by Litt-Yee Hiew
Our immune system is naturally gifted with remarkable specificity, potency and memory. So far, no pharmacological treatment for any diseases could possibly provide comparable level of safety, efficacy and lasting effects as the human immune response. In the treatment of cancer, after the primary therapies – i.e. surgery, radiotherapy and chemotherapy – immunotherapy is being explored as a fourth option, especially for advanced stage cancers. To date, several kinds of novel immunotherapeutics, including cancer peptide vaccines, dendritic cell vaccine, immune checkpoint blockade and adoptive cell therapy are increasingly being introduced in clinics. This article peeks into the brief history of immune-stimulation, state-of-the-art advances, as well as some limitations of these approaches.
Of spontaneous cancer regression and Coley’s toxins
In the unavailing search for a cancer cure, it has been interestingly noted that certain cancers could spontaneously regress. Albeit rare and only anecdotally reported, kidney, brain (neuroblastoma), uterine and skin cancer are amongst the four frequent cancers associated with cancer regression, according to a review of 176 published cases from 1900 to 1960 . The sudden disappearance of not only the primary tumour but also their metastatic foci has been hypothesised to be a result of unexpected activation of the immune system leading to the recognition of non-self-proteins and subsequently the destruction of these cancer cells .
Indeed, Dr. William Coley, an orthopaedic surgeon, was one of the first physicians to relate the concept of immune system and its interface with cancer by leveraging on the serendipitous discovery of the record of an immigrant patient with a recurring sarcoma that spontaneously regressed following an extended postoperative surgical wound infection with Streptococcus pyogenes . It was documented that the tumour, despite only partially removed, shrank over several months and finally disappeared completely. Following discharge, he remained cancer-free in the subsequent reviews. Penned in his paper dated back in 1893 , Coley attributed the patient’s unexpected cure to the infection which likely led to stimulation of some type of immune responses. In an attempt to rationalise his hypothesis, series of experiments to deliberately infect cancer patients with S. pyogenes were carried out and resulted, at times, in failed infections or even death. Eventually, a version containing a mixture of killed S. pyogenes and Serratia marcescens was developed into what became known as the ‘Coley’s toxins’. While remarkable recoveries were documented in several cases of advanced diseases , its use eventually faded for various reasons.
Harnessing the natural capacity of immune system
However, it was not until the middle of 20th century when the term “immune-surveillance” was coined by immunologists Lewis Thomas and Macfarlane Burnet [6, 7]. Accordingly, the host immune system is deemed capable of constantly monitoring and blocking cancerous cell development by detecting mutated cells and eliminate them through various mechanisms. Nonetheless, with emerging mutations, some may incidentally evade immunosurveillance and continue to expand. Such consequent development of resistant mutant selected by the continuous pressure from the host immune system are referred to as “immune-editing” . In other words, cancer that present as clinically detectable mass are likely to have progressed beyond the initial stage of carcinogenesis that render them capable of evading the imposed immune equilibrium to achieve invasion and metastasis.
At its core, the human immune system is composed of both innate and adaptive immunity. Phagocytes and natural killer cells are effectors of innate immunity which recognise target antigens in a non-specific manner. Cytotoxic T-cells (CTLs) and antibodies, on the other hand, are effectors of cellular and humoral immunity, respectively, and function in an antigen-specific manner. Although various innate and adaptive immune cells contribute to anti-tumour immunity, current available evidences strongly suggest T-cell responses specific to tumour antigens can mediate spontaneous tumour clearance [9, 10]. To evoke T-cell activation, two signals are indispensable: one of which is the signal through T-cell receptor (TCR) induced by the complex of antigenic peptide and major histocompatibility complex while the other is through surface molecules termed stimulatory co-receptors, such as CD28, 4-1BB and OX-40.
