THE body’s immune system is programmed to recognise “self” from “non-self” cells—foreign molecules such as bacteria, viruses and parasites—and mount a response to get rid of them. Critical to this response are certain white blood cells called T-cells. T-cells have receptor molecules on their surface that bind to structures recognised as non-self, and this binding triggers the immune response. There are also two kinds of other proteins on the surface of the T-cells, one kind that switches on the immune response and the other that switches it off. These are called “checkpoint proteins”.
The former set of checkpoints make the T-cells active when an infection is present and the latter set puts the brakes on the immune system and helps it switch off when the immune cell response is no longer required. Just like in a motor car, an intricate balance between acceleration and braking is essential because otherwise the immune system will go on an overdrive and begin to respond to things that it should not respond to and potentially destroy healthy cells and tissues. Autoimmune disorders are the result of the body’s immune system reacting to the body’s self cells as if they were non-self or antigenic.
Cancer cells, by producing certain proteins, can often trick the immune system to switch off its T-cells when they should actually be attacking them. These proteins on cancer cells are thus actually activating a certain switch-off molecule on the surface of the T-cells. This year’s Nobel Prize in Physiology or Medicine has been awarded to two key scientists who identified these brake checkpoint molecules on T-cells and invented biological techniques to prevent the cancer cells from activating these, which otherwise would have put brakes on the immune response to cancer. The molecular brakes on T-cells thus remain inactive and the surcharged immune system can now attack the cancer cells.
James Allison, 70, of MD Anderson Cancer Centre at the University of Texas, Houston, and Tasuku Honjo, 76, of Kyoto University are the two immunologists whom the Nobel Academy has chosen for the prestigious award for, as the citation says, “their discovery of cancer therapy by inhibition of negative immune regulation”. The two scientists will share the prize money of $1.1 million equally.
Paradigm shift
The research of these two laureates has marked a paradigm shift in the approach to cancer therapy. Before their groundbreaking work, the approach was mainly aimed at attacking cancer cells directly through different kinds of drugs that were specific to types of cancer. Here the approach is more universal, where the immune system itself is being primed, or reprogrammed, to unleash T-cells to fight all types of cancer. The technique is referred to as cancer immunotherapy and the corresponding therapeutic agents—drugs that block checkpoint proteins—are called “checkpoint inhibitors”. Given the way it works, the technique is also sometimes called “checkpoint blockade therapy”.
Their seminal work has laid the foundation for an entirely new clinical approach to treating cancer. The first “checkpoint inhibitor” drug was approved by the United States’ Food and Drug Administration (FDA) in 2011 and within a short period, a number of drugs came into use that have either completely cured thousands of patients who had no hope, or have prolonged many lives by a decade and more where traditional interventions would have at best extended their lives by a year or two.
“The laureates’ discoveries have added a new pillar in cancer therapy [Figure 1]; it represents a completely new principle because, unlike previous strategies, it is not based on targeting the cancer cells but rather the brakes—the checkpoints—of the host immune system,” said Klaus Karre, a member of the Nobel Committee for the Physiology or Medicine and an immunologist at the Karolinska Institute in Stockholm, while describing the work during the announcement. “It’s a big step in the fight against cancer. We can now see the long-term outcome of this treatment [since 2011], and it is quite convincing,” he added.
In an interview to the popular science magazine Scientific American in 2015, soon after winning the Lasker Award, Allison said: “Traditional therapies are typically drugs that attack mutations causing the cancer… but there are a few problems there including the fact that the tumour becomes resistant to the drug. What we call ‘checkpoint blockade’ therapy differs in a few ways. We get the immune system to attack the process of carcinogenesis itself… T-cells detect mutant or foreign peptides on the surface of cells to give the immune cells an idea of what is going on. With lung cancer or melanoma [a type of skin cancer] there are hundreds of thousands of mutations…. Then once you have T-cells, unlike with chemotherapy or radiation, there is memory. You have T-cells for the rest of your life…. The last difference is adaptability. Since your immune system is a dynamic thing, if the tumour changes, the immune system can change its response itself.”
