Transcript Coronaviruses

Coronaviruses
24 Muharram 1430H
20 February 2009
SBM 2044
Coronaviruses
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Large enveloped RNA; ss positive-sense
Cause common cold.
Cause of SARS = severe acute respiratory
syndrome
Replication occurs in cytoplasm, the mature form
is budded into ER and golgi.
In humans, coronaviruses are limited to upper
resp T.
SARS
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2003 – outbreak of SARS inc serious resp disease,
pneumonia and progressive resp failure. Begun in rural
southern China, where people, pigs and domestic fowl
live close together and widespread use of wild species
for food and traditional medicine – conditions that
promote the emergence of new viral strains.
Transmitted by close contact with SARS patients or
recent travel to area infected with SARS
Incubation period ~6 days
Symptoms: headache, dizziness, sorethroat, shortness
of breath and 10% death.
Diagnosis: by serological test, passive
haemagglutination – RBC coated with coronavirus are
agglutinated by Ab-containing sera.
Rx: No vaccine. Precaution and less travelling.
Viruses that invade
the CNS
Rabies &
Prion Diseases
Rabies viruses:
Do VAMPIRES exist?
Rabies
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Rabies viruses invade CNS, causing an acute infection
of CNS.
Rabies is a rhabdovirus; rod and bullet-shapped
particles; membranous envelope with protruding spikes.
Ss RNA negative-sense; virions containing RNAdependent RNA polymerase.
Natural hosts are all warm-blooded animals inc foxes,
coyotes, bats, skunks.
Virus is widely distributed in nervous system, saliva,
urine, lymph, milk and blood. Recovery is rare except in
certain bats. Vampire bats may transmit the virus for
months without any clinical symptoms.
Pathogenesis
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Rabies virus multiplies in muscle and connective tissue
at the site of inoculation, and then enters peripheral
nerves at NMJ and spreads up the nerves to CNS.
Multiplication in CNS leads to encephalitis.
Then virus spreads to salivary gland and other tissues.
Highest titres of the virus is detected in submaxillary
salivary gland.
Rabies virus produces specific eosinophilic cytoplasmic
inclusion, the Negri body, in infected nerve cells. These
Negri bodies are filled with viral nucleocapsids.
Negri body in rabies-infected nerve
cells
Clinical Findings
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Incubation period is 1-2 months, but could be as
short as 1 week.
Symptoms: Malaise, anorexia, headache,
photophobia.
During acute neurologic phase, which lasts 2-7
days, patients show signs of NS dysfunction,
such as nervousness, apprehensions,
hallucinations and bizarre behaviour.
Rabies should be considered in any case of
encephalitis or myelitis of unknown cause. Long
incubation period – people may forget a possible
exposure incident, no recollection of being bitten
by bat.
Treatment
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Treatment is no benefit after the onset of clinical
illness. Therefore prompt post-exposure treatment is
vital – clean wounds, administration of rabies
immunoglobulin and vaccination.
Prompt vaccination to depress viral replication and
stop virus from invading CNS as well as to lower
concentrations of virus. Vaccine also inactivate rabies
virus.
Rabies Abs are administered either as pre-exposure
prophylaxis (for high risk people); or post-exposure
prophylaxis.
Vaccination also for dogs, and eliminating stray
animals
So, do VAMPIRES exist?
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Actually human-to-human infection is very
rare. The only documented cases involve
rabies transmitted by corneal transplants,
whereby the cornea was donated from a
patient with undiagnosed CNS disease.
Recipients died from rabies 50-80 days
later.
Prions
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Some chronic degenerative diseases of CNS are caused
by “slow” or chronic, persistent infections by classic
viruses eg. transmissible spongiform encephalitis – CJD
caused by prions. Other egs are Kuru, scrapie, BSE of
cattles, Gerstmann-Sträussker-Scheinker syndrome.
Prion = an agent of proteinaceous material devoid of
detectable amount of nucleic acid.
