Introduction to VirologyCasey D. Morrow, Ph.D.

Department of Cell Biologycaseym@uab.edu

• Viral replication: How viruses exploit cells to make new viruses

• Viral pathogenesis: How virus replication causes disease and how viruses escape/interact with the immune system

http://www.bioteach.ubc.ca/MolecularBiology/AMonksFlourishingGarden/

The DNA -> RNA -> Protein Pathway

RNA Polymerase = enzyme that

makes mRNA from the DNA gene template

Characteristics of Viruses• “Filterable agents” – Pass through filters that capture bacteria

• Obligate intracellular parasites

• Minimal genetic information, efficient

• Rely on host cell machinery to fulfill replication cycle

• Assembled from building blocks encoded by the virus (don’t divide like cells)

• Absolutely require host cell for replication to produce proteins required to synthesize new viral genomes and the building blocks of the virus structure

For a Virus to be Successful:

• Capable of transmission through potentially harsh environmental conditions

• Traverse skin or other barriers of the host

• Must adapt to the biochemical machinery of the host cell for replication

• Escape elimination by the host immune response

Basic Components of a Virion

Virus Classification• Size• Morphology• Genome Type (DNA or RNA)• Means of Replication

Relative Sizes of Viruses

General Structure of Viruses

Genome Contents of Viruses

• RNA• Single-strand• Double-strand• Linear• segmented

• DNA• Double-strand• Single-strand• Linear• Circular

http://gsbs.utmb.edu/microbook/images/fig41_6.JPG

Naked vs. Enveloped Viruses• Naked Capsids

• Withstand harsh environmental conditions

• Resistant to drying, acids, detergents

• Many are transmitted fecal-oral route

• Enveloped Viruses• Can’t dry out• Not stable in acid• General must remain in body fluids (respiratory, blood)

http://www.cat.cc.md.us/courses/bio141/lecguide/unit2/viruses/images/u2fig2b.jpg

http://www.tarvacin.com/media/gif/EnvelopedVirusStructure.gif

Icosahedral Capsid Assembly

Examples of Icosahedral Capsids

1. Equine Herpesvirus Nucleocapsid

2. Simian Rotavirus

3. Reovirus type 1 virion

4. Intermediate partice: Reovirus

5. Inner core particle (Reovirus)

6. Human Papillomavirus type 19

7. Mouse Polyomavirus

8. Cauliflower Mosaic Virus

General Enveloped Virus Structure

http://www.schoolscience.co.uk/content/5/biology/mrc/hiv/page2.html

HIVInfluenza

Example Envelope Glycoprotein: Influenza Hemagglutinin

Steps of Virus Replication Cycle

Recognition of Cells and Attachment

Sialic acid: bound by Influenza virusHIV

Steps of Virus Replication Cycle

Cell Entry:

Naked Viruses: Typically endocytosis

Enveloped Viruses: Typically cell fusion

Synthesis of New Viral Components

• Viral Nucleic Acids• mRNA’s encoding viral proteins• New viral genomes for encapsidation into new virions (viral particles)

• Viral Proteins• Enzymes and other proteins required for viral transcription and genome replication

• Structural proteins (capsid proteins, viral glycoproteins)

http://www.bioteach.ubc.ca/MolecularBiology/AMonksFlourishingGarden/

The DNA -> RNA -> Protein Pathway

RNA Polymerase = enzyme that

makes mRNA from the DNA gene template

DNA Virus Transcription• Generally use the host cell’s DNA-dependent RNA Polymerase II to make mRNA’s

• Generally the DNA genomes go to the nucleus (some integrate into chromosomal DNA)

• One exception: Poxvirus family – Replicates only in the cytoplasm of cell so can’t use host cell’s RNA Polymerase II; instead, makes its own enzymes for transcription of mRNA’s

RNA Virus Transcription• Many replicate entirely in the cytoplasm of the cell

• Must encode their own enzymes (RNA-dependent RNA polymerases) for transcription of mRNA and to replicate their full-length RNA genomes

• REASON: The host cell has no enzymes for generating new viral RNA genomes using an RNA template