In the late 1980s, a protein molecule expressed by T-cells, called CTLA-4 (cytotoxic T-lymphocyte associated protein 4), was isolated and cloned. However, its function was unknown. It is now known that CTLA-4 protein resides intracellularly within resting T-cells but translocates itself rapidly to the surface after T-cell activation and is constantly expressed. In 1994, work by researchers, including Allison, led to the conclusion that CTLA-4 reined in the T-cell activation; that is, it acted as a negative regulator, a brake. This discovery led researchers to find ways of enhancing its expression as a method to develop therapy for autoimmune diseases.
But Allison, then working at Berkeley University, thought otherwise. In a bid to find a cure for cancer, he intended to block the negative regulatory effects of CTLA-4, thereby unleashing a full-scale immune response. He had already developed an antibody that could bind to CTLA-4 and block its function (Figure 2). He also realised that, in contrast to other approaches to cancer therapy, here there was no need to understand which “non-self” molecules the T-cells would recognise, and the strategy was not selective for a particular type of cancer but universal.
Allison and his co-workers performed the first experiment at the end of 1994, which was immediately repeated after the Christmas break. The results of these were spectacular. Mice transplanted with tumours were cured when injected with antibodies that blocked the brake CTLA-4 and unlocked anti-tumour T-cell activity. In 1996, Allison and colleagues also showed that antibodies against CTLA-4 in mice models not only got rid of tumours but also prevented new tumours from forming. Such dramatic results had never been seen before in any therapy for cancer.
In the post-announcement telephonic interview with the official website of the Nobel Prize, Allison said that he had not actually set out to find a cure for cancer. “I was trying to understand how T-cells work. We figured out this one thing about this negative regulator. [I] had this idea that if we just took that off, you know, maybe it would do a better job of killing cancer cells, and sure enough it works!”
“We patented it and I thought everybody would jump at it,” the magazine The Scientist quoted him as saying earlier this year. Despite the pharmaceutical industry showing little interest in this crucial finding, Allison was determined in his intense efforts to develop the strategy into a therapy for humans.
That persistence came from his innate interest in cancer, as he told Scientific American in his 2015 interview. “I became particularly interested in cancer because my mother died from lymphoma when I was a kid. I saw the effects of the radiation treatment she was receiving. I also saw the ravages of the treatment on her brothers who also died from cancer. One of her brothers had lung cancer and another had melanoma. I saw personally the toll that the disease takes. More recently my older brother died from metastatic prostate cancer. Both me and my other brother had prostate cancer, too, but we caught it quickly with me and I’m fine. It’s had a big impact on my family.”
It took two years of peddling his checkpoint blockade idea to pharmaceutical companies before Allison managed to establish collaboration with a small biotech firm, Medarex, where producing human monoclonal antibodies became possible. In 1999, an anti-CTLA-4 monoclonal antibody, called MDX-010, was developed, which was later named Ipilimumab. Bristol-Myers Squibb of New York City subsequently acquired Medarex and advanced the clinical programme for the new checkpoint inhibitor.
In 2003, Allison and colleagues tested the anti-CTLA-4 antibody in 14 patients with advanced metastatic melanoma (who usually are given one year of life, at best) and found the tumours disappeared in three of them, but still scepticism prevailed. Except for a few immunologists, even the larger medical community did not show much interest.
Michael Curran, an immunologist who worked in Allison’s laboratory for a decade, told The Scientist how a journal review of a paper that he, Allison and others had submitted, said: “It is well known that immunotherapy works only in mice.” Recognition of Allison’s work had to wait until another important clinical study in 2010, when he showed similar striking effects of the therapy in patients with advanced melanoma.
In a 2013 interview to Nature , Allison said: “It was very frustrating. They [the companies] said, ‘It may work in mice, but it’ll never work in people.’ The concept was new and it was so unusual.” Finally, in 2011, the FDA approved the drug Ipilimumab, now commercially known as Yervoy, as a treatment for advanced melanoma.
