Block 3 - TME Cancer Hallmarks

Question 1

What are the four main tissue types in the human body?

Answer 1

1. EPITHELIAL (protective layer that covers the body, lines organs and forms glands)
2. CONNECTIVE (provides support, protection and structure - bone, cartilage, blood, adipose, fibrous tissue etc.)
3. MUSCULAR (responsible for movement - skeletal/smooth/cardiac muscle)
4. NERVOUS (communication between different parts of the body - neurons, glial cells)

BUT every organ is different in terms of cell type content, cell arrangement, structure, stiffness and content of ECM, nutrients, pH, oxygen levels etc. -> all of this can affect signalling of cells within a tissue, and there are always caveats and exceptions to models when it comes to TME

Question 1

What are the main cell types in the immune system, what is the main distinction within them and how do they relate to cancer?

Answer 1

INNATE IMMUNITY (i.e., cells which do not require “learning” to show a response) - macrophages, neutrophils, eosinophils, basophils and dendritic cells

ADAPTIVE IMMUNITY (i.e., cells which are activated by immunogenic antigens such as bacterial proteins) - e.g., T cells, B cells

Natural Killer T Cells exhibit BOTH (they can react to antigens, AND to stress within cells themselves)

All of these cell types are present in most tumours, and the balance between them is a major factor in tumour development and outcome - some have pro-tumour effects, others anti-tumour

Question 2

What do experiments in mice demonstrate about why we can’t “catch” cancer?

Answer 2

If tumour cells from a particular mouse line are injected into a syngeneic host, a tumour will form - however, when injected into an ALLOGENEIC host, no tumour forms

This is because the immune system in each line is tailored to its specific genetic background, and the genetic differences between individuals mean that the immune system prevents foreign tumour cells from being accepted or allowed to grow

MCH Class I proteins are expressed on the cell membranes of all cells, and display parts of intracellular proteins to passing cells - certain mutations will be recognised by the immune system (e.g., by T cells) and induce an attack response

Question 3

How do cancer cells evade antigen detection?

Answer 3

In many cancer types (especially breast and prostate) it is common for cancer cells to downregulate MHC1, preventing T cells from detecting them
-> This is partly why some cancers respond better to immunotherapy than others

Question 4

In terms of the immune system, explain what is meant by “the good and the bad TME”

Answer 4

Some immune cells are tumour supportive (e.g., Tumour-associated macrophages, Myeloid-derived Suppressor Cells, Tumour-infiltrating Dendritic Cells)

Others act as tumour suppressor cells (e.g., T cells, Dendritic Cells, M1 macrophages, Natural Killer Cells)

Some therapies aim to tip this balance in favour of the tumour suppressing cells

Question 5

In terms of macrophages specifically, explain how and why some are pro-tumour, while others are anti-tumour

Answer 5

Macrophage polarization:

M1 macrophages (often activated by LPS/IFN-y) have anti-tumour effects (e.g., immunostimulatory cytokine activation, chemokines for lymphocytes, tumour cell lysis, ROS and RNS, MMPs 7/9/12)

M2 macrophages (often activated by hypoxia, IL10, TGFB, IL4) have pro-tumour effects (e.g., pro-angiogenic cytokines and enzymes; ROS and RNS, mitogens and immunosuppressive cytokines, a wide range of MMPs)

Question 6

What are the actual effects/processes promoted by pro- and anti-tumorigenic macrophages?

Answer 6

M1-like TAMs:
-> IMMUNE ACTIVATION (TNFa/IL1ß/IFNy)
-> PHAGOCYTOSIS OF CANCER CELLS
-> Apoptosis of cancer cells (TNFa, ROS, NO)
-> Tissue damage (ROS)
-> Maturation of antigen presenting cells (IL12)

M2-like TAMs:
-> IMMUNE SUPPRESSION (TGFß, PDL1)
-> PROLIFERATION AND SURVIVAL OF CANCER CELLS (EGF, FGF, PDGF)
-> ANGIOGENESIS (VEGF, FGF, CXCL8)
-> Invasion and metastasis, EMT (TGFß)
-> Tissue remodelling and fibrosis (MMPs)

Overall: M2-like TAMs secrete key ligands (e.g., VEGF, FGF, TGFß, CXCL8) which drive the processes that promote tumour growth, e.g., angiogenesis, proliferation, tissue remodelling, metastasis, angiogenesis and immune suppression

Question 7

What interesting point was made about TNFa in terms of cancer development?

