A commentary on the Nobel Prize award for Physiology or Medicine – for the discovery of cancer therapy by inhibition of negative immune regulation

The promise of harnessing the immune system to fight cancer has long been dreamt of. In the late 19th century William Coley, a New York cancer surgeon, found that inflammation caused by purposely injecting streptococcal bacteria into sarcoma lesions could control the spread of the disease, at least temporarily. Over time, his ideas were left by the wayside as techniques in cancer surgery and radiotherapy were honed and the new science of chemotherapy was developed.


The cytotoxic T lymphocyte as an effector of cancer immunotherapy
In recent years, the focus of much research into cancer immunotherapy has focused on cytotoxic T lymphocytes and their interactions with cancerous cells. This class of white blood cells possess highly specific T cell receptors that can bind to fragments of proteins presented by cells at their surface on their MHC (major histocompatibility complex) class I molecules. During their development in the thymus, a gland sitting above the heart in humans, lymphocytes are removed if they have the ability to identify fragments of self-proteins (i.e., normal proteins produced by the individual themselves). As such, healthy individuals should possess a repertoire of lymphocytes that can detect cells possessing non-self peptides, such as those infected by viruses or containing mutated proteins early in cancer development. When engaged, these lymphocytes initiate a process of cell death in the affected cells to control and eliminate the danger.
This highly effective mechanism is continuously at work, deleting cells showing signs of dysfunction that could lead to malignant change. However, when affected Immunotherapy was not entirely abandoned though, and throughout the last century, cancer researchers continued to explore the interaction between cancer and the immune system. Small gains were made. The injection of the bacillus Calmette-Guérin (BCG) organism, commonly used as a vaccine for tuberculosis, into the bladder of patients with early-stage bladder cancer, could control disease and became common practice in the 1970s. Similarly, the administration of cytokines, immune system signalling molecules such as interferon-α and interleukin-2, were shown to improve survival after surgical removal of melanoma and in disseminated kidney cancers. Unfortunately, the numbers who benefited were small and the side effects were considerable.
Substantial progress was achieved following the invention of monoclonal antibody production and the use of genetic modification to change parts of the molecules from mouse to human form, a fact acknowledged by the co-awarding of the 2018 Nobel Prize for Chemistry to Sir Greg Winter for his work in the field of antibody development and evolution. These laboratory-manufactured molecules can be produced in large quantities to bind to a specific target. This has been exploited using antibodies recognizing cancer cell surface molecules to direct the immune system against the target cell, or to block crucial cellular signalling pathways.
In 1997, Rituximab became the first anti-cancer antibody approved for use in humans. This resulted in a marked improvement in survival for patients with non-Hodgkin's lymphoma, an increasingly prevalent cancer. It was quickly followed by the development of a host of other monoclonal antibody therapies including Trastuzumab for breast cancer, and as we will come to see, the checkpoint inhibitors. Among those who came to realize the promise of cancer immunotherapy were The promise of harnessing the immune system to fight cancer has long been dreamt of. In the late 19th century William Coley, a New York cancer surgeon, found that inflammation caused by purposely injecting streptococcal bacteria into sarcoma lesions could control the spread of the disease, at least temporarily. Over time, his ideas were left by the wayside as techniques in cancer surgery and radiotherapy were honed and the new science of chemotherapy was developed. Like all body systems, the immune system has complex mechanisms to prevent excessive activation and correct itself after clearing a danger. This is important to prevent the risk of autoimmune disease, where a person is harmed through excessive immune attack on healthy tissues. It is these brake mechanisms or checkpoints, which Honjo, Allison and others have investigated and targeted to re-energize the immune response to cancer.

The discovery of PD-1
While working at Kyoto University, Tasuko Honjo and his team were investigating the mechanism by which cells undergo programmed cell death, or apoptosis, and identified the programmed cell death-1 (PD-1) receptor. This member of the immunoglobulin (antibody) superfamily, which also includes CTLA-4 (see below), was found to be an immune system regulator, rather than simply a trigger of apoptosis, as first thought. When mice deficient in the PD-1 gene developed a lupus-like autoimmunity, displaying arthritis and kidney impairment, Honjo recognized the significance of the finding as a fundamental control process. The team pursued the function of this new receptor and its targets were identified, the PD-1 ligand 1 and 2 (PD-L1 and PD-L2). They considered how this biological process would affect other situations where immunity is important such as inflammatory and infectious diseases, as well as cancer. Altered versions of these molecules were found in a range of autoimmune diseases and high levels of PD-L1 and 2 were found on the surface of tumour cells.
PD-1 blocking antibodies showed an upregulation of tumour-active T cells in mice and tumour regression. Honjo pursued this as a potential human cancer treatment, leading ultimately to the development of Nivolumab, the first of the PD-1/PD-L1 targeting antibody treatments to reach the clinic.

