Tumour Microenvironment (TME) Flashcards
what constitutes a tumour?
cancer cells and the tumour microenvironment
- immune cells
- stromal cells
crosstalk between cancer and stroma can support tumour growth
what are organs comprised of?
Parenchyma
stroma
what is parenchyma?
Parenchyma comprises the tissues that confer function
- tissue of organ that functions
- e.g. kidney parenchyma is epithelial
- e.g. spleen: parenchyma is connective tissue
- e.g. heart: parenchyma is muscle tissue (cardiac muscle cells)
what is the stroma?
Stroma supports the parenchyma:
- Without stroma, parenchyma cannot function - no organ can function without the mechanical and nutrient support of the stroma
Without stroma, tumour cannot grow
what is an example of a parenchyma and its stroma?
In the skin:
- Parenchyma – squamous epithelia
- Stroma: Dermis, fibrous connective tissue
the epithelial cells are tightly connected
the connective tissue is much looser
the blood vessels in the skin connective tissue (not the epithelia)
Both needed for function
how is the stroma involved in cancer?
Stroma is non-malignant cells that support the malignant parenchymal cells
- stroma is essential for tumour growth
tumour cells have the same needs as normal tissues
how are tumour cells complex ecosystems?
Multiple non-malignant cell types are found within
- Cancer-associated fibroblasts associated with cancer cells
- ECM
- Endothelial cells – blood vessels
- Myeloid and lymphocyte cells
where does cancer originate?
sustained proliferation
in a milieu rich in inflammatory cells, growth factors, DNA damaging agents
potentiates/promotes neoplasia
- originates at sites of chronic, persistent inflammation
- may be due to autoimmune inflammation e.g. inflammatory bowel disease, or due to infectious agents
what are examples of inflammation inducing cancer?
Barret’s oesophagus leads to chronic reflux of stomach acid into oesophagus
- This induces inflammation and metaplasia in oesophagus
– DNA damage and malignancy – oesophageal cancer
Hep B causes chronic inflammation in liver – increases risk of liver cancer
how can anti-inflammatory drugs reduce cancer risk?
Aspirin is anti-inflammatory and can decrease risk of CRC by dampening inflammation
- decreases the risk of metastasis
and decreases incidence after a few years
do certain cancers metastasise to certain tissues?
yes:
e.g. CRC tends to metastasise to liver
e.g. Ovarian cancer metastasise to peritoneal cavity
e.g. breast cancer preferentially metastasises to lungs and bone marrow
why is it strange that cancers metastasise in certain tissues?
Venous shunts allow large numbers of ovarian cancer cells to circulate yet metastasis outside of the peritoneal cavity is still rare
- cancers are constantly in circulation, but are specific in where they extravasate
- Transformed cells can’t grow anywhere
why do certain cancers preferentially metastasise to certain tissues?
2 theories:
- seed and soil theory
- alternative theory - due to anatomy of vascular and lymphatic drainage
both mechanisms/theories are likely important and true
what is the seed and soil hypothesis?
Proposes certain cancers metastasise to particular organs because metastasis is the result of favourable interactions between tumour cells (‘seed’) and the organ microenvironment (‘soil’)
- Secreted factors from primary tumour may influence the soil to be a preferable environment
what is the alternative theory of cancer metastasis?
Alternative theory – organ preference is due to anatomy of vascular and lymphatic drainage from site of primary tumour (e.g. lung is common site for metastasis)
- Hepatic portal vein takes blood into liver to be processed
- Tumour in colorectal tissue enters blood supply and reaches liver – why liver is one of the first metastatic tissues
what is an example of primary tumours altering the environment of the ‘soil’?
primary tumours can induce VEGFR-1 on distal lung cells, triggering MMP9 expression, making the lung more receptive to metastases
what are the key stages of wound healing?
- hemostasis
- humoural inflammation (vascular permeabilisation)
- cellular inflammation (immune cells infiltrate – neutrophils and macrophages)
- angiogenesis (formation of new blood vessels)
- generation of mature connective stroma
why does the body provide tumours with the stroma that is essential for tumour growth?
Tumours disguise themselves as wounds and call upon the host to heal them:
- Tumours continuously secrete factors that cause persistent angiogenesis and stroma formation
- Body provides support for tumour as it acts like a wound
- wound healing is usually self-limiting, but cancer is uncontrolled, so ‘wound’ never heals, and body continues to support
what tumour growth limited by?
