No abstract

Replication in SARS-CoV-infected macaques of pneumonia similar to that in human beings with SARS, combined with the high prevalence of SARS-CoV infection in SARS patients, fulfill the criteria required to prove that SARS-CoV is the primary cause of SARS.

Influenza A is a highly contagious single-stranded RNA virus that infects both the upper and lower respiratory tracts of humans. The host innate immune Tolllike receptor (TLR) 3 was shown previously in cells of myeloid origin to recognize the viral replicative, intermediate double-stranded RNA (dsRNA). Thus, dsRNA may be critical for the outcome of the infection. Here we first compared the activation triggered by either influenza A virus or dsRNA in pulmonary epithelial cells. We established that TLR3 is constitutively expressed in human alveolar and bronchial epithelial cells, and we describe its intracellular localization. Expression of TLR3 was positively regulated by the influenza A virus and by dsRNA but not by other inflammatory mediators, including bacterial lipopolysaccharide, the cytokines tumor necrosis factor-␣ and interleukin (IL)-1␤, and the protein kinase C activator phorbol 12-myristate 13-acetate. We also demonstrated that TLR3 contributes directly to the immune response of respiratory epithelial cells to influenza A virus and dsRNA, and we propose a molecular mechanism by which these stimuli induce epithelial cell activation. This model involves mitogen-activated protein kinases, phosphatidylinositol 3-kinase/ Akt signaling, and the TLR3-associated adaptor molecule TRIF but not MyD88-dependent activation of the transcription factors NF-B or interferon regulatory factor/interferon-sensitive response-element pathways. Ultimately, this signal transduction elicits an epithelial response that includes the secretion of the cytokines IL-8, IL-6, RANTES (regulated on activation normal T cell expressed and secreted), and interferon-␤ and the up-regulation of the major adhesion molecule ICAM-1.

. This suggests that the mechanism of SARS-CoV assembly differs from that of other studied coronaviruses, which only require M and E proteins for VLP formation. When coexpressed, the native envelope trimeric S glycoprotein is incorporated onto VLPs. Interestingly, when a fluorescent protein tag is added to the C-terminal end of N or S protein, but not M protein, the chimeric viral proteins can be assembled within VLPs and allow visualization of VLP production and trafficking in living cells by state-of-the-art imaging technologies. Fluorescent VLPs will be used further to investigate the role of cellular machineries during SARS-CoV egress.Coronaviruses are positive-sense RNA enveloped viruses that belong to the Coronaviridae family in the Nidovirales order. These viruses are found in a wide variety of animals and can cause respiratory and enteric disorders of diverse severity (11,18). In the past 5 years, several human and animal coronaviruses have been discovered, including the highly pathogenic virus responsible for the severe acute respiratory syndrome (SARS-CoV) (34,58, 60,64,68,69). Coronavirus particles consist of a helical nucleocapsid structure, formed by the association between nucleocapsid (N) phosphoproteins and the viral genomic RNA, which is surrounded by a lipid bilayer where three or four types of structural proteins are inserted: the spike (S), the membrane (M), and the envelope (E) proteins and, for some coronaviruses only, the hemagglutinin-esterase (HE) protein (for a review, see reference 14). Once sufficient amounts of new genomic RNA and structural proteins have been produced, assembly of particles occurs. Assembly and release of virions are the last stages of the virus life cycle.The triple-spanning membrane glycoprotein M drives the assembly of coronavirus, which bud into the lumen of the endoplasmic reticulum-Golgi intermediary compartment (ERGIC) (32,33,62,63). M is the most abundant envelope protein that sorts viral components to be incorporated into virions. M oligomerization, mainly driven by its transmembrane domain, is believed to allow the formation of a lattice of M proteins at ERGIC membranes (16, 41). S and E membrane proteins are integrated into the lattice through lateral interactions with M, whereas N and viral RNA interact with M C-terminal domain, which is exposed to the cytosol (4,8,15,19,30,36,48,54,55). The coronavirus S protein, responsible for receptor binding and membrane fusion, does not seem to have any major role in coronavirus assembly. Recent studies show that E is a viroporin that forms ion channels (46,66,67). Despite its minor incorporation into virion particles (7,22,40), the small E protein plays an important but not fully understood role in virus morphogenesis and budding (20,35,70). Studies performed on coronaviruses, including the SARS-CoV, demonstrate that depletion of the E gene from coronavirus genome strongly diminish virus growth and particle formation (9,12,35,37,57). The N protein self-associates and encapsidates the RNA genome for incorporation i...