Viruses of Humans

DNA VIRUSES

Parvoviridae

Densivirinae Parvovirinae(Invertebrates)

Bocavirus

Parvovirus Dependoviruses(AAV)- Erythrovirus -Feline PV Helpers are * B19 parvovirus -Canine PV - adenoviruses * Semian PV -Porcine PV - Herpesviruses -Mice PV - HPV

- Vaccinia virus

A negatively stained electron micrograph of AAV

•Parvoviruses are the simplest DNA viruses.

•They are icosahedral in symmetry with a size of 18-26 nm .

•The capsid consists of 60 capsomeres each of which is composed of two proteins; VP1 and VP2.

•VP2 constitutes approximately 80% of the total protein mass of the virus.

•VP1 and VP2 share significant sequence identity but VP1 is longer and has a unique region external

to the capsid which is accessible to antibody binding.

•VP1 is necessary for stable confirmation and induction of neutralizing antibodies.

•The genome of B19 is 5.6 kb in length with a molecular weight of 1.5-1.8x106 Daltons.

•Of all the DNA viruses, Parvoviruses seem to be among the most dependent on cellular function.

•Dependoviruses (Adeno -associated viruses)

commonly infect humans.

•They do not appear to cause illness nor do they modify infection by their helper viruses .

•AAV can become integrated at a specific site on the q-arm of chromosome 19 establishing latency with

no known consequence.

•Parvovirus B19 was the only human pathogen until September 2005 when a new member, Bocavirus,

was described.

•Parvovirus B19 was Discovered by Yvonne Cossart in 1975.

•In 1981, It was linked to transient aplastic crisis(TAC), and in 1983 with fifth disease.

•It has been classified in the genus Erythrovirus .

•Bocavirus is a cause of respiratory tract infection

Papovaviruses

Papilloma Polyoma

Vacoulating viruses

•Human pathogens of the family belong to Papillomaviruses and Polyomaviruses only.

• Virions are naked, icosahedral with a double stranded superhelical (circular) closed DNA

genome .

•They range in diameter form 40 to 45 nm for polyomaviruses and from 52 to 55 nm for

papillomaviruses .

•Their icosahedral capsid is composed of 72 pentamers .

•Each capsomere is composed of 2 (L1, L2) peptides for papillomaviruses or 3 (VP1, 2, and 3) peptides

for polyomaviruses.

•Their genome is 5 - 8 Kbp

•They are capable of causing lytic, chronic, latent, and transforming infections depending on the host

cell .

Papillomaviruses •Induce warts (papillomas) in a variety of higher

vertebrates, including man . •The viral nature of human warts was first indicated

in 1907 by Ciuffo. •>100 different HPVs have been described on the

basis of DNA sequence homology (> 50%) which fall into 16(A-P) groups that differ antigenically.

•Genome: Two large (E1 and E2) and several smaller (E4-E7) ORFs and a late region with two large genes

(L1 and L2) .

•The E region codes for proteins concerned with DNA replication, transcription and transformation .

•The E1 and E2 proteins are both essential for replication .

•The long control region (LCR) is concerned with the control of transcription .

Transformation

•Binding of the E2 protein to an enhancer site on the LCR can upregulate transcription of E6 and E7.

•Integration of the viral genome results in disruption of E1 and E2 with subsequent increase in expression

of E6 and E7.

•E7 binds Rb (P 105) and E6 binds to the tumor suppressor gene product P53 causing its rapid

degradation.

Polyomaviruses

•They were named so because of their capacity to cause different types of tumors .

•Most of them display a narrow host range and do not productively infect other species .

•Two viruses are definitely associated with human disease, JC virus )JCV (and BK virus (BKV).

•The genome codes for large T and small t antigens .

•They accumulate in the nucleus and stimulate cellular growth and are important for replication .

•The early proteins are associated with immortalization and transformation.

•Large T antigen binds to both Rb and P53 and prevents the induction of cell death .

Adenoviruses

Introduction

•Rowe and colleagues (1953)

•Hilleman and Werner (1954) •First given different names, these agents were

called adenoviruses in 1956.

•Today well over 100 members of the adenovirus group have been identified which infect a wide

range of mammalian and avian hosts

Introduction•Mastadenoviruses: infect mammals

•Aviadenoviruses: infect birds.

•49 human adenovirus serotypes •Six groups (A-F)

Structure•Naked icosahedral viruses with a diameter of 70-

75 nm .

