CoronavirusesChapter 40

Structure and Composition• Enveloped

• Spike proteins resemble solar corona or crown

• 120-160 nm

• Positive-strand RNA (27-32 kb)

• Cytoplasmic replication

• Budding into ER and Golgi

• Notoriously difficult to propagate in culture

• High frequency of recombination

• Cause colds and severe acute respiratory syndrome (SARS)

Classification• Family Coronaviridae

• Genus Coronavirus

• Genus Torovirus

• Replication

• Details are largely unknown because viruses are difficult to grow in cell culture

• Mouse hepatitis virus is model for coronavirus replication• Viral spike proteins mediate attachment

• Aminopeptidase N is a cell receptor target for many coronaviruses

• Endocytosis is thought to mediate infection

• After uncoating, the viral genome (mRNA) is translated to produce RNA polymerase

• Subgenomic RNAs are synthesized for each viral polypeptide

• Genomic RNA is cosynthesized with nucleocapsid

• Results in nucleocapsid binding immediately to genomic RNA

• Progeny virus buds from ER and Golgi and are packaged into vesicles

• Vesicles travel to and fuse with plasma membrane, releasing viral particles from cell

Coronavirus Infections• Pathogenesis

• Limited knowledge

• Highly species-specific

• Typically mild upper respiratory infections (“colds”) that remain localized

• Exception: SARS

• Immunity is not durable

• Many people become resusceptible after a few years

• Laboratory Diagnosis

• ELISA - may not discriminate past infections

• HA

• PCR

• Virus isolation is difficult (often impossible) and requires great expertise

Severe Acute Respiratory Syndrome

• Initial outbreak in SE Asia

• Hong Kong and Singapore first reported

• Disease originated in China

• Originally thought to be from wild game markets• Palm civet cat (which isn’t a cat) - Paradoxurus

hermaphroditus

• Raccoon dog (which isn’t a dog) - Nyctereutes procyonoides

• It is a bat virus

• Chinese horseshoe bats (Rhinolophus sinicus)

• No virus isolation• Amplification of coronavirus RNA from anal swabs

• Serology

• It is highly-similar, but not identical to SARS-CoV

• Mutations have most likely occurred in transmission from bats to civets to humans

• Reverse genetics of SARS-CoV and some bat viruses has been done

• No animal pathogenesis model

SARS CoV

Coronavirus Phylogeny

Chymotrypsin-like protease (3CLpro), RNA-dependent RNA polymerase (Pol), spike (S), and nucleocapsid (N)

Coronaviruses Are Bat Viruses

SARS Pathogenesis

• Virus is transmitted by respiratory and fecal routes

• Infection is mediated by human angiotensin-converting enzyme 2 (hACE2) receptor

• High expression

• Lung alveolar epithelial cells

• Intestinal enterocytes

• Low expression

• Blood vessels (virtually all organs)

• Pneumonia

• Cause of death is lung failure

Pulmonary Inflammation of SARS

Pathologic findings of lung tissue sections. A: Pulmonary congestion and edema (H&E stain, original magnification x100). B: A mild degree of interstitial lymphocytic infiltration. Intra-

alveolar organizing exudative lesion was occasionally found. Detached atypical pneumocytes indicated by arrow (H&E stain, original magnification x200). C: Atypical multinucleated

pneumocytes were occasionally identified. Definite viral inclusion was not apparent (H&E stain, original magnification x400). D: Fibrin thrombi were frequently noted in small

pulmonary arteries and arterioles (H&E stain, original magnification x200).

Is SARS an Immunopathogenesis?

Criterion Evidence in SARS Precedent in other viral infections

Worse disease with decrease in viral load

Controversial; viral titres, measured in nasopharyngeal-aspirate samples, decrease as clinical disease worsens; but high viral loads have been

detected in lungs and immune cells after death

MHV-induced demyelination increases as virus is cleared; MHV-3-induced hepatitis correlates with macrophage activation and not viral load; and IBV-induced nephritis is detected in chickens

with very low viral loads

Macrophage or DC infection

Infection is abortive but induces expression of pro-inflammatory

mediatorsMHV and FIPV productively infect macrophages

Macrophage infiltration into sites

of inflammation

Macrophages are present in large numbers in infected lungs

In MHV infection, macrophages infiltrate the CNS coincident with demyelination (thought to be the

final effector cell); and in FIPV infection, macrophages are the main cell type in

granulomas and are crucial for pathogenesis

High concentration of pro-inflammatory mediators in serum

or at site of infection

Controversial; anti-inflammatory mediators might

contribute to delayed viral clearance

MHV-3-induced FGL2 expression is crucial for liver necrosis; in MHV-JHM-infected mice, IFN- is required for CD8+ T-cell-mediated responses;

and in FIPV infection, increased cytokine concentrations are present in blood and tissues

during exacerbation of disease

Inhibition of type I IFN induction in infected cells

Shown using isolated macrophages, DCs and

fibroblastsMHV does not induce type I IFN expression

Lymphopenia and neutrophilia

Present in most severe cases; and lymphocytic infection has

been detected

In FIPV infection, lymphopaenia is present during clinical relapses; and in MHV-3 infection, lymphopaenia is present and lymphocytic

infection has been detected

Haemophagocytosis Present in severe cases Not reported