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SEVERE ACUTE RESPIRATORY SYNDROME (SARS) Winnie A. Apidi Student No. 7643853 March 22, 2011 Introduction Coronaviruses are large, enveloped, positive sense, single-stranded RNA viruses Responsible for upper repiratory tract infections e.g. common cold Coronavirus genus is divided into 3 groups: SARS stipulated to be in Group II Severe Acute Respiratory Syndrome Feb 2003, cases of atypical pneumonia of unknown aetiology in Guangdong, China Outbreak spread to Asia, Europe, North America ~8000 cases and 774 deaths Transmission stopped after four months Rapid control globally: ◦ Easy I’D ◦ Isolation of infected SARS 2004: 3 cases of lab-based acquired infections, and re-introduction from animals Not sufficient secondary human-human transmission to generate outbreak Epidemiology Nosocomial infectcion strikingly (22-40% were healthcare workers) Incubation period 2-10 days Transmitted through respiratory droplets Generated through hospital procedures Fecal-oral transmission also possible as SARS-CoV can survive in feces for 1- 2days w’out losing infectivity Disease SARS is characterized by: ◦ Fever ◦ Chills ◦ Malaise ◦ Headache ◦ Cough ◦ Dyspnea ◦ Pneumonia (radiologically) Immune response Pro- andanti-inflammatory cytokines detected during SARS-CoV infection ◦ Interleukin 13, 16,TNFα and TGFβ ◦ IL-18 found to be suppressed Serum antibodies detected 20 days after onset of infection: IgM then IgG Antibodies recognize N protein Diagnosis of SARS-CoV Lab confirmation based on virus isolation, detection of viral RNA and serologic assays SARS-CoV replicates in Vero-E6 cells and fetal rhesus monkey kidney cells Viral RNA can be extracted and RT-PCR or nucleic acid-based amplification done Antibodies can be detected by ELISAs using substrates to N protein and Western Blots of infected cells Biological assays such as microneutralization test can be used to measure specific immunoglobulin inhibiting growth of SARS-CoV in culture Animal reservoirs Non-SARS cases lacked SARS-CoV antibodies Horse-shoe bats Palm civet cats SARS-CoV isolated from animals Genetically… SARS-CoV is genetically & structurally a typical coronavirus Genome encodes 4 structural proteins: ◦ nucleocapsid (N) ◦ Membrane (M) ◦ Envelope (E) ◦ Spike (S) ◦ S gene has been associated with disease progression Spike (S) protein •S protein induces virus binding, fusion and entry •Consists of signal peptide and extracellular domain with 2 subunits S1 & S2 •S1 enhances virus binding to receptor angiotensin-converting enzyme ACE2 via its RBD •Forms core with S2 thereby initiating fusion & entry into cell SARS-CoV replication SARS-CoV replication Cell entry by attachment of S to ACE2 receptors Triggers fusion of viral & plasma membranes Entry of nucleocapsid into cytoplasm ORFs 1a & 1b of replicase translation Viral Proteins synthesized Undergo translational proteolytic processing into key enzymes SARS origin & evolution Originally reported as a result of zoonotic shift from palm civet cat or raccoon dog However, origin is still unclear Indication of recombination shown phylogenetically Recombination useful in eliminating frequent deleterious mutations in RNA viruses Lack of homology between CoVs indicates that not a single recombination event. Translational frame shifts occur leading to additional ORFs with deletions. Not clear whether deletion precede or follow transfer of CoV into humans Phylogeny and recombination Phylogenetic studies show that SARS-CoV has a mammalian ancestry It is most closely related to Group II CoVs Intermediary host is still unclear Recombination of RNA-dependent RNA polymerase with those of CoVs suggests that SARS may be old, diverse and changeable not yet discovered in its natural hosts Recombination nature of SARS Antigenic variation SARS may not have emerged due to a single recombination event but due to genetic drift Accumulation of genetic mutations over time Signature variation residues in S protein observed in civet cats Overall mutation rate is low-moderate Variation causing SARS-CoV to be unique still being investigated Comparison of SARS-CoV and civet COV Antigenic variation SARS-CoV shown to have evolving heterogeneity This questions how protective a specific vaccine strain could be and possibility of imune escape Some S glycoprotein variants have been found to be resistant to antibody neutralization Others show enhanced entry in presence of certain antibodies Others have lower affinity to to ACE2 domain Antigenic variation Amino acid substitutions also detected in S and M proteins Y442C & L472F have been related to incorporation of protein to virions and this alters antigenic structure Vaccines Vaccines targeting CoVs are in existence Various approaches to a SARS-CoV vaccine: ◦ Inactivated SARS-CoV-based- easy to generate as involve whole killed virus particles ◦ S-protein-based vaccines- S protein is a type 1 transmembrane glycoprot. Induces neutralizing antibodies, virus binding, fusion & entry Has 2 subunits: S1 & S2. S1 binds to ACE2, angiotensin converting enzyme causing RBD in S2 to induce fusion between virus and target cells & entry Vaccines S-protein based vaccines include: ◦ Use neutralizing antibodies raised against the entire S proteins or its fragments ◦ Use strong neutralizing antibodies recognizing different epitopes of RBD, thus blocking RBD- ACE2 interaction therby blocking attachment ◦ Antibodies raised are generated thru monoclonal/polyclonal technology or vaccination ◦ Used to control severe disease vaccines Asante! Questions???
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