•They have a characteristic Sputnik appearance .

•Capsid is composed of 252 capsomeres ( 240 hexons and 12 pentons .) Of the hexons

60 a re peripentons.

Structure•Each penton contains a base and a projecting fiber

(10-30 nm). •The penton base and Fiber are cytotoxic and

contain type-specific antigen. •The Fiber contains the VAP and acts as a

hemagglutinin.

Structure•At least 11 polypeptides

•The outer shell is composed of seven known polypeptides of which polypeptide II is the most

abundant.

•The hexon protein is comprised of three tightly associated molecules of polypeptide II .

Structure•Polypeptides VI, VIII, and IX are associated with

the hexon protein as they likely stabilize the hexon capsomere lattice.

•Five copies of polypetide III associate to form the

penton base protein. •Polypeptide IV forms the trimeric fiber protein .

•Peptide IIIa is penton associated

Structure •Polypeptide V bridges the core to the capsid

•Polypeptide VII is a major core protein around which DNA is wrapped forming a histone -like center

•Polypeptide Mu

•A terminal protein which is attached to the 5-end of viral DNA (two copies per virion) and serves as a primer for

DNA replication.

Structure•DNA polymerase uses the terminal protein and a

cytosine monophosphate as a primer .

•Genome - ds DNA of 34-45 Kbp with a molecular weight of 20-25 x 106 Daltons .

- A terminal protein (55kd) covalently attached to the 5-end (TP) .

Gene expression•Occurs in three phases termed pre-early, early, and

late .

•Most regulatory events in adenovirus gene expression occur at transcriptional or

posttranscriptional steps.

•Pre-early phase: Transcription of E1a. The E1a transcript is spliced to yield two E1a mRNAs. One

of the proteins is a transcriptional activator that induces transcription of the early phase proteins.

Gene expression•Early phase: Transcription of (E1b, E2, E3, E4) and

(L1) followed by splicing of RNA transcripts. Progression to the late phase of gene expression

with the help of protein products of at least 6 early genes then takes place.

•Late phase: Coincident with DNA replication and directed by the so-called major late promoter (20 distinct viral mRNAs). Transport of mRNAs to the

cytoplasm. Splicing and transport involves proteins produced in the early phase .

Protein Functions•Adenoviruses employ both virally encoded and host

proteins in replication.

•E1A and virus-associated RNAs (VA RNAs) afford protection from α and ß interferons.

•E1A and E1B activate host cell growth (S phase) and

mediate transformation of cells in vitro

•E1A inhibits apoptosis whereas E2 activates viral DNA replication and E3 antagonizes TNF and CTLs.

Behavior In Vitro•Replicate best in cells of human origin (HEK, A549,

Hep-2 and Hela).

•A group reactive epitope on the hexon is used to identify all adenovirus isolates .

•Adeno CPE Consist of rounding and grape-like clustering of swollen infected cells .

•Group and type- specific antigens exist in hexons and type specific antigen is present in pentons.

HERPESVIRIDAE

•Membership in the family herpesviridae is based on the architecture of the virion.

•Enveloped, icosahedral capsid (100-110nm)

•Average size of the enveloped virus is 200 nm

•Herpes viruses are highly disseminated in nature.

HSV particle

•Eight human herpesviruses; HSV-1, HSV-2 VZV, HCMV, EBV, HHV-6, HHV-7 and HHV-

8

•Predominantly neurotropic(HSV-1, HSV-2, VZV) or predominantly lymphotropic

( EBV, HHV-6, HHV-7 )

•Virion size: 120-300 nm.

•Variability is due to variable tegument thickness.

•Genome: 80-150x106 (120-230 Kbp). Circularizes immediately upon release

from capsid into nucleus.

Biological Properties•They specify a large array of enzymes. •The synthesis of viral DNA and assembly of capsids

occur in the nucleus .

•The irreversible destruction of the infected cell.

•They are able to remain latent in their natural hosts .

•Herpesviruses also vary greatly in their biological properties .

- Host cell range.

- Efficiency and speed of replication.

- Site of latency.

- Clinical manifestations of diseases they cause.

Classification

•Three subfamilies, alpha, beta, and gamma on the basis of biological properties .

•Classification into genera is based on : - DNA sequence homology . - Similarities in genome sequence rearrangement. - Relatedness of viral proteins.

Alphaherpesvirinae

•HSV –1, HSV-2 and VZV.

