Chapter 9

Outcomes of Infection for the Host

Productive Infection• Infected cell releases some new viruses • Host cell may lyse or may survive for a short

period like in HIV-1 or long time like HepB• Some virus infections are not productive– may be latent infection – genome persists for the

cell’s lifetime or pass on to daughter cell– may be abortive – neither productive or latent – may

be due to a mutation of genome or virus particle may be defective• cannot undergo complete replication cycle unless another

virus infects the cell to provide the missing function = helper virus, provides good copy of the gene

Persistance

• Virus may persist for long times in the host and are productive like HIV-1 or may persist with periods of latency and productive infection like HSV

• Some long term infections may cause cancer• Some phages including the filamentous

phages initiate productive infection of bacterial hosts – appropriate host can shed virus for long periods

Effect of Virus on Host

• Effect on host can range from harmless to deleterious (death)– outcome varies on a complex interplay between

host, virus and environmental factors– hosts have evolved anti-viral defenses with viruses

have found ways to evade these mechanisms

• We will discuss the factors affecting the outcomes of infection

Innate Immunity in Vertebrates

• Interferons and natural killer (NK) cells• Interferons are proteins made and secreted by

cell in response to viral infection– induced by dsRNA from these viruses and in the

replication process of ssRNA– protect adjacent cells from infection– activates T-cell mediated immunity

Interferon• Most cells make both and

that diffuse to nearby cells to trigger various antiviral activities by binding receptors– activation of genes that

encode antiviral proteins – dsRNA dependent protein kinase R and Rnase L

– stimulate MHC I molecules and proteosome proteins – enhance presentation of viral peptides on surface of cell to T-cells

– activate NK cells– induction of apoptosis

Interferon

• Made mainly by T-cells and NK cells triggered by certain molecules such as interleukin-2 during the immune response– stimulates antigen presentation – activation of phagocytes and NK cells

Viral Countermeasures

• Viruses try to prevent the production of the these proteins or try to interfere with their activities– NS1 of influenza and NS3-4A of hepatitis C block

pathway of production– poliovirus prevents synthesis of interferon as

result of general inhibition of cellular gene expression

Natural Killer (NK) Cells• Throughout the body but

mostly in the blood• Recognize changes in surface

molecules of virus-infected cells but not the specific antigens like B- and T-cells

• NK cells bind to infected cell and then kills them and can also release interferon

• 2 mechanisms– release perforins – proteins

that when inserted into plasma membrane make pores and cell dies

– induce apoptosis

Viral Countermeasures

• HIV particles in the blood alters the expression of a number of molecules on surface of NK cells, reducing effectiveness in killing viral-infected cells and to make/secrete interferon

APOBEC 3 Protiens

• Enzymes in cells – humans and animals – that interfere with retrovirus replication– make lethal mutations by deaminating

deoxyctidine to deoxyuridine during reverse transcriptase

• Several of these proteins in human cells can interfere with replication of HIV – APOBEC 3F and 3G – incorporated into HIV virion– wreaks havoc in the next cell infected

Viral Countermeasures

• HIV infected cell has Vif protein that binds APOBEC 3G and causes its degradation– cannot be incorporated into HIV particle

Adaptive Immunity in Vertebrates

• Outcome of virus infections in vertebrate host is development of virus – specific immune response

• Regions of antigens known as epitopes, bind to specific receptors on lymphocytes and activating a cascade of events that result in an immune response

• 2 classes of lymphocytes – each is specific for a particular epitope as a result of presence of epitope-specific receptor on cell surface– B-cells – develop in the Bursa of Fabricuis in birds and bone

marrow in mammals– T-cells – develop in the thymus

• Naïve lymphocytes – have not encountered their epitope – keep circulating and have different surface molecules– distinctly different than cells exposed to epitope

Antibodies

• Glycoprotein known as immunoglobulins (Ig)• Basic structure looks like a Y and is made of 2 heavy

chains and 2 light chains with 2 binding sites for antigen (Fab) and a region known as the FC for fragment crystallizable

