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Home , Viral replication

Steps in Viral Replication: Attachment (First Step)

• Surface on attaches to specific Viral Replication receptor(s) on surface - May be specialized with limited tissue distribution or more widely distributed - Virus specific receptor is necessary but not sufficient Scott M. Hammer, M.D. for to infect cells and complete replicative cycle

Viral Replication: Basic Concepts Selected Virus Receptors

• Viruses are obligate intracellular parasites Adenovirus CAR Coxsackievirus CAR, CD55 • Viruses carry their (RNA or DNA) and Echovirus Integrin VLA-2, CD55 sometimes functional proteins required for early Epstein-Barr Virus CD21 steps in replication cycle HIV-1 CD4, CCR5, CXCR4 Measles virus CD46 • Viruses depend on host cell machinery to Parvovirus Erythrocyte P Ag complete replication cycle and must commandeer Poliovirus PVR that machinery to successfully replicate Rhinovirus ICAM-1

Viral Replication: Basic Concepts Steps in Viral Replication: Penetration (Second Step)

• Replication cycle produces • Enveloped viruses penetrate cells through fusion - Functional RNA’s and proteins of with host - Genomic RNA or DNA and structural proteins - May or may not involve receptor mediated

• 100’s-1,000’s new particles produced by each cycle • Non enveloped viruses penetrate by - Referred to as burst size - Receptor mediated endocytosis - Many are defective - Translocation of the virion across the host cell - End of ‘eclipse’ phase membrane

• Replication may be cytolytic or non-cytolytic

1 Virus Replication Cycle Steps in Viral Replication: Basic Strategies of and Translation (Fourth and Fifth Steps)

• (+) RNA Æ Proteins

• (-) RNA Æ (+) RNA Æ Proteins

• RNA Æ DNA Æ RNA Æ Proteins

• DNA Æ RNA Æ Proteins

From Fields

Steps in Viral Replication: Steps in Viral Replication: Uncoating (Third Step) Assembly and Release (Sixth and Seventh Steps) • Makes viral nucleic acid available for transcription to permit multiplication to proceed • Process involves bringing together newly formed genomic nucleic acid and structural proteins to • Mechanism variably understood depending upon form the nucleocapsid of the virus the virus • Nonenveloped viruses exhibit full maturation in the or nucleus with disintegration of cell

Uncoating of Influenza Virus Steps in Viral Replication: Endosome Assembly and Release (Sixth and Seventh Steps)

• Many enveloped viruses exhibit full maturation as the virion exits the cell - Viral proteins are inserted into the host cell membrane - Nucleocapsids bind to these regions and bud into the extracellular space - Further cleavage and maturation of proteins may occur after viral extrusion - Cytolytic activity of these viruses varies

From Fields Virology

2 Influenza Virus Herpes Simplex Virus

From Fields Virology From Fields Virology

Retroviruses Schematic of Replication Cycle of (+) RNA Single Strand Viruses Coding for One Sized RNA

Genomic RNA binds to and is translated into polyprotein

Polyprotein is cleaved

Genomic RNA’s serve as templates for synthesis of complementary full length (-) RNA’s by viral

(-) strand RNA serves as template for (+) strand RNA’s; these serve to produce more polyprotein, more (-) strand RNA’s or become part of new From Fields Virology virions From Fields Virology

Schematic of Replication Cycle of Steps in Viral Replication: (+) RNA Single Strand Viruses Coding for Assembly and Release Genomic and Subgenomic RNA’s (Sixth and Seventh Steps) Genomic RNA binds to • Herpesviruses (enveloped) assemble ribosomes but only a portion nucleocapsids in the nuclei of infected cells and of 5’ end is translated into non-structural proteins mature at the inner lamella of the nuclear membrane (-) strand RNA is synthesized. - Virions accumulate in this space, in the ER and in Different classes of (+) RNA’s are produced. One is trans- vesicles lated into a polyprotein which - Virion release is associated with cytolysis is cleaved to form structural proteins. Another is full length and serves as genomic RNA for new virions

