Human Coronaviruses and Other Respiratory Viruses: Underestimated Opportunistic Pathogens of the Central Nervous System?

Abstract

Respiratory viruses infect the human upper respiratory tract, mostly causing mild diseases. However, in vulnerable populations, such as newborns, infants, the elderly and immune-compromised individuals, these opportunistic pathogens can also affect the lower respiratory tract, causing a more severe disease (e.g., pneumonia). Respiratory viruses can also exacerbate asthma and lead to various types of respiratory distress syndromes. Furthermore, as they can adapt fast and cross the species barrier, some of these pathogens, like influenza A and SARS-CoV, have occasionally caused epidemics or pandemics, and were associated with more serious clinical diseases and even mortality. For a few decades now, data reported in the scientific literature has also demonstrated that several respiratory viruses have neuroinvasive capacities, since they can spread from the respiratory tract to the central nervous system (CNS). Viruses infecting human CNS cells could then cause different types of encephalopathy, including encephalitis, and long-term neurological diseases. Like other well-recognized neuroinvasive human viruses, respiratory viruses may damage the CNS as a result of misdirected host immune responses that could be associated with autoimmunity in susceptible individuals (virus-induced neuro-immunopathology) and/or viral replication, which directly causes damage to CNS cells (virus-induced neuropathology). The etiological agent of several neurological disorders remains unidentified. Opportunistic human respiratory pathogens could be associated with the triggering or the exacerbation of these disorders whose etiology remains poorly understood. Herein, we present a global portrait of some of the most prevalent or emerging human respiratory viruses that have been associated with possible pathogenic processes in CNS infection, with a special emphasis on human coronaviruses.

Keywords: CNS infection; acute and chronic neurological diseases; encephalitis; encephalopathy; human coronavirus; human respiratory virus; neuroinvasion; respiratory viral infection.

Figure 1

Illustration of the principal route of infection used by HCoV-OC43 for neuroinvasion in the central nervous system (CNS). (A) Schematic representation of intranasal injection of HCoV-OC43 in susceptible mice. (B) Histological examination of decalcified whole head allows to visualize virus spread in the CNS at 3 dpi. Left top panel represents the nasal cavity and right top panel represents a higher magnification of infected olfactory receptor neurons (ORN) in the neuroepithelium. Bottom panel represents viral dissemination in several regions of the brain from the olfactory bulb to the brainstem. The inset on the right represents a zoomed image of the area delimited by the red frame in left panel. The red arrow indicates the enlarged region in the red frame. In all regions of the brain, neurons are the main target of infection. Detection of viral S glycoprotein (green) and cell nucleus (DAPI; blue). Magnification is 20× and 63× for upper panels and 4× for the bottom panel. (C) Corresponding schematic representation of intranasal infection in humans. HCoV may infect the ORN, pass through the neuroepithelium and gain access to the olfactory bulb (OB) and eventually to other regions of the brain. The blue arrows indicate the direction of viral spreading. Schematic representations were assembled using the Motifolio Neuroscience Toolkit 2007.

Figure 2

Model of axonal transport and neuron-to-neuron or neuron-to-non-neuronal cells propagation. (A) Murine primary mixed neuronal cultures (PMNC) grown in Xonachip microfluidic compartmentalized chambers. These devices allow fluidic isolation of axons by establishing a volume difference between cell bodies (1) and axonal end (3) compartments and the high fluidic resistance of the microchannels (2) (where the axons grow) produces a sustained flow that counteracts diffusion. The blue dotted lines are there to separate the three different parts of the microfluidic device. This situation blocks the migration of free virus through the microchannels and makes it possible to use this system in viral infection studies in neuronal cultures. PMNC were infected in the cell body portion (1) and viral S glycoprotein (yellow/green) and neuronal marker (MAP2 protein; red) detection was performed on fixed cells at 24 hpi. Data are indicative of viral antigens and/or viral particles going from cell body (1) through the axon in the microchannel (2) and then towards the axonal end portion (3). (B) Electron microscopy images of infected PMNC grown on Aclar-33C embedding film (Electron Microscopy Sciences) and infected at an MOI of 0.03 for 48 h at 37 °C. Sliced embedded samples (EPON RESIN 828; Polysciences Inc., Warrington, PA, USA) observed with a Hitachi H 7100 electron microscope show viral particles (white arrows) in both cell bodies and axons at 48 hpi. Upper panel is a complete neuron (magnification 5000×), left lower panel is a representative neuronal cell body (magnification 10,000×) and lower right panel is a representative axonal portion (magnification 20,000×). Pictures were taken with an AMTXR-111 camera (Advanced Microscopy Techniques, Woburn, MA, USA). (C) Model of HCoV-OC43 propagation (neuron-to-neuron or neuron-to-non-neuronal cells) based on our data [269] and adapted from Tomishima and Enquist [291]. In this model, solid arrows represent fully assembled virus transport and dashed arrows represent subvirion assemblies [291]. Schematic representations were assembled with the Motifolio Neuroscience Toolkit, 2007.

Figure 3

Non-invasive imaging of viral neuroinvasion and dissemination within the CNS in living infected mice and associated clinical scores. A recombinant HCoV-OC43 harboring a luciferase reporter gene [292] was injected intra-nasally (I.N.) into mice. Virus spread was assessed by bioluminescence imaging (BLI) with the Xenogen VIVO Vision IVIS 100 imaging system (Perkin-Elmer) in infected anaesthetized mice placed in a light proof specimen chamber after intraperitoneal injection of d-luciferin. Images were taken with a CCD camera mounted in a light-tight imaging chamber, using the acquisition software Living Image version 4.3.1 (Caliper-LifeSciences). Evaluation of associated clinical scores: (levels 0 to 4: 0 is asymptomatic; 1 is mice with early hunched back; 2 is mice presenting slight social isolation, weight loss and abnormal gait; 3 is mice presenting total social isolation, ruffled fur, hunched back, weight loss and almost no movement; and 4 is mice moribund or dead (presented elsewhere; [266]), indicate that only mice with a positive signal at both the level of the brain and spinal cord were evaluated to be at level 2 to 3.