Transl Oncol: Radiation therapy – light and darkness in the age of immunotherapy

The introduction of immunotherapy into cancer treatment has fundamentally changed the clinical management
of tumors.
However, only a small number of patients (about 10% to 30%) showed a long-term response
to immunotherapy monotherapy.
In addition, there are many cancer types, including pancreatic cancer and glioma, that are resistant to immunotherapy
.
Due to the immunomodulatory effect of radiotherapy, the combination of radiotherapy and immunotherapy has achieved good therapeutic effects
in some clinical trials.

However, radiation therapy is a double-edged sword, and at a certain dose and fractionation, radiation therapy can also weaken the immune system, and not all clinical trials have shown that a combination of radiation therapy and immunotherapy can improve survival
.
Therefore, elucidating the interaction between radiation therapy and the immune system helps to optimize the synergistic effect
of radiation therapy and immunotherapy.

Radiation therapy can be used alone or in combination with
other treatments such as surgery, chemotherapy and immunotherapy.
On the one hand, radiotherapy enhances the immunogenicity of tumor cells and improves anti-tumor immunity
.
On the other hand, in some cases, radiation therapy may enhance the patient's local and systemic immunosuppression
.

Radiation therapy induces local and systemic antitumor immunity

Ionizing radiation promotes the release of double-stranded DNA (dsDNA) in the nucleus, increases the permeability of the outer mitochondrial membrane, and triggers exposure
to mitochondrial DNA (mtDNA) in the cytoplasm.
Both dsDNA and mtDNA are efficient mediators for initiating the cGAS-STING pathway and subsequent transcription of type I interferons
.

Type I interferon signaling is critical in activating DCs, thereby promoting T cell activation and tumor control
.
The accumulation of dsDNA in tumor-derived exosomes after radiotherapy also promotes DC recruitment and directly induces DC's type I IFN response, which further promotes the recruitment of CD8+ T cells and provides a third signal
for T cell activation.

To escape the attack of T cells, MHC class I molecules are either missing or low expressed
in many cancer cells.
Radiation therapy upregulated MHC class I molecules on the surface of tumor cells and enhanced TAA production, thereby expanding the antigen repertoire
available for presentation.
In addition, calreticulin exposure on the cell surface, secretion of HMGB1 and ATP enhance immunogenicity and promote immune cell infiltration, thereby promoting an immune response
in the tumor microenvironment.

Radiation therapy also increases the abundance of tumor infiltrating immunostimulating cells and stimulates tumor cells and stromal cells to release pro-inflammatory mediators
.
The release of chemokines, such as CXCL9, CXCL10, CXCL11, and CXCL16, leads to infiltration of DC, macrophages, and T cells
.

In conclusion, by altering tumor cell phenotypes, triggering the signature release of immunostimulatory DAMP, and increasing the number of pro-inflammatory immune cells, radiotherapy enhanced the sensitivity of tumor cells to T cell-mediated anti-tumor effects and facilitated the recognition and removal
of cancer cells.

Radiation therapy triggers immunosuppression and lymphopenia

Radiation therapy leads to the accumulation of dsDNA in cancer cells, thereby activating cGAS/STING signaling and promoting the transcription
of the type I IFN gene.
However, interferon signaling can also have harmful effects, leading to treatment resistance
.
Repeated irradiation of tumor cells induces chronic type I interferon and interferon-stimulated gene expression, thereby mediating radiation resistance and metastatic propagation
through multiple inhibitory pathways.

Both IFN-γ and type I IFN are responsible for upregulating the expression of PD-L1 on tumor cells, thereby further inducing T cell depletion and resisting anti-tumor immunity
.
IDOs are also upregulated by type I IFN and IFN-γ and function
as immunosuppressive factors.
In addition, activated STING signals enhance the mobilization of Tregs, MDSCs, and eliminate tumor immunogenicity
.
Local radiotherapy upregulates the secretion of CCL2 and CCL5, which is associated with
the recruitment of Tregs and monocytes.
Recruited monocytes activate Tregs in a TNF-α-dependent manner, thereby reducing the efficacy
of radiation therapy.
In addition, by secreting IL-10 and TGF-β, Tregs enhance the immunosuppressive effect of MDSCs and inhibit the function of
effector T cells.

