Lighting up the tumor fire with low-dose irradiation
Authors: Fernanda G. Herrera, Pedro Romero, George Coukos
Journal: Trends in Immunology (2022)

Low T cell infiltration of tumors is a dominant feature of advanced cancer patients who do not respond to immune checkpoint inhibitors.Numerous efforts to promote antitumor immune responses by combining high-dose tumor-targeted radiation to a few tumor deposits with immune checkpoint blockade have been attempted to induce immune-mediated systemic tumor regression. However, abscopal effects are uncommon in cancer patients, and high-dose irradiation of large tumor volumes is curtailed by potential local and systemic toxicity.In mouse and human studies, three recent publications demonstrate the ability of low-dose radiation directed at large metastatic deposits to enhance immunotherapy by reprogramming the tumor microenvironment, facilitating T cell priming via innate immune stimulation in the lymph node, increasing the frequency of tumor-specific T cells, and modulating the immune suppressive stroma in favor of tumor eradication.These findings pave the way for the clinical development of innovative and effective combinatorial radioimmunotherapies. Current efforts combining immunotherapy and radiation have focused on high-dose radiation delivered to few tumor lesions, aiming to generate diffuse abscopal effects; however, these effects are uncommon in patients. Three recent studies in mouse tumor models and human cancer patients show that low-dose radiation (LDRT) delivered to all tumor lesions effectively mobilizes innate and adaptive immunity and synergizes with immunotherapy. These new findings suggest LDRT's potential as an immune amplifier capable of reprogramming the tumor microenvironment, instigating inflammation, and sensitizing 'cold' tumors to immune checkpoint blockade responsiveness. Current efforts combining immunotherapy and radiation have focused on high-dose radiation delivered to few tumor lesions, aiming to generate diffuse abscopal effects; however, these effects are uncommon in patients. Three recent studies in mouse tumor models and human cancer patients show that low-dose radiation (LDRT) delivered to all tumor lesions effectively mobilizes innate and adaptive immunity and synergizes with immunotherapy. These new findings suggest LDRT's potential as an immune amplifier capable of reprogramming the tumor microenvironment, instigating inflammation, and sensitizing 'cold' tumors to immune checkpoint blockade responsiveness. Cancer treatment has been transformed by the introduction of immune checkpoint inhibitors (see Glossary) that target negative regulators of T cell function, such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 (PD-1) and its ligand (PD-L1). However, less than 50% of patients are eligible for immune checkpoint blockade (ICB) therapies and less than 50% of these will benefit [1.Haslam A. Prasad V. Estimation of the percentage of US patients with cancer who are eligible for and respond to checkpoint inhibitor immunotherapy drugs.JAMA Netw. Open. 2019; 2e192535Crossref PubMed Scopus (365) Google Scholar]. Low tumor mutation burden (TMB) and low T cell infiltration, for example, are associated with low responses to various ICBs [2.Ochoa de Olza M. et al.Turning up the heat on non-immunoreactive tumours: opportunities for clinical development.Lancet Oncol. 2020; 21: e419-e430Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar,3.Tumeh P.C. et al.PD-1 blockade induces responses by inhibiting adaptive immune resistance.Nature. 2014; 515: 568-571Crossref PubMed Scopus (3972) Google Scholar], while the frequency of CD8+PD-1high tumor-infiltrating lymphocytes (TILs) may predict therapy response [4.Thommen D.S. et al.A transcriptionally and functionally distinct PD-1+ CD8+ T cell pool with predictive potential in non-small-cell lung cancer treated with PD-1 blockade.Nat. Med. 2018; 24: 994-1004Crossref PubMed Scopus (435) Google Scholar]. Similarly, a subset of TILs known as precursor exhausted CD8+ T cells sustain the tumor response to ICB [5.Im S.J. et al.Defining CD8+ T cells that provide the proliferative burst after PD-1 therapy.Nature. 2016; 537: 417-421Crossref PubMed Scopus (796) Google Scholar, 6.Miller B.C. et al.Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade.Nat. Immunol. 2019; 20: 326-336Crossref PubMed Scopus (500) Google Scholar, 7.Kallies A. et al.Precursor exhausted T cells: key to successful immunotherapy?.Nat. Rev. Immunol. 2020; 20: 128-136Crossref PubMed Scopus (104) Google Scholar, 8.Siddiqui I. et al.Intratumoral Tcf1+PD-1+CD8+ T cells with stem-like properties promote tumor control in response to vaccination and checkpoint blockade immunotherapy.Immunity. 2019; 50: 195-211.e110Abstract Full Text Full Text PDF PubMed Scopus (431) Google Scholar, 9.Chu T. Zehn D. Charting the roadmap of T cell exhaustion.Immunity. 2020; 52: 724-726Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar]. Furthermore, T cell clonal expansion in peripheral blood or in tumor samples has been associated with durable responses to ICB therapy [10.Fairfax B.P. et al.Peripheral CD8+ T cell characteristics associated with durable responses to immune checkpoint blockade in patients with metastatic melanoma.Nat. Med. 2020; 26: 193-199Crossref PubMed Scopus (92) Google Scholar]. Thus, pre-existing immunity can help to predict responses to ICB; however, only a fraction of tumors is infiltrated by relevant T cells at the steady state. A key goal of cancer immunotherapy remains to overcome tumor primary resistance by promoting T cell infiltration of 'cold' tumors. To promote antitumor immune responses, to date, many efforts have focused on combinations of high-dose external-beam irradiation (EBRT) to a few tumor deposits (e.g., >5 Gy per fraction; oligoRT) and immunotherapy. Essentially, irradiation is meant to trigger an in situ tumor vaccination effect, inducing systemic immunity and the so-called abscopal effect, which refers to the immune-mediated destruction of metastatic lesions in distant non-irradiated organs [11.Herrera F.G. et al.Radiotherapy combination opportunities leveraging immunity for the next oncology practice.CA Cancer J. Clin. 2017; 67: 65-85Crossref PubMed Scopus (221) Google Scholar]. However, clinical emerging evidence in randomized studies testing oligoRT combinations with ICB suggests that such abscopal effects may be rare [12.McBride S. et al.Randomized Phase II trial of nivolumab with stereotactic body radiotherapy versus nivolumab alone in metastatic head and neck squamous cell carcinoma.J. Clin. Oncol. 2021; 39: 30-37Crossref PubMed Scopus (82) Google Scholar]. The expectation that oligoRT to one lesion would change the tumor microenvironment (TME) in distant lesions may be unrealistic, because the latter is determined by local interactions between the host and tumor cells, guided by the local genetic background and molecular states of the latter [13.Wellenstein M.D. de Visser K.E. Cancer-cell-intrinsic mechanisms shaping the tumor immune landscape.Immunity. 2018; 48: 399-416Abstract Full Text Full Text PDF PubMed Scopus (273) Google Scholar,14.Dentro S.C. et al.Characterizing genetic intra-tumor heterogeneity across 2,658 human cancer genomes.Cell. 2021; 184: 2239-2254.e2239Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar]. Although oligoRT may induce in situ vaccination, decades of cancer vaccine failures [15.Rosenberg S.A. et al.Cancer immunotherapy: moving beyond current vaccines.Nat. Med. 2004; 10: 909-915Crossref PubMed Scopus (2443) Google Scholar] demonstrate that, in addition to eliciting a systemic immune response, local reprogramming of the TME in all distant lesions is required (Figure 1, Key figure ). The inherent toxicity and immune suppression resulting from high-dose radiation directed at many tumor deposits precludes the use of oligoRT in this context. Three new studies, however, illustrate the feasibility of low-dose radiotherapy (LDRT) (e.g., doses below 3 Gy) directed at large metastatic deposits to stimulate innate and adaptive immunity, which we posit may be a promising anticancer therapeutic strategy moving forward. In the first study [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar], targeted radionuclide therapy (TRT) using 90Y-NM600 was shown to render immunologically cold syngeneic B78 melanoma tumors sensitive to ICB therapy. After low-dose TRT, a significant increase in tumor-infiltrating myeloid (CD11b+) and natural killer (NK) cells, as well as an increase in the ratio of effector CD8+ to suppressor CD4+CD25+FOXP3+ T regulatory (Treg) cells, was observed compared to controls, along with improved responses to ICB. The radionuclide 90Y-NM600 is a theranostic alkylphosphocholine radiometal chelate that binds preferentially to tumors and can be used for cancer therapy via β-particle-emitting 90Y-NM600 [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar], delivering radiation to the TME within a path length of 3.9 mm. The dose is absorbed almost entirely in the tumor, without significant exposure of the bone marrow, spleen, or draining lymph nodes (dLNs). A study looking at the biodistribution, tumor selectivity, and safety of 90Y-NM600 in several preclinical syngeneic or xenograft mouse tumor models (including melanoma and lymphoma as well as lung, pancreatic, prostate, and colon adenocarcinomas) has shown that 90Y-NM600 is safe and suitable for human applications [17.Grudzinski J.J. et al.Preclinical characterization of 86/90Y-NM600 in a variety of murine and human cancer tumor models.J. Nucl. Med. 2019; 60: 1622-1628Crossref PubMed Scopus (9) Google Scholar], with clinical studies under way. This work in B78 melanoma [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar] revealed that low 90Y-NM600 doses delivering 2.5 Gy to tumors produced important immune reprogramming of the TME. It upregulated type I interferon (IFN) signatures, which were dependent on cGMP-AMP (cGAMP) synthase (cGAS)/stimulator of interferon genes (STING), as evidenced by activation of the pathway post-irradiation and by the loss of the therapeutic activity in a STING knockout (Tmem173−/-) B16 melanoma mouse tumor; the treatment also resulted in reprogrammed immune and endothelial cells, as evidenced by high-dimensional phenotypic flow cytometry and transcriptomic analyses [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar]. Consistent with a radionuclide half-life of 2.5 days, these changes were short lived and returned to baseline by day 14. However, when combined with CTLA-4 monoclonal antibody (mAb) blockade in the same mouse model, TRT led to increased total CD8+ as well tissue-resident memory T cells and innate γδ T cells, while CD8+ T cells exhibited more clonal expansion and less phenotypic and functional exhaustion than either treatment alone, as shown via flow cytometry and T cell receptor (TCR) sequencing [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar]. The combination of TRT with ICB significantly improved the survival of treated mice compared to either treatment alone. Furthermore, the co-administration of oligoRT (12 Gy to one lesion) and 90Y-NM600 together with anti-CTLA-4 mAb blockade further enhanced the therapeutic efficacy and induced more abscopal effects compared to either treatment alone. In addition, mice that achieved long-term tumor clearance after combinatorial therapy developed protective immune memory [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar]. Hence, low-dose 90Y-NM600 and oligoRT were reported to be complementary in their ability to augment the response to ICBs. Overall, this study indicated that low-dose TRT could reprogram the TME and convert cold tumors into immunoreactive and ICB-responsive ones. These findings provide the foundations for clinical testing of the sequence of TRT in combination with immune checkpoint inhibitors to treat cold tumors. In a second study, our research group investigated the use of LDRT in the syngeneic ID8 ovarian cancer mouse model [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar] mimicking the low T cell infiltration seen in many human epithelial ovarian carcinomas [19.Zhang L. et al.Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer.N. Engl. J. Med. 2003; 348: 203-213Crossref PubMed Scopus (2482) Google Scholar] which are naturally resistant to ICB. We found that external-beam whole-abdominal irradiation at 0.5–2 Gy could inflame cold tumors by increasing the frequency of lymphocytes, monocytes, dendritic cells (DCs), and NK cells in the TME, with 1-Gy irradiation resulting in the greatest immune cell infiltration and the highest CD8+ T:Treg cell ratio. This was relevant because, to our knowledge, this was the first time that EBRT was administered at low doses to large abdominal volumes without causing toxicity, with the goal of reprogramming the TME and promoting immune cell infiltration rather than causing direct tumor cell elimination. The effects were again short-lived; T cell inflammation subsided after 1 week. However, repeat 1-Gy administration weekly resulted in ongoing immune cell recruitment into ovarian tumors. Importantly, we identified 'druggable' immune targets upregulated within tumors by LDRT on day 5 post-treatment, including genes encoding the co-inhibitory T cell receptors PD-1 and CTLA-4 expressed by exhausted TILs, the hallmark of Treg cells FOXP3, and CD40, a key stimulatory receptor expressed in myeloid antigen-presenting cells. These findings enabled the design of rational combinatorial therapy including ICB to reinvigorate effector T cells, along with CD40 ligand to activate antigen-presenting cells, and low-dose cyclophosphamide to attenuate Treg cells. A 3-week course of orthogonal radiocombinatorial immunotherapy (RACIM) comprising LDRT (1 Gy to the whole abdomen), antibodies inhibiting PD-1 and CTLA-4, an agonistic antibody targeting CD40, and low-dose cyclophosphamide resulted in an 83.5% tumor response and a 15% cure in this model, which was otherwise completely refractory to the immunotherapy combination alone. Deconvolution experiments (RACIM without one of the components) and comprehensive characterization of the immune TME revealed that all components of the cocktail were required for survival benefit and, mechanistically, coordinated innate and adaptive immunity activation was required for cancer eradication [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar]. RACIM was associated with an influx of TILs TCF1−PD-1+CD4+ and CD8+ effector T cells with high expression of IFNγ, granzyme B, and perforin. RACIM also induced a new population of conventional DCs type 2 (cDC2) (Il1r2hi) and monocyte-like DCs (MoDCs) (Clec10ahi) expressing the NK group 2D (NKG2D) ligand RAE1 [20.Zilionis R. et al.Single-cell transcriptomics of human and mouse lung cancers reveals conserved myeloid populations across individuals and species.Immunity. 2019; 50: 1317-1334.e1310Abstract Full Text Full Text PDF PubMed Scopus (390) Google Scholar]. Moreover, in RACIM-treated tumors CD4+ and CD8+ TILs with an exhausted phenotype upregulated the NKG2D co-stimulatory receptor. This was relevant as these NKG2D-expressing TILs defined a subset of canonical exhausted T cells with high effector and proliferative capacity. In support of a role of NKG2D, the therapeutic effect of RACIM was abrogated by NKG2D antibody blockade. Furthermore, inhibiting T cell migration from dLNs with fingolimod (FTY720) also counteracted RACIM's effect as evidenced by a lack of overall survival advantage, and indicated that effector cells were indeed recruited to tumors from lymph nodes [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar]. This suggested that cross-presentation of DCs (Batf3-dependent) located in dLNs was required to maintain the pool of terminally differentiated effector TCF1− cells. A companion Phase I clinical trial (NCT03728179i) translated the above findings [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar]: eight heavily pretreated, immunotherapy-naïve patients with metastatic ovarian, prostate, gallbladder, or colon carcinoma whose tumors were classified as 'immune desert' (here with a cutoff of fewer than five intraepithelial CD8+ T cells per high-power field) were treated with a similar combination of LDRT (0.5 or 1 Gy per fraction every 2 weeks, total dose 6 or 13 Gy, respectively), low-dose cyclophosphamide, ipilimumab, nivolumab, and aspirin (RACIN). The combination resulted in radiographic regression of irradiated lesions in three patients on CT or 68Ga-prostate-specific membrane antigen positron emission tomography (PSMA-PET), with an immune-Response Evaluation Criteria in Solid Tumors (iRECIST) objective response (OR) of 12.5%, while overall, seven patients achieved disease control (partial responses and stable disease) and one had disease progression. The rate of grade ≥3 immune-related adverse events was 25%, including colitis, hepatitis, and myocarditis, but no new safety signals emerged. Similar