Understanding CTLA-4
Concurrently, while investigating the activation of cytotoxic T lymphocytes by a specialized class of immune cells called dendritic cells, Allison identified a role for the cytotoxic T lymphocyte-associated protein 4 (CTLA-4) receptor, which had recently been characterized. While most were thinking about interactions between T cells and dendritic cells as activating, Allison identified the interaction with CTLA-4 to be inhibitory. CTLA-4 is expressed at the surface of activated T lymphocytes and this mechanism creates a restraint on the immune system, to prevent uncontrolled activation and thus damage by autoimmunity.
Allison built upon this new way of thinking about T lymphocyte activation and, in a mouse model of cancer, blocked the interaction between the inhibitory CTLA-4 and its trigger on dendritic cells by using a specific antibody. The results of this experiment were striking, with mice previously destined to succumb to their tumours surviving and even clearing the disease. On seeing the results, the excited researchers immediately set about repeating the experiment over their Christmas holidays.
Like Honjo, Allison appreciated the significance of his results. At the same time as further characterizing the cellular interactions, he sought a pharmaceutical partner to develop an antibody drug product to move into clinical trials. When the resulting drug, named Ipilimumab, was trialled in humans the results were remarkable. This proved to be the first treatment to significantly improve survival for patients with metastatic melanoma, with around 20% seeing sustained disease control.

Building on early promise
The results of Honjo and Allison's work have been combined most strikingly in the treatment of patients with malignant melanoma. This aggressive skin cancer is increasingly prevalent, and when spread and not removable by surgery it can quickly prove fatal. Until 10 years ago, patients with disease involving their liver, bones or brain could expect only brief survival, with less than half surviving 5 months.
The notable response of some tumours to the antibody Ipilimumab led to an interest in combining this with a drug blocking PD-1, hence trials into the use of Ipilimumab and Nivolumab in combination immunotherapy were established.
The CheckMate 067 study, launched in 2013, recruited 945 patients with previously untreated metastatic melanoma. They received either one of the drugs or both. Early reports showed that the response rate was highest in the combination treatment arm, and when the most up-to-date results were presented in October 2018, 4-year survival in this arm was at 51%. Many of these patients have no measurable disease on repeat computerized tomography (CT) scans, representing a dramatic improvement in their outlook.
The results from this trial provide proof that treating cancers with combination immunotherapies is feasible and can be very effective. However, the study also demonstrated that these treatments come at the cost of significant autoimmune-like side effects. Most people treated with combination immunotherapy will experience some side effects, more than half being severe enough to warrant hospital treatment. This can include severe diarrhoea and inflammation of the bowel, similar to aggressive inflammatory bowel disease. These patients require steroid treatment, often for a long duration, and occasionally other immune suppression and even surgery. Damage to the lungs, skin and even destruction of the hormoneproducing glands such as the thyroid and pituitary gland are commonly seen.
It is important to remember that not all individuals will have a meaningful response to immunotherapy. Malignant melanoma is an unusually immunogenic cancer and the dramatic results observed in some patients with melanoma are much less commonly seen in other cancers. Checkpoint inhibitors have received licenses for use in non-small cell lung cancer, squamous cell cancers of the head and neck, kidney and bladder cancer as well as Hodgkin's lymphoma. However, a large number of common cancers have shown minimal responses to this strategy, indicating the need for different approaches.
Allison and Honjo's work provided a proof-of-principle for the large-scale deployment of cancer immunotherapy and has led to renewed enthusiasm in the field. A range of new checkpoint inhibitors are in clinical development and are being trialled in many different settings. These include adjuvant therapy, for metastatic disease, and in combination with chemotherapy, radiotherapy and targeted agents. Other methods of boosting the anticancer immune response are also being investigated.

Broadening the approach to cancer immunotherapy
The technique of adoptive cell therapy was pioneered by Rosenberg and colleagues at the National Cancer Institute in Bethesda, Maryland. This technique requires the harvesting of T lymphocytes from a sample of a patient's tumour, before increasing their number in the laboratory. These cells are then re-administered to the patient in the hope that their high numbers will overcome any inhibitory immune mechanisms and kill residual disease.
Initially this approach had limitations, most notably the need to access a sample of the patient's tumour, and the blunted ability of harvested lymphocytes to control the disease. This has been circumvented by the alteration of a patient's T lymphocytes, collected from peripheral blood, through the addition of an engineered T cell receptor targeting tumour-specific cell surface markers. This socalled chimeric antigen receptor therapy T-cell (CAR-T) allows production of highly tumour-specific T cells that can be re-infused into patients with striking results in some cases. Patients with advanced-stage lymphoma or leukaemia may achieve complete and durable responses. The side effects however are significant, as are the costs, and at present this technology has only proven effective against a limited range of haematological cancers.

Immunology
In recent years we have come full circle and interest in vaccines has again been rekindled. Modern studies have utilized our ability to rapidly and accurately sequence a patient's genome as well as that of their cancer, and develop unique personalized vaccines. Studies published in 2017 showed an ability to prime a T lymphocyte response against a tumour to dramatic effect and we look forward to future trials in larger cohorts of patients.

Finally
As we reflect on the advances in understanding the interaction of cancer with the immune system over the last decade and how this has altered patients' chances of survival, we cannot help but consider those less well served by these developments. A survival rate of 50% at 4 years in patients with malignant melanoma is remarkable, but it does mean that half of those treated have died either of their disease or the effects of treatments in the same time frame. Response rates to immunotherapies in other cancer types are lower still. As we gain a greater understanding of the complex tumour microenvironment and how its components interact we can hope to truly understand why some patients will respond while others do not, and how to better predict and manage toxicity.
The work done to raise the profile and prospects of cancer immunotherapy by those honoured in Stockholm, as well as their colleagues, collaborators and competitors, is highly deserved, but is just the start. There is now concerted international effort, of which we are excited to be a part, to realize the immune system's potential for the benefit of many more patients with advanced cancers. ■