Cancer immunotherapy

• What is the rationale for targeting cancer through the immune system?

  • Traditional chemotherapy is not specific to cancer cells - rapidly dividing healthy cells are often affected too
  • Targeted therapy via. oncogenes/tumour suppressor genes allows lower doses of chemotherapy but tend to build up resistance
  • In comparison, the immune system is trained to ignore self-cells and avoid autoimmunity - better target
• What are the problems in targeting cancer through the immune system?

  • Cancer cells are not immunogenic enough to elicit recognition by the immune system
  • Cancer cells cleverly secrete factors which switch off the immune system and allow the tumour to grow

  Need to find a way to reactivate immune cells to target cancer cells
• How does the immune system affect susceptibility to cancer?

  Increases susceptibility to cancer induced by chemicals and viruses

  
  

  Deficiency in cytotoxic and NK T cells (which are largely responsible for killing virally infected cells) = increased sizes of tumour volume

  
• How could the immune system be programmed to recognise cancer cells?

  Target the tumour antigens on the surface of cancer cells

  • Works similarly to virally infected cells that express viral antigens
• What are the key principles of the immune system?

  1. Self-tolerance - during T cell development in the thymus, T-cells are screened prior to maturation to eradicate those which are self-reactive (avoid autoimmunity)
  2. Generate an immune response against foreign (non-self) antigens - body must detect harmful foreign antigens and attract immune cells to attack it
• What is the problem with targeting tumour-specific antigens (neoantigens)?

  There are few tumour-specific antigens which exist
• What are tumour-specific antigens know as?

  Neoantigens
• How are neo-antigens produced?

  Produced as abnormal protein (peptide) as a result of cancer cell mutation

  • Non-self peptides - not produced in normal tissues
  • Generate a targeted immune response when presented by MHC complexes on the cell surface
  • Detected by T-cells which eliminate the cancer cell
• What is an example of a tumour-associated antigen targeted in breast cancer?

  HER2 receptor

  • Overexpressed in breast cancer
  • Targeted by Herceptin (trastuzumab) antibody
  • However, HER2 receptor exists amongst other non-cancer cells in the body which will also be susceptible to Herceptin
• In what scenario are neoantigens detected?

  • Detected by T cells
  • Neotigens are released from destroyed cancer cells - most proto-oncogene to oncogene transformations tend to occur intracellularly
  • Need enough neoantigens that can be detected by immune cells to activate immune cells - but struggling to find these

  
• What is cancer immunoediting?

  Characterised by changes in the immunogenicity of tumors due to the anti-tumor response of the immune system, resulting in the emergence of immune-resistant variants

  • Phase I = elimination - innate and adaptive immune responses to tumour cells
  • Phase II = equilibrium - tumor cells that have escaped the elimination phase and have a non-immunogenic phenotype are selected for growth
  • Phase III = escape - tumor cells continue to grow and expand in an uncontrolled manner and may eventually lead to malignancies
• Role of NK cells during phase I of cancer immunoediting

  NK cells

  • Healthy cells express inhibitory ligands against NK cells
  • Reduced expression of these ligand or up-regulation of stress-induced ligands in tumour cells leads to production of IFN-y and direct cytotoxic activity
• Role of Helper T cells (CD4+) during phase I of cancer immunoediting

  Th1

  • Macrophage activation - tumour cells polarise macrophages to make them tumour promoting - try to manipulate polarisation to favour target the tumour
  • Also suppress Th2 responses and promote cell-mediated cellular cytotoxicity

  Th2

  • Antibody production via. B-cells
  • Also suppress Th1 responses and promote mast cell and eosinophil function
• Role of Cytotoxic T cells during phase I of cancer immunoediting?

  Cytotoxic T cells (CD8+)

  • Induce cell killing
• What occurs during phase I of cancer immunoediting?


  • Tumour recognised and eliminated by the innate (kill cancer cells) and adaptive (recognise tumour-associated neoantigens) immune response
  • Innate = macrophages, NK cells
  • Adaptive = T-cells and B-cells
  • Phase I is not extensive enough - not enough enough of stress ligand nor immunogenic enough to provoke a strong immune response

  
• How are new genetic variants induced during phase II of cancer immunoediting?