Prion agent is usually resistant to standard means of
inactivation, such as 3.7% formaldehyde, dry heat,
boiling and 50% ethanol. But prion is sensitive to 90%
phenol, household bleach and autoclaving (1hr 121°C).
Guanidine thiocyanate is highly effective in
decontaminating medical supplies.
Incubation periods may be years before clinical
manifestations.
Distinguishing hallmarks of prion
disease
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Confined to the nervous system
Amyloid plaques may be present
Long incubation periods (monthsdecades)
precede clinical and chronic illness
Always fatal, no known cases of remission or
recovery
Host shows no inflammatory/immune response –
agents are not antigenic. Hence no production of
IFN and no effect on B/T lymphocytes.
Amyloid plaques
Scrapie
Animals are susceptible to this disease
 High titres of prion are found in brain,
spinal cord and eye
 Development of amyloid plaques in CNS
 PrP, is a protease-resistant protein 2730kDa, purified from scrapie-infected
brain.
 PrPc
PrPsc
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normal protein
larger host-encoded protein
BSE and new vCJD
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1986 – Bovine spongiform encephalopathies
“Mad cow disease”, emerged in cattle in Great
Britain.
Was traced to the use of cattle feed that
contained contaminated bone meal from
scrapie-infected sheep and BSE-infected cattle
carcasses.
1993 – peak. 1 million cattle was infected.
1996 – new variant CJD formed, occurs in
younger people.
BSE-infected brain cells
Kuru and classic CJD
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These are 2 human spongiform encephalopathies.
Kuru only found in eastern highlands of New Guinea, spread
by customs surrounding ritual cannibalism of dead relatives.
Since the practice has ceased, disease disappeared.
CJD develops gradually in humans, progressive dementia,
ataxia, myoclonus. Death in 5-12 months time.
Iatrogenic CJD transmitted accidentally by contaminated
growth hormone preparations from human cadaver pituitary
gland, by corneal transplant, by contaminated surgical
instruments and cadaveric human dura mater grafts used for
surgical repair of head injury. No suggestions of CJD
transmission by blood or blood products.
Human Cancer
Viruses
Virus Family
Virus
Human Cancer
Paillomaviridae
Human
papillomaviruses
Genital tumours, squamous cell
carcinoma, oropharyngeal
carcinoma
Herpesviridae
Epstein-Barr v Nasopharyngeal carcinoma,
African Burkitt’s lymphoma, B
cell lymphoma
Hepadnaviridae Hepatitis B
virus
Hepatocellular carcinoma
Flaviridae
Hepatitis C
virus
Hepatocellular carcinoma
Retroviridae
HTL virus
Adult T cell leukaemia
HIV
AIDS-related malignancies
Features of Viral
Carcinogenesis
1. Different types of tumour viruses
 DNA tumour viruses encode viral oncoproteins
that are important for viral replication
 RNA tumour viruses carry and RNA-directed
polymerase (reverse transcriptase) that
constructs a DNA copy of the RNA genome. The
DNA copy (provirus) integrates into host DNA
and it is from this integrated DNA copy that all
proteins of the virus are translated.
Features of Viral
Carcinogenesis
2. Multistep Carcinogenesis:
 Multistep genetic changes to convert normal cell
into a malignant one.
 Intermediate stages are designated
“immortalisation”, “hyperplasia”, and
“preneoplastic”.
 It appears that a tumour virus usually acts as a
co-factor, provide in only some of the steps
required to generate malignant cells ie. viruses
are necessary – but not sufficient – for
development tumours with a viral aetiology.
Genes
Oncogene = gene that causes cancer. In
normal cells, known as proto-oncogenes.
 The molecular mechanisms responsible
for activating a benign proto-oncogene
and converting it into a cancer gene vary –
but all involved genetic damage.
 Tumour suppressor genes = negative
regulators of cell growth, also known as
the second class of human cancer gene.
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Genes
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Prototypes of tumour suppressor genes
are Rb (retinoblastoma) and p53 genes.
Inactivation of tumour
suppressor genes
Rb retinoblastoma
p53  tumour growth
Activation of oncogenes
Cancer
Retroviruses
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Retroviruses contain an RNA genome and an
RNA-directed DNA polymerase (rev transcrp).