Cancer immunotherapy: from experimental to mainstream
Over the years, intense research effort to identify immunogenic targets recognisable by the T-cells has identified several human tumour-associated antigens (TAAs) such as cancer-testis (CT) antigens. Like the predecessors of existing immunotherapies, cancer immunotherapy was in favour of the humdrum concept of reinforcing the host immune response to eliminate cancer cells and produce lasting immunity . It, therefore, seems logically sound to evoke T-cell responses against these tumour antigens through vaccination or similar mechanisms. One option for developing vaccines for infectious diseases includes using the inactivated form of pathogen to stimulate immune response. However, when a similar approach was employed to make tumour vaccines, it proved ineffective. One prominent example is the whole-cell melanoma vaccine known as Canvaxin which, despite a seemingly promising phase II studies , revealed no benefit in the subsequent phase III testing , ultimately leading to its discontinuation . When such powerful-but-blunt approach failed, it led to the realisation that targeting tumour-specific antigens that are solely expressed on the cancer is crucial. Though this ideal is rarely achieved, certain TAAs e.g. CT antigens and mutated antigens were found to be expressed only in certain cancer cells and are therefore plausible target as well . Given the propensity of viruses to efficiently induce CTL production, these antigens were delivered using selected recombinant viral vectors and adjuvants. One such example is PROSTVAC, a pox virus-based prostate cancer vaccine containing prostate-specific antigen  which demonstrated an improved overall and median survival in phase II testing in advanced stage patients  and is currently tested in an ongoing phase III trial .
Dendritic cell-based (DC-based) vaccine is another antigen-specific approach but with the advantage of bypassing the vectored delivery step. The DCs are presented to the antigen directly ex vivo and, following priming, readministered back into the patient. Notably, the only therapeutic cancer vaccine that has been licensed for clinical use so far is the DC-based sipuleucel-T (Provenge®) for used in the treatment of advanced prostate cancer . Specifically, sipuleucel-T involves autologous cell transplantation of peripheral blood monocytes primed with a fusion protein consisting of recombinant prostate acid phosphatase (another TAA expressed in prostate tumour cells), as well as granulocyte-macrophage colony-stimulating factor. On the other hand, mutated antigens, also known as neo-antigens, refer to antigens derived from tumour-specific genomic DNA mutations. Because they possess epitopes that are specific to individual tumour , hence the induction of neoantigen-specific effector T-cells may be less affected by T-cell tolerance compared with non-mutated self-antigens. Notably, a recent publication has revealed an exciting finding that complex tumours with multiple mutations was found to have an increased chance of being spotted by the immune system .
Overcoming the inherent immune resistance in established cancers
Vaccines aside, alternative approaches include genetically engineering patient’s own immune cells via tumour-infiltrating lymphocytes therapy or chimeric antigen receptor T-cells therapy to directly target cancer cells . While some cancers demonstrated satisfactory clinical response [23-27], they are relatively uncommon . In other words, it appears that these potent immune cells are switched off by some tumour defence mechanisms. Indeed, the immunosuppressive condition in the tumour microenvironment is amongst the most crucial factors that account for ineffectiveness even when the therapeutic balance has a significant stimulation-dominant side, simply because providing adequate stimulatory co-signals to exceed the heavy inhibitory conditions in the tumour microenvironment is particularly challenging without introducing significant adverse effects to the patient.
The current understanding of tumour immunology proposes that tumours could generate an immunologically-restrained milieu, typically by interfering with any one of the following steps, including the priming, recruitment, trafficking, entry and accumulation of activated T-cells through various signalling pathways that facilitate immune escape. In order for tumours to grow, the immune system is often prevented from mounting an effective antitumour response. In the tumour microenvironment, this cancer-specific milieus are formed by several cellular populations, including tumour cells, stromal cells and infiltrating immune cells. Accordingly, this response can be broadly categorised into a few phenotypes . Notably, the T-cell infiltrated phenotype has been demonstrated to confer solid tumour a positive prognostic value in colon, breast, skin (melanoma) and ovarian cancer [30-33]. Nonetheless, subsequent studies looking at advanced melanoma revealed that, notwithstanding their presence, the T-cell response is in actuality blunted albeit being largely reversible . Further research eventually led to the discovery of inhibitory receptors such as immune checkpoint molecules, which were found to be highly expressed in tumour tissues and contribute to this immunosuppressive conditions [35, 36]. As a transducer of co-inhibitory signals, these immune checkpoint molecules inherently exist to maintain immunological homeostasis to limit over-activation of the host immune systems. Hence, the concept of immune checkpoint blockade is to induce therapeutic benefit by counteracting the immunosuppression in the tumour microenvironment.