Hypoxia
- oxygen only diffuses a distance of 0.2mm, so newly formed tumour hits hypoxia in the centre early on - pH and oxygen tension drops within the growing tumour
what are the two possible results of hypoxia in tumours?
- Can’t grow due to lack of blood supply and therefore lack of oxygen
OR
- cancer can change stroma to form new blood vessels for oxygen delivery to grow beyond fixed size – angiogenic switch
- Switch enables growth from small tumour to large solid tumour – this is supported by immune cells
how functional are the blood vessels that are produced in tumour angiogenesis?
New blood vessels are altered:
- Areas of hypoxia
- Areas of low blood supply
- Areas of necrosis due to lack of oxygen
Angiogenesis for cancer isn’t perfect – there are dead ends - not optimal
what kind of metabolism do tumours undergo?
low oxygen favours anaerobic respiration
- Warburg effect
- less efficient ATP production but increased production of biosynthesis factors for cell growth
- growth/survival advantage
- but anaerobic metabolism is less efficient, so more glucose is required to provide energy
what pH is in a TME?
Anaerobic metabolism forms lactic acid:
- there is build-up of lactic acid due to poor vascularisation, so waste products are not removed
- TME therefore has lower pH – favours tumour growth, unfavourable for immune cell function
TME also uses up lots of glucose – Warburg effect
why is interstitial fluid pressure higher in tumours?
Malformed blood vessels are leaky:
- IFP is similar to atmospheric pressure in normal tissues
- IFP is highly increased in solid tumours
- Caused by vascular abnormalities e.g.
high endothelial vessel permeability (promoted by VEGF) - vessels become leaky which reduces lymphatic function
- High IFP has important consequences reducing drug concentrations in tumours
why does high IFP reduce drug concentrations in tumours? How can this be overcome?
Difficult for therapy – drugs can’t access tissues as the vessels are malformed
- Drugs that target VEGF alone to stop angiogenesis in tumours are not effective
- When VEGFi given in combination with chemotherapy, there are better effects – VEGFi can restore blood vessels to improve chemotherapy access to tumour
what is the extracellular matrix (ECM)?
Within tissues, cells are surrounded by a network of proteins and proteoglycans termed ‘the extracellular matrix’.
- ECM serves as a basis for cell anchorage
- Collagen, the most abundant structural protein in the human body, is a major ECM component
what is collagen?
Collagen is most abundant protein in body:
- important for cell-cell adhesion – maintains tissue structure
- Collagen makes up ECM to anchor cells
how do tumours affect the ECM?
For tumours to grow, the ECM needs to be remodeled, as the pressure exerted by the ECM prevents tumour expansion and metastasis
- tumours induce ECM remodeling via high secretion of MMPs which break down the ECM
- this enables metastasis
- collagen can also promote cell growth via signalling
how do MMPs effect patient prognosis?
In most human cancers, high MMPs correlate with more invasive phenotype and worse outcome
what cells exist within the TME?
- cancer cells
- cancer stem cells
- endothelial cells
- cancer-associated fibroblasts
- immune inflammatory cells
- tumour-associated macrophages
- MDSCs
why are the mixture cells in the TME important?
Tumour growth depends on interactions between many different cell types
- The mixture of cells changes as tumours develop, invade and metastasise to new sites
- metastatic cancer forms own TME at the distal site
what are cancer-associated fibroblasts (CAFs)?
Part of stromal environment and help cancers via crosstalk
- produce factors to promote angiogenesis for tumour growth and metastasis
- attract pro-inflammatory cells to alter TME
what happens when a cancer is injected with cancer-associated fibroblasts?
Injecting cancer cells with different fibroblasts into mouse models shows CAFs:
i) increase the metastatic capacity of breast cancer cells.
ii) increase the progression of pancreatic cancer
how are cancer-associated fibroblasts important in prognosis?
High CAF abundance correlates with worse outcome in Pancreatic cancer
High CAF abundance and high levels of CAF marker FAP (Fibroblast activated protein) correlate with worse outcome in colorectal cancer
CAF proteins worsen prognosis
how can tumours be classified?