A couple of years before Allison’s discovery, across the Pacific, Honjo and his colleagues were looking at another T-cell surface protein called PD-1 (programmed death-1), which they had isolated and cloned in 1992. This, too, was a basic research programme, and not aimed at finding a cure for cancer.
In fact, initially Honjo and colleagues at Kyoto believed that the protein was involved in biochemical pathways regulating apoptosis (cell death that occurs as part of normal growth and development). Hence the name PD-1. Over the next nearly 10 years, Honjo’s group studied the protein using a series of elegant experiments and discovered that, like CTLA-4, it was also a checkpoint protein that acted as a brake but operated by a different mechanism (Figure 2).
Alongside, Honjo and colleagues were also attempting to find the ligand (a molecule that binds on to a target protein) that binds with the protein PD-1. Together with the groups of Gordon Freeman at Harvard Medical School and Clive Wood at the Genetics Institute in Massachusetts, they identified the associated ligand, which is called PD-L1. On the basis of the observation that the molecule was expressed not only by macrophages, dendritic cells and additional immune cells but also by certain cancer cells, Freeman and others suggested in a paper that some tumours may use PD-L1 to switch off the anti-tumour response in T-cells. They were the first to do so. It has been reported by Nature that Freeman felt disappointed by the Nobel Committee passing over his contribution to the prize-winning work.
Checkpoint therapy
The proposition that the PD-1/PD-L1 pathway could be associated with immune responses to tumours was tested in two studies in 2002, one at the laboratory of Lieping Chen at Mayo Clinic (who actually had identified the ligand PD-L1 as B7-H1 in 1999 but had not studied its binding to PD-1) and another at the laboratory of Honjo. In animal model studies by Honjo and colleagues, PD-1 blockade was also shown to be a potential strategy to fight cancer. This paved the way for utilising PD-1 as a target for treating cancer patients. In fact, their work was the first to also discuss the possible synergistic effects in combination therapy based on CTLA-4 and PD-1 pathways to blockade, which strategy is now being clinically investigated.
After the initial studies that showed the effects of CTLA-4 and PD-1 blockade clinical development has been both dramatic and rapid. Immune checkpoint inhibitor therapy has fundamentally changed the outcome for patients with many types of advanced cancer.
Although an MD (Doctor of Medicine), Honjo had not set out to find a cure for cancer. “Well, you know, biology is such a complex system. We cannot design. Many people tried to find therapy for cancer, but all failed. I never expected my research, working on immune system, would lead to cancer therapy. But, in a sense, I am also fortunate that I thought about it. You know you have to try many things and if you are lucky you can hit, but you have to pursue,” he told the Nobel Prize website in his post-announcement interview.
Of the two strategies, checkpoint therapy against PD-1 has proven to be more effective and it has shown positive results in many types of cancer, including lung cancer, renal cancer, lymphoma and melanoma. The checkpoint proteins CTLA-4 and PD-1 are found on T-cells, whereas the ligand PD-L1 is found on cancer cells.
As Freeman has pointed out to Nature , CTLA-4 inhibitors have so far proved to be effective only in melanoma, whereas the FDA has approved drugs that target PD-1 and PD-L1 to treat 13 different cancers. The checkpoint inhibitors against PD-1 include nivolumab (Opdivo) and pembrolizumab (Keytruda), which are used for patients with melanoma, Hodgkin’s lymphoma, non-small cell lung cancer (NSCLC) and cancer of the urinary tract. Those against PD-L1 include atezolizumab (also known as MPDL3280A), which is used as treatment for some people with lung cancer and urothelial cancer. It is also being currently studied for breast cancer and other cancers.
“PD-1 and PD-L1 are what work in a really wide variety of people, and our discoveries were foundational there,” Freeman has been quoted as saying in Nature . “But CTLA-4 was the first success and Jim Allison had been a real advocate and champion of the idea of immunotherapy,” Freeman added.