•Classified on the basis of: - A variable host range - Relative short reproductive cycle - Rapid spread in culture - Efficient destruction of infected cells - Capacity to establish latency primarily but not exclusively in sensory ganglia

Betaherpesvirinae•CMV, HHV-6, HHV-7.

•Classified on the basis of: - Restricted host range - Long reproductive cycle - Slow progress of infection in culture - Enlargement of infected cells (cytomegalia) - Latency in secretary glands, lymphoreticular cells, kidneys and other tissues.

Gammaherpesvirinae

•EBV, HHV-8

•Classified on the basis of: - Limited host range . - Replication In vitro in lymphoblastoid cells - Cause lytic infections in some types of epitheloid cells and fibroblasts - Specificity for either T or B lymphocytes - Latent virus is frequently demonstrated in lymphoid tissue

Herpes Simplex Viruses

•The first of the human herpesviruses to be discovered and the most intensively investigated of

all viruses

•Their attractions are their biologic properties •HSV-1 and HSV-2 have identical morphology and

cross- reacting antigen

•HSV infections of humans have been documented since ancient Greek times

Viral DNA

•It is composed of about 150 kbp with a molecular weight of 100x106 Daltons .

•Two segments; short (S) and long (L), with reiterated sequences resulting in four isomeric

forms. •It codes for more than 70 proteins .

Gene Expression•Three gene classes

- Alpha (immediate early) - Beta (early) - Gamma (late)

•Alpha genes are responsible for the initiation of replication (5 genes) .

•Beta genes products include enzymes necessary for replication (e.g TK), and the replication proteins. They require functional alpha gene products for

expression.

•Initially, five HSV immediate-early genes are transcribed with the assistance of an activating

protein carried in the tegument.

• In turn, some of these five genes then activate expression of about another dozen early genes

whose protein products are needed to replicate the viral DNA.

•Following DNA synthesis, about another five dozen

genes are activated. These genes encode structural components.

Virion Polypeptides

•The virion contains more than 30 proteins designated as virion polypeptides (VP).

•At least 11 are envelope glyocproteins

•These are gB, gC, gD, gE, gG, gH, gI, gJ, gK, gL and gM .

•gC, gE, gG, gI, gJ, and gM appear unnecessary for entry and egress in most infected cells.

•gB, D and H are essential for the production of infectious virions

•gB and gD are involved in adsorption and penetration into cells

•gB and D share antigenic determinants between HSV-1 and HSV-2

•gH is involved in the release of virus.

•gG provides antigenic specificity to HSV (distinction between HSV-1 and HSV-2)

•gC binds to C3b, whereas gE binds to Fc of IgG .

•gI is thought to be involved with gE at the Fc receptor.

•The role of gJ, gK, gL and gM are not well

appreciated.

Replication

•Fusion of the envelope with the plasma membrane

•Nucleocapsid is then transported to the nuclear pores where DNA is released into the nucleus .

•Viral DNA synthesis by a rolling circle mechanism, yielding concatemers which are cleaved into

monomers and packaged into capsids.

•Viruses utilize more than one attachment pathway

•HSV infects and replicates in two different cell

types, epithelial cells and neurons . •It must be able to attach and infect at least

three very different types of membranes. •It seems that multiple receptors exist

Fate of the Infected Cell

•Structural Alterations - Chromatin changes - Cell membrane alterations - Polykaryocytosis

•Host Macromolecular Metabolism

Latency•In latently infected neurons, the viral genome acquires

the characteristics of an endless or circular DNA .

•The virus expresses no functions, which are required for the establishment or maintenance of the latent

state.

• In a fraction of neurons harboring latent HSV the virus is periodically reactivated

•Latency of HSV does not appear to depend on the synthesis of virus-encoded proteins .

•RNA transcripts in lieu of proteins, may be involved in HSV latency (latency associated-transcripts- LATs).

•One possible mechanism is that LATs suppress and act as anti-sense to ICP0 (infected cell protein α 0) which is

expressed early during viral replication.

•Current research suggests that the ability to hide may be achieved via modification to the cellular enzymes histone

deacetylases (HDACs), namely HDAC1 and HDAC2 .

•Hypothetically, by interfering with the HDAC enzymes' effectiveness, it may be possible to block the virus's ability

to hide from the immune system, leading to a complete elimination of the virus by the immune system.

Varicella- Zoster Virus(VZV)

•Morphologically similar to HSV but antigenic relatedness is minimal .