- Infiltrate-excluded (cold)
- Infiltrate-inflamed (hot)
- Infiltrated-tertiary lymphoid structures (TLS)
what are cold tumours?
immune cells ‘trapped’ at tumour:stroma interface
- Very low numbers of TILs – immune system is cold at this tumour
- Likely to respond poorly to immunotherapies as effector cells cannot enter tumour
what are hot tumours?
adaptive anti-immune mechanisms may be overcome by e.g. anti-PD1 checkpoint blockade:
- Rich infiltrate of immune cells – inflamed tumours
- These patients do better with checkpoint blockade
how can tertiary lymphoid structures be formed in tumours?
TLS generally present at invasive margin of tumour:
- may suggest ongoing immune priming - a good sign as cancer is being recognised by immune system
- Generally associated with better prognosis
do immune cells infiltrate Hodgkin lymphomas?
Hodgkin lymphoma – EBV-associated:
- Can become large in size
- 99% of Hodgkin is non-malignant, infiltrating immune cells
- Only a small number of malignant cells
- but the TILs are not performing their effector function
what innate cells can be found in tumours?
dendritic cells
eosinophils
granulocytes
macrophages
mast cells
myeloid derived suppressor cells
NK cells
what adaptive immune cells are found in tumours?
CD4+ T-cells (Th1 & Th2 & Treg)
CD8+ T-cells
how are macrophages plastic?
Circulating monocytes enter tissues and develop into macrophages:
- Macrophages are highly versatile and ‘plastic’ cells and can alter their phenotype to suit the environment they find themselves in
- M1 = pro-inflammatory
- M2 = wound healing
what are tumour-associated macrophages?
Tumour Associated Macrophages (TAMs) are M2-like (although this varies depending on the tumour, and the location of the TAMs in the tumour)
- support tumour growth and angiogenesis
what are the features of M1 macrophages?
- induced by LPS and IFN-gamma
- pro-inflammatory, bactericidal
- Promote CD4 T cell differentiation into Th1 and Th17 cells that support cellular immunity
- Phagocytic
what are the features of M2 macrophages?
induced by IL4, IL10, IL13 - immunosuppressive – tissue healing
- Promote CD4 T cells to become Th2 and Tregs
(Th2 promote antibody production –
not helpful here – and suppress cellular immunity).
- Pro-angiogenic – key in tumour growth
which macrophage phenotype is associated with poorer prognosis?
More M2s associated with poor prognosis – less inflammation in tumour so less immune activation`
- Aid angiogenic switch
what do TAMs do in the tumour?
Tumour hypoxia promotes release of chemoattractants that recruit macrophages into the tumour:
- TAMs then promote angiogenesis, by releasing VEGF, MMP9 and many other pro-angiogenic factors for angiogenic switch - enables tumour expansion
- suppress other immune cells via IL-10 (prevent T cell and DC maturation and arginase (T cells become unresponsive)
- release factors that promote metastasis
what are myeloid derived suppressor cells?
MDSCs are induced by inflammation and are highly immunosuppressive
- release IL-10 and TGFb
- Inhibit NK and DC function
- Induce Treg cells
- Produce arginase, depleting arginine in the tumour, thereby interfering with T cell function - alter metabolic environment of TME
- low arginine causes nitrotyrosinylation
- hijacked by tumours to dampen anti-tumour immune response
how do MDSCs effect prognosis?
Levels of MDSCs often elevated in patients.
- Accumulation of MDSCs is driven by markers
of inflammation present in the tumour
- 20% MDSCs present in blood of cancer patients
- Persistent immunosuppression in tumour
how are MDSCs implicated in prognosis?
Multiple studies show MDSCs correlate with poorer survival e.g. in stage IV melanoma, breast cancer and pancreatic cancer
how are NK cells affected by tumours?
NK cells recognise cells that downregulated MHCI, so should recognise cancers which deplete their MHC expression
- but, NK cells cannot function properly in TME due to altered conditions
- NK cells can become anergic and unresponsive to tumour stimuli - can’t release cytokines or kill targets
- IL-10 and TGFb produced by the tumour suppress the NK cell function
how does T cell infiltration affect prognosis?
