Radiotherapy and Immunotherapy for Cancer: From “Systemic” to “Multisite”

The discovery of the important role of the immune system in the therapeutic effect of ionizing radiation (IR) and the development of cancer immunotherapy has led to increased interest in combining the two modalities. The first clinical trials of radio-immunotherapy were initiated on the basis of encouraging case reports and preclinical results but lacked a robust framework for establishing the optimal radiotherapy dose, fractionation, target selection, and timing. Nonetheless, many clinical trials testing this combination are currently ongoing. Accumulating data will guide future trial design to optimize the therapeutic ratio of radio-immunotherapy in the treatment of metastatic cancer. The correlation of patient outcomes with peripheral blood analyses and tissue biopsies from these studies and further preclinical investigation may enable new mechanistic insights into the interaction between IR and cancer immunity that could potentially have important clinical applications. Here, we review some clinical and preclinical data that are reshaping our understanding of the interaction between radiotherapy and immunotherapy.

In 1995, Hellman and Weichselbaum proposed the oligometastatic state as an intermediate phenotype between locoregionally confined malignancy and widespread metastatic disease, largely characterized by clinical features, including a limited number of lesions and a slow pace of progression (1). The implication of this hypothesis involves the possibility of significantly benefitting and potentially curing a subset of metastatic patients with localized therapies (e.g., surgery or radiotherapy). In several recent phase II clinical trials, patients with oligometastatic disease received standard of care treatment with or without metastasis-directed ablative radiotherapy to all visible sites of disease (2–5). Although the primary site, number of lesions treated, and radiation dose and fractionation differed across these studies, they all showed an improvement in meaningful endpoints, including but not limited to progression-free survival (PFS) and overall survival (OS).

Expanding these results to the setting of radio-immunotherapy, two recently published prospective studies in metastatic non–small cell lung cancer (NSCLC) treated patients with Pembrolizumab with or without locally ablative therapy including stereotactic body radiotherapy (SBRT; refs. 6, 7). The first study, which treated only one metastatic site with SBRT, did not significantly improve PFS or OS, although there was a trend towards improved objective response rates. The second, which treated all metastatic sites as in the above oligometastatic studies, did demonstrate a significant increase in PFS of approximately 12 months as compared with historic controls. We will discuss the implications of these differing conclusions within the context of the abscopal and preclinical discussions below.

It is becoming increasingly clear that cancer immunotherapy is more effective when treating patients with limited disease burden. Analysis of the KEYNOTE-001 trial, in which patients with advanced melanoma were treated with Pembrolizumab, revealed that the baseline sum of lesion(s) size below the median (10.2 cm) was independently associated with OS (8), and most complete responses occurred in patients with tumors that were smaller yet (<5 cm; ref. 9). Translational data provide a mechanistic underpinning for these findings: in patients with stage IV melanoma treated with Pembrolizumab, it is the ratio of T-cell reinvigoration (fold change in %PD1⁺ki67⁺CD8⁺ T cells posttreatment vs. pretreatment) to baseline tumor burden that best predicts clinical outcomes, rather than either factor alone (10).

Patients treated with immunotherapy in the adjuvant setting for malignancies with a high metastatic propensity often see the greatest clinical benefits. In stage III to IV completely resected melanoma, adjuvant treatment with anti-PD1 agents markedly improved recurrence-free survival in several studies (11, 12). Similarly, patients with locally advanced NSCLC treated with definitive chemoradiation experienced significantly improved PFS and OS with the addition of adjuvant Durvalumab versus placebo in the PACIFIC trial (13).

Studies other than the PACIFIC trial in the setting of combined immunotherapy and radiotherapy support the assertion that less disease leads to better outcomes. In a trial in which patients with metastatic solid tumors were treated with radiotherapy to one metastatic site concurrently with daily injections of GM-CSF, systemic responses were observed only in patients with less than six lesions (14). In another trial of patients with metastatic prostate cancer treated with one 8 Gy fraction to a single bone lesion with or without systemic ipilimumab, patients were more likely to benefit from the addition of ipilimumab if they had one compared with two or more bone metastases (15).

The abscopal effect (from the Latin ab- for “away from” and scopus for “target”) is the regression of unirradiated tumors in a patient treated with radiotherapy and was first described in 1953 (16). In the intervening decades, the abscopal effect had largely been viewed by radiation oncologists as an interesting but exceedingly unusual phenomenon with little hope for application in the clinic. In the era of cancer immunotherapy, a new appreciation of the role played by the immune system in governing the therapeutic effect of ionizing radiation has caused a major spike in interest in this phenomenon. Underlying this enthusiasm is the hope that immunotherapy would amplify the rare systemic effects of radiotherapy, while radiotherapy would serve as an “in situ vaccine” (17), rendering immune-excluded tumors responsive to immunotherapy. Indeed, a systematic analysis of the literature on the abscopal effect (18) found 46 case reports described in 31 studies that spanned 50 years (1964–2014), with the majority published within the last decade.