• PLUS-STRAND RNA VIRUSES: Genomes same sense as mRNA

• MINUS-STRAND RNA VIRUSES: Genomes opposite sense as mRNA

Replication of Viral Genomes• DNA Viruses:

• DNA-dependent DNA polymerases to make new DNA copies from DNA templates

• Some use cellular DNA polymerases• Others encode their own DNA polymerases

• RNA Viruses:• Use RNA-dependent RNA polymerases to make new RNA copies from RNA templates

• Encoded by the virus• Plus-strand versus minus-strand

• Retroviruses (example: HIV):• Genome in the viral particle is single-strand RNA• Packages Reverse Transcriptase• RNA DNA Integrated into chromosomal DNA• New viral RNA genomes transcribed in the nucleus by host cell polymerase

Viral Protein Synthesis• All viruses depend on host cell translation machinery (ribosomes, tRNA, post-translational modifications) to generate viral proteins from mRNA templates

• Different strategies for compactness/efficiency:

• Separate mRNA’s for each viral protein

• Polyprotein strategy• Multiple proteins encoded on one mRNA• Individual proteins are derived from polyprotein by enzymatic cleavages catalyzed by proteases

Polyprotein Strategy

Assembly and Release

• Enveloped Viruses: typically exit by budding from the cell

• Naked viruses: typically exit through cell lysis

Lytic Virus Growth Curve

Viral Budding from Cell Membrane

http://www.itech.pjc.edu/fduncan/mcb1000/micc6ppt_files/slide0004_image012.jpghttp://en.wikipedia.org/wiki/AIDS http://www.med.wayne.edu/immunology/department/roberts2.html

HIV budding from a cultured lymphocyte

Budding Influenza Virus

Viral Pathogenesis• Interaction between the virus and the host

• General Steps:

• Entry into the body (ex. Fecal-Oral, Inhalation)

• Primary Site of Replication

• Viremia

• Secondary Site of Replication in target tissues

Example: Ebola

Primary Target

Secondary Targets

Outcomes of Virus Infection at the Cellular Level

• Failed infection (abortive)

• Cell death:• Lytic viruses• Apoptosis (programmed cell death)

• Infection without cell death

• Chronic infection: no cell lysis, new viruses are produced

• Latent infection: limited production of viral components; no new viruses produced

• Cellular properties may change later resulting in viral production (ex. Herpes Simplex Virus)

• Transformation: Virus infection results in cell immortalization (conversion to a tumor cell)

• Oncogenic viruses

Host Defenses Against Viruses• Natural Barriers of the Body (ex. Skin)

• Innate Immune Defenses (not antigen dependent)• Interferon response• Macrophages• Dendritic cells• Natural Killer Cells

• Antigen-specific immune responses• Antibodies• Helper T-cells

• Cell-mediated immunity• Recognition of virus-infected cells• Lysis of infected cells

Viral Strategies to Evade Host Defenses

• Preventing interferon action:• Presence of double-stranded RNA in cell causes interferon response

• Shuts down cellular translation (suspended animation)

• Degradation of viral RNA

• Changing viral antigens• High error-rate in many viral polymerases• Allows rapid change of protein sequences and, therefore, antigenic characteristics

• Cell-to-cell spread: Evade antibodies

• Suppression of antigen presentation and lymphocyte function (hide from immune system)

Viral immunopathogenesisImmunopathogenesis Immune Mediators Examples

Flulike symptoms Interferon, cytokines Respiratory viruses, arboviruses (viremia-inducing viruses)

Delayed-type hypersensitivity and inflammation

T cells, macrophages, and polymorphonuclear leukocytes

Enveloped viruses

Immune complex disease Antibody, complement Hepatitis B virus, rubella

Hemorrhagic disease T cell, antibody, complement

Yellow fever, dengue, Lassa fever, Ebola viruses

Postinfection cytolysis T cells Enveloped viruses (e.g., postmeasles encephalitis)

Immunosuppression - Human immunodeficiency virus, cytomegalovirus, measles virus, influenza virus