to mouse models, RACIN led to increases in CD4+, Th1, and CD8+ T cells as well as T effector-memory signatures localized to tumor islets in responsive tumors, as evidenced by GeoMx spatial transcriptional profiling. By contrast, non-responding tumors displayed anti-inflammatory (M2-like) macrophage upregulation signatures mainly in the tumor stroma, as evidenced by GeoMx immune profiling. Underscoring the key contribution of locally delivered LDRT, responder patients showed regression of all irradiated metastases, whereas subsequent progression was observed only in lesions outside the irradiated field [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar]. These findings suggested that the observed synergy between LDRT and immunotherapy was not due to a systemic abscopal effect but rather to the local effects mediated by the direct modulation of the TME; presumably, the TME was rendered permissive by LDRT to enable tumor control by immune cells, although the exact mechanisms of this modulation in patients remain to be further examined. Of note, this study documented the systemic mobilization of T cell immunity in peripheral blood (showing via TCR sequencing of blood lymphocytes significant changes in the circulating T cell repertoire), which was not sufficient in lesions unexposed to LDRT and which clearly showed tumor progression [18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar]. These two parallel studies in human and mouse have shown that LDRT can be safely delivered through large tumor volumes to reprogram multimetastatic cold tumors, triggering features of inflammation and improving responses to immunotherapy. A third study [21.Barsoumian H.B. et al.Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma.J. Immunother. Cancer. 2020; 8e000537Crossref PubMed Scopus (34) Google Scholar] examined the combination of oligoRT to a single metastatic lesion (three 12-Gy fractions) given together with LDRT (two 1-Gy fractions) to secondary metastases and ICB in a murine lung tumor (344SQ-P) model in 129Sv/Ev mice. Low-dose irradiation favored inflammatory (M1-like) macrophage polarization and downregulation of immunosuppressive transforming growth factor beta (TGFβ) in the TME of metastatic lesions, enabling impressive systemic tumor regression when combined with oligoRT and anti-CTLA-4/anti-PD-1 therapy [21.Barsoumian H.B. et al.Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma.J. Immunother. Cancer. 2020; 8e000537Crossref PubMed Scopus (34) Google Scholar]. Control of metastatic tumors was mediated by the engagement of both innate and adaptive immunity, specifically NK, CD4+, and CD8+ T cells, as evidenced by a significant increase in their frequencies using flow cytometry. In a companion Phase I clinical study (NCT02710253ii), patients with tumors refractory to anti-PD-1/PD-L1 therapy, including squamous cell carcinoma of the head and neck, nasopharyngeal carcinoma, anaplastic thyroid carcinoma, Merkel cell carcinoma, salivary gland cancer, neuroblastoma, and melanoma, were treated by a combination of oligoRT/LDRT, while anti-PD-1/PD-L1 antibody therapy was allowed to continue (following documented progression on the latter). The combination was found to be safe and feasible. Five of eight patients experienced a partial response of LDRT-treated lesions. Of note, a patient with multimetastatic oropharyngeal squamous cell carcinoma was given oligoRT (four 12.5-Gy fractions) to a lung lesion and LDRT (four 1.5-Gy fractions) to abdominal lesions while anti-PD-1 mAb therapy was continued. A remarkable response was observed whereby the patient's LDRT-treated lesions were in near-complete remission 6 months later. A similar observation was made in a patient with advanced melanoma [21.Barsoumian H.B. et al.Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma.J. Immunother. Cancer. 