Immunotherapy and radiotherapy – How can they work together to defeat cancer?

Dr Martin Ebert PhD
Director of Physics Research
Radiation Oncology
Sir Charles Gairdner Hospital

Introduction

More than half of all cancer patients receive radiotherapy as part of their treatment and, for patients receiving radiotherapy alone, the average cure rate is 40%. Cancer cells are killed by X-ray ionisation of the DNA and other associated biological effects. But, under certain circumstances, the radiation exposure can also stimulate the immune system and enhance further elimination of the malignancy (see: Immune system and radiotherapy).

Immunotherapy is a drug-based cancer treatment that alters the interaction between cancer cells and a patient’s immune system. Immune Checkpoint Inhibitors (ICI) comprise one of the most recent immunotherapy techniques, representing a bold new hope for cancer treatment. ICIs resulted from discoveries by the 2018 Nobel Prize winners, Jim Allison from the USA and Tasuku Honjo from Japan. They showed that cancer can evolve the ability to deceive the patient’s immune system. ICIs enable that deception to be exposed.

However, research has since shown that ICIs are more effective in some cancers than others and that the overall proportion of patients who benefit from this treatment is still less than half. Current research is aiming to improve treatment outcome. We hope to achieve this through combining the immunotherapy treatment with radiotherapy.

Cancer and the Immune System

Under normal living conditions, the population at large can frequently be exposed to potential carcinogenic effects to varying degrees. Examples of cancer causing effects are ultraviolet light , alcohol, processed meat, cigarette smoke, and many others. Even when cells replicate, the production of the new DNA can potentially fail and cause cell mutations. So, why isn’t there a higher incidence of cancerous tumours formed in the body?

We now know that the body’s immune system plays a key role identifying and eliminating mutated or cancer cells. The process is referred to as immunosurveillance. The immune system can detect and destroy cancer cells in various ways. One of the major mechanisms, T-cell mediated adaptive immunity, is illustrated in Figure 1. In this process, the body is under constant surveillance by immune cells called “dendritic cells” which find foreign cells and identifying markers for them. The dendritic cells take those markers to other immune cells – “T cells” – which become “activated”, increase in number and launch an immune attack on the foreign cells, seeking them out and destroying them.

Figure 1 – T cell mediated immunity. Specialised immune cells called ‘dendritic cells’ continuously collect cellular debris. Protein fragments, known as ‘antigens’, are then transferred to the surface of the dendritic cell, and displayed there by major histocompatibility complex (MHC) molecules where they can be seen by other types of immune cell e.g. T cells. If a T cell comes into contact with that displayed antigen and recognises it (termed ‘antigen-specificity’), the T cells may then become activated, and rapidly grow in number. These can then circulate throughout the body – finding any cells bearing the recognised antigen and destroying them.

This is all fine in theory, but things can go wrong. The reason that many cancers occur is due to mutations which allow tumour cells to ‘escape’ immune detection. One of the hallmarks of cancer is its ability to evade the immune system and avoid cell death. The process of cancer cell immunity occurs via the “3 E’s of immunoediting” – elimination, equilibrium, escape – and is illustrated in Figure 2.

*Figure 2 – The 3 E’s of immunoediting – elimination, equilibrium and escape – and how a cancer can develop the ability to evade the immune system.*

Immune Checkpoint Inhibitors

Cancer cells can escape the immune system using immune checkpoints, which are part of the body’s control that prevent autoimmune reactions. Under these circumstances, immune checkpoints tell the immune system that “there’s nothing unusual detected here”, thus deceiving the immune system and allowing the cancer cells to avoid destruction.

Immunotherapy uses monoclonal antibodies, (i.e. ICIs), to block the effect of immune checkpoints. ICIs block the PD-1 and CTLA-4 pathways and have shown clinical benefits in the treatment of cancers such as melanoma, non-small cell lung cancer, renal cell carcinoma, bladder cancer and head and neck squamous cell carcinoma (Nature). However, clinical results have shown that less than half of patients adequately respond to treatment and that cancers can become resistant to further treatment.

Figure 3 shows how a tumour cell can deceive a T cell into thinking it is not bad by utilising the PD-1 pathway checkpoint, which normal cells use to prevent an auto-immune response. An ICI (in this case, “anti-PD-L1”) will block that pathway, exposing the true nature of the cancer cell, leading to an immune response and killing of the cancer cell.

There are several reasons why cancer immunotherapy with checkpoint inhibitors may not always be effective (see Frontiers). One potential reason is that the T cells may not be able to infiltrate into the tumour to reach the cancer cells. Cancer cells are supported by a tumour microenvironment (TME), which helps the cancer to survive and, potentially, to spread. The TME includes very complex and chaotic blood vessels which can leave many parts of a tumour with a poor blood supply and depleted in oxygen (i.e. hypoxic). These areas tend to be very difficult for T cells and drugs to infiltrate which likely leads to reduced efficacy or failure of immunotherapy in these regions.