Lymphopenia is one of the most common side effects of
radiation therapy.
Because the bone marrow is extremely sensitive to radiation, severe bone marrow damage
can occur during radiation therapy.
Even relatively low radiation doses can cause temporary functional ablation
of the bone marrow.
When the bone marrow is exposed to moderate doses of radiation, it takes several years to restore active hematopoietic function
.
Higher radiation doses can cause irreversible damage
.
Circulating peripheral blood mononuclear cells are also highly sensitive
to ionizing radiation.
Repeat routine fractionation radiation therapy daily for cytotoxicity sufficient to deplete migrating immune effector cells
.

Another mechanism by which radiation therapy leads to lymphopenia is through irradiation
of lymphoid organs.
Naïve T cells are extremely sensitive to radiation, and even low doses of lymphoid tissue can lead to p53-mediated rapid apoptosis
.
In addition, radiotherapy-induced secretion of Gal-1 by tumor cells exhibited T cell apoptotic activity and was associated
with lymphopenia and poor survival outcomes.

Over the past decade, several immunotherapy drugs
have been approved for clinical treatment of cancer.
Many of these immunotherapy drugs have been tested
in combination with radioimmunotherapy.

Synergistic effect of radiotherapy and immunotherapy

Due to the significant immunostimulating effect of radiotherapy, it provides a theoretical basis
for combining immunotherapy with different forms of radiotherapy.

Preclinical studies of HNSCC have shown that radiotherapy sensitizes unresponsive HNSCC tumors to PD-L1 inhibition
by transforming the tumor microenvironment into an inflammatory environment.
Radiotherapy and anti-PD-L1 antibodies synergistically reduce the accumulation of tumor-invasive MDSCs, release T cells from the suppressive tumor immune microenvironment, and enhance anti-tumor effects
.
Radioimmunotherapy also induces a strong CD8+ T cell-mediated antitumor response
outside the illuminated area.
With the combination of immunotherapy and radiation therapy, patients can achieve local control and regression
of metastatic tumors.

In addition, when fully responsive mice are attacked by reimplanted tumor cells, the combination of radiotherapy and anti-PD-L1 drugs induces immune memory effects
.
Since recurrence and distant metastasis are the leading causes of cancer-related death, generating a memory immune response can effectively prevent tumor recurrence and metastasis, thereby prolonging patient survival
.

Clinical studies have shown that radioimmunotherapy has an encouraging benefit
on patient survival.
Patients with advanced non-small cell lung cancer who were given Nivolumab after radiotherapy had a median PFS (4.
4 versus 2.
1 months, p=0.
019) and median OS (10.
7 versus 5.
3 months, p=0.
026) significantly longer
than those who did not receive radiotherapy.

In conclusion, radioimmunotherapy reshapes the suppressive tumor immune microenvironment, promotes the generation of T cell responses, and identifies and clears cancer cells
at irradiated tumor sites and distant metastases.

In some cases, radiotherapy does not enhance the combined effect with immunotherapy

Immunotherapy eliminates cancer cells by stimulating an anti-tumor immune response, which relies primarily on activating CD8+ T cells or utilizing immune cells
.
Radiation therapy may effectively reduce the efficacy
of immunotherapy by upregulating the infiltration of suppressive immune cells and directly damaging circulating lymphocytes, including CD8+ T cells.
Therefore, radiotherapy and immunotherapy do not always show synergistic effects
.

Although anti-PD-1 antibody combined with 8Gy×2 irradiation enhanced the control of primary and metastatic tumors in a CD8+ T cell-dependent manner and reversed adaptive immune resistance, anti-PD-1 antibodies after 2Gy×10 irradiation inhibited the expression of IFN in tumor-specific CD8+ T cells within DLN, resulting in a lack of efficacy
.
This may be due to lymphopenia and immunosuppression
caused by low-dose daily fractionated radiation therapy.
Selective nodal radiotherapy is often used in the treatment of local tumors to address potential subclinical lymph node micrometastases
.
However, when combined with anti-CTLA-4 antibodies, selective lymph node irradiation contributes to immunosuppression of the tumor microenvironment, and survival time is not prolonged
compared to anti-CTLA-4 antibody monotherapy.
In addition, although radiation therapy combined with anti-CTLA-4 can lead to tumor regression, PD-L1 expression is upregulated at the same time, leading to T cell depletion and drug
resistance.