Ss RNA, linear, positive-sense. Enveloped.
Retroviruses have been isolated virtually from all
vertebrate species. Most viruses of a given type
are isolated from a single species, though
natural infections across species barriers may
occur.
DNA Tumour Viruses
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RNA tumour virus, eg. Retroviruses carry
transduced cellular oncogenes that have no role
in viral replication.
However, DNA tumour viruses carry
transforming genes which encode functions
required for viral replication and do not have
normal homolog in cells.
For eg. Human papillomavirus has viral
oncoproteins E6 and E7 that interacts with
cellular p53 and pRb respectively.
Polyomaviruses
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Ds DNA, circular; the genome enclosed in a nonenveloped.
SV40 virus is from humans and monkeys.
Polyomavirus genome expresses “early” and “late”
proteins. Early proteins – eg. SV40 large tumour antigen,
is necs for the replication of viral DNA. The late genes
encode for the synthesis of coat protein.
SV40 DNA has been detected in human brain tumours,
mesotheliomas, bone tumours and lymphomas.
Other human polyomaviruses are BK and JC, have been
isolated from immunocompromised patients.
Papillomaviruses HPV
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Are highly tropic for epithelial cells of
the skin and mucous membrane.
Are the cause for skin warts, genital
condylomas, laryngeal papillomas,
as well as cervical cancer.
HPV-16 and HPV-18 are the common cause of
cancer. HPV-6 and HPV-11 (which cause benign
genital condylomas) are often found in laryngeal
papillomas in children.
The early proteins (HPV transforming proteins) are
synthesised in cancer tissues and able to complex
with pRb and p53.
Adenoviruses
Commonly infect humans, and cause mild
acute respiratory and intestinal infections.
 E1A early protein complexes with pRb;
E1B and E40RF1 bind with p53.
 No association of adenoviruses with
human neoplasms has been found.
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Herpesviruses - HHV
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EBV causes acute infectious mononucleosis
when it infects B lymphocytes  B_______
l_______
EBV encodes a viral oncogene protein (LMP1)
that mimics activated growth factor receptor.
LMP1 can transform B lymphocytes. EBNAs
(nuclear Ags) are important for immortalisation
of B cells, and consistently seen in BL cells.
HHV-8 is suspected as being the cause of
K_________ s____________.
Hepatitis Viruses – HBV & HCV
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HBV is a risk factor in the development of liver
cancer in humans (hepatocellular carcinoma).
The mechanism of oncogenesis ? Persistent
viral infection leads to necrosis, inflammation 
cirrhosis - - -> cancer
Chronic infection of HCV also leads to
hepatocellular carcinoma. Indirect mechanims..?
Human cancers
Other factors also may attribute to the
oncogenesis in humans, besides the
tumour viruses.
 Lifestyles: diet, smoking.
 Availability of vaccines.
 Genetics/hereditary
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Human
Immunodeficiency
Virus
Lentivirus
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The virus contains the 3 genes required for a replicating
retrovirus – gag, pol and env.
Classification: Lentiviruses have been isolated from
many species including at least 26 different African nonhuman primate species. There are 2 distinct types of
human AIDS viruses : HIV-1 and HIV-2. The 2 types are
distinguished on the basis of genome organisation and
phylogenetic relationships with other lentiviruses.
Simian immunodeficiency virus (SIV) is a type of monkey
lentivirus. The SIVs appear to be nonpathogenic in their
host species of origin (chimpanzee etc.)
Humans probably get infected with HIV from direct
contact with infected primate blood.
HIV
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Human
immunodeficiency
virus (HIV) is a
retrovirus, a
member of the
Lentivirus genus.
Ss positive-sense
RNA, linear;
enveloped.
Disinfection and Inactivation
HIV is completely inactivated by treatment
for 10mins at room temp with any of the
following: 10% household bleach, 50%
ethanol, 35% isopropanol, 0.5% paraform
and 0.3% H2O2.