  • Tumour cells are constantly mutating - also applies to tumour surface antigen
  • This induces resistance as the immune system cannot keep up
  • The clones which survived elimination and are capable of evading the immune system still remain, learning to be less immunogenic
• What occurs during phase II of cancer immunoediting?


  • Rare tumour subclones with further mutations survive elimination (phase I)
  • Tumour progresses to equilibrium (phase II) where selection pressures instigate new tumour cell genetic variants
  • Net tumour growth is limited and stalled - tumour enters dormancy (neither shrinking nor growing rapidly)

  
• What occurs during phase III of cancer immunoediting?

  Tumours with reduced immunogenicity grow progressively in an immunologically unrestrained manner, establish an immunosuppressive tumour microenvironment, and become clinically detecatable

  
• What processes occur during phase III of cancer immunoediting?

  • Poor antigenic expression - tumours fail to produce tumour neoantigens and mutations in MHC genes leads to loss of MHC complex (invisible presentation to immune system)
  • Increased release of immune suppressive factors, e.g. TGFb inhibits T-cell activation
  • Increased number of immunosuppressive cell types that are tumour promoting
  • Increased expression of checkpoint proteins - sustain anti-immunogenic environment around tumour - downregulation of T cells
• What is the role of regulatory T cells?

  Suppress exaggerated immune response, thereby maintaining homeostasis and self-tolerance. Inhibits the anti-tumour immune response through inhibition of tumour-suppressing immune cells
• Regulatory T cells in tumours

  Upregulation of regulatory T cells to promote tumour development, i.e. overexpression of IL-10, TGFb, M2 macrophages, myeloid derived suppressor cells
• What is the role of myeloid-derived suppressor cells (MDSCs)?

  • Heterogeneous population of immature myeloid cells with immunosuppressive properties
  • Produce high levels of IL-10, TGF-b and other cytokines as well as both reactive oxygen and reactive nitrogen species (ROS and RNS), which together inhibit NK cells, and CD8+ and CD4+ effector T cells, and promote the expansion of regulatory T cells
  • Supports environment and suppresses activation of immune cells
• What is the difference between hot and cold tumours?

  Hot

  • Infiltrate the tumour microenvironment (high Teff to Treg ratio)
  • Immune cells tend to be non-functional (exhausted phenotype)
  • Aim to reactivate those immune cells since they are already inside the tumour

  Cold

  • Unable to infiltrate the tumour microenvironment
  • Immune cells cannot simply be reactivated since they are not inside the tumour
• What is the race to make "cold" tumours "hot"?

  • Cold tumours struggle to be detected by immune cells as they cannot penetrate inside the tumour microenvironment and detect tumour antigens
  • In contrast, hot tumours that have high mutational load and often have higher levels of neoantigens that can be recognised by the immune system
  • Making cold tumours hot will allow for more effective treatment
• What are examples of hot and cold tumours?

  Hot = melanoma and lung cancer

  Cold = prostate and pancreatic cancer
• Why is tumour profiling necessary?

  Response is too low just using checkpoint inhibitors

  
  

  Characterising and profiling tumours provide information on gene mutations, proto-oncogene to oncogene transformation, loss-of-function tumour suppressor genes, angiogenic factors, likelihood to metastasis etc.
• What are further categoies of hot and cold tumours?

  Immune-inflamed

  • High degree of cytotoxic T-cell infiltration
  • Mechanism of immune evasion: checkpoint activation, T-cell exhausation

  Immune-excluded (immunogenic)

  • Presence of T cells at invasive margin, absent in central tumour mass
  • Mechanism ": stromal barriers, aberrant vasculature, hypoxia, lack of chemokines, oncogenic pathways

  Immune-desert (resistant to immunotherapy)

  • Absence of T cells within tumour and at margins
  • Mechanism ": insufficient priming, defects in antigen presentation, lack of antigen
• Which further category of hot and cold tumours are prime candidates for checkpoint inhibitor therapy?