Consistent with preclinical studies, there are several clinical trials that have shown no combined effect
between radiation therapy and immunotherapy.
A phase 3 trial involving prostate cancer showed no significant difference
in OS between 8Gy radiotherapy plus ipilimumab and radiotherapy alone in patients who progressed after docetaxel treatment.
Preclinical evidence suggests that hypofractionated ionizing radiation in combination with ICIs significantly enhances tumor control
.
However, clinical trials aimed at evaluating the synergistic effects of nivolumab and SBRT (9Gy×3) in HNSCC failed to improve ORR
.
In addition, as the first FDA-approved autologous cell immunotherapy, sipuleucel-T improved median survival by 4.
1 months
compared to placebo.
However, after 300 cGy × 10 doses of radiation therapy in patients with metastatic castration-resistant prostate cancer, the median PFS in the sipuleucel-T group was comparable
to that in the sipuleucel-T alone group.
In addition, cumulative antigen-presenting cells and IFN-γ+ T cells were even higher
in the Sipuleucel-T group alone.

In conclusion, radiation therapy may in some cases weaken the efficacy
of immunotherapy.
The synergy between immunotherapy and radiation therapy is related
to a variety of factors.
When combining radiotherapy with immunotherapy, the immunomodulatory effect
of radiotherapy should be fully considered.
Additional investigations are required to determine the optimal radiotherapy regimen, target checkpoints, and patient cohorts to fully assess the efficacy
of radioimmunotherapy.

In order to optimize the anti-tumor effect of radioimmunotherapy, several aspects should be considered, including exploring appropriate radiation treatment regimens, considering the schedule when immunotherapy is combined with radiation therapy, reducing the immunosuppressive effects of radiation therapy, and the application of
some new radiation therapy technologies.

Choose the right radiation treatment plan

To optimize the effectiveness of radioimmunotherapy, radiotherapy regimens should be designed to include two factors: dose and segmentation
.
Clinicians empirically apply conventional radiation therapy to achieve local tumor control and tolerability toxicity
.
However, even tolerable lymphopenia can diminish the efficacy
of combining radiation therapy with immunotherapy.
The development of conventional radiation therapy does not take into account the potential therapeutic effects of the immune system, which often adversely affects the immune system to the detriment of immunotherapy
.
The combination of hypofractionated radiation with ICIs enhances anti-tumor immunity and thus significantly controls tumors
.
In addition, hypofractionated radiotherapy also has superior efficacy
compared with a single high-dose radiotherapy.

Studies have reported improved
efficacy of hypofractionated radiotherapy combined with immunotherapy.
For example, anti-PD-1 antibodies combined with 8Gy×2 instead of 2Gy×10 enhance the control
of primary and metastatic tumors.
The efficacy of a single dose of 20 Gy topical radiation therapy was comparable
to 8 Gy×3 and 6 Gy×5.
However, when combined with anti-CTLA-4 antibodies, fractionated radiotherapy alone significantly improves tumor growth
at primary and secondary tumor sites.

Radiation therapy regimens have a specific treatment window
when used in combination with ICIs.
The final effect of radioimmunotherapy depends on the combined effect
of complex effects on the immune system.
A combination of hypofractionated radiotherapy and immunotherapy is more appropriate
.
However, clinical trials of radiotherapy combined with immunotherapy are often based on classical protocols, and the optimal radiotherapy regimen for combination immunotherapy remains to be explored
.