 Extreme pHs (1 and 13) inactivate the
virus.
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Common Features of HIV
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Transmission by exchange of body fluids
Virus persists indefinitely in infected hosts
High mutation rates
Virus infection progresses slowly
It may take years for disease to develop
The virus receptor is the CD4 molecule expressed on
macrophages and T lymphocytes. The CD4 molecule has a
high affinity for the viral envelope.The second co-receptor is
required for fusion of the virus with the cell membrane. The
binding of virus with the 2 receptors cause conformational
changes in the viral envelope, activating the gp41 fusion
peptide and triggering membrane fusion.
Chemokine receptors (CCR5 on mØ and CXCR4 on
lymphocytes) serve as HIV-1 second receptors. These
chemokine receptors used by HIV are found on lymphocytes,
macrophages, thymocytes as well as neurons and cells in
colon and cervix.
CD4 molecule of T cells
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Is the major receptor for HIV
Number of molecules depleted during HIV
infection.
The consequences of CD4 T cell dysfunction are
devastating because the CD4 T lymphocyte
plays a critical role in the human response,
which inc activation of macrophages, induction
of cytotoxic T cells, NK cells and B cells.
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Individuals who possess homozygous deletions of CCR5 may be
protected from infection by HIV-1; mutations in the CCR5 gene
promoter appear to delay disease progression.
A dendritic cell-specific lectin, DC-SIGN, appears to bind HIV-1 but
not to mediate cell entry. Rather, it may facilitate transport of HIV by
dendritic cells to lymphoid organs and enhance infection of T cells.
Replication of HIV
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HIV binds to CD4 cell surface molecules, entry into the cell also
requires binding to co-receptorsCXCR4 and CCR5). This step can
be inhibited by fusion/entry inhibitors.
HIV is uncoated inside the cell and reverse transcriptase copies
genomic RNA into DNA, making errors at a frequence of about
one per replication cycle. Reverse transcriptase inhibitors were
the first class of HIV inhibitors to be used as drugs.
Viral DNA can integrate into DNA and become a part of the
cellular genome. This step makes the infection irreversible, and
may mean that eliminating the virus from an infected individual is
not possible. Integrase inhibitors are designed to block this step
of infection.
The virus uses cellular machinery to synthesize viral proteins.
Several of these are long amino acid chains which must be
cleaved by a specific viral protease before new viral particles can
become active. Protease inhibitors block viral maturation at this
step.
Course of HIV infections
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Following primary infection, it takes 8-12 weeks
for the viraemia. Virus is widely disseminated
throughout the body during this time, and the
lymphoid organs become seeded.
An acute mononucleosis-like syndrome
develops 3-6 weeks after primary infection,
during which a significant drop in circulating CD4
T cells is also detected.
An immune response occurs 1 week to 3 months
after infection, however this was not enough to
clear the infection completely, and the HIVinfected cells persist in the lymph nodes.
Course of HIV Infection
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The period of clinical latency may last for as long as
10 years. During this time, there is a high level of
ongoing viral replication.
There is a high turnover rates of HIV, and rapid viral
proliferation and high mutation rates.
Eventually, the patient will develop constitutional
symptoms and clinically apparent disease eg.
opportunistic infections or neoplasms.
HIV found in patients with late-stage disease is usually
much more virulent and cytopathic than strains
recovered early in infection.
Clinical Findings
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AIDS is characterised by pronounced
suppression of the immune system and
development of a wide variety of severe
opportunistic infections or unusual neoplasms
(Kaposi’s sarcoma).
Symptoms: weight loss, candidiasis, hairy
leukoplakia.
Disease symptoms in GIT from the oesophagus
to the colon are a major cause of debility.
With no treatment, the interval between primary
infection with HIV and the first appearance of
clinical disease is usually long in adults; 8-10
years.
Plasma viral load
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The amount of HIV in the blood (viral load) is of
significant prognostic value.
Apparently a single measurement of plasma viral load
about 6 mths after infection is able to predict the
subsequent risk of development of AIDS in men several
years later.
Difference in gender – women may be less predictive.
