15. Cancer immunology 2 Flashcards
What receptor expression can be altered to suppress immunity?
- MHC1 downregulation
- Inhibitory receptor expression: CLTA-4, PD-1, LAG-3, TIM-3 and TIGIT.
How does loss of MHC1 cause immunosuppression in the TME?
- MHC1 is required for antigen presentation to T cells.
- Down regulation of MHC1 makes T cells less effective at killing tumour cells.
- However it also causes the activation of NK cells.
- However the increase in NK cell killing doesn’t really effect tumours so downregulation of MHC1 is immunosuppressive.
- This is common for tumours to use to escape CD8 mediated killing.
- Tumour cells can also disrupt TAP loading to prevent effective T cell activation.
Where and how is MHC1 normally expressed?
- Expressed on all nucleated cells.
- A classic response to tumours is downregulation of MHC1.
- MHC1 is upregulated during inflammation and by the type 1 IFN response.
- This is a key way inflammation aids the immune response.
What are the key features of inhibitory receptors?
- They are common expressed on multiple immune cell types.
- They are part of normal T cell activation and biology.
- Critical for maintaining immune homeostasis
- Commonly unregulated in T cell exhaustion
What role do inhibitory receptors play in normal T cell biology?
- They are upregulated on T cell activation.
- They are used for negative feedback and to limit the T cell response.
- PD-1 is an inhibitory receptor and T cell activation marker.
What role do inhibition receptors play in immune homeostasis?
- Inhibitory receptors limit T cell activation to prevent T cell mediated tissue damage.
- In PD-1 KO mice, they die very quickly due to immune damage.
- Inhibitory receptors don’t just have roles in disease so when you block them in disease you effect their role in homeostasis.
Why are inhibitory receptors upregulated in persistent in immune response?
- It time limits how long the effective immune response can last.
- This is done to limit immune mediated tissue damage.
- This is of interest in tumour immunity as anti tumour responses are persistent and characterised by very high expression of inhibitory receptors.
What are the 5 key inhibitory receptors in tumours?
- PD-1
- LAG3
- TIM3
- TIGIT
- CLTA4
What is the only inhibitory receptor restricted to T cells?
CTLA4
What are the 2 main ways inhibitory receptors work?
- Inhibitory signalling
- Ligand competition/depletion
What inhibitory signalling do inhibitory receptors normally do?
- Inhibitory receptors normally recruit phosphatases to counter kinase activity of the activating receptors.
- This is inhibition the signal transduction.
- PD-1 is a good example of this as it recruits SHP2 that counters Lck and ZAP-70 activity.
How do inhibitory receptors inhibit signalling through competition?
- They inhibitory receptors competes for the same ligand as the activating receptor.
- CLTA4 and TIGIT do this
- CLTA4 and CD28 both bind CD80/86
How does CTLA-4 do ligand competition?
- CTLA4 is unregulated on every activated T cell and is constitutively expressed on Tregs.
- CTLA-4 binds to CD80/86 on the APC at higher affinity then CD28.
- This prevents co-stimulation and activation of naive T cells.
- CTLA-4 can also be soluble.
How does CTLA-4 do ligand depletion?
- CTLA-4 can deplete CD80 from the membrane of the APC and internalise it for degradation in the T cell.
- This is called transendocytosis.
- This deprives the activating T cells.
- Treg sit near DCs to reduce co-stimulation and reduce activated T cells
- Using CTLA-4 manipulation to cancer can be tricky as it targets activating T cells and Tregs
What is PD-1?
- It is an inhibitory receptor that is upregulated as part of default T cell activation.
- It is expressed on CD4+ effector, CD4+ regulatory and CD8+ T cells.
- It is further upregulated on T cell exhaustion.
- Also expressed on myeloid cells.
- Ligands are PD-L1 and PD-L2.
What happens if you target PD-1?
You effect CD4 cells, CD8 cells and Tregs
What effect does deleting PD-1 have on myeloid cells?
It drives tumour immunity
Where is PD-L1 expressed?
