Non-canonical NF-κB Antagonizes STING Sensor-Mediated DNA Sensing in Radiotherapy.

The NF-κB pathway plays a crucial role in supporting tumor initiation, progression, and radioresistance of tumor cells. However, the role of the NF-κB pathway in radiation-induced anti-tumor host immunity remains unclear. Here we demonstrated that inhibiting the canonical NF-κB pathway dampened the therapeutic effect of ionizing radiation (IR), whereas non-canonical NF-κB deficiency promoted IR-induced anti-tumor immunity. Mechanistic studies revealed that non-canonical NF-κB signaling in dendritic cells (DCs) was activated by the STING sensor-dependent DNA-sensing pathway. By suppressing recruitment of the transcription factor RelA onto the Ifnb promoter, activation of the non-canonical NF-κB pathway resulted in decreased type I IFN expression. Administration of a specific inhibitor of the non-canonical NF-κB pathway enhanced the anti-tumor effect of IR in murine models. These findings reveal the potentially interactive roles for canonical and non-canonical NF-κB pathways in IR-induced STING-IFN production and provide an alternative strategy to improve cancer radiotherapy.

STING-Dependent Cytosolic DNA Sensing Promotes Radiation-Induced Type I Interferon-Dependent Antitumor Immunity in Immunogenic Tumors.

Ionizing radiation-mediated tumor regression depends on type I interferon (IFN) and the adaptive immune response, but several pathways control I IFN induction. Here, we demonstrate that adaptor protein STING, but not MyD88, is required for type I IFN-dependent antitumor effects of radiation. In dendritic cells (DCs), STING was required for IFN-? induction in response to irradiated-tumor cells. The cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthase (cGAS) mediated sensing of irradiated-tumor cells in DCs. Moreover, STING was essential for radiation-induced adaptive immune responses, which relied on type I IFN signaling on DCs. Exogenous IFN-? treatment rescued the cross-priming by cGAS or STING-deficient DCs. Accordingly, activation of STING by a second messenger cGAMP administration enhanced antitumor immunity induced by radiation. Thus radiation-mediated antitumor immunity in immunogenic tumors requires a functional cytosolic DNA-sensing pathway and suggests that cGAMP treatment might provide a new strategy to improve radiotherapy.

Tumour ischaemia by interferon-γ resembles physiological blood vessel regression.

The relative contribution of the effector molecules produced by T cells to tumour rejection is unclear, but interferon-γ (IFNγ) is critical in most of the analysed models. Although IFNγ can impede tumour growth by acting directly on cancer cells, it must also act on the tumour stroma for effective rejection of large, established tumours. However, which stroma cells respond to IFNγ and by which mechanism IFNγ contributes to tumour rejection through stromal targeting have remained unknown. Here we use a model of IFNγ induction and an IFNγ-GFP fusion protein in large, vascularized tumours growing in mice that express the IFNγ receptor exclusively in defined cell types. Responsiveness to IFNγ by myeloid cells and other haematopoietic cells, including T cells or fibroblasts, was not sufficient for IFNγ-induced tumour regression, whereas responsiveness of endothelial cells to IFNγ was necessary and sufficient. Intravital microscopy revealed IFNγ-induced regression of the tumour vasculature, resulting in arrest of blood flow and subsequent collapse of tumours, similar to non-haemorrhagic necrosis in ischaemia and unlike haemorrhagic necrosis induced by tumour necrosis factor. The early events of IFNγ-induced tumour ischaemia resemble non-apoptotic blood vessel regression during development, wound healing or IFNγ-mediated, pregnancy-induced remodelling of uterine arteries. A better mechanistic understanding of how solid tumours are rejected may aid the design of more effective protocols for adoptive T-cell therapy.

Enhancing T cell therapy by overcoming the immunosuppressive tumor microenvironment.

Immune response to tumors can be successfully oriented for therapeutic purposes, as shown by the clinical efficacy of checkpoint blockade in extending the survival of patients with certain solid and hematologic neoplasms. Nonetheless, numerous patients do not benefit from these new treatments. Tumor-specific CD8(+) T lymphocytes, either endogenously revived by checkpoint interference or adoptively transferred after in vitro expansion and retargeting, can be extremely efficient in controlling metastatic disease but have to overcome a number of restraints imposed by growing tumors. This immune escape relies on a profound modification of the tumor environment, which is rendered less permissive to lymphocyte arrival, persistence, and functional activity. We review here emerging findings on the main negative circuits limiting the efficacy of cancer immunotherapy, as well as novel and conventional approaches that can translate into rational combination therapies.

Tumor-associated fibroblasts predominantly come from local and not circulating precursors.

Fibroblasts are common cell types in cancer stroma and lay down collagen required for survival and growth of cancer cells. Although some cancer therapy strategies target tumor fibroblasts, their origin remains controversial. Multiple publications suggest circulating mesenchymal precursors as a source of tumor-associated fibroblasts. However, we show by three independent approaches that tumor fibroblasts derive primarily from local, sessile precursors. First, transplantable tumors developing in a mouse expressing green fluorescent reporter protein (EGFP) under control of the type I collagen (Col-I) promoter (COL-EGFP) had green stroma, whereas we could not find COL-EGFP(+) cells in tumors developing in the parabiotic partner lacking the fluorescent reporter. Lack of incorporation of COL-EGFP(+) cells from the circulation into tumors was confirmed in parabiotic pairs of COL-EGFP mice and transgenic mice developing autochthonous intestinal adenomas. Second, transplantable tumors developing in chimeric mice reconstituted with bone marrow cells from COL-EGFP mice very rarely showed stromal fibroblasts expressing EGFP. Finally, cancer cells injected under full-thickness COL-EGFP skin grafts transplanted in nonreporter mice developed into tumors containing green stromal cells. Using multicolor in vivo confocal microscopy, we found that Col-I-expressing fibroblasts constituted approximately one-third of the stromal mass and formed a continuous sheet wrapping the tumor vessels. In summary, tumors form their fibroblastic stroma predominantly from precursors present in the local tumor microenvironment, whereas the contribution of bone marrow-derived circulating precursors is rare.

Adoptively transferred immune T cells eradicate established tumors despite cancer-induced immune suppression.

Myeloid-derived CD11b(+)Gr1(+) suppressor cells (MDSCs) and tumor-associated macrophages (TAMs) are considered a major obstacle for effective adoptive T cell therapy. Myeloid cells suppress naive T cell proliferation ex vivo and can prevent the generation of T cell responses in vivo. We find, however, that adoptively transferred immune T cells eradicate well-established tumors in the presence of MDSCs and TAMs, which are strongly immunosuppressive ex vivo. These MDSCs and TAMs were comparable in numbers and immunosuppressive capacity among different tumor models. Longitudinal microscopy of tumors in vivo revealed that after T cell transfer, tumor vasculature and cancer cells disappeared simultaneously. During T cell-mediated tumor destruction, the tumor stroma contained abundant myeloid cells (mainly TAMs) that retained their suppressive properties. Preimmunized but not naive mice resisted immune suppression caused by an unrelated tumor burden, supporting the idea that in vivo, myeloid immunosuppressive cells can suppress naive but not memory T cell responses.

Epitranscriptional regulation of TGF-ß pseudoreceptor BAMