Transcript NT-HostDefense2-2005..

HOST DEFENSE AGAINST VIRAL INFECTIONANIMALS
• Primary defenses- physical and chemical barriers
-skin
-mucous secretions
-tears
-acid pH
-surface cleansing mechanisms (swallowing,
blinking)
IMMUNE DEFENSES
• Three critical processes in the immune defense:
-recognition
-amplification
-control
• The immune response to viral infection consists of:
Innate (nonspecific) defense: first line of immune
defense, responds to any infection, recognizes
characteristics common to microbial invaders,consists
of interferons, complement, natural killer cells, dictates
the adaptive response
Adaptive (specific) defense: antibody response and the
lymphocyte-mediated response also called the humoral
and cell-mediated responses
Innate and adaptive
immune responses:
The innate immune response:
• Can be activated rapidly and functions within hours
of a viral infection.
• Continued activity is damaging to the host.
• Considerable interplay occurs between the adaptive
and innate immune defenses.
Important components are:
-cytokines
-complement
-collectins
-natural killer (NK) cells
The adaptive immune response:
• Differentiates self from nonself, tailored to the
particular invader
• Has ‘memory’; subsequent infection by the same
agent are met with a robust and highly specific
response that stops the infection
• Consists of the:
antibody response - humoral response
lymphocyte mediated response- cell-mediated
response
O’Neill, Scientific American, Jan 2005 pp. 38-45
The inflammatory response:
• Essential in initiating immune defenses
• Cell and tissue damage caused by infection
induces the inflammatory response
• Provides communication with the components of
the immune system
• Characterized by redness, heat, swelling and pain
The inflammatory assault is initiated by Toll-like receptors
Inflammation can be initiated in
several ways:
• By interferon released by
immature dendritic cells
• Locally produced cytokines,
such as interleukin-1, tumor
necrosis factor- a and
interferon-g control the
reactions that occur during
inflammation
• Inflammatory cytokines also
activate B and T cells that are
needed for the adaptive
response
Cytokines:
• Regulatory proteins that mediate intercellular
communication during an antiviral defense.
• Their presence is one of the first indicators that the
host has been infected.
• They act locally, near the cells that make them.
• They control inflammation, induce and antiviral state
in cells and regulate the adaptive immune response.
• They exert their activities by binding to specific
receptors and activating gene expression.
• Three types of interferons are the most important
cytokines in the innate response to viral infection.
Interferons
Ifn-g is induced only when certain lymphocytes are
stimulated to replicate and divide after binding a foreign
antigen
Ifn-a and Ifn-b are induced by viral infection of any cell
type
Interferons
• Ifn is induced by accumulation of double stranded
RNA (dsRNA).
• Ifn induces gene expression at the transcriptional
level after binding to specific cell surface receptors.
• A cell that is bound to interferon and responds to it
is in an antiviral state.
• Ifn induces expression of more that 100 genes,
products of many of these genes possess broad
spectrum antiviral activity.
• They lead to cell death by apoptosis or programmed
cells death, limiting cell to cell spread of virus.
• Production of large amounts if Ifn causes common
symptoms such as fever, chills, nausea, etc.
Interferon induced antiviral responses:
• Both viral and cellular protein synthesis stops in Ifn
treated cells.
• This is dues to two cellular proteins, ds-RNA activated
protein kinase (Pkr) and ribonuclease L (RNase L).
• Pkr is a serine/threonine kinase that has antiviral
properties, as well as antiproliferative and antitumor
functions.
• Activated Pkr phosphorylates the alpha subunit of the
translation initiation factor eIF2, inhibiting translation.
• RNase L is a nuclease that can degrade cellular and viral
RNA; its concentration increases after Ifn treatment.
Interferon induced antiviral responses:
• RNase L concentration increases 10-1,000 fold after
Ifn treatment, but is inactive unless 2’-5’-oligo(A)
synthetase is produced.
• 2’-5’-oligo(A) synthetase produces 2’, 5’ oligomers of
adenylic acid, only when activated by dsRNA.
• These poly(A) oligomers then activate RNase L,
which degrades all host and viral mRNA in the cell.
• RNase L participates not only in Ifn-mediated
antiviral defense, but also in apoptosis.
• Ifn is a broad spectrum, highly effective antiviral
agent. However, viruses have developed numerous
mechanisms for inhibiting interferon action.
The adaptive immune response:
• Humoral response
Consists of lymphocytes of the B-cell lineage
Interaction of a specific receptor on precursor B
lymphocytes with antigens promotes differentiation
into antibody secreting cells (plasma cells).
• Cell-mediated response
Consists of lymphocytes of the T-cell lineage
Cytotoxic T cells (Tc cells) and T-helper cells (Th
cells) are the key effectors of this response.
Cell-mediated response cont.
• T lymphocytes recognize antigens on the surface of
self cells.
• The antigens on self cells can be recognized only by a
receptor on the surface of T cells when they are bound
to the MHC family of membrane proteins.
• The Th cells recognize antigens bound to MHC class II
molecules and produce powerful cytokines that affect
other lymphocytes (B and T cells) by promoting or
inhibiting cell division and gene expression.
• Once activated by Th cells, Tc cells differentiate into
CTLs that can kill virus infected cells.
The antigen receptors on the surface of B and T cells
B cells have about 100,000 molecules
of a single antibody receptor per cell,
which has specificity for one antigen
epitope.
T cells bearing the surface membrane
protein CD4 always recognize
peptides bound to MHC class II
proteins and function as Th cells.
