This mini review describes some of the key interactions between cancer cells and the immune system. This includes the concept of tumour cell immunosurveillance, mechanisms of immune evasion by tumour cells and some of the novel immunology-based anticancer therapies that have recently been introduced. The latter are also set into the context of the enlarging spectrum of immunohistochemistry-based and molecular testing that can now be performed on formalin-fixed and paraffin-embedded tissues for predicting response to both well-established and newly developed agents. The emerging field of cancer immunotherapy requires and encourages close working between cellular and molecular pathology and clinical cancer treatment, while providing new hope for patients with cancers that may not have responded to conventional oncological treatments.
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Immunotherapy is becoming an increasingly important technique for cancer treatment. The clinical response is variable but can be spectacular for some cancers. This review will focus on the mechanisms of action of emerging immune-based cancer treatments and their relationship to the pathways of interaction between cancer cells and immune effector cells. The potential side effects of these treatments will also be described.
Interactions between cancer cells and immune effector cells
The molecular mechanisms by which cancer cells and the immune system interact are highly complex but are gradually becoming elucidated. Some cancers are typically associated with a variable but often dense infiltrate of lymphocytes and phagocytic cells, and the best example of this is malignant melanoma, although this phenomenon may be seen in other tumours too, for example, breast and colorectal cancer, and appears to be related to clinical outcome.1 2 In this situation, the immune effector cells may be present mainly at the advancing edge of the tumour or be seen to infiltrate the tumour itself. The latter pattern in particular suggests the presence of intimate cell–cell interactions between tumour cells and immune effector cells. Indeed, it is well known that segmental or complete regression of primary malignant melanoma lesions can occur—presumably as a result of immune-mediated tumour cell destruction.3 This process has also rarely been observed in metastatic malignant melanoma, as well as in other primary tumour types, for example, Merkel cell carcinoma.4 5 Tumour regression of this nature is thought to represent a consequence of the natural immunosurveillance mechanisms that kill neoplastic cells at a very early stage, before they can develop into clinically evident tumours.6 7 It is well established that mutations within individual cells that may induce neoplastic transformation are common, but that the vast majority of these cells are identified and destroyed in individuals with a competent immune system. States of immunocompromise are associated with an increased risk of neoplasia (eg, cutaneous squamous cell carcinoma in organ transplant recipients, lymphoma and Kaposi sarcoma in HIV-positive individuals). Highly immunogenic early neoplastic cells may be eliminated by host cytotoxic T-cells or natural killer cells, while those that are poorly immunogenic may escape immunosurveillance and develop into clinically evident tumours. The term ‘immunoediting’ has been suggested to describe this range of interactions between early neoplastic cells and the immune system.8
Cancer cell evasion of immune-mediated destruction
Several mechanisms exist by which cancer cells may evade immune-mediated destruction (box 1). These include loss of cell surface antigens that immune effector cells would require in order to recognise a cell as ‘self’—but abnormal, enhanced tumour cell proliferation/reduced apoptosis, or the creation of a state of anergy towards the cancer cell via the expression of cell surface molecules that actively promote immune tolerance.8 9
Mechanisms by which cancer cells may evade immune destruction8 9
Reduced immune recognition of tumour cells.
Absence of strongly antigenic tumour epitopes.
Loss of MHC class 1 expression.
Mutations in CD58 and B2M genes.
Increased tumour cell survival.
Increased expression of STAT-3 or Bcl-2.
Creation of an immunosuppressive environment within the tumour.
Cytokine expression, for example, VEGF, IL-10 and TGFβ.
Expression of immunoregulatory molecules, for example, B7 family checkpoint inhibitors—PD-1, PD-L1, CTLA-4, VISTA, B7-H4, BTLA; IDO, TIM3/galectin9, LAG-3 and sMICA.
Expression of CD73, adenosine receptors and HLA-G.
Alterations of CD70–CD27 axis.
B2M, beta-2-microglobulin; Bcl-2, B cell lymphoma 2 (inhibits apoptosis); BTLA, B-lymphocyte and T-lymphocyte-associated; CD27, a member of the TNF (tumour necrosis factor) family, expressed on certain B- and T-cells; CD70, ligand for CD27, expressed on tumour cells; CTLA-4, cytotoxic T-lymphocyte-associated protein 4; HLA-G, a non-classical HLA class 1 molecule; IDO, indoleamine-pyrrole 2,3-dioxygenase; IL-10, interleukin 10; LAG-3, lymphocyte-activation gene 3; MHC, major histocompatibility complex; PD-1, programmed death 1; PD-L1, programmed death ligand 1; sMICA, major histocompatibility complex class I-related chain A; STAT-3, signal transducer and activator of transcription 3 (promotes proliferation and inhibits apoptosis); TGFβ, transforming growth factor β; TIM-3, T-cell immunoglobulin and mucin-domain containing 3; VEGF, vascular endothelial growth factor; VISTA, V-domain immunoglobulin suppressor of T-cell activation.