When asked whether immunotherapy may not be effective against certain types of cancers, Allison said in his Lasker Award interview to Scientific American : “Theoretically, since we are not treating the cancer, the cancer doesn’t really matter. Melanoma and lung cancer, where most of this work has been done, both have a lot of mutations. They have hundreds of thousands of mutations per cell and it’s the mutations the immune system recognises. When you get to cancers like breast, prostate and kidney, which have smaller numbers of mutations, the drugs aren’t quite as effective. Our job now is to figure out how to make immunotherapy effective against those tumours with small numbers of mutations. I think there may be some that don’t respond to [immunotherapy] but I’m pretty optimistic that we will be able to deal with a very large number of types of cancer.”
Echoing these thoughts, Honjo said in his interview: “Well, there are still several problems, but two are most important. One is, still only 30 per cent of patients are responding. So we wish to have some biomarkers to predict whether he or she is responsive or not. Secondly, definitely we wish to improve the efficacy of this treatment… I believe these two problems will be solved in the near future.”
Also, at present, similar to other cancer therapies, adverse side effects are seen to checkpoint inhibitor therapy as well, which can be serious and even fatal. They are caused by a hypercharged immune response leading to autoimmune response, but it is believed that they are usually manageable. Intense continuing research is currently focussed on elucidating mechanisms of action, with the aim of improving therapies and reducing side effects.
Although checkpoint inhibitor therapy has revolutionised the treatment of tumours, many patients still do not respond, as Honjo has pointed out. But there are also many long-term survivors. In other forms of cancer treatment, the Nobel Assembly observed in its description of the scientific background to the discovery, durable recurrence-free survival is extremely rare, particularly among patients with certain forms of cancer like advanced melanoma and NSCLC (Figure 3).
New clinical studies have indicated that combination therapy, targeting both CTLA-4 and PD-1, can be even more effective, as demonstrated in patients with melanoma. Thus, Allison and Honjo have inspired efforts to combine different strategies to release the brakes on the immune system with the aim of eliminating tumour cells even more efficiently. A large number of checkpoint therapy trials are currently under way against most types of cancer, and new checkpoint proteins are being tested as targets.
Immunotherapy is actually not new. It goes back to 150 years ago when infectious agents were used to stimulate immune responses to cancer. The first such attempts were made by German clinicians. But the most well known for such studies in this field are by the American surgeon William Coley, who inoculated live, cultured Streptococci bacteria to treat malignant tumours in the early 1890s. However, the practice fell from favour with the medical community as the clinical outcomes varied wildly. Today the treatment for bladder tumours is akin to this approach where the bacillus Calmette-Guerrin is administered.
In the early 20th century, even though the mechanism underlying the principle of stimulating the immune system with infectious agents to cure cancer was not understood, the idea that the immune system could be boosted to influence tumour development prevailed. As Allison has pointed out: “The fact that the immune system could be used to treat cancer was first proposed in 1909 by Paul Ehrlich, and he had the idea about antibodies. He thought the immune system had antibodies that could eliminate tumours. The question was, what antibodies could we use? The reason I think there is so much excitement about checkpoint blockade is it’s relatively easy. You inject an antibody into a person. And you target the immune system and unleash it, and there are many different ways to do it that can be combined. It’s the renaissance of immunotherapy. I would say it’s the rebirth of immunotherapy rather than it’s new.”
The future course of development in this field, in which researchers across the world are seriously engaged in, is to improve the understanding of the mechanisms of the therapy, in particular the ones leading to adverse events.
As the Nobel Assembly’s descriptive note points out, the identification of CTLA-4 and PD-1/PD-L1 is only the beginning, and additional molecules with similar functions are currently under investigation in different laboratories. Despite attempts by scientists to engage the immune system to fight cancer, clinical development towards a general approach to treating all types of cancer was absent until checkpoint inhibitor therapy arrived thanks to the seminal discoveries by Allison and Honjo. With the unprecedented research in immune checkpoint therapy that their discoveries have unleashed, one can expect major discoveries at all levels in the field that should benefit humankind a great deal more in the years to come.
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