PATHOLOGY 505
Lecture # 19
Infectious Disease: Viruses
A. General Characteristics
1. Small size
a. pass bacterial filters
b. largest viruses (e.g. poxviruses) visible by light microscopy same size as smallest prokaryotic cells (e.g. chlamydiae, mycoplasmas)
2. Chemical composition
a. variable
b. smallest viruses (parvoviruses, picornaviruses) possess only 3-4 proteins together with small nucleic acid genome
c. largest viruses code for 20-50 proteins
3. structure
a. mature particle is virion
b. nucleic acid surrounded by protective protein coat (capsid) composed of repeating subunits (capsomeres) in precise symmetrical arrangement
c. geometry either helical or icosahedral (20-sided)
d. package of nucleic acid within its capsid is nucleocapsid
e. may be surrounded by envelope
(1) lipid bilayer, derived from host cell membrane into which viral glycoproteins are inserted
(2) spikes projecting from envelope consist of organized aggregates of viral glycoproteins
f. particles, not cells
(1) lack such intracellular structures as ribosomes or mitochondria
(2) although may be surrounded by lipid membrane (envelope), does not have outer wall structure
4. genetic information
a. Either DNA or RNA, not both
b. genome (of RNA v.) can be in fragments, rather than continuous thread, with each.fragment constituting a single gene
c. genetic information can be in 1 of 4 forms
(1) double-stranded DNA
(2) single-stranded DNA
(3) double-stranded RNA
(4) single-stranded RNA
5. Replication
a. obligate intracellular parasites
b. Stages
(1) Adsorption (attachment)
(a) must bind to host cell
(b) requires binding protein on virus and corresponding cell membrane receptor
(c) if receptor lacking, host cell cannot be naturally infected by virus
i) reason for cell selectivity which many viruses show
ii) e.g., poliovirus- cells without receptor cannot be naturally infected, only gray matter CNS cells (neurons)
iii) e.g.2, influenza virus-envelope contains specific glycoprotein, haemagglutinin, which is present as spikes, which bind specifically to (NANA) N-acetylneuraminic acid on cell membrane glyoproteins. If treat cells with neuraminidase to remove NANA, cells resistant to infection
(2) penetration
(a) non-enveloped v. enter by "viropexis", resembles phagocytosis
(b) enveloped v. enter by fusing their lipid membrane with host cell. n.b., viral envelope does not enter cell.
(3) uncoating of particle
(a) removal of capsid proteins leading to release of viral nucleic acid
(b) with some viruses (e.g. reoviruses), uncoating incomplete, with template copies extruded into cytoplasm
(4) transcription of mRNA (host) from viral nucleic acid
(a) most DNA viruses (except poxv.) transcription occurs in nucleus
i) uses host DNA-dependent RNA polymerase to produce "early mRNA"
[a] to cytoplasm to direct synthesis of viral DNA polymerase
[b] used to replicate viral DNA
ii) "late mRNA"
[a] to cytoplasm to direct synthesis of capsomere subunits
[b] subunits migrate to nucleus to from virions
[c] intranuclear inclusion bodies
iii) poxviruses
[a] replicate in cytoplasm
[b] cytoplasmic inclusion bodies
carry own DNA-dependent RNA polymerase within virion
(b) most RNA viruses, transcription occurs in cytoplasm
i) RNA-dependent RNA polymerase
[a] used by all except retroviruses
ii) retroviruses use reverse transcriptase
[a] RNA-dependent DNA polymerase
[b] production of DNA intermediate, which is integrated into host DNA
[c] provirus, capable of directing synthesis of both mRNA and virion RNA
(5) translation of mRNA into viral proteins, structural and non-structural, on host ribosomes
(6) replication of viral nucleic acid
(a) commences as soon as necessary viral polymerases are available
(b) either nuclear or cytoplasmic
(7) assembly of particles into virions
(a) either nuclear or cytoplasmic
(b) icosahendral or helical
(8) release of mature virions
(a) lysis of host
(b) budding through specifically prepared sites in host cell membrane
i) acquisition of envelope
ii) cell membrane lipid plus viral glycoprotein
c. once virus in host cell, can not identify virus particle
(1) eclipse period
(2) viral nucleic acid is released from protein coat, and is directing host cell synthetic machinery to produce more new viral particles
B. Classification and Nomenclature
1. criteria
a. nature of nucleic acid
(1) DNA or RNA
(2) double- or single-stranded
b. symmetry of virion
(1) icosahedral
(2) helical
(3) other
C. Pathogenesis of Viral Disease
1. Effects of viruses on host cells
a. cytocidal or lytic infection
(1) direct lysis of cell
(2) shut-down or usurpation of host cell macromolecular synthesis
(3) cytopathic effect (CPE)
(a) plaque assay-each area of cell death identifiable-presumably one virion
(b) quantitation of # virions (e.g. titer
b. alterations in cell membrane
(1) if virus produces hemagglutinin
(a) will be inserted into host cell membrane before viral release
(b) can detect increased ability to bind rbc by hemadsorption assay
(2) some viruses promote cell fusion (e.g. paramyxoviruses-measles, some herpesv.-H.simplex, RSV)
note syncytia as CPE
c. inclusion bodies
(1) nuclear
(2) cytoplasmic
d. persistent infection
(1) virus does not kill host, but rather host continues to produce infective virus
e. latent infection
(1) viral information persists in cell, but mature viral particles not detectable
(2) DNA tumor viruses
(a) can identify presence of viral genome by nucleic acid hybridisation or presence of viral protein as antigen in/on cell
(3) Herpes viruses-
(a) H. simplex-cold sores
(b) latent in sensory nerve ganglia
f. Transforming infection
(1) tumor viruses transform susceptible host cell such that proliferates free from normal regulatory mechanisms.
(a) free from contact inhibition
(b) immortalize cell
(2) DNA tumor viruses only transform cells which they are unable to infect productively-
(a) normally reproduction is cytocidal
(b) become latent by integrating genome into host cell genome
(3) RNA tumor viruses (oncornaviruses)
(a) continue to replicate in cells they transform
(b) establish persistent infection
(c) continue to bud virions from cell surface
(d) viral genome also integrated into host cell genome
g. Abortive infections
(1) virus fails to replicate completely in host cell
(2) can still cause CPE, though no complete virions released
(3) can be rescued by coinfection with helper virus (e.g. SV-40)
D. Host antiviral response
1. Cellular response: Interferons
a. discovered in 1957 (Isaacs & Lindenmann)
(1) multiplication of one virus type inhibits that of a second virus added subsequently to same cell culture
(2) infected cells had produced a soluble substance which was capable of inhibiting viral multiplication in other cells.
b. can be produced not only by infection with virus, but also treating cells with synthetic polyribonucleotides
c. some are products of activated T-lymphocytes
d. Characteristics of action
(1) interferon induced by one virus active against range of other viruses (i.e. is not virus specific)
(2) act via cell-surface receptors
(3) do not enter "target" cell
(4) antiviral state produced within cell following binding of interferon
(a) inhibition of translation of viral protein from mRNA
i) stimulation of protein kinase, inhibiting synthesis
ii) activation of a ribonuclease to destroy viral mRNA
2. Host immune response
a. antibody
(1) neutralize virus in blood and ECF
(2) blocks binding of virus to host cell
(3) Active
(a) induction of antibody
(b) requires presence of antigenically active materials in sufficient quantity to stimulate B-cell proliferation
(4) Passive
(a) maternal IgG via placenta or mild
(b) maternal IgA in milk
b. cell mediated
(1) cytotoxic T cells
(2) recruitment/activation of macrophages