It is currently controversial whether or not the addition of radiotherapy targeting one or two sites with the intent of inducing an abscopal effect significantly adds to the systemic efficacy of immunotherapy. Most available data have been presented in the form of case reports (19, 20) or single-arm, early-phase clinical trials (21, 22) testing different radioimmunotherapy combinations. In the abscopal study by Theelen and colleagues (6), the addition of single-site SBRT to pembrolizumab in metastatic NSCLC demonstrated a trend towards improved response rates but did not meet its other efficacy endpoints; however, the PD-L1-negative patient subgroup did derive significant benefit from SBRT. Similarly, in a study by Kwon and colleagues in metastatic prostate cancer (15), there was no difference in OS between those treated with single-site radiotherapy plus ipilimumab versus those who received single-site radiotherapy alone. However, patients with a small tumor burden defined as one versus two or greater bony metastases did demonstrate a benefit.

These results are contrasted with the two radioimmunotherapy trials using radiotherapy to treat all sites of known disease, for example, eschewing the abscopal approach for a cytoreductive one. The PACIFIC trial as noted above and the study by Bauml and colleagues in oligometastatic NSCLC both demonstrated 12-month improvements in their endpoints (time to distant metastasis and death in the PACIFIC and PFS over historical controls in the study by Bauml and colleagues) (7).

Taken together, the clinical observations to date strongly suggest that a potentially effective way to apply radioimmunotherapy is to treat as many sites with local therapy as possible (23), with the goal of increasing the potentially synergistic local and systemic effects of both modalities. We have previously suggested that combinations of radiotherapy and immunotherapy are likely to be most effective in patients with fewer metastases who have all sites ablated (24) but this needs to be proven.

Although the abscopal effect was defined in 1953, it remained largely limited to case reports for most of the following five decades. In 1999, Chakravarty and colleagues reported that in a metastatic murine lung cancer model, co-administration of high-dose local IR (60 Gy) in the primary tumor and daily injections of Flt3L to expand dendritic cells in vivo dramatically increased survival due to a reduction in the number of spontaneous lung metastases (25). The Weichselbaum and Schreiber labs demonstrated that intratumoral injection of an adenoviral vector with IL12 enhanced local antitumor effects of irradiation and suppressed microscopic tumor growth at a distant site (26). Demaria and colleagues expanded the early observations with IR+Flt3L to breast cancer models (27) and also pioneered the use of immune checkpoint inhibitors (specifically anti-CTLA-4) in combination with IR (28). These early studies showed that the addition of immunotherapy was critical for increasing the distant effect of IR, and that host T cells were required for this effect. However, it is important to note that the preclinical data are mixed with regard to the optimal dosing and fractionation of IR in combination with immunotherapy. A widely cited preclinical study by Dewan and colleagues in 2009 reported that fractionated IR, specifically three doses of 8 Gy each, was more efficacious in controlling distal, unirradiated tumors when combined with anti-CTLA-4 therapy than a single high dose of 20 Gy in 2-flank TSA mammary and MCA38 colon carcinoma models (29). However, this might not be applicable to all tumor types and immunotherapies combined with IR. Doses as low as 2 Gy (27) and as high as 60 Gy (25) effectively mediated abscopal responses in 67NR mammary carcinoma and LLC lung cancer models, respectively. A single 12 Gy dose, which was shown to cause the upregulation of PD-L1 in the tumor microenvironment (30) or 5 doses of 2 Gy (31), in combination with anti-PD-L1 blockade were sufficient to cause an abscopal response in different tumor models (30, 31). In recent years, attempts to potentiate the abscopal response have focused on the simultaneous combination of IR with multiple therapies. These therapies include T cell checkpoint blockade and costimulatory antibodies, for example, anti-41BB (32) or anti-CD40 (33), TGFβ blockade (34, 35), chemotherapy (36), and immunotherapeutics that modify macrophage function (37). In this context, the combination of IR with approaches that allow for localized delivery of immunotherapeutic drugs (38, 39) are of special interest, since they might promote efficacy while minimizing adverse effects of the multiple drugs used. As an example, a study by Schrand and colleagues used IR to induce intratumoral VEGF expression, which caused accumulation of VEGF-41BB aptamer conjugates in the tumor, leading to increased antitumor efficacy and decreased toxicity (39). A full discussion of optimal dosing, fractionation and timing of radiation with immunotherapy is beyond the scope of this review but can be found elsewhere (see review by Vanpouille-Box et al. 40).

