Lecture 8: Cancer and the immune system

Question 1

How are cancers classified (giving 4 examples)?

Answer 1

Classified according to their embryonic tissue origin

Examples:
1. Carcinomas = epithelial origin
2. Sarcomas = mesodermal connective tissue origin
3. Lymphomas, myelomas and leukaemias = haematopoietic stem cells
4. Mesotheliomas = mesothelial lining cells of body cavities

Question 2

Describe the four stages of tumour development

Answer 2

1. Initiation = a single cell develops an altered growth phenotype (initial mutation which may be due to UV or carcinogen exposure)
2. Promotion = the cell proliferates forming a benign tumour (build up of mutations that may turn on proto-oncogenes or turn off tumour suppressor genes)
3. Progression = the tumour becomes invasive and is now malignant
4. Metastasis = via local blood vessel or lymph duct to colonise secondary site

Question 3

What is transformation?

Answer 3

the process by which a normal cell acquires the properties of a tumour cell - can be benign or malignant

Question 4

What can transformation be initiated by?

Answer 4

Exposure to carcinogens that damage cell’s DNA (E.g. UV, tobacco smoke, asbestos)

Infection with tumour-causing virus (E.g. human papillomaviruses cause cervical cancer)

Question 5

Give an example of a genetic pre-disposition that means these people have cells that are more likely to become transformed?

Answer 5

people that already have mutations in the BRCA1 tumour suppressor gene are predisposed to breast cancer

Question 6

Describe some genetic changes that can occur in colon cancer

Answer 6

Loss of tumour suppressor genes APC, DCC and p53

Activation of K-Ras oncogene that is linked to proliferation

Question 7

What are some cancer-associated genes that change during transformation?

Answer 7

Proto-oncogenes (can be turned into oncogenes that promote uncontrolled growth when mutated or overexpressed - E.g. K-Ras, Src, Myc)

Tumour suppressor genes (can be mutated or lost during transformation to promote uncontrolled cell growth - E.g. p53, BRCA1/2, APC, Rb)

Apoptosis regulators (can be inactivated or altered - E.g. Bcl-2)

Question 8

What cancer-causing factors can be expressed by tumour-causing viruses?

Answer 8

Viral oncogenes - often similar to cellular oncogenes and drive host cell growth (E.g. vSrc)

Viral inhibitors of tumour suppressor genes (E.g. E6 protein from HPV inhibits p53, HPV E7 inhibits Rb)

Question 9

What is immune surveillance?

Answer 9

the detection and elimination of transformed cells or tumour cells by the immune system

Question 10

How can tumours be detected by the immune system?

Answer 10

mutations in a cell can result in expression of ‘non-self’ or ‘altered-self’ antigens that out T-cells can recognise as they’re not tolerant to them.

Question 11

What are the different types of tumour antigens that can be detected by the immune system?

Answer 11

Tumour specific antigens (TSA) which are unique to the tumour (E.g. mutated oncogenes, viral proteins such as HPV E6 or E7 in cervical carcinoma)

Tumour associated antigens (TAA) which are normal proteins that are over-expressed in cancer cells (E.g. HER2 epithelial growth factor receptor in breast cancer) or re-expressed embryonic genes call oncofoetal antigens (E.g. AFP in liver cancer)

Question 12

How are TSAs and TAAs detected by the immune system?

Answer 12

These antigens are presented on MHC I molecules on tumour cells
Replication stress and DNA damage may also cause expression of ligands for NK cell activation

Th1 immune response promoted - dendritic cells and macrophages present tumour-derived antigens to T- and B- cells, secrete cytokines such as TNF-alpha and interferons

If detected, tumour cells are killed by NK cells, inflammatory macrophages and cytotoxic T-cells

Question 13

How can the immune system promote tumour growth?

Answer 13

immunoediting of cancer in which the immune system shapes tumour development by eliminating some tumour cells by immunosurveillance, causing selection of tumour cells that escape immune recognition, and creating an inflammatory microenvironment that actually promotes tumour growth (TGF-beta)

Question 14

What are the three sequential phases of immunoediting?

Answer 14

Elimination (tumour cells are killed by the immune system)

Equilibrium (immune system restricts tumour growth)

Escape (selection pressures cause the outgrowth of tumour cells that are no longer sensitive to immune attack)

Question 15

How does immuno-editing lead to cancer progression?

Answer 15

with time and further mutation, selective pressures allow escape mutants to develop which allow invisible cancer cells to outgrow
- loss of MHC I and NK ligands
- cancer cells express ligands for co-inhibitory receptors that turn off T-cells.

Question 16

How can tumour cells evade immune detection?

Answer 16

• may not express any appropriate antigen
• often lack co-stimulatory molecules CD80/86 for T-cell activation
• genetically unstable so can evolve to escape selection pressure from immunosurveillance (escape mutants)
• establish an immunosuppressive micro-environment

Question 17

How can tumour cells establish a immunosuppressive microenvironment?

Answer 17

Downregulate expression of MHC, tumour antigens or ligands for NK cells

Secrete immunoregulatory cytokines (TGF-beta which inhibits NK cell and Cytotoxic T-cell activity)

Question 18

Explain how escape mutants arise in immune avoidance

Answer 18

Immunosurveillance results in killing of tumour cells that express MHC class I and tumour antigens by CD8+ T-cells

Selection pressures of immunosurveillance means the few cells with low MHC expression survive and proliferate - no longer detected by the immune system and can go on to form a tumour

Question 19

What are six current cancer treatments?

