Challenges to an obligate intracellular parasite

1. Challenges to an obligate intracellular parasite • Gain entry to host cell • Survive host defenses • Do not destroy cell while you need it • Compete for host resources to multiply • Exit and survive passage to new host cell

2. Virus-cell interactions may vary with type of cell infected • Productive - progeny produced • Cytopathogenic: results in cell death • Persistent: cell survives • Transforming: cell survives and changes properties • Non productive - no progeny produced • Latent: can revert to productive • Transforming: cell changes properties • Abortive: Lacking needed factors • Null: No receptors

3. PRODUCTIVE INFECTIONS - CYTOPATHOGENIC, CYTOCIDAL, VIRULENT, LYTIC • Vegetative virus replicates and cell dies • virus product kills cell - nuclease • virus takes over host machinery - access to ribosomes • virus kills cell during lysis/release • virus induces apoptosis - programmed cell death

4. Useful for assay/count of viruses - plaques, CPE Cucumber mosaic virus

5. PERSISTENT, TEMPERATE: Replication without cell death • Release typically by budding - fewer per unit time but longer time period • Mechanisms leading to persistence • Makes little demand on host resources • Inhibits apoptosis • Progeny reduced due to interferon (IF) or antibody so rate of cell replication matches destruction • Production of defective interfering particles (DI) - deletion mutants that compete with and dilute concentration of parent helper virus

6. NONPRODUCTIVE INFECTIONS LYSOGENY, LATENCY • Virus nucleic acid maintained • If host cell reproduces so does virus nucleic acid • May be integrated as prophage or provirus or may be cytoplasmic episome • May give host new characteristics due to some gene expression (Lysogenic conversion; transforming viruses) • Viral product may prevent apoptosis in certain cells • Ability to return to vegetative state for many in same or different cell (different disease - VZV) • Presence shown by PCR or probes

7. Transformation • May or may not produce progeny depending on type of virus • Virus product/presence turns on expressions of oncogenes • Abortive infections: Permissive versus nonpermissive cells • Infection but no functional progeny • Missing factors for replication • Failure to process proteins • Null infections • Lack receptors • May be permissive if transfection or other means to infect

8. Attachment: Specificity begins hereVirus attachment site and cell receptor • envelope glycoproteins • capsid proteins • ends of filaments of bacterial rods (all filamentous animal viruses are enveloped) • penton proteins (poliovirus - canyon formation) • penton fibers (12 per virion) • A proteins (single copies) • Tailed phages - tail fiber tips (6 on Teven); tail pins (reversible vs irreversible)

9. May be multiple sites or one • Attachment sites are highly conserved • Adjacent ab binding sites may not be • Polio vaccine blocks near attachment site • Virus can escape detection but remain infectious (HIV)

10. Plant viruses bypass this step Aphid on bean leaf transmitting virus

11. What is the value of knowing the receptor for a virus?

12. Receptor sites • Essential to host cell function (Achilles pore) • Present in multiple copies • Narrow vs broad host range (universality of receptor) • HIV vs rabies • Tissue tropism

13. Receptor sites • LPS - T4,T3 • OmpF porin - T2 • TonA iron transport - T1,T5 • Nucloside transport - T6 • Maltose transport - Lambda • Flagellum, pilus, teichoic acid, capsule, membrane

14. Receptors and co-receptors • Unrelated viruses may use the same receptor • Adenovirus (fiber knob) vs coxsackie (groove) • HIV and GBV - C(HepG)

15. How do we determine the receptor? • Susceptible cells • Removal and loss of binding • Examine resistant mutants • Inhibition assays • Monoclonal antibodies directed against cells surface • compete with soluble receptors • Transform resistant cells to sensitive cells with receptor DNA