2020; 8e000537Crossref PubMed Scopus (34) Google Scholar]. The encouraging preclinical and clinical data obtained in this study suggest that LDRT delivered to metastatic lesions might enable the long-sought-after abscopal effect in certain cancers, and which, to date, clinicians hoped to trigger via oligoRT delivered to a single lesion in combination with ICB. These preliminary observations offer renewed hope that such effects might be successfully achieved in cancer patients. We argue that these findings support future research development of LDRT-based approaches in patients who were previously refractory to ICB. The study also highlights the effects of external-beam LDRT in reprogramming the TME, which in this model, also downregulates TGFβ. These studies [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar,18.Herrera F.G. et al.Low-dose radiotherapy reverses tumor immune desertification and resistance to immunotherapy.Cancer Discov. 2021; 12: 108-133Crossref PubMed Scopus (13) Google Scholar,21.Barsoumian H.B. et al.Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma.J. Immunother. Cancer. 2020; 8e000537Crossref PubMed Scopus (34) Google Scholar] provide important preclinical and pilot clinical data geared towards a potentially paradigm-changing approach in the irradiation of metastatic cancers. They build on previous work initially showing that LDRT-induced myeloid and T cell infiltration in murine neuroendocrine pancreatic tumors (RIP1-Tag5 model), could polarize tumor-associated macrophages towards an anti-tumoral M1-like phenotype, normalize tumor vasculature, and enable T cell homing and antitumor responses following adoptive T cell transfer [22.Klug F. et al.Low-dose irradiation programs macrophage differentiation to an iNOS+/M1 phenotype that orchestrates effective T cell immunotherapy.Cancer Cell. 2013; 24: 589-602Abstract Full Text Full Text PDF PubMed Scopus (593) Google Scholar]. We argue that low doses delivered to all detectable tumors using broad EBRT fields, or systemic TRT, promise to effectively mobilize innate and adaptive immunity against cold tumors while also targeting immune suppressive pathways and are apparently ideally suited for combination with ICB (Figure 1). Furthermore, as the first and third studies show [16.Patel R.B. et al.Low-dose targeted radionuclide therapy renders immunologically cold tumors responsive to immune checkpoint blockade.Sci. Transl. Med. 2021; eabb3631Crossref PubMed Google Scholar,21.Barsoumian H.B. et al.Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma.J. Immunother. Cancer. 2020; 8e000537Crossref PubMed Scopus (34) Google Scholar], conventional oligoRT can be combined with diffuse low-dose EBRT or systemic TRT to trigger in situ vaccination (by the former) while cold tumor lesions may be reprogrammed (by the latter), thus enabling the rare abscopal effects of oligoRT. The choice of LDRT could be dictated by the tumor type and distribution. While oligometastatic tumors of any kind can be effectively treated by low-dose EBRT, the major advantage of TRT is that it is systemic; therefore, it can theoretically deliver RT to all metastatic deposits irrespective of whether they are visible by imaging, which is a clear shortcoming of EBRT. Future research will need to evaluate such approaches to improve this response by optimizing the choice of therapeutic radionuclides. Specifically, it will be essential to test whether available alpha-emitters (e.g., radium-223 [23.Parker C. et al.Alpha emitter radium-223 and survival in metastatic prostate cancer.N. Engl. J. Med. 2013; 369: 213-223Crossref PubMed Scopus (2129) Google Scholar]) or other beta emitters [e.g., lutetium-177 (177Lu)-PSMA-617] [24.Sartor O. et al.Lutetium-177-PSMA-617 for metastatic castration-resistant prostate cancer.N. Engl. J. Med. 2021; 385: 1091-1103Crossref PubMed Scopus (170) Google Scholar] that are already in clinical use can induce the same effect when delivered at low doses in combination with ICB, as well as in combination with alternative immunotherapies. Examination of whether immune resistance mechanisms similar to those reported following high-dose oligoRT also intervene in response to low-dose TRT and ICB will also be of key importance [25.Barker H.E. et al.The tumour microenvironment after radiotherapy: mechanisms of resistance and recurrence.Nat. Rev. Cancer. 