Preclinical investigation led to high expectations regarding the induction of systemic antitumor effects through the combination of immunotherapy and single site-directed radiotherapy. Though some positive studies have been reported, abscopal responses have generally been limited in both number and duration. Two notable studies treated mostly 1–2 sites of disease with IR and identified a small population (10–20%) of patients who appear to benefit from this abscopal approach. In a heavily pre-treated patient population with advanced solid tumors, the combination of pembrolizumab with SBRT resulted in objective response rates of 13.2% (22), whereas in a cohort of patients with chemo-refractory and anti-CTLA4 antibody-refractory NSCLC treated with radiotherapy and CTLA4-blockade, objective response rates were 18% and there were some long-term survivors (41). These results are in contrast to the very promising outcomes seen in the preclinical murine models and there may be several reasons behind this discrepancy. One potential explanation is the limited ability of the murine models to faithfully recapitulate metastatic cancer in patients. Preclinical studies of the abscopal effect most frequently used “two-flank” mouse tumor models (27, 29, 30, 32, 33, 36) in which the same transplantable cell line is injected subcutaneously in two distant locations in the mouse, and only one tumor is irradiated; the abscopal effects are followed in the untreated lesion. In these models the unirradiated tumors are small and not “established” with minimal recruitment of vasculature and are therefore more analogous to microscopic metastases in the clinical setting. Furthermore, the genetic and environmental factors defining these “abscopal” tumors are almost identical to those of the “primary” tumor, and therefore, immune responses directed to antigens present in one of the tumors can sometimes also recognize the abscopal tumor. While some transplantable cell lines exist that give rise to spontaneously metastatic tumors (25), it is likely that even these relatively quickly established metastases fail to recapitulate the complexity of human metastatic cancer. Two recent studies using whole-exome sequencing and immunohistochemistry-derived data highlight the presence of high intermetastasis heterogeneity within the same patient (42, 43), with different metastasis demonstrating variable genomic alterations, clonal composition and immunogenic potential (43). One of these studies longitudinally collected metastases from two patients with advanced CRC and found that T cell receptor (TCR) diversity (alpha and beta V-J recombinations) was high among metastases in individual patients (43). In each patient, a single synchronous metastasis appeared capable of expanding the metastatic lineage, proving a different dissemination potential. Interestingly, low recurrence risk among individual tumor clones was associated with high immunoscore (reflective of CD8⁺ T-cell density) and low tumor burden, providing further support for the contention that immunotherapy works best in the setting of limited tumor burden. Another important observation from this study is that mutational load by itself did not determine T-cell reactivity against tumor as measured by the immunoscore (43). In another study in which regressing metastatic lesions from a patient treated with chemotherapy were compared with progressing lesions in the same patient (42), it was found that regressor versus progressor behavior was not explained by the presence of specific mutations or neoepitopes. However, regressor lesions showed higher TCR clonotype diversity and expansion, implicating the individual metastasis microenvironments as possible determinants of the generation of effective T-cell responses. One caveat should be mentioned: the three patients studied in the Jimenez-Sanchez (42) and Angelova (43) reports had an exceptionally long survival, which could potentially increase tumor heterogeneity in a way that is not generalizable to the entire spectrum of patients with metastatic disease. Nonetheless, the existence of mixed responses and general intermetastasis tumor heterogeneity could account for the low rates of abscopal responses observed in the clinic and highlight the potential benefits of ablating non immunogenic lesions.

Lessons learned from the clinical trials performed to date and preclinical investigations into the interactions among IR, the tumor microenvironment, and intratumoral T cells should guide the design of more effective radio-immunotherapy combinations. Here we propose some specific ways this could be achieved:

On the basis of the observations discussed above and other evidence, we have recently proposed that potential synergies between immunotherapy and radiotherapy require treatment of all or most sites of metastatic disease (44). This conclusion is shared by others in the field (23) and is based on recent technical advances that allow for the delivery of high doses of IR with high precision and lower toxicities. This approach is also supported by observations from an exploratory subset analysis in our recent clinical trial combining Pembrolizumab and SBRT, which demonstrated that tumors that had to be partially irradiated (due to organ-at-risk considerations) showed similar tumor control compared with those that were completely irradiated (22). These suggest that, even in cases where irradiation to all sites might not be possible, partial irradiation of all lesions could potentially suffice. Furthermore, excellent control of large, partially irradiated tumors on this study also suggest that immunotherapy itself may improve the radiotherapy-related local control. One argument against this complete ablative approach is that T cells might be radiosensitive and radiotherapy in this context might be immunosuppressive, which we discuss below.

