Basic Principles of Infectious Diseases L15

Question 1

Infectious disease is caused by which pathogenic microorganisms? (3)

Answer 1

• Viruses
• Bacteria
• Parasites

Question 2

List the routes of transmission of infectious disease. (6)

Answer 2

• Contact
• Aerosol
• Faeco-oral
• Sexual
• Animal vector
• “Mechanical” (e.g. needle-stick, insect bite)

Question 3

Define obligate intracellular parasites.

E.g.?

Answer 3

Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite’s reproduction is entirely reliant on intracellular resources. Can only live within living cells.

E.g. Viruses

Question 4

What do viruses not have that they need from living cells? (3)

Answer 4

• Organelles
• Raw material for reproduction
• Enzymes

etc.

Question 5

A virus basically consists of a nucleic acid packaged up in a ____1____, and in some cases wrapped up in an ____2____.

Answer 5

1. Protein
2. Envelope

Question 6

Disease outcome depends on type of pathogen and ability of immune response to clear the infection. An ____1____ viral infection is characterized by rapid onset of disease, a relatively brief period of symptoms, and resolution within ____2____. It is usually accompanied by early production of infectious ____3____ and elimination of infection by the host immune system.

Answer 6

1. Acute
2. Days
3. Virions

Question 7

A chronic infection is a ____1____-lasting infection Chronic infection occurs when the host immune response is able to ____2____, but not eliminate, the infecting organism In chronic infection the infecting organism continues to ____3____ at a low level In latent infection the infecting organism is dormant and does not ____3____. Some infections may have both a chronic and latent phase.

Answer 7

1. Long
2. Control
3. Replicate

Question 8

What is the longest a chronic infection can last?

Answer 8

A lifetime.

Question 9

Give example of bacterial chronic infections. (invading organism, disease caused) (2)

Answer 9

• Mycobacterium tuberculosis - tuberculosis
• Mycobacterium leprae - leprosy

Question 10

Give example of parasitic chronic infections. (invading organism, disease caused) (2)

Answer 10

• Plasmodium falciparum - malaria
• Schistosoma – schistosomiasis

Question 11

Give example of viral chronic infections. (3)

Answer 11

• HIV-1
• Hepatitis B virus
• Hepatitis C virus

Question 12

Hep _1_ 90% cases = Acute infection 10% cases = Develop a chronic infection

Hep _2_ 20% cases = Acute infection 80% cases = Develop a chronic infection

Answer 12

1. B
2. C

Question 13

____1____ live inside the macrophage and are able to resist the immune response. They replicate inside the endosomes.

Answer 13

1. Mycobacterium

Question 14

Mycobacterium tuberculosis form a ____1____ in the lung leading to tuberculosis. Mycobacterium leprae live in cooler regions (such as skin and nose) and also infects ____2____ cells leading to nerve degeneration in leprosy.

Answer 14

1. Granuloma
2. Schwann

Question 15

Question 16

Question 17

Read

Cycle of viral infection

Entry

• Virus attaches to cell by binding a cell surface receptor.

Penetration

• Either through receptor-mediated endocytosis or by fusion of viral envelope to membrane and subsequent release of rest of virus.

Uncoating

• Viral genome is unwrapped (protein coat is removed).

Biosynthesis

• Of new nucleic acid (genome) copies.
• To produce new viral proteins.

Assembly

• New viral proteins package up viral genome copies.

Release

• Via exocytosis or lysis of cell.

Viruses require the host cell ribosomes, enzymes, precursors etc. to generate the new proteins and genomes. This is why they must live inside cells.

Question 18

Describe an asymptomatic/subclinical infection.

Answer 18

No adverse symptoms – virus cleared by immune response

or

Establishes persistent infection

Question 19

Describe an acute infection

Answer 19

Illness lasts a few days/weeks – virus then cleared by immune response

or

Establishes persistent infection or death

Question 20

Describe an chronic infection

Answer 20

Virus not cleared by immune response - continuous production of virus progeny

Question 21

Describe latent infection

Answer 21

Viral genome is maintained in host cells for many years until re-activated (e.g. by stress). No progeny during latent period.

Question 22

How the acute infection progresses is dependent on how well your immune response is able to control the infection.

Question 23

Enzymes of virus genome replication and transcription

dsDNA and ssDNA can often be replicated by ______ ___1___ ______ and transcribed by ______ ___2___ ______.

Answer 23

1. Host DNA polymerase
2. Host RNA polymerase

Question 24

Enzymes of virus genome replication and transcription

dsRNA and ssRNA require an ______ ______ 1 ______ ______ (RdRp) for replication and translation: must be encoded by ____2____.

Answer 24

1. RNA dependent RNA polymerase
2. Virus

Question 25

Enzymes of virus genome replication and transcription

Positive stranded ssRNA genome can be immediately ______ to produce RdRp.

Answer 25

1. Translated

Question 26

Enzymes of virus genome replication and transcription​

Negative stranded ssRNA genome cannot be immediately translated and so requires RdRp to be carried in ______.