• Several classes of Ig – most imortant in view of antiviral immunity being IgG and IgM in the blood and IgA in mucosal surfaces– IgG is a monomer and IgA and IgM are dimers and

pentamers respectively

Plasma Cell

• Ag specific Ab are made by plasma cells – made from B-cells after it has been stimulated by interaction between Ag and specific receptor at the cell surface

Role of Antibodies• Play important roles in several aspects of anti-viral

immunity based on its structure – Ab binds to Ag and then the cell or virus is destroyed by various mechanisms– neutrophils and macrophages will phagocytize Ab-coated

cells and viruses because have an FC-receptor on surface, may kill without phagocytosis

– NK cells may be activated to kill cell with perforins– activation of the complement system which has a number

of antiviral effects• insert complement proteins complexes into membrane a virus

infected cell or enveloped virus – kills them• complemented coated virions can trigger phagocytosis by

neutrophils and macrophages – have receptors for complement proteins

Additional Ab Effects

• Ab binds to virion and can neutralize infectivity by a variety of mechanisms– release of nucleic acid from virions – poliovirus = Ab

attaches and when detaches, the capsid is empty of genome

– prevent attachment to cell receptors – Ab masks virus attachment sites, not all sites are accessible to Ab – picornovirus in deep canyons

– release of virions that have attached to cell receptors– inhibition of entry into cell – Ab coating fusion proteins on

enveloped virion may inhibit fusion with cell membrane– inhibition of genome uncoating

T-Cells• After Ag stimulation naïve or memory T-cells develop

into effector T-cells – 2 classes– helper T-cells – secrete specific cytokines and

characterized CD4 on surface – essential roles in initiation of immune response• help trigger B-cells in Ab secreting cells and maturation of

cytotoxic T-cells– cytotoxic T-cells – kill virus infected cells, characterized by

CD8 on surface – viral Ag must be expressed on surface of target cell• surface proteins like envelope glycoproteins but most often

internal virion proteins or non-structural proteins• CTL can remove early infection cells before more virus made

• Ag displayed on infected cells on MHC I on surface and trigger CTL action – insert perforin into membrane or induce apoptosis

Viral Countermeasures

• Some viruses like HSV reduce level expression of MHC I – makes more difficult for CTLs to recognize infected cells

Immunological Memory

• Quantity and quality of adaptive immune response depends on whether virus is encountered for the first time

• B and T cells can serve as memory cells long after first or subsequent encounters– memory – return to resting state to be reactivated

when exposed to Ag again– can be from natural Ag or from Ag in a vaccination

• If host has memory – signs and symptoms may be less severe or absent on subsequent exposures

RNA Silencing

• Also known as post-transcriptional gene silencing or RNA interference (RNAi)

• Intracellular processes induced by dsRNA – causes destruction of mRNA that have the same sequence as the inducing dsRNA – both cellular and viral mRNAs can be destroyed

RNAi Mechanism• Cleave dsRNA into small interfering dsRNA (siRNA) 21-25 bp

using a complex with Dicer (Rnase III family)– leaves a 3’ overhang of 2-3 nt

• siRNA binds to complex to make RISC (RNA induced silencing complex)

• siRNA is unwound and (-) strand stays with RISC causing on active complex

• (-) strand finds complementary mRNA, mRNA degraded where double-stranded

• Found in plants, fungi, invertebrates, vertebrates (animals)– important anti-viral defense mechanism, may be for rest also

• Induced when add synthetic dsRNA to cell

Viral Countermeasure

• Some plants make proteins to inhibit silencing– helper-component proteinase of potyvirus and

P19 protein of tombusvirus – both strong suppressors

Programmed Cell Death

• Kill the cell before can release new viruses – called apoptosis

• Also functions when cell reaches end of life span

• Bacteria has mechanism to cause cell death as well – again prevents spread to other bacteria

Viral Countermeasure

• Viruses can make proteins that can control the process of apoptosis – some DNA viruses make a protein similar to BCL-2

that controls apoptosis = block the process

Non-Productive Infection

• Replication cycle is not completed – 2 outcomes– latent infection – genome persists but no virus– abortive infection – any other thing