2015; 15: 409-425Crossref PubMed Scopus (973) Google Scholar, 26.Vanpouille-Box C. et al.TGFβ is a master regulator of radiation therapy-induced antitumor immunity.Cancer Res. 2015; 75: 2232-2242Crossref PubMed Scopus (327) Google Scholar, 27.Lan Y. et al.Simultaneous targeting of TGF-β/PD-L1 synergizes with radiotherapy by reprogramming the tumor microenvironment to overcome immune evasion.Cancer Cell. 2021; 39: 1388-1403.e1310Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar]. Further work is required to fully define immunologically optimal doses and schedules of radiation as well as the tolerability of repeat administrations – a required step given that the effects are transient (see Outstanding questions). Future studies will also need to identify optimal drug combinations that most benefit from the addition of LDRT. Furthermore, whether there are tumor-intrinsic properties driving immune desertification that might also drive resistance to radiation remains an open question.Outstanding questionsWhich low-dose irradiation (LDRT) technique is preferred? External-beam LDRT can be used to treat metastatic lesions that are visible on imaging. However, therapeutic radionuclides would be extremely beneficial in the treatment of microscopic systemic disease. Further research must be conducted to ascertain the most effective method of irradiation for a given clinical situation.What is the optimal dose and schedule for LDRT? The studies presented here evaluated doses ranging from 0.5 to 2.5 Gy administered once or three times at varying intervals. Additional rigorous work is necessary to fully define the immunologically optimal radiation dose and schedule as well as the tolerability of repeat administrations, which are required due to the transient nature of the effects.Which immunosuppressive mechanisms are activated in response to LDRT? High-dose oligoRT induces a wound-healing response characterized by inflammation, remodeling of the extracellular matrix, and modulation of cancer-associated fibroblasts, all of which may contribute to tumor recurrence; thus, studies should examine whether LDRT in combination with immune checkpoint blockade (ICB) can induce the same type of immune suppression that must be counterbalanced in the TME to obtain more tumor responses to immunotherapy.Which drug combinations benefit most from the addition of LDRT? A pivotal question for future research is whether LDRT-induced precursor exhausted T cells can be further manipulated with other immune checkpoint inhibitors to broaden their effector phenotype or whether new immunotherapies targeting the innate and adaptive immune systems can result in the adoption of less terminally differentiated effector states, thereby prolonging overall survival and eventually increasing curability.Do intrinsic tumor features that lead to immune desertification and resistance to ICB also contribute to resistance to LDRT and combinatorial immunotherapy? Gain-of-function mutations in the WNT/β-catenin or MYC pathways, as well as loss-of-function mutations in PTEN or deletion of the LKB1 pathway, can inhibit T cell recruitment and result in a noninflamed TME for certain tumors. Thus, the question of whether such mechanisms also impart resistance to LDRT in combination with orthogonal immunotherapy remains unanswered. Which low-dose irradiation (LDRT) technique is preferred? External-beam LDRT can be used to treat metastatic lesions that are visible on imaging. However, therapeutic radionuclides would be extremely beneficial in the treatment of microscopic systemic disease. Further research must be conducted to ascertain the most effective method of irradiation for a given clinical situation. What is the optimal dose and schedule for LDRT? The studies presented here evaluated doses ranging from 0.5 to 2.5 Gy administered once or three times at varying intervals. Additional rigorous work is necessary to fully define the immunologically optimal radiation dose and schedule as well as the tolerability of repeat administrations, which are required due to the transient nature of the effects. Which immunosuppressive mechanisms are activated in response to LDRT? High-dose oligoRT induces a wound-healing response characterized by inflammation, remodeling of the extracellular matrix, and modulation of cancer-associated fibroblasts, all of which may contribute to tumor recurrence; thus, studies should examine whether LDRT in combination with immune checkpoint blockade (ICB) can induce the same type of immune suppression that