Commonly upregulated on tumour cells
Where is PD-L2 expressed?
On immune cells
When is TIM-3 expressed?
- It is not expressed on normal T cell activation.
- It is upregulated in persistent T cell activation.
What are the functions of TIM-3?
- TIM-3 limits the activation of cGAS-STING in dendritic cells.
- It restrains anti-tumour immunity by regulating inflammasome activation in migratory DCs.
- TIM-3 allows recognition and phagocytosis of apoptotic bodies via binding phosphatidylserine which prevents macrophage activation.
- TIM-3 is highly expressed on exhausted cytotoxic T cells and prevents killing of tumour cells.
How does TIM-3 inhibit cGAS-STING?
- TIM-3 recruits binding proteins to the cell surface.
- These sequester the DNA and prevent ti entering the dendritic cell.
- This prevents dendritic cell activation
What are the key types of tumour immunotherapies?
- Antibody drug conjugates.
- Cytokines
- Vaccination
- Check point blockade
- T cell therapies
How do antibody drug conjugates work?
- Take an antibody against a tumour specific antigen and add cytotoxic payloads.
- These payloads are chemotherapy drugs but being used in a more targeted way.
- These drugs interfere with replication or damage DNA.
- It more specifically targets tumour cells with reduced side effects.
What are the problems with antibody drug conjugates?
- Finding an antigen that is only expressed on tumour cells has so far proved impossible.
- So there are still off target side effects.
- Due to targeting surface antigens immune escape is easy as the tumour cell can just down regulate expression.
How do the drugs carried on antibody drug conjugates get into the tumour cells?
- The default response of the ligand receptor engagement is to internalise the receptor and the drug gets internalised with it.
- Then the drug enters and escapes the lysosome.
- Then the drug can work
What are the mechanisms of resistance to antibody drug conjugates?
- Antigen down regulation gives massive growth advantage to the cells that down regulate it.
- Efflux pump activation
- Drug specific resistance
What cytokine is given to treat cancer?
IL-2
How can IL-2 be used to treat cancer?
- More IL-2 should mean more T cells to aid the anti tumour response.
- IL-2 was approved for treatment of metastatic renal cell carinoma in 1992.
- Had 20% tumour regression rate
- Had wide spread toxicities due to Treg activation caused by IL-2 and suppression of immunity.
- includes leukopenia, nausea, hypotension and fatigue.
What is the function IL-2?
- IL-2 drives proliferation of activated T cells and Tregs.
- The high affinity IL-2 receptor is only expressed on activated T cells and Tregs.
- Tregs are highly dependent on IL-2
What could make IL-2 treatment more effective?
- If you restrict the IL-2 delivery to just the tumour resident cytotoxic T cells.
- This could make the treatment work a lot better.
- This could reduce toxicity and activation of Tregs.
- This is still under development.
What are the key considerations for designing a cancer vaccine?
- choice of antigen.
- Delivery platform
- Potential for resistance.
What is the idea behind cancer vaccines?
Use the vaccine to enhance the immune response to cancer cells.
What antigen choices need to be considered when designing cancer vaccines?
- Do you target 1 or multiple antigens.
- Targeting a conserved antigen would treat more people but be subject to partial tolerance.
- Targeting patient specific neoantigens so an individualised vaccine. Requires sequencing and lots of money
How could cancer vaccines be subject to resistance?
- Anti tumour responses need to last for months to be effective.
- The generation of the immune response by the vaccine needs to be sustained or the tumours could suppress it and make the vaccine pointless.
- This is a challenge for cancer vaccine design.
Cancer vaccine examples: Advanced melanoma
- Designed against 4 antigens commonly expressed in melanoma: NY-ESO, MAGE-A3, tyrosinase and TPTE.
- Screen patients for expression of at least one of these.
- mRNA given to patiens.
- Small faction of patients responded.
- Lots still have disease progression and don’t respond at all.
Cancer vaccine examples: Glioblastoma
- Glioblastoma doesn’t generate much of an endogenous immune response so the hope is to generate one.