Latent Infection• Initiated when virus

genome is maintained is maintained in the infected cell – integrate into cell DNA or maintained copies of covalently closed circular DNA (episomes)

• Eukaryotic cells – viral DNA associated host cell histones – help with latency

Retrovirus Latency

• Retroviruses – early in infection integrate into host genomes, infection doesn’t progress, if intracellular environment changes to favorable the latent infection can become a productive infection

Bacteriophage Latency

• Phenomenon of latent phage infection in bacterium = lysogeny – phage is said to be temperate– prophage (genome of the phage) persists in cell –

can integrate into genome or as non-integrated circular DNA

– temperate phage genomes – may encode gene to have selective advantage on host – sometimes have virulence factors like Shiga toxin in E coli

During Latency• Genome may be shut down or may have a few genes

expressed – proteins or non-coding RNA– if cell divides, nee daughter cell gets viral genome too –

replicate without hurting cell• Latent cells can become productive infection =

induction can occur if following happen– eukaryotic host cell moves into another phase of cell cycle– exposure to UV light – phage to lyse bacteria or HSV to

cause cold sore– immunocompromised host – HSV– host infected with 2nd virus (helper virus) – provide

function lacking in 1st virus (satellite virus)

Examples of satellite/helper viruses

Satellite virus Helper virus

Hepatitis delta virus Hepatitis B virus

Adeno-associated viruses Adenovirus

Abortive Infection

• Non-productive and genome doesn’t persist• May be abortive because of cell,

environmental conditions and/or virus• May be productive in permissive cells but

abortive in non-permissive cells• Infection may kill cell before make more virus• Virus may be mutant – good enough to start

infection but not to finish – virus is said to be defective

Types of Defective Viruses

• Number of different types• Defective interfering particle (DIP) – happens in

lab after animal virus passed several times in cell culture at high MOI – see also in chick embryos, in mice and in plant viruses– DIPs have less genome than normal – are either non-

infectious or can do only abortive infections– if cell infected with normal virus and DIP – DIP can

replicate but “interferes” with replication of normal viruses

Productive Infections• Spread of infections within multicellular hosts

– virus may spread to nearby cells – common cold virus and rotavirus may infect additional respiratory and gastrointestinal epithelial cells – direct cell to cell spread

• Plant viruses move thru plasmodesmata – virus encodes 1 to 4 proteins to enable this = movement proteins (MP) – different functions– complex with viral RNA– or with RNA + coat protein– form tubular structures thru which encapsidated RNA is

transported, also roles in viral replication• May move distances in host – blood and nerves in animals

and phloem in plants

Disease• Many viral infections result in no disease to host

while others are fatal (rabies and HIV)• Many manifest as symptoms (subjective like pain;

recognized by patient only) or signs (objective like skin rash, blood in stool; recognized by others)

• Infections that do not cause disease are said to be subclinical or asymptomatic

• Outcome is based on complex interplay with virus factors, host factors and human intervention

• Not all viruses are pathogenic and some only pathogenic under certain conditions

Pathogenic Virus Factors

• 1 – virulence of strain – measure of severity of disease it causes– influenza A H5N1 more virulent than H1N1 and

H3N2 - H5N1 is more severe cause of disease

• 2 – dose of virus – large dose can shorten incubation period – time between infection and 1st signs/symptoms

Host Factors

• Immune system effectiveness based on age and nutritional status of host– strong immune may not mean complete

elimination of virus• HIV still replicates in high Ab titer and T-cells

– symptoms/signs may be response of immune system• measle rash and HSV lesion – clinical manifestation of

attempts to destroy virus-infected cell

Human Intervention

• Anti-viral Ab or anti-viral drugs• Recovery – may clear virus completely or may

establish long-term infection– may be persistant or latent– no further problem or cause problems later, may

lead to cancer as well• Varicella-zoster – chicken pox as kid; shingles as adult