Immune checkpoint inhibitors
These agents are designed to inhibit mechanisms of immune tolerance to cancer cells (table 1) and represent the latest advances in cancer immunotherapy. This is achieved via the use of monoclonal antibodies to block one of a family of molecules that modulate T-cell function. This family includes cellular receptors such as cytotoxic T-lymphocyte-associated protein-4 (CTLA-4, also known as CD152), its ligand B7; programmed death-1 (PD-1), its ligand programmed death-ligand 1 (PD-L1); CD28; and inducible costimulator.10
CTLA-4 was one of the first molecules to be used as a therapeutic target for cancer immunotherapy, and this was in malignant melanoma. CTLA-4 is important in the modification of T-cell function. In health, CTLA-4 binds its ligand B7 (present on antigen-presenting cells) with a higher avidity than the stimulatory molecule CD28. CTLA-4-B7 binding therefore has an inhibitory effect on T-cell function. Therapeutic blockade of CTLA-4 removes this inhibitory effect and potentiates T-cell activity, including cancer cell lysis.10 CTLA-4 blockade may also preferentially inhibit the action of regulatory T-cells (Treg cells) within the tumour microenvironment and therefore lessen the inhibitory effect of these cells on effector T-cell (Teff cell) mediated cancer cell killing.10 This secondary mechanism has been suggested because CTLA-4 may be expressed at a higher level on Treg cells than Teff cells.
PD-1 and PD-L1
The PD-1/PD-L1 receptor-ligand system is important in health for maintaining a state of immune tolerance. This system was the subject of a recent and detailed review in this journal.11 Low-level PD-L1 expression is normally present on resting lymphocytes and antigen-presenting cells, as well as within tissues at ‘immunoprivileged’ sites such as the testis, cornea and syncytiotrophoblast. PD-L1 expression is also induced within haematopoietic cells, endothelial cells and epithelial cells in response to infection or inflammation. This occurs via pathways involving toll-like receptors that are highly expressed by antigen-presenting cells, for example, in response to the presence of pathogens, or involving interferon-γ receptors.11
Interactions between PD-1 and PD-L1 induce tolerance via suppression of immune effector cell function. This can occur centrally, for example, within the thymus, where PD-L1 expression leads to the deletion of autoreactive naïve T-cells; or at peripheral sites, for example, in association with PD-L1 expression by dendritic cells (antigen-presenting cells). The PD-1/PD-L1 system is also important in the development of ‘immune exhaustion’. This is suppression of the T-cell response in chronic inflammatory states and acts to limit immune-mediated tissue destruction.11
Of particular importance to this review, PD-L1 expression by malignant cells is found in several cancers, for example, malignant melanoma and carcinomas arising within the lung (non-small cell), ovary, breast and gastrointestinal tract.11 The proportion of cancers that express PD-L1 varies between tumour sites and types (table 2). This mechanism is believed to promote immune tolerance towards the cancer via interaction with PD-1 expressed on tumour-infiltrating lymphocytes. Various different assays for PD-L1 are available (eg, 22C3, 28–8, SP142, SP263, 73–10) and the staining patterns achieved vary between some of these.12
Defective DNA mismatch repair and immunotherapy
Cancer cells that express an increased number of abnormal proteins may represent enhanced targets for immune cell interactions. For example, cancers characterised by defective DNA mismatch repair (MMR) mechanisms (ie, tumours exhibiting a high level of microsatellite instability—‘MSI-high’) contain neoplastic cells bearing increased numbers of abnormal proteins. These tumours may occur sporadically or as a result of germline mutations within DNA MMR-encoding genes, that is, Lynch syndrome. One of the histological appearances of Lynch syndrome-associated cancers is the presence of lymphoid aggregates at the advancing edge of the tumour (a ‘Crohn’s-like’ reaction) and/or a tumour-infiltrating lymphocyte response.13 It is possible that this histological pattern may result from the presence of increased numbers of antigenic proteins on the surface of the cancer cells. Immune-mediated cancer cell destruction may be a mechanism underlying the improved clinical prognosis of such tumours compared with those that do not harbour defective DNA MMR.
