asia, and is characterised by neurological involvement, which can range from mild to severe, and in 33-60% of cases, it leads to a post-encephalitis syndrome and long-term morbidity. While TBE virus, now identified as Orthoflavivirus encephalitidis, was originally isolated in 1937, the pathogenesis of TBE is not fully appreciated with the mode of transmission (blood, tick, alimentary), viral strain, host immune response, and age, likely helping to shape the disease phenotype that we explore in this review. Importantly, the incidence of TBE is increasing, and due to global warming, its epidemiology is evolving, with new foci of transmission reported across Europe and in the UK. As such, a better understanding of the symptomatology, diagnostics, treatment, and prevention of TBE is required to inform healthcare professionals going forward, which this review addresses in detail. To this end, the need for robust national surveillance data and randomised control trial data regarding the use of various antivirals (e.g., Galidesivir and 7-deaza-2'-CMA), monoclonal antibodies, and glucocorticoids is required to improve the management and outcomes of TBE.

Powassan Viruses Spread Cell to Cell during Direct Isolation from Ixodes Ticks and Persistently Infect Human Brain Endothelial Cells and Pericytes

Powassan viruses (POWVs) are neurovirulent tick-borne flaviviruses emerging in the northeastern United States, with a 2% prevalence in Long Island (LI) deer ticks (Ixodes scapularis). POWVs are transmitted within as little as 15 min of a tick bite and enter the central nervous system (CNS) to cause encephalitis (10% of cases are fatal) and long-term neuronal damage. POWV-LI9 and POWV-LI41 present in LI Ixodes ticks were isolated by directly inoculating VeroE6 cells with tick homogenates and detecting POWV-infected cells by immunoperoxidase staining. Inoculated POWV-LI9 and LI41 were exclusively present in infected cell foci, indicative of cell to cell spread, despite growth in liquid culture without an overlay. Cloning and sequencing establish POWV-LI9 as a phylogenetically distinct lineage II POWV strain circulating in LI deer ticks. Primary human brain microvascular endothelial cells (hBMECs) and pericytes form a neurovascular complex that restricts entry into the CNS. We found that POWV-LI9 and -LI41 and lineage I POWV-LB productively infect hBMECs and pericytes and that POWVs were basolaterally transmitted from hBMECs to lower-chamber pericytes without permeabilizing polarized hBMECs. Synchronous POWV-LI9 infection of hBMECs and pericytes induced proinflammatory chemokines, interferon-β (IFN-β) and proteins of the IFN-stimulated gene family (ISGs), with delayed IFN-β secretion by infected pericytes. IFN inhibited POWV infection, but despite IFN secretion, a subset of POWV-infected hBMECs and pericytes remained persistently infected. These findings suggest a potential mechanism for POWVs (LI9/LI41 and LB) to infect hBMECs, spread basolaterally to pericytes, and enter the CNS. hBMEC and pericyte responses to POWV infection suggest a role for immunopathology in POWV neurovirulence and potential therapeutic targets for preventing POWV spread to neuronal compartments. IMPORTANCE We isolated POWVs from LI deer ticks (I. scapularis) directly in VeroE6 cells, and sequencing revealed POWV-LI9 as a distinct lineage II POWV strain. Remarkably, inoculation of VeroE6 cells with POWV-containing tick homogenates resulted in infected cell foci in liquid culture, consistent with cell-to-cell spread. POWV-LI9 and -LI41 and lineage I POWV-LB strains infected hBMECs and pericytes that comprise neurovascular complexes. POWVs were nonlytically transmitted basolaterally from infected hBMECs to lower-chamber pericytes, suggesting a mechanism for POWV transmission across the blood-brain barrier (BBB). POWV-LI9 elicited inflammatory responses from infected hBMEC and pericytes that may contribute to immune cell recruitment and neuropathogenesis. This study reveals a potential mechanism for POWVs to enter the CNS by infecting hBMECs and spreading basolaterally to abluminal pericytes. Our findings reveal that POWV-LI9 persists in cells that form a neurovascular complex spanning the BBB and suggest potential therapeutic targets for preventing POWV spread to neuronal compartments.

Neuroprotective versus Neuroinflammatory Roles of Complement: From Development to Disease

Complement proteins are ancient components of innate immunity that have emerged as crucial regulators of neural networks. We discuss these roles in the context of the CNS development, acute CNS viral infections, and post-infectious and noninfectious CNS disorders, with an emphasis on microglia-mediated loss of synapses. Despite extensive examples that implicate classical complement proteins and their receptors in CNS dysfunction, recent data suggest that they exert neuroprotective roles in CNS homeostasis through continued refinement of synaptic connections. Thorough understanding of the mechanisms involved in these processes may lead to novel targets for the treatment of CNS diseases involving aberrant complement-mediated synapse loss.

Aristolochic acid I aggravates renal injury by activating the C3a/C3aR complement system

Previous studies have reported that the complement system is unconventionally activated in many kinds of glomerulonephritis. Multiple complement components participate in the pathogenic process by triggering immune response or other intracellular signaling pathways. Here, we have investigated the role of C3a and its receptor C3aR in aristolochic acid nephropathy (AAN), which, is featured with progressive interstitial fibrosis. Over release of C3a and increased expression of C3aR parallels to the up-regulation of α-SMA and TGF-β1 in AAN, which appeared to promote epithelial-mesenchymal-transition (EMT). To identify the role of complement activation in AAN, we used an inhibitor of C3aR (C3aRA) to block the coupling of C3a to its receptor. Our results confirmed from decreased EMT, the protective effect of C3aRA in cell apoptosis and inflammatory response induced by aristolochic acid I. These results showed that C3a and its receptor C3aR played pathogenic roles in AAN, and renal tubular epithelial cells were potentially pivotal targets of complement activation that could cause pro-fibrotic effects.