Explore the timeline for combining immunotherapy with radiotherapy

A significant effect
of immunotherapy dosing regimens has been reported on tumor suppression in radiation therapy.
Simultaneous administration of radiation therapy and anti-PD-L1 therapy, rather than sequential therapy, is associated with
long-term tumor control.
However, a phase 1 clinical trial of KEYNOTE-001 showed that patients who received any form of radiation prior to pembrolizumab showed prolonged PFS
.
In addition, in patients with metastatic HNSCC, concomitant reception of SBRT by nivolumab did not improve ORR
compared with nivolumab monotherapy.

Therefore, the optimal sequence may be related to a variety of factors, such as the type of immunotherapy, radiation regimen, tumor characteristics, and individual differences
.
All of these factors should be taken into account for a better prognosis
.

Narrowing the scope of radiation therapy

Complete cancer immune circulation is essential in immunotherapy, and radioimmunotherapy is important
to protect lymphocytes during radiation therapy.
To reduce the negative effects of radiotherapy on immune cells, irradiation
to the lymph nodes, bone marrow and blood must be avoided.

Over the past two decades, IMRT and other techniques have made it possible to apply conformal doses to radiation therapy, thereby reducing radiation doses to non-malignant tissues while ensuring a tumicicidal dose
for local tumors.
Conventional radiotherapy regimens for various indications, such as NSCLC, cervical cancer, and HNSCC, usually formulate a full dose of 50-70 Gy for the tumor site and a dose of 45-50 Gy for DLN to achieve prevention coverage
.
Since DLN is the main platform for T cell cross-activation, T cells in lymphoid tissue are extremely sensitive to radiotherapy, and irradiating tumor-free lymph nodes may interfere with the initiation
of anti-tumor immune responses.
Therefore, cervical node-negative patients do not need prophylactic lymph node irradiation
.
The protection of
lymph nodes should be given great attention.
In addition, T cells circulating in the blood and bone marrow are extremely sensitive
to radiation.
During radiation therapy, it is necessary to reduce the exposure
of blood vessels and bone marrow.

Application of new technologies for radiotherapy

The development of new technologies is also a great option
to minimize the side effects of radiotherapy on the immune system.
Proton radiation therapy reduces the dose delivered to non-malignant tissues and induces upregulation
of histocompatibility antigens and TAAs.
In addition, proton radiotherapy increased the exposure of calreticulin on the surface of tumor cells, thereby increasing the killing effect
of CTL on tumor cells.
Combining immunotherapy with proton radiation therapy is promising
.

Heavy ion radiation therapy (HIRT) is becoming a cutting-edge technology
for the treatment of malignant tumors.
HIRT can alter the energy and superimpose multiple Bragg peaks, providing a more precise treatment
.
Compared to proton radiation therapy and photon-based IMRT, carbon-ion radiation therapy is associated
with higher OS.
During treatment, HIRT will significantly reduce lymphocyte damage
.
Thus, HIRT enhances the role
of the immune system in radioimmunotherapy.

Compared to the usual dose rate of about 5 Gy/min for clinical use, FLASH radiotherapy can deliver doses
at ultra-high dose rates (>40 Gy/s).
Thus, it can reduce normal tissue toxicity while maintaining local tumor control
.
By providing higher dose rates, FLASH radiotherapy shortens treatment time
.
The short exposure time significantly reduces radiation
from circulating immune cells.

The efficacy of immunotherapy is highly dependent on the immune system
.
The combination of immunotherapy with other therapies should take full account of immunomodulatory effects
.
Radiotherapy is an ideal companion
due to its stimulating effect on the immune system.
However, radiation therapy has far-reaching immunomodulatory effects, and not all radiation regimens work synergistically with immunotherapy
.
The underlying immune mechanisms of liberation therapy are essential
to improve the synergistic effect of radiotherapy and immunotherapy.

Available evidence suggests that simultaneous hypofractionated radiotherapy and immunotherapy are more likely to improve outcomes
.
Reducing the amount of exposure also shows the potential to
improve patient outcomes.
In addition, technological breakthroughs in radiotherapy technology offer an important opportunity
to create the maximum efficacy of radioimmunotherapy.

References:

1.
Radiotherapy: Brightness and darkness in the era of immunotherapy.
Transl Oncol.
2022 May; 19:101366.