Figure 3 – Immune checkpoint blockade therapy. a). Activated T cells kill the cancer cells through the interaction of surface receptors. However, this process is inhibited due the PD-1 checkpoint which tells the tumour cell not to attack, b). If a checkpoint inhibitor antibody is introduced, the checkpoint is blocked, and the T cell will attack.

How can radiotherapy improve immunotherapy?

(i) Radiotherapy can change the immune system

The dose required to kill cancer cells, grown in petri dish experiments, is higher than what is needed to kill equivalent cancer cells in a patient. This suggests that DNA damage is not the only mechanism of cancer cell kill in the body.

In 1979, Stone et al. identified an association between radiation therapy and the immune system. They showed that, after removing or killing the mouse’s immune cells, a higher dose of radiation therapy was needed to control its sarcoma. These results suggested that the immune system played a role and subsequent experimental evidence confirmed this.

Experimental evidence also now shows that radiation can act as an in-the- body ‘tumour vaccine’ which changes or modulates the TME. Radiation can kill cells in a manner that:

A) releases antigens specific to tumour cells and can make them discernible from healthy cells; and

B) releases ‘danger signal’ molecules from dead cells – alerting nearby immune cells that these antigens represent a threat and must be acted against.

This dual combination can ‘prime’ the immune system to hunt down and destroy other tumour cells that harbour those same antigens.

(ii) Radiotherapy kills cancer cells but needs oxygen for efficiency

Traditionally, radiotherapy has been applied as a cancer cell-killing agent. The importance of oxygen supply to the cancer for effective treatment has long been recognised. Cancer cells in poorly perfused tumour regions are more difficult to kill by the X-ray exposure with a “conventional” dose of around 2 Gy/fraction. A routine daily treatment protocol (usually given over three to six weeks) shrinks the tumour mass and restores blood supply making the cancer cells more susceptible to treatment.

(iii) Cancer cells and the cancer environment react together in response to radiation

The 1990’s and 2000’s saw the emergence of a complex model of tumour behaviour, including a co-dependence of cancer cells and the TME. Cancer cells thrive by modifying their environment, and radiation can influence that environment and how cancer cells interact with it.

(iv) A key to activating immunotherapy may lie in radiation-modulation of the cancer environment

It may be possible to achieve a synergistic effect between radiotherapy and immunotherapy by deliberately changing the TME to make it more susceptible to immune cell infiltration. ‘Conventional’ radiotherapy of ~2 Gy/fraction dose can normalise previously abnormal (irregular and fragile) tumour blood vessels which then leads to improved blood flow in hypoxic areas of the tumour.

Working out how to combine radiotherapy and immunotherapy

Our multi-disciplinary group in Western Australia received Cancer Australia funds to investigate how radiotherapy fractionation can be used to “prime” the tumour microenvironment. Our work is based on two simple hypotheses:

Immunotherapy may fail when immune cell infiltration into the TME is inadequate.
Low-dose radiotherapy can enhance immune cell infiltration by normalising tumour vasculature.

These hypotheses suggest that a judicious schedule of low dose-per-fraction radiotherapy will prime the environment to enable the infiltration of helpful T cells, and will set the scene for the optimal effect of an introduced checkpoint inhibitor (Figure 4). This could be followed by high-dose fractions (~ 6 Gy per treatment) to enhance cancer cell killing. Candidate drugs are available that can normalise blood vessels to some extent, although the ability to use low-cost radiotherapy, targeted with millimetre accuracy, is very attractive.

Figure 4 – Reduced vessel length and diameter and increased helper T cell infiltration following low dose radiotherapy (right) compared with an untreated group (left). Vessels are marked in green (using the “CD31” label) and T cells in red (CD3).

Our investigation is using mesothelioma to test the above hypotheses. Mesothelioma is amongst the most lethal and poorly investigated cancers, with Western Australia having the highest incidence world-wide. The National Centre for Asbestos Related Diseases (NCARD, https://ncard.org.au/) has worked extensively on treating mesothelioma, via basic laboratory studies and clinical trials. NCARD is providing facilities, expertise and mesothelioma cells for this study.

Figure 5 – Imaging being used to assess vessel and perfusion changes inside irradiated tumours. Doppler ultrasound (left) is being used to monitor the changes in vessels, whilst photoacoustic imaging (right) is being used to track changes in oxygen perfusion.

Using a series of carefully crafted experiments to yield in-depth information on the interaction of radiotherapy fractionation and immunotherapy, we are assessing:

The response of cancer cells in mouse tumours.
Methods for imaging changes in mouse tumour oxygenation and vasculature, as demonstrated in Figure 5.
Optimisation of low-dose radiotherapy to remodel the tumour microenvironment.
Quantification of immune cell infiltration into tumours following treatment.
Building a mathematical model encompassing microenvironment changes, T-cell infiltration, and tumour cell dynamics, which can then be used to inform the best dose-fractionation schedules.

Acknowledgements:

The authors acknowledge funding support from the Cancer Australia Priority-driven Collaborative Cancer Research Scheme (APP1163065) and specify that this material is the responsibility of the authors and does not reflect the views of Cancer Australia.

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23 December 2021