Plasma viral load is best at predicting long-term clinical
outcome.
CD4 lymphocyte counts are best predictor of short-term
risk of developing an opportunistic disease.
Plasma viral load is critical at assessing the
effectiveness of antiretroviral therapy.
Paediatric AIDS
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Neonates acquired the HIV from mothers.
Clinical symptoms appear at 2 years of age. Death
follows 2 years later.
Devastating for the neonates since immune system has
not developed at the time of primary infection.
Symptoms: pneumonia, oral candidiasis.
Untreated children will have very poor prognosis.
A high rate of disease progression occurs in the first few
years of life. Initially, low viral RNA load at birth, which
then rapidly rise within the first 2 mths of life, followed by
a slow decline until the age of 24 mth – suggesting that
the immature immune system has difficulty containing
the infection.
Other Associated Diseases
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A) Neurological disease – in 40-90% patients. Severe
symptoms inc aseptic meningitis, dementia,
toxoplasmosis. Paediatric AIDS patients display
seizures, loss of behavioral developmnt and attention
deficit disorder.
B) Opportunistic infections – infections by agents rarely
cause a problem. Infections inc Toxoplasma, Candida
albicans, Mycobacterium tuberculosis, Listeria
monocytogenes,HSV, VZV.
C) Cancer is another consequence of immune
suppression. AIDS-associated cancers inc nonHodgkin’s lymphoma (Burkitt’s lymphoma), Kaposi’s
sarcoma,
Immunity
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HIV-infected persons develop both humoral and cellmediated responses.
Most individuals make low level of neutralising Abs
against HIV, directed against envelope gp.
The envelope gp has great variability which allows
resistant virus to escape recognition by the host.
Cellular responses develop against HIV proteins. CTLs
recognise env, pol and gag gene products.
NK cells act against HIV-1 gp120.
It is not clear which host responses are crucial against
HIV infection.
Epidemiology
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AIDS was first recognised in 1981.
By 2002, WHO estimated that more than 24
million people worldwide had died of AIDS.
The WHO estimates that of the 5 million new
HIV infections each year, 90% are occurring in
developing countries.
Based on 2002 data, sub-Saharan Africa had
the highest number of HIV infections. High
prevalence – as many as 1 of every 3 adults was
infected.
Routes of Transmission
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High titres of HIV are found in blood and semen. HIV is
transmitted sexually. The presence of other sexually
transmitted diseases such as syphilis, gonorrhoea or
HSV-2 increases the risk of sexual HIV transmission
because the inflammation and sores facilitate the
transfer of HIV across mucosal barriers.
Blood transfusions
Injection drug use
Mother-to-infant in utero, during birth process, or through
breast-feeding (common, highest way of infection in
Africa)
Health care workers may accidentally get infected
following a needlestick with contaminated blood, though
the risk is very low 0.3%.
Prevention & Treatment
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Antiviral drugs: nucleoside and non-nucleoside inhibitors
and protease inhibitor.
Highly active antiretroviral therapy (HAART) – to
suppress viral replication, decrease viral load in
lymphoid tissues and to allow recovery against
opportunistic infections. However, HAART has failed.
Monotherapy usually results in the rapid emergence of
drug-resistant mutants of HIV.
Triple-drug therapy has proven effective.
Current drug-therapy may have toxic side effects.
Prevention & Treatment
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Zidovudine (azidothymidine, AZT) can reduce
the HIV transmission from mother to infant.
Vaccine development is difficult because HIV
mutates (envelope antigens) rapidly, HIV is not
expressed in all cells that are infected and is not
completely cleared by the host immune system.
Vaccine using recombinant viral proteins
(envelope gp) are likely candidates.
Gene therapy to genetically alter target cells to
make them resistant to HIV.
Control Measures
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Maintain an ad-Deen healthy lifestyle.
All donor blood should be screened for Ab.
Educating HIV-infected individuals.
Careful when dealing with contaminated
syringes, needles and other equipment.
Health care professionals should take
precautions when performing invasive
procedures to HIV-infected patients.