For tumors with the “excluded” phenotype, it has been frequently proposed that irradiation attracts T cells to the tumor, turning “cold” tumors that cannot respond to immunotherapy into “hot” tumors that can be treated with immunotherapy (45). While preclinical studies have convincingly shown that irradiation can increase T-cell infiltration of tumors (46–50), there is currently a paucity of complementary clinical evidence. For murine tumors that have a detectable T-cell population at baseline (comparable to the “inflamed” phenotype), we have recently reported that many T cells present in the tumor before irradiation survive even after high doses of IR (51). In addition, we found that blockade of T-cell infiltration by the S1P1 inhibitor FTY720 did not affect the radiation response if the tumor was established before administration of the drug, but if the drug was started at the time of implantation, irradiation was ineffective. These suggest that preexisting T cells are in certain conditions sufficient to mediate the local cytotoxic effects of radiotherapy; this is in contrast to the mechanisms of some immunotherapeutics, which required newly infiltrating T cells (52). This is relevant because some investigators have proposed that intensity modulated radiotherapy, which results in a higher integral dose than 3D-conformal radiotherapy, might be immunosuppressive by depleting T cells. The high radio-resistance of T cells in tumors is not the only exception to the rule that T lymphocytes are one of the most radiosensitive cells in the organism; indeed, tissue-resident memory T cells from certain solid tissues (e.g., intraepithelial lymphocytes in the gut) are also partially radio-resistant. It is likely that solid tissue/tumor residence imparts some common characteristics to the T cells that reside within them, and the transcriptomes of intratumoral and tissue-resident T cells seem to have much in common (51). It is possible that molecular factors present both in transplantable subcutaneous murine tumors and normal skin or the epithelium of the gut are responsible for these similarities. TGFβ was tested as a possible candidate that would explain both the general similarities and the radio-resistant phenotype based on the transcriptional analysis of intratumoral T cells, which implicated TGFβ as a master regulator of their phenotype. Together with IL15, TGFβ present in tissues is required for the formation and maintenance of tissue-resident memory T cells (53). Furthermore, exposure to TGFβ has been shown to increase the radioresistance of nonmalignant and malignant cells (54–57). Accordingly, we found that treatment with anti-TGFβ antibodies resulted in higher T-cell densities in the tumors; however, these T cells were more sensitive to irradiation than intratumoral T cells in IgG-treated mice (51). Not all T cells were destroyed after IR in anti-TGFβ treated mice, suggesting that other mechanisms might contribute to an increased radio-resistance in intratumoral T cells.

Little is known about how classic modulators of tumor radio-resistance, including hypoxia and integrin-signaling, interact with the radio-resistance of T cells. Hypoxia has long been known to be an obstacle to effective radiotherapy (58), and many approaches to radiosensitize tumors by targeting hypoxia have been developed over the years (59). In addition, cellular contact with extracellular matrix proteins can increase radio-resistance by promoting DNA damage repair and activation of Akt/MAPK signaling pathways (60). It will be important to discern which of these mechanisms favor radioresistance of both cancer and T cells or are specific for one or the other. This might be very relevant for combinatorial strategies where drugs with a radio-sensitizing effect are used, such as anti-TGFβ agents. In strategies featuring TGFβ blockade, treatment has usually been initiated before radiation treatment, with the goal of radiosensitizing cancer cells (54). To what extent the possible radiosensitization of preexisting intratumoral T cells using this approach might limit the potential of combinatorial strategies involving radiation and TGFβ-targeting drugs remains to be elucidated. It will also be helpful to determine whether treatments could overcome any potentially harmful interactions. Use of strategies that radiosensitize cancer but not T cells, if these exist, would be preferable.

In summary, we propose a model in which radiotherapy and immunotherapy synergize (Fig. 1) to exert more potent local effects in the irradiated tumors rather than to elicit a systemic immune response. We apply lessons from preclinical studies into the mechanisms of interaction of IR and anti-tumor immunity, translational results highlighting heterogeneities in tumor immune microenvironments, and emerging clinical outcomes to suggest that the path forward for radio-immunotherapy is near complete ablation with multi-site therapy.

R.R. Weichselbaum has stock and other ownership interests with Boost Therapeutics Inc., Coordination Pharmaceuticals Inc., ImmunoVir LLC, Magi Therapeutics, Oncosenescence and Reflexion Pharmaceuticals. He has served in a consulting or advisory role for Aettis Inc., Astrazeneca, Coordination Pharmaceuticals, Genus, ImmunoVir LLC, Merck Serono S.A., Nano proteagen, NKMax America Inc, and Shuttle Pharmaceuticals. He has a patent pending entitled “Methods and Kits for Diagnosis and Triage of Patients With Colorectal Liver Metastases.” No royalties, he has received compensation including travel, accommodations, or expense reimbursement from Astrazeneca and Merck Serono S.A. No potential conflicts of interest were disclosed by the other authors.

The authors would like to thank Sean P. Pitroda for helpful suggestions and Amy K. Huser for editorial help. This work was supported by funds from the Ludwig Foundation for Cancer Research and Regeneron Pharmaceuticals (to R.R. Weichselbaum) and NIH R21 CA226582 (to R.R. Weichselbaum and A. Arina).