CD8 cells present in tumour = better prognosis
- Hot tumour - active anti-tumour immune response
TH1 cells provide pro-inflammatory response against tumours
- Good prognosis
how do CD8 T cells kill cancers?
cytotoxic effector cells:
- cytotoxic granules containing perforin and granzymes trigger apoptosis in target (tumour) cells
- also produce IFNy and TNFa
why do Th1 cells improve prognosis?
Normal role, defence against viruses - secrete IL2, IFN-g, TNF-a
- these cytokines support cytotoxic CD8 T cells – CD4 cells are essential for long-lived memory.
- Th1 cells can activate macrophages.
- Evidence they can also kill tumour cells e.g. melanoma - cytotoxic activity
how are Th2 cells implicated in cancer?
Normal role – defence against parasites. Produce IL4 and IL10.
- Generally though to be a poor prognostic indicator – although some reports disagree.
In Hodgkin’s Lymphoma & breast cancer – Th2 correlate with better outcome
(role for antibodies?)
- May indirectly eliminate cancer cells by activating eosinophils
how are Th17 cells implicated in cancer?
CD4 T cells that express IL17 (may express other cytokines including IFN-g, IL2, TNF-a) ,
- May be involved in cancer development via IL-17 inducing early inflammation
-In some cancers Th17 may promote angiogenesis and attract MDSCs
- Th17 linked to poor prognosis in colorectal cancer
but may be beneficial in ovarian cancer - nuanced
what are Tregs and their main subsets?
About 5% of total CD4 T cells :
- Important in preventing autoimmunity-
- K/O Treg genes = autoimmunity
Two subsets, natural T-regs and induced T-regs – typically express FoxP3 and CD25
( identification can be complicated – these markers can also be expressed by activated effector cells)
how are Tregs implicated in cancer?
Most studies show increased numbers of Tregs correlate with poorer prognosis
- Increased Tregs in blood and tumours of patients
- frequency of Tregs elevated in ovarian cancer – poorer prognosis
But studies in other cancers (e.g. colorectal) suggest increased numbers are beneficial!
(in these cases – were they Treg cells? only suppression assays definitively show cells downregulate immunity)
how are B cells implicated in cancer?
B-cells are present in diverse tumour types.
- Intratumoural B cells can act as APCs – triggering T-cell responses.
- Evidence for direct tumour cell killing by B-cells within tumour
- improves prognosis
how are B-regs implicated in cancer?
Intratumoural B-cells can also be immunsuppressive B-regs
- B-regs can express PD-L1 (inactivating PD1+ T-cells)
- B-regs can secrete immunosuppressive
cytokines (IL10 TGF-beta)
why are Tregs enriched in tumours?
Tumours produce CCL22 & CCL28, which attracts Tregs.
Hypoxia can increase CCL28 – thus increasing T-reg infiltration
how do Tregs suppress immunity in the TME?
High CD25 is marker for Tregs, and is a high-affinity IL-2 receptor
- Tregs mop up IL2 in TME from T cells – T cells are starved of IL-2
Tregs produce TGFb and IL-10
Tregs produce adenosine to suppress T cells
how does the production of adenosine by Tregs dampen down T cells?
Adenosine is a metabolic inhibitor of T cells within tumours:
-Tregs express CD39 ATPase
- In TME, ATP is broken down to AMP
- CD73 on Treg or cancer cell breaks down AMP to adenosine
- adenosine binds to its receptor on T cells and switches them off
how do MDSCs deprive amino acids in the tumour?
MDSCs express arginase enzyme which depletes arginine levels:
- Low levels of arginine (<60mmol/L) leads to decreased T-cell receptor signalling by decreasing CD3zeta chain expression
- systemic effect in blood – arginine depleted in patient blood
how can patients be stratified based on their immune infiltration?
There is variation in numbers and types of immune cells, between disease but also between patients
- we can use TME markers to predict outcomes e.g. for Tregs, CD8s, MDSCs
how are cancers currently scored?
based on their metastasis
- stage 1 = primary/localised
- stage 4 = metastatic
what is immunscore? is it a good prognostic system?
Immunoscore for CRCs
- Numbers of CD3, CD45RO (mature effector cells) positive effector cells
- More of these cells in tumour core = better prognosis
Poor immunoscore leads to poor prognosis, even in stage 1 tumour
Immunoscore possibly better prognostic marker than cancer stages