This phenomenon may also represent a mechanism by which cancers can be identified as particularly suitable for immunotherapy, due to the presence of increased antigenicity within the tumour, that is, the existence of greater numbers of abnormal proteins that could be identified as ‘foreign’ by the immune system.14 For this reason, assessment of DNA MMR status (using immunohistochemistry or PCR-based assessment of MSI status) is likely to become increasingly important as the potential clinical applications of cancer immunotherapy widen. This represents an interesting expansion of the indications for DNA MMR testing, from assessment primarily due to possible Lynch syndrome, to predictive testing in cancer treatment. In a related development, measurement of the tumour mutational burden—expressed as the number of single nucleotide variants per megabase (Mb—million base pairs)—is showing promise as a predictive tool when considering the use of immunotherapy.15
Side effects of cancer immunotherapies
The side effects of immunotherapies result from immune-mediated damage to normal tissues (table 3).16–18 This is because these treatments are based on alteration of the interactions between cancer cells and the immune system. The most commonly affected sites include the skin and mucosae, the luminal gastrointestinal tract, the liver and the endocrine system. Skin rashes are usually the earliest side effect and may be erythematous, reticular or maculopapular in nature, while mucositis may present with a dry mouth. Diarrhoea is a common side effect, especially of anti-CTLA-4 antibodies. Luminal gastrointestinal tract biopsies show features that are similar to those of graft-versus-host disease, for example, villous blunting (small intestine), increased crypt epithelial cell apoptosis, together with lymphoplasmacytic lamina propria expansion and neutrophilic infiltration of epithelium with cryptitis and crypt abscess formation.18 Increased intraepithelial lymphocyte numbers are less common, although ipilimumab therapy may be associated with features in the duodenal mucosa that mimic coeliac disease.18 19 Interestingly, immunotherapy-associated colitis can respond well to antitumour necrosis factor-α therapy, for example, infliximab.20 Hepatitis most commonly becomes evident as raised serum transaminase concentrations, while a raised serum bilirubin concentration can also be seen. Liver biopsies may show lobular inflammation with perivenular (zone 3) accentuation or portal tract inflammation surrounding bile ducts. Endocrine involvement typically results in non-specific symptoms such as headache, nausea and fatigue.21
Immunotherapy predictors in the spectrum of predictive cancer tests
Elucidation of cancer characteristics that may be associated with differential responses to immunotherapy (eg, using immunohistochemistry to define PD-1/PD-L1 and DNA MMR enzyme expression patterns) represents just one, although increasingly important, evaluation during the increased ‘personalisation’ of cancer treatments (table 4). Some of these other tests are very well established and based on immunohistochemistry, for example, hormone receptor expression status in breast cancer. This was followed by HER-2 receptor status assessment—either via immunohistochemistry or in situ hybridisation—in breast cancer and now oesophagogastric cancer. HER-2 status assessment is an example of a test that was initially only required for a subset of patients with breast cancer (those with advanced disease who had not responded to conventional chemotherapy) but whose indications subsequently expanded such that it is now performed at diagnosis on all new invasive breast cancers. ALK expression can be assessed using immunohistochemistry in lung cancer.22 More recently, assessments based purely on molecular techniques on paraffin-derived DNA have become established, for example, RAS (ie, KRAS and NRAS) mutation status in colorectal cancer, BRAF mutation status in malignant melanoma, non-small cell lung cancer and colorectal cancer, ALK and EFGR mutation studies in non-small cell lung cancer, KIT and PDGFRA mutational studies in gastrointestinal stromal tumours, and Oncotype DX analysis in breast cancer.22–28 The introduction of these predictive tests strengthens the links between pathology and oncology. However, there are clear staffing and consumables resource implications for laboratories that require appropriate funding to enable the tests to be delivered in a timely and reliable manner.
Interactions between tumour cells and the immune system are complex. Some tumours are known to be commonly associated with an inflammatory cell infiltrate and these may undergo spontaneous regression—consistent with the result of an antitumour immune response. As well as leading to regression of clinically evident tumours, constant immunosurveillance takes place and is believed to lead to destruction of tumour cells at a very early (preclinical) stage. Tumours use a variety of strategies that can result in evasion of the immune response. These include creating reduced tumour cell antigenicity (eg, via loss of major histocompatibility complex class 1 expression), developing mechanisms enhancing tumour cell survival (eg, Bcl-2 expression) and inducing a suppressed immune state within the tumour (eg, checkpoint inhibitor expression). Of these, modulation of the immunosuppressed state via the use of immune checkpoint inhibitors (eg, PD-1, PD-L1 and CTLA-4 blockers) has so far been the most studied and applied in clinical practice. Immunohistochemical assessment of PD-1 and PD-L1 expression is an important step when deciding whether a tumour may respond to blockade of these molecules. Identification of tumours with potentially increased antigenicity via immunohistochemical demonstration of DNA mismatch enzyme deficiency or identification of MSI allows targeting of these treatments to patients who are most likely to benefit. Alongside immunohistochemistry, DNA extracted from formalin-fixed and paraffin-embedded tumour samples provides an important basis for genetic analyses (eg, RAS and BRAF mutation status) that are key determinants for therapy choices in cancer treatment. These emerging forms of personalised medicine are exciting developments that increasingly closely link cellular pathology, molecular pathology and oncology and that provide renewed optimism for cancer treatment.
Handling editor Des Richardson.
Contributors This is a sole author script. ACB created the concept for the review, prepared the script and checked it.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Not commissioned; externally peer reviewed.