Week 15

Question 1

How are T cells activated?

Answer 1

T cells are activated to proliferate by binding to antigens that are presented to them by dendritic cells, professional antigen presenting cells.

Question 2

T cell activation process (innate immune response)

Answer 2

• Pathogens enter through break in skin and are phagocytosed by dendritic cell
• Activated dendritic cell carries pathogen peptides to local lymph node

Question 3

T cell activation process (adaptive immune response)

Answer 3

Activated dendritic cell activates specific T cells to respond to pathogen peptides bound to MHC proteins on dendritic cell surface.

Question 4

An antigen must be bound to what for a T cell to bind to it?

Answer 4

An MHC protein.This allows T cells to recognize infection inside a cell because the dendritic cell presents the antigen on the surface.

Question 5

Class I MHC proteins

Answer 5

MHC proteins made by dendritic cells

Question 6

Functional structure of Class II MHC proteins

Answer 6

Composed of two proteins that form a heterodimer. The antigen binds in the peptide-binding groove.

Question 7

How do MHC proteins bind peptides?

Answer 7

MHC proteins bind peptides promiscuously but weakly

Question 8

What type of proteins do activated cytotoxic T cells bind to?

Answer 8

Activated cytotoxic T cells identify infected host cells by looking for antigens bound to Class I MHC proteins

Question 9

In the human genome, what encodes for MHC genes?

Answer 9

MHC genes are encoded in the HLA complex in the human genome.

Question 10

Which alleles that encode for MHC proteins are polymorphic?

Answer 10

Alpha and the beta chains of the MHC class II proteins and the alpha chain of the MHC class I protein are most polymorphic

Question 11

Which type of cells trigger apoptosis of infected cells and cancer cells?

Answer 11

Cytotoxic T cells and Natural Killer cells (NK cells) trigger apoptosis of infected cells and cancer cells

Question 12

Role of perforin molecules

Answer 12

Perforin molecules form a pore in the membrane of the host cell.

Question 13

Role of granzyme B in apoptosis

Answer 13

Granzyme B moves through the pore and activates executioner caspases, forcing the host cell to undergo apoptosis.

Question 14

Macrophages

Answer 14

• Long-lived
• Already present in tissue
• Survive after engaging

Question 15

Neutrophils

Answer 15

• Short-lived (few hours)
• Recruited to site of infection from blood
• Die via apoptosis during engagement (make up majority of pus)

Question 16

Two categories of viruses

Answer 16

1. Enveloped
2. Noneneloped

Question 17

Packaging of nonenveloped viruses

Answer 17

Viral genome is packaged in a protein coat called capsid

Question 18

Packaging of enveloped viruses

Answer 18

Viruses with a capsid coat are enclosed within a lipid bilayer

Question 19

Which type of virus is SARS-CoV-2?

Answer 19

Enveloped

Question 20

What can receptors be composed of that viruses bind to to enter cells on the host cell?

Answer 20

• Viruses enter cells by binding to receptors on the host cell (can be proteins, but also lipids)
• This can serve as a species barrier if there are differences in the receptor between species

Question 21

Which type of viruses can sometimes enter a host cell by fusing with the plasma membrane?

Answer 21

Enveloped viruses. An enveloped virus will fuse with the host membrane, uncoat, and be released into the cytoplasm.

Question 22

Besides fusing with the membrane, what is the alternative way an enveloped virus can enter the host cell?

Answer 22

Endocytosis. After endocytosis, the virus’ own lipid will fuse with the cell’s lipid and allow the genome to be released.

Question 23

How do viruses leave cells? (enveloped vs nonenveloped)

Answer 23

Nonenveloped viruses usually leave cells by lysing it, enveloped viruses use membrane-based strategies

Question 24

Which type of virus is more fragile outside the cell?

Answer 24

Enveloped viruses are more ‘fragile’ outside the host because of their lipid coats and hence easier to kill

Question 25

What type of genetic material are viral genomes composed of?

Answer 25

RNA and DNA

Question 26

Why don’t dsDNA viruses not need to encode their own DNA polymerase?

Answer 26

A viruses do not necessarily need to encode their own DNA polymerase as they can rely on the host DNA polymerase for their replication. But since DNA polymerase is only expressed during S phase, these viruses often promote the cell cycle (e.g., by inactivating Rb). Consequently, these viruses (e.g., papillomavirus) can promote cancer.

Question 27

Why would a virus make the host cell enter the cell cycle?

Answer 27

The DNA of the host is only replicated during the cell cycle. The virus must find a way to have the host cell express DNA polymerase by forcing it to enter the cell cycle. Then the virus can use it to make more copies of its own genome.

Question 28

How is -ssRNA used?

Answer 28

-ssRNA used as a template by RNA-dependent RNA polymerase to make +ssRNA. This +ssRNA can be used for translation, as well as a template for replication to make -ssRNA.

Question 29

Why do -ssRNA viruses have to bring RNA-dependent polymerase along as part of the viral particle?

Answer 29

Since cells don’t have RNA-dependent RNA polymerases, -ssRNA viruses have to bring this protein along as part of the viral particle

Question 30

How does +ssRNA get RNA-dependent RNA polymerase?

Answer 30

In contrast, +ssRNA viruses have to encode RNA-dependent RNA polymerase gene but they can make the protein once inside the host.

Question 31

What additional enzyme do retroviruses have to encode?

Answer 31

Have to encode a reverse transcriptase in their genome.

Question 32

Three reasons viruses can overcome host’s immune system

Answer 32

1. Massive population size
2. Rapid proliferation
3. High mutation rates

Question 33

Diversity in humans and chimpanzees vs in viruses

Answer 33

• Human and chimpanzee genomes differ by 1-2% after evolving separately for ~5-8 million years. Polio viruses in a single host acquire this much diversity in about 5 days.
• HIV strains can differ by as much as 30% between different isolates.

Question 34

Antigenic variation

Answer 34

Diversity of antigens, resulting from evolution of the virus to evade host’s adaptive immune system.