Two most representative immune checkpoint molecules, at present, are the cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) and programmed cell death-1 (PD-1) [37, 38]. Accordingly, antibodies against those molecules have been developed such as ipilimumab (anti-CTLA-4 antibody), nivolumab and pembrolizumab (anti-PD-1 antibody) which have been approved for use in a number of countries. Antibodies against programmed cell death ligand-1 (PD-L1) are also under development . So far, treatments with these antibodies have shown promising results. Durable clinical response were noted in 15–20% of the patients treated with CTLA-4 blockade while anti-PD-1 notably reported a 3-year survival rate of up to 40% in advance stage cancers [40-43]. While not all agents necessarily confer an overall survival (OS) advantage [44, 45], the long-term survival benefit is unobserved in conventional therapies for which the clinical responses, though immediate, are commonly transient. Despite longer OS has been reported in skin, lung, kidney and bladder cancer [46-48], other types of cancers unfortunately do not respond well to these therapies, probably due to insufficient numbers or repertoires of neo-antigens to evoke host immunity. Apart from CTLA-4, PD-1 and PD-L1, other immune checkpoint molecules with potential as clinical targets include lymphocyte-activation gene-3, T-cell immunoglobulin mucin-3, and B- and T-lymphocyte attenuator [49, 50]. Research and development of those molecules are actively carried out at present.
Conclusions and future directions
The progression from Coley’s toxins to the myriad of immunotherapeutic approaches under development today speaks profoundly of the contribution of immunology in the evolution of cancer therapeutics. Ultimately, whether T-cells are activated or inactivated upon TCR ligation will depend on the delicate balance between the stimulatory and inhibitory co-signals. On the other hand, multiple lines of evidence indicate that cancer stem-like cells or cancer-initiating cells (CSC⁄CIC) – which are notoriously resistant to the current standard therapies, including molecular-targeting therapy – also express several TAAs that are recognisable by CTLs both in vitro and in vivo . Given their susceptibility to CTL and high level of expression of TAAs, it is likely that CSC⁄CIC would be the next target for future cancer immunotherapy. Indeed, several trials of both pre-clinical and clinical settings have been reported, with DNAJB8-derived antigenic peptide being one example of a promising candidate for both colon and kidney CSC⁄CIC-targeting immunotherapy .
Indeed, the research on cancer therapy has come a long way in the past few decades and each step of this journey has been marked by milestones that shaped the current clinical approach . Still, the outstanding value of Coley’s immunotherapy regimen, perhaps more than any existing therapeutics, stemmed from the remarkable clinical recoveries even in advanced diseases, with such patients in remission for life. While cases of spontaneous regression have been controversial, disregarding these exceptional examples may risk losing valuable opportunities to learn about the intricacies of our immune system.
Alongside the rapid advances in sequencing technologies, the knowledge of tumour immunology has been greatly expanded at the molecular level. More than just providing an incentive for the development of novel immunotherapies, a better mechanistic understanding will allow clinician to stratify individual patients accordingly and exploit their inherent immune capacity to complement the standard-of-care treatment. The synergy of modulating various arms of immunity for potential incorporation into the existing surgery, chemotherapy, targeted therapy and radiotherapy may hold great promise as the curative combination for advanced stage malignancies and will probably form the foundation for future personalised treatment.
About the Author
Litt-Yee Hiew is currently studying towards an MSc in Molecular Medicine at the International Medical University. She finds great fulfilment from unravelling the wonders of science as well as in creative writing that could foster meaningful dialogue and bridge the gap between the scientific community and society. Find out more about Litt-Yee by visiting her Scientific Malaysian profile at http://www.scientificmalaysian.com/members/lyhiew/
This article first appeared in the Scientific Malaysian Magazine Issue 12. Check out other articles in Issue 12 by downloading the PDF version for free here: Scientific Malaysian Magazine Issue 12 (PDF version)
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