Answer 26

1. Virion

Question 27

Enzymes of virus genome replication and transcription​

Retroviruses and hepadnaviruses carry ______ 1 ______ in virion, in order to convert RNA genome to ____2____ intermediate.

Answer 27

1. Reverse transcriptase
2. cDNA

Question 28

Enzymes of virus genome replication and transcription

Host RNA polymerase is a DNA dependent RNA polymerase.

If the virus requires something different then they must either ____1____ it or transcribe and translate it themselves using the host cell ____2____.

Answer 28

1. Carry
2. Machinery

Question 29

Some virus genomes are translated to produce a single large viral polyprotein which must be post-translationally cleaved to release individual viral proteins.

Give examples (3)

Answer 29

• Poliovirus
• Rhinovirus
• Hepatitis C virus

Question 30

Viral proteins undergo the same range of ______ ______ ______ as the host cell proteins, e.g. glycosylation, acylation, lipoylation.

Answer 30

Post translational modifications

Question 31

Viruses don’t carry their own ribosomes, therefore all viruses must compete for the host cell ribosomes. ____1____ of most viral mRNAs (like host proteins) is dependent on 5’ terminal cap. Eukaryotic initiation factors (eIFs) bind to cap and ____2____ ribosomal subunit. When viruses infect cells they often down-regulate or cause down-regulation (by triggering the immune response) of the ____3____ of the messenger RNA.

Answer 31

1. Translation
2. 40S
3. Capping

Question 32

Some viruses (e.g. Hep C) have evolved a mechanism for translation of its genome in the ____1____ of the 5’ cap.

So when the cells translation is effectively shut down by the ____2____ response due to the infection of the virus, these viruses contain an internal ribosome entry site (IRES) to allow the initiation of translation (3D shape acts like the 5’ cap).

Answer 32

1. Absence
2. Immune

Question 33

Question 34

Hep B and C are very good at evading the immune response, which is how they can maintain a ____1____ infection. This is partly due to both being RNA viruses, of which there isn’t a checking mechanism on the RNA genome so they are quite prone to mutation which causes the constant change of ____2____ proteins allowing them to evade the immune response.

Answer 34

1. Chronic
2. Surface

Question 35

Read

Hep C Genome

Hep C is a very simple virus – consists of an RNA genome which is packaged up in an icosahedral nucleocapsid.

All viral envelopes originally come from the cell membrane.

E1 & E2 = cell surface glycoproteins that allow attachment by binding to the target cells membrane

Core = protects genome and provide structure of the virus

NS proteins = non-structural but are needed in order for the virus to replicate

In order to generate all these proteins, the large polyprotein must be cleaved at each cleavage site (above). Cleavage (white triangles) are carried out by host cell proteases. Once these cleavages have taken place, the newly cleaved NS2 acts as a protease itself (autoprotease) to cleave (red triangle). Newly cleaved protease NS3-NS4A cleaves (green triangles).

Each of the cleaved proteins also has a role in the assembly of the virus.

Cleaved NS5B is the RNA-dependent RNA-polymerase which, when cleaved, is able to start the process again by making more copies of the viral genome.

miR-122 = short non-coding RNA that is present in hepatocytes and is important in stabilising the IRES structure and preventing it from being degraded once it enters the hepatocyte cell.

Question 36

Read

Hepatitis C virus replication

E1 & E2 (viral surface glycoproteins) interact with target cell surface proteins leading to entry of the virus by receptor-mediated endocytosis.

SRB1, CD81, CLDN1 and OCLN have all been implicated in binding the virus and getting it into the cell. CLDN1 and OCLN are proteins that make up tight junctions.

Virus is released from the endosome and the RNA genome is released into the cytoplasm. The polyprotein is translated and cleaved (blobs on ER represent the individual cleaved proteins).

Then inside the cell a membranous web (MW) is developed. The MW is a complex system of membranes surrounding the nucleus, where the viral particles start to assemble and associate with the lipid droplets.

The viral particle-lipid droplet complex is then released from the cell by exocytosis and the virus can then enter back into the bloodstream to infect more hepatocytes or to be transmitted to a new host.

Question 37

HCV “hides” in lipoviroparticles.

By being in a lipoviroparticle, the virus is less accessible to antibodies in the blood stream, therefore evading the ____1____ response.

Also by being in a lipoviroparticle, the virus may be aided in entering cells due to the target cells displaying lipoprotein ____2____.

Answer 37

1. Immune
2. Receptors

Question 38

Read

HCV Summary

• Hepatitis C virus (HCV) infects hepatocytes in the liver, establishing a chronic infection in the majority of patients
• HCV has a positive sense single stranded RNA genome which is translated into a single polyprotein, using an Internal ribosome entry site (IRES) sequence.
• The HCV polyprotein is cleaved into individual viral proteins by a combination of host and viral proteases.
• HCV is able to evade the immune response to establish a chronic infection. This is in part due to a combination of mutational escape and inhibition of the interferon response.
• HCV virions associate with lipoproteins. This may aid cell entry and evasion on the immune response.