From Fields Virology

3 Schematic of Nonsegmented (-) RNA Strand HIV-1 Virion Virus Replication Cycle

Transcription of (-) strand occurs after entry and mediated by virion packaged transcriptase

(+) strand RNA’s produced; proteins synthesized

Full length (-) strand RNA’s produced and packaged into new virions

Transcription and translation take place entirely in cytoplasm

From Fields Virology

Schematic of Segmented (-) RNA Strand HIV Life Virus Replication Cycle Cycle

mRNA’s are synthesized from each segment

Viral proteins are synthesized

(+) strand RNA’s are synthesized and serve as templates for (-) strand genomic RNA’s

From Fields Virology

Schematic of Herpesvirus Replication Cycle HIV Entry (DS DNA Virus Which Replicates in Nucleus) HIV Co-receptor interaction HIV

Sequential, ordered gp41 rounds of mRNA and HIV Anchorage protein production regulate replication gp120 HIV CD4 CXCR4 Structural proteins CCR5 produced during last Attachment CD4 gp41 cycle of replication Cell

Fusion HIV HR1-HR2 Complete interaction From Fields Virology

4 Primary HIV Infection: The Variable Course of HIV-1 Infection Typical Progressor Rapid Progressor Pathogenetic Steps Primary HIV Primary HIV Infection Clinical Latency AIDS Infection AIDS CD4 Level CD4 Level • Virus – dendritic cell interaction - Infection is typically with R5 (M-tropic) strains - Importance of DC-SIGN Viral Replication • Delivery of virus to lymph nodes A B Viral Replication months years months years Nonprogressor • Active replication in lymphoid tissue Primary HIV Infection • High levels of viremia and dissemination Clinical Latency CD4 Level • Downregulation of virus replication by immune response • Viral set point reached after approximately 6 ? months C Viral Replication months years Reprinted with permission from Haynes. In: DeVita et al, eds. AIDS: Etiology, Treatment and Prevention. 4th ed. Lippincott-Raven Publishers; 1997:89-99.

PHI: Early Seeding of Lymphoid Tissue Primary HIV Infection: Determinants of Outcome

• Severity of symptoms • Viral strain - SI (X4) vs. NSI (R5) viruses • Immune response - CTL response - Non-CTL CD8 responses - Humoral responses? • Viral set point at 6-24 months post-infection • Other host factors - Chemokine receptor and HLA genotype • Gender and differences in viral diversity? • Antiviral therapy - Near vs. long-term benefit?

Schacker T et al: J Infect Dis 2000;181:354-357

Primary HIV Infection: Natural History of Untreated Clinical Characteristics HIV-1 Infection

1000 • 50-90% of infections are symptomatic • Symptoms generally occur 5-30 days after 800 exposure 600 • Symptoms and signs CD4+ Early Opportunistic Infections - Fever, fatigue, myalgias, arthralgias, headache, nausea, Cells Late Opportunistic Infections vomiting, diarrhea 400 - Adenopathy, pharyngitis, rash, weight loss, mucocutaneous ulcerations, aseptic meningitis, occas. 200 oral/vaginal candidiasis - Leukopenia, thrombocytopenia, elevated liver 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 • Median duration of symptoms: 14 days Infection Time in Years

5 Antiviral Agents for HIV

Entry Inhibitors

RNA Nucleus

Protease

Reverse DNA transcriptase

Reverse transcriptase inhibitors Protease inhibitors

Mechanism of T20/T1249 Mediated Fusion Inhibition Modified from Weissenhorn et al., Nature 387, 426-430 (1997) T20 T1249 and Furuta et al., Nature structural 5, 276-279 (1998).

gp120 Fusion Blockade

Cell Membrane

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------Fusion peptide HR1 “Ensnared” Transition State Intermediate Receptor X

Binding

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Δ Conformation Membrane

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Native Form Fusion Intermediate Core Structure

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