T cells bearing the surface membrane
protein CD8 always recognize peptide
antigens bound to MHC class I
proteins and function as cytotoxic T
cells.
Endogenous antigen processing: MHC class I peptide
presentation
• Intracellular proteins of host and virus are marked for
degradation by ubiquitination and are degraded by the
Proteasome.
• The resulting viral peptides are transported into the ER
lumen by the Tap1-Tap2 heterodimeric transporter.
• In the ER lumen, viral peptides associate with newly
synthesized MHC class I molecules.
• MHC class I-peptide complex is transported to the cell
surface via the golgi compartments.
• On the cell surface, the MHC class I-peptide complex
interacts with the T- cell receptor of a Tc cell carrying the
CD8 coreceptor.
Endogenous antigen processing: MHC class I peptide presentation
Exogenous antigen processing: MHC class II peptide
presentation
• MHC class II complex is prevented form binding to viral
peptides in the ER by association with the invariant chain.
• The complex is transported through golgi where the
invariant chain is removed, activating the MHC class II
complex.
• The peptides are derived from extracellular proteins
that enter the cell by endocytosis.
• Viral proteins are degraded in the lysosomes by
proteases that are activated by low pH.
• Endosomes fuse with vesicles containing MHC class II.
• On the surface of the cell the MHC class II complex
interacts with the T cell receptor of a Th cell carrying the
CD4 coreceptor.
Exogenous antigen processing: MHC class II peptide
presentation
ANTIVIRAL DRUGS
•Because viruses are obligate intracellular parasites,
identification of safe and effective antiviral therapies is
difficult.
•The best antiviral drugs inhibit a specific step in viral
replication or pathogenesis.
•Drug discovery can be accomplished by screening or
rational design.
•The emergence of virus mutants resistant to antiviral
drugs is a serious problem.
•Combination of targeted delivery strategies to control
toxicities and resistance.
The pathway for drug discovery
Antiviral compounds
Many well-known antiviral
compounds are
nucleoside and nucleotide
analogs:
• Acyclovir is a nucleoside
analog similar to
guanosine, but contains
an acyclic sugar group
• AZT is a nucleoside
analog similar to
thymidine, but contains an
azide group
Acyclovir:
•Close to a perfect antiviral drug (specific, nontoxic).
•Highly effective against herpes simplex virus (HSV),
less so against varicella-zoster virus (VZV).
•Higly selective and extremely safe.
•Acyclic guanine derivative that inhibits viral DNA
synthesis.
•It is a prodrug, a precursor of the antiviral
compound.
•Activation of the drug requires three kinase
activities to be present in the cell to convert
acyclovir to a triphosphate derivative, the actual
antiviral drug.
Activation of acyclovir:
• Acyclovir has no effect
on host DNA replication
because the first kinase
activity is not found in an
noninfected cell.
• Cellular enzymes
complete the synthesis.
• Incorporated into viral
DNA, lacks the 3’ OH of
the sugar ring.
• Growing DNA chain
terminates, replication is
blocked.
Chemical structures of effective antiviral compounds:
Commercially available antiviral drugs:
Steps in replication of HIV targeted by antiviral drugs:
Antivirals against HIV:
• Azidothymidine (AZT)
Dideoxy analog of thymidine
Inhibits viral DNA synthesis
• Efficiently phosphorylated
to triphosphate by cellular
kinases
• AZT monophosphate competes with thymidine
monophosphate
• Much less selective than acylovir and has side effects
Does not eliminate previously incorporated provirus
Protease inhibitors
•Uncleavable mimics of gag-pol polyprotein
•Inhibits HIV protease
•Does not eliminate previously incorporated provirus
but does prevent further spread
•Resistance due to protease alterations
Prevention & Treatment of Influenza:
Beyond Chicken Soup
Zanamivir (Relinza): Blocks active
site of all NA proteins. Must be
inhaled as a spray, but prevents flu
symptoms if given before infection
& reduces symptoms if given
shortly after infection. Has no side
effects.
Other Anti NA Drugs in
development can be taken as pills
and appear to be very effective in
reducing illness if taken early in
infection. These drugs look very
promising in clinical trials and have
no adverse effects.
Neuraminidase Inhibitors:
Zanamivir & Oseltamivir
• Mechanism: blocking of the active site of
neuraminidase; prevents removaal of sialic acid
residues and results in clumping of viral progeny
• Effective against influenza A & B.
• Effective when flu symptoms are < 2 days old.
• Inhibitors reduce disease syndrome by 1 day.
• May decrease influenza secondary complications
• Antiviral resistance can occur, but much less
frequently than with the ion channel blockers
amantadine or rimantadine
Influenza Treatment with Ion Channel
Blockers Amantadine & Rimantadine
• Effective against influenza A but not B.
• Mechanism: inhibit ion channels & viral uncoating.
• Can reduce severity & duration of illness if started
within 48 hrs of onset of symptoms.
• Treated persons may shed resistant virus after 5-7
days of treatment (sometimes as early as 2-3
days).
• Treatment should be discontinued after 3-5 days
of treatment or within 24-48 hrs after
disappearance of signs/symptoms).
Antiviral Agents for Influenza
• Neuraminidase inhibitors appear to have similar
efficacy to the amantidine & rimantidine ion
channel blockers for prevention & treatment of
influenza
• Neuraminidase inhibitors have Less Central
Nerveous System side effects, but more GastroIntestinal effects
• Neuraminidase Inhibitors are more expensive, but
there is less risk of inducing virus resistance.
Very rapidly growing field of research.