Answer 19

1. Surgery to remove discrete tumours
2. radiotherapy to destroy discrete tumours
3. Chemotherapy to selectively block tumour cell growth
4. Hormone therapy to interfere with tumour cell growth
5. Targeted therapy with small molecule inhibitors of signalling pathways important in cancer development and progression
6. Immunotherapy - reviving, initiating or supplementing the anti-tumour response (reactivate CD8+ T cells to kill tumour)

Question 20

What is the goal of cancer immunotherapy?

Answer 20

promote a strong tumour-specific cytotoxic T-cell response (want a population of T-cells with TCR that recognises tumour-specific antigen)

Question 21

How can antibodies be used in cancer immunotherapy?

Answer 21

antibodies can be used to:
- target drugs, toxins and radioisotopes specifically to cancer cells (antibody drug conjugates)
- direct the innate immune system to cancer cells (complement, NK cells - ADCC, macrophages)
- block receptors on cancer cells required for signalling in progression

Question 22

How are monoclonal antibodies produced?

Answer 22

mice injected with antigen to produce B cells against the antigen
- B-cells that recognise the tumour-specific antigen are selected and fused with a cancer cell to form a hybridoma
- mouse hybridoma cell lines are grown in culture and all antibodies from a particular cell line generate antibodies that all have the same specificity.

Question 23

Why are genetically engineered humanised antibodies more effective than mouse monoclonal antibodies?

Answer 23

The humanised antibodies have been genetically engineered to to be more similar to our own antibodies (such as having an Fc region more similar to human antibodies) so are more able to be recognised by Fc receptors of immune cells
- they activate the human immune system more effectively and are not eliminated as quickly

Question 24

Give two examples of monoclonal antibodies that have been licensed for cancer treatment

Answer 24

1. Herceptin against the TAA HER2 receptor in breast cancer
2. Rituximab against B-cell marker CD20 for Non-Hodgkin’s lymphoma

Question 25

How can monoclonal antibodies be used against cancer cells?

Answer 25

Target tumour antigens (alert complement, NK cells - ADCC, macrophages, antibody drug conjugates)

Target surface receptors to block interaction between surface receptors and ligands (block growth factor receptors - HER2, block vascular endothelial growth factor (VEGF) signalling - limit angiogenesis)

Immune checkpoint inhibitors that block the binding of co-inhibitory receptors on T-cells

Question 26

Give two examples of immune checkpoint inhibitors

Answer 26

1. Ipilimumab against CTLA-4 blocks binding to CD80/86 and released T-cell inhibition
2. Nivolumab against PD-1 (of CD8+ T-cell) or Atezolizumab against PD-L1 (of tumour cell and macrophages) to prevent interaction that inhibits T-cell activation

Question 27

What is Chimeric Antigen Receptor T-cell therapy (CAR-T cell therapy)?

Answer 27

Adoptive T-cell transfer of in vitro modified autologous T-cells from patients with cancer:
- isolate patient’s peripheral T-cells
- insert gene for a CAR that is specific for a defined tumour antigen
- re-infuse tumour specific CAR-T cells into patient

Question 28

What is the general structure of a CAR?

Answer 28

Externally like a BCR so can bind to specific tumour antigen

Internally signals like a TCR (has the CD3ζ component of TCR) and includes a domain for co-stimulation

Question 29

What was a the major downside to CAR-T cell therapy?

Answer 29

Cytokine release syndrome (the systemic activation of highly proliferative cytokine secreting CAR T-cells resulting in high fever, flu like symptoms with neurological impact and resulted in some deaths

Question 30

How can vaccines be used in cancer treatment?

Answer 30

Prophylactic vaccines that aim to prevent disease (only one - against HPV to protect from cervical carcinoma)

Therapeutic vaccines that treat cancer by enhancing appropriate immune responses to kill tumour cells

Question 31

What are the three main types of therapeutic vaccines?

Answer 31

Virus-based vaccines
- engineered to express cancer antigens and co-stimulatory molecules

Cell-based vaccines
- immune cells isolated from patient, stimulated to respond to tumour and injected back into the body

mRNA based vaccines
- mRNA delivered to cells to encourage expression of tumour-specific antigen and mount an immune response against it

Question 32

Give an example (and details) of a virus-based vaccine to treat cancer

Answer 32

ProstVac VF
- treatment of prostate cancer
- consists of vaccinia virus (large DNA virus) particle expressing PSA and co-stimulatory molecules (LFA3, ICAM1 and CD80)
- Injected intradermally, infected epithelial cells undergo necrosis and release virus components including PSA
- PSA released is taken up by APCs, which induces a cytotoxic T cell response against tumour cells expressing PSA

Question 33

Give an example (and details) of a cell-based vaccine to treat cancer

Answer 33

Sipuleucel-T (Provenge)
- prostate cancer vaccine
- immature monocytes isolated from patient
- cultured with cytokine GM-CSF to promote differentiation to Dendritic cells, which are fused to the antigen PAP (Prostatic Acid Phosphatase)
- PAP is presented on MHC on DCs
- cells are injected back into the patient where they induce a cytotoxic T-cell response to the PAP antigen on tumours

Question 34

How can immunotherapies improve effectiveness of conventional cancer therapies?

Answer 34

Chemo- and radio-therapies cause DNA damage and tumour cell death
- Dendritic cells take up dead tumour cell debris and present antigens to cytotoxic T-cells
- Tumour-specific cytotoxic T-cells can eliminate metastases not originally targeted by the treatment since they are mobile
- Therefore, the combination of immunotherapies such as T-cell checkpoint inhibitors can make conventional therapies more effective by improving the T-cell response.