  • Immune-inflamed - prime candidate due to high cytotoxic T-cell infiltration
  • Immune-excluded - not a great candidate because of T-cell exclusion from centre mass, but also poor blood supply to this tumour
  • Immune-desert - not a candidate as there are no immunes at all
• What are the main types of immunotherapy?

  • Monoclonal antibodies
  • Immune checkpoint inhibitors (ICIs)
  • Chimeric antigen receptors (CARs)
  • Immunotherapy combinations
• What was the problem with initial immunotherapies?

  Stimulated the immune system non-specifically

  • Expanded T cell population and drove T cell activation
  • Made immune system more sensitive to kill tumour cells generally
  • Short-term responses
  • Not targeted so had risks of autoimmunity
• What is antibody-based immunotherapy?

  • Attaches an antibody to an antigen on tumour cell (must locate first)
  • Can conjugate many things to antibodies, e.g. radionucleotides, toxins
  • Recruits immune cells, e.g. NK T cells recognise stalk (Fc) region of antibody and secrete perforins and granzymes to digest tumour cell
  • e.g. Herceptin antibody blocks GF ligands from binding to HER2 receptor dimer and stop activation
• What is monoclonal antibody-based immunotherapy?

  • Developed against checkpoint proteins, e.g. PD1, PDL1, CTLA4
  • Blocks the downregulation of T cells - allowing T cells to target tumour cells
  • Can use bispecific antibodies - link to both tumour neoantigen and T cell receptor to bring T cells into TME
  • Often bind extracellularly as they are large molecules not readily taken up by cells - manipulate them to bind
  • If the disease is ligand dependent, antibody-based therapy will not work
• What are examples of monoclonal antibody-based immunotherapy?

  Anti- suggests function as inhibitors of specific molecular targets

  • Herceptin (trastuzumab) - anti-HER2 receptor (breast cancer)
  • Rituximab - anti-CD20 on B-cell surface (non-Hodgkin's lymphoma, chronic lymphocytic leukaemia etc.)
  • Cetuximab - targets anti-EGFR (metastatic colorectal cancer, non-small cell lung cancer etc.)
  • Pembrolizumab - anti-PD-1 (melanoma etc.)
• What are examples of immune checkpoint inhibitor-based immunotherapy?

  1st generation antibody - ipilimumab

  • Anti-CTLA4 antibody
  • Used specifically for melanoma
  • Over time, gradually had elimination/regression of tumours due to reactivating T cells
  • Eventually, resistance builds up and the tumours come back - then switch to a different agent

  2nd generation antibodies - nivolumab and prembrolizumab

  • Anti-PD-1 and anti-PDL-1 antibodies
• What is the cascade for chimeric antigen receptor-based immunotherapy?

  • CAR-T cells recognise and bind with ScFv region tumour cells
  • Binding causes intercellular signalling in CAR-T cells
  • Activation of CAR-T cells and rapid proliferation
  • CAR-T cells attack and kill cancer cells
• What are examples of chimeric antigen receptor-based immunotherapy?

  Kymriah (tisagenlecleucel)

  • Treatment of acute lymphoblastic leukaemia (ALL)

  Yescarta (axicabtagene ciloleucel) 

  • Treatment of diffuse large B-cell lymphoma (DLBCL)

  Both of these treat blood cancer - single circulating leukaemic cells are easier to target than solid tumour mass cells.
• What is chimeric antigen receptor-based immunotherapy?

  • Rationale is to engineer T cell receptors to recognise tumour antigens with high affinity, rather than relying on the immune system to to recognise the limited/slightly immunogenic neoantigens - made to attach more strongly
  • Main structural features - ScFV of antibody (provide specificity), hinge and transmembrane region, co-stimulatory domain, T-cell activation domain
  • Potential to manipulate material and sequences for enhanced receptor efficacy, e.g. gene editing via. TALEN, CRISPR
• What is the difference between tumour-associated antigens and tumour-specific antigens (neoantigens)?

  Tumor-associated antigens may be found at low levels elsewhere in the body but neoantigens are completely unique to the cancer cells

  
  

  Targeting neoantigens for immunotherapy comes with a lower risk of accidentally harming healthy cells along the way