- This took the patient specific neoantigen aproach so each vaccine is different for every patient.
- Used a classic delivery approach of antigen + adjuvant.
- Gave a vaccine and a booster dose.
- All patients needed secondary treatment after vaccination lots died quickly.
- Challenges to get this to work but still has good potential.
What is the most effective cancer immunotherapy?
Checkpoint blockade
What is checkpoint blockade?
- Using antibodies to target inhibitory receptors to prevent their function.
- Most antibodies in clinic are anti-PD1 and anti-CTLA4.
- Antibodies that block LAG3 have recently been approved for use in combo with anti-PD-1.
- Anti-PD-1 treatment has been around for 10 years
- Anti-CTLA-4 treatment has been around for 15 years.
What are example of anti-PD-1 treatment?
Anti-PD-1 treatment is Nivolumab and Pembrolizumab
Anti-PD-L1 treatment is Atezolizumab.
What is an example of anti-CTLA-4 treatment?
Ipilimumab
What are the considerations of using checkpoint blockade to treat cancer?
- Type of cancer
- Previous treatment failure vs first line treatment.
- Use in isolation vs in combination with other checkpoint inhibitors or conventional therapy.
How was the use of anti-PD-1 tested and expanded?
- Start with limited cancer types and then expand once its been proven to work.
- Initially use it as final line treatment on patients that have failed everything else then move it forward to be 1st line treatment once its proven.
- Then try it in combination with other treatments.
- Anti-PD-1 is now a very common 1st line treatment for lots of different tumours.
Does checkpoint blockade work?
- Tested anti-PD-1 and anti-PD-1/anti-CTLA4 treatment and showed it works very well.
- Study tested it on melanoma patients that failed all previous therapies.
- 12 weeks of long repeated treatments.
- Around 1/3 of patients had 80% tumour reduction which is a massive improvement on existing treatment and proved evidence for the expansion of anti-PD-1 therapy.
- The odds are still not great with a lot better then other things
How did we think checkpoint blockade anti-PD-1 treatment worked?
- Exhausted T cells express lots of PD-1.
- It was thought that blocking PD-1 would just reactivate these exhausted cells.
- This was not what was happening.
What did the experiment that determined how checkpoint blockade works measure?
- It looked at patients pre and post anti-PD-1 treatment.
- Comparison of the transcriptomes of the T cells in the TME
- T cells in the TME have clonally expanded so if you have lots of the same T cell you know they are involved in the anti tumour response.
How does checkpoint blockade work?
- Transcriptomics found that pre anti-PD-1 treatment T cell clones were disappearing and the t cells present post treatment were different T cells.
- These T cell weren’t in the TME before treatment. This is called clonal replacement.
- Anti-PD-1 treatment allowed wave after wave of new T cell clones to be activated and keep the immune response going.
- This avoids exhaustion and there is no reactivation of exhausted T cells.
- Anti-PD-1 can also play a role in driving precursor exhausted T cells into proliferation. This is a secondary role of anti-PD-1 treatment.
- Treatment with anti-PD-1 = more new T cells.
What are the side effects of checkpoint blockade?
- Autoimmunity is the key side effect as PD-1 is important in immune homeostasis.
- Treatment is over months so these side effects need to be minimised and controlled.
- You can use steroids to manage autoimmune side effects but this is not ideal.
- If we improve these therapies to have less autoimmune side effects this would be great.
Does checkpoint blockade fail?
- Resistance rates are around 50%
- But this varies based on cancer type
- can be reduced by using combinations of therapies.
Why does checkpoint blockade fail?
- By using mechanisms of immune suppression
- Down regulation of MCH1 is very common to reduce T cell activation.
- Compensatory upregulation of other inhibitory receptors that are not being blocked like TIM-3 and LAG-3
- This shows how important inhibition is in tumours.
How does PD-1 work?
- It contains an ITSM.
- This recruits a tyrosine phosphatase called SHP2.
- SHP2 counters the kinase activity of Lck and ZAP70.
- This prevents T cell activation.