Answer 7

While TNFa often acts as an anti-tumour ligand, promoting immune activation, it is also required for initial cancer development:
-> Tumours failed to grow in TNFa -/- mutant background, as some inflammation is required for cancer development

Question 8

What do NK cells do and how does this relate to cancer?

Answer 8

Natural Killer cells induce contact-mediated apoptosis in target cells, including cancer cells

They respond to “alarm proteins” signalling stress, e.g., MICA/MICB family, which get “flipped” to the extracellular side of the membrane as a response to cellular stress

However, cancer cells can downregulate these alarm proteins to avoid detection by NK cells

Interestingly, MHC1 acts as an inhibitor of this response, interacting with an the inhibitory receptor on NK cells -> this is the opposite to its role in activating T cells (which is why not ALL cancers downregulate MHC1, as this can make them more vulnerable to NK cell attack)

Question 9

How exactly do NK cells induce cell death?

Answer 9

NK cells can induce cancer cell death via several different pathways, including:
-> PERFORIN/GRANZYMES (make pores in the membrane, granzymes enter and trigger apoptosis)
-> Fas/FasL (activation of caspase 8 + 10 and apoptosis)
-> TRAIL pathway (caspase 8 and 3 and apoptosis)
-> Antibody-dependent cell-mediated cytotoxicity, ADCC (Fc receptors on the surface of NK cells bind the Fc portion of antibodies which have already bound the tumour cells, leading to granzyme release and apoptosis)

Question 10

Summarise how T cells are involved in cancer and the TME

Answer 10

There are multiple types of T cells:
-> CD8 cytotoxic T cells induce cell death and are an important part of anti-cancer immunosurveillance
-> Treg cells inhibitor effector T cells, and can promote suppression of the anti-tumour response if there are too many of them

The more Tregs are present, the faster a tumour generally progresses

Question 11

How can the TME favour Treg’s?

Answer 11

Tumour cells or M2 macrophages secrete TGFß which activates T cells to Treg’s, as well as CCL22 which inhibits CD8 T cells -> this creates an environment favouring Tregs over CD8+ T Cells, thereby promoting tumour growth

Question 12

How exactly do CD8 cells kill cancer cells (and what can prevent this)?

Answer 12

The TCR protein of CD8 cells interacts with the MHC1 protein of tumour cells, and promotes cell death via granzymes or cytokines such as IFN
-> HOWEVER, IMMUNE CHECKPOINT PROTEINS, e.g., PD1/PDL-1 and CTLA-4 can inhibit this, and these are commonly overexpressed in cancer

Immune Checkpoint INHIBITORS aim to target these immune checkpoint proteins and prevent immunosuppression, thereby promoting cancer cell death via CD8 T Cells:
-> PD-1 inhibitors (e.g., Nivolumab, Cemiplimab)
-> PDL-1 inhibitors (e.g., Avelumab, Durvalumab)
-> CTLA-4 inhibitors (e.g., Ipilimumab)

However, not all patients respond positively to these therapies, and biomarkers are needed to better predict patient responses

Question 13

Summarise the main mechanisms by which cancer cells avoid immune surveillance

Answer 13

1. Suppression of MHC expression to prevent antigen detection
2. Overexpression of PDL1 to inhibit the T-cell/tumour checkpoint
3. Overexpression of BCL2 family members (e.g., BCL2, BCL-xL, MCL1, CED-9)
4. Manipulate the balance of immune cell types -> more Tregs and more M2-like TAMs (e.g., via TGFß and IL10)