must be counterbalanced in the TME to obtain more tumor responses to immunotherapy. Which drug combinations benefit most from the addition of LDRT? A pivotal question for future research is whether LDRT-induced precursor exhausted T cells can be further manipulated with other immune checkpoint inhibitors to broaden their effector phenotype or whether new immunotherapies targeting the innate and adaptive immune systems can result in the adoption of less terminally differentiated effector states, thereby prolonging overall survival and eventually increasing curability. Do intrinsic tumor features that lead to immune desertification and resistance to ICB also contribute to resistance to LDRT and combinatorial immunotherapy? Gain-of-function mutations in the WNT/β-catenin or MYC pathways, as well as loss-of-function mutations in PTEN or deletion of the LKB1 pathway, can inhibit T cell recruitment and result in a noninflamed TME for certain tumors. Thus, the question of whether such mechanisms also impart resistance to LDRT in combination with orthogonal immunotherapy remains unanswered. This work was supported by the Ludwig Institute for Cancer Research and grants from Bristol-Myers Squibb , the Prostate Cancer Foundation Challenge Award ( 18CHAL08 ), and the Cancera, Biltema, and Paul Matson Foundations (to G.C.). No interests are declared. ihttps://clinicaltrials.gov/ct2/show/NCT03728179 iihttps://clinicaltrials.gov/ct2/show/NCT02710253 PET with 68Ga-PSMA; noninvasive diagnostic technique to image prostate cancer with increased PSMA expression. PET measures the 3D distribution of the radiopharmaceutical, allowing semiquantitative images for noninvasive assessment of PSMA expression. refers to tumor regression observed outside the field of radiation. key professional antigen-presenting cells for CD8+ T cell activation, including cross-presentation; ontogeny is dependent on the transcription factors IRF-8 and BATF3. cytosolic DNA sensor that binds to microbial DNA as well as self-DNA and catalyzes cGAMP dinucleotide synthesis. tumor classification based on the presence or absence of T cells in the TME. Cold tumors are devoid of T cells (noninflamed or immune desert). Mechanisms leading to poor T cell infiltration of tumors include impaired T cell priming and deficient T cell homing to tumor beds. Immune checkpoint inhibitors are not effective in 'cold tumors'. therapeutic strategy based on the blocking of T cell-intrinsic immune checkpoint pathways that maintain self-tolerance and are co-opted by cancer to evade immune rejection. Currently approved drugs (antibodies) block the immune checkpoint receptors CTLA-4, PD-1, and/or PD-L1. consensus guideline developed (2009) for the use of modified Response Evaluation Criteria in Solid Tumors (RECIST version 1.1) in cancer immunotherapy trials; defines when tumors in cancer patients improve ('respond'), stay the same ('stabilize'), or worsen ('progress') during treatment. IgG1 mAb that binds CTLA-4 and blocks its interaction with CD80 and CD86. It prevents negative signaling on activated T cells; first approved immunotherapy in melanoma in 2011. radioligand therapy targeting PSMA-expressing cells and the surrounding microenvironment with beta-particle radiation. human IgG4 monoclonal antibody that blocks PD-1, a negative co-receptor that reinvigorates effector CD8+ T cells; second immunotherapy approved by the FDA for the treatment of a growing number of tumor types. precise high-dose irradiation delivered to a few (one to three) tumor deposits. CD8+ T cell differentiation state arising on chronic antigen exposure, characterized by the expression of TCF1, which confers a memory-like phenotype; subset associated with effective immunity against chronic viral infection and cancer. energy deposited by ionizing radiation per unit mass, measured in Gy; 1 Gy = 1 J/kg. radium isotope; half-life, 11.4 days. Radium-223 dichloride is a radiotherapy agent that emits alpha particles and forms complexes with hydroxyapatite in areas of increased bone turnover, such as bone metastases. protein residing in the endoplasmic reticulum; activated by cGAMP produced by cGAS and by other cyclic dinucleotides of bacterial origin. STING activates type I IFN and NFκB pathways.