•One serotype with strains showing little variation.

•Heberden distinguished varicella from smallpox in 1867

•VZV has the smallest genome of all human herpes viruses (encodes 69 proteins).

Major Antigenic Proteins •gE (Gp1), gB (GpII), gH (GpIII), gI (GP IV), and gC (GP V).

•gE (GpI) - Most abundant in VZV-infected cells . - Binds to the FC of IgG - Induces antibodies that have neutralizing activity in the presence

of complement

•gI (gpIV) is non covalently linked to gE in infected cells. Induces neutralizing antibodies in the presence of complement

•gB (GPII) elicits complement independent neutralizing antibodies. It appears to have a role in virus entry .

•gH (Gp III) and gC (GpV) also induce antibodies with complement independent neutralizing function.

Epstein – Barr Virus (EBV or HHV- 4)

•Discovered by Epstein, Barr and Achong in 1964 in lymphoma cells provided by Burkitt .

•Two EBV types; 1 and 2

•EBV differs in the predominance of a single surface glycoprotein (gp 350/220) .

•An additional minor protein (gp 85) also exists .

•Characteristic features of EBV genome include:

- A size of 172 kbp

- A single over all format and gene rearrangement.

- Reiterated 0.5 Kbp terminal repeats.

- Reiterated 3 Kbp internal repeats, that divide the genome into short and long unique sequences

( US, UL.)

•EBV has a very limited host range and tissue tropism defined by the limited cellular expression of its receptor.

•This receptor is the CD21 molecule (CR2), the receptor for

C3d. It is expressed on B cells and on epithelial cells of the oropharynx and nasopharynx.

•EBV causes productive (lytic) infection of epithelial cells and latent infection in B lymphocytes .

•EBNAs 1, 2, 3A, 3B and 3C, LPs, LMPs 1 and 2, EBER 1 and 2 are expressed in latently infected cells.

•EBNAs and LPs are DNA binding proteins that are essential for establishing an infection (EBNA-1), immortalization

(EBNA-2) and other purposes .

•LMPs have oncogenic like activities

•EBERs are most abundant in latently infected cells.

•The viral gene products maintain the latent infection and cause the previously resting B lymphocyte to continuously

proliferate .

•Permissive cells allow the transcription and translation of the ZEBRA peptide, which activates the early genes of the

virus and the lytic cycle.

Replication of EBV

Cytomegalovirus (CMV, HHV-5)

•Widely distributed in nature

•Share common growth characteristics

•Characteristic nuclear as well as cytoplasmic inclusions .

•Distinguishing characteristics - salivary gland tropism - species-specificity - slow replication in cultured cells.

•The largest genome (240 kbp) that exists in four isomeric forms.

•It is rich in direct and inverted repeats.

•Purified CMV virions consist of at least 30 readily detected polypeptides

•Two capsid, 8 envelope and as many as 20 tegument proteins.

Genomes of Herpes viruses

Envelope Glycoproteins

•Glycoprotein B (150 kd) is the major envelope glycoprotein of CMV that has the following

functions :

- Virion penetration into cells - Transmission from cell to cell - Fusion of infected cells . - a prominent target for neutralizing

antibodies

•Glycoprotein H (gP75) has the following functions:

- Target for complement-independent neutralizing antibodies

- Cell-to-cell transmission of virus

- Membrane fusion .

- Essential for viral replication (entry) .

•Glycoprotein C II or gp47-52 - An abundant envelope glycoprotein - Target for complement dependent neutralizing antibody.

•Glycoprotein 48; a minor envelope constituent .

•Numerous strains with an overall genomic DNA sequence homology of approximately

95%.

•CMV replication is slow and does not immediately kill cells or shut off host cell

metabolic processes

•Attachment proceeds rapidly and efficiently in both permissive and non permissive cells

•The highly restricted host cell range of CMV is determined by a post penetration block

involving restriction to viral gene expression (β2G)

Poxviridae

•The general properties of poxviridae include-:

-A large complex virion containing transcriptase and polymerase.

-A genome composed of a single linear ds DNA of 130-260 Kbp with a hairpin loop at each

end (fused).

-A cytoplasmic site of replication.

•The largest (240-300nm) and most complex of animal viruses.

• Brick – shaped or ovoid .

•Many of these viruses share antigenic determinants.

•A biconcave core (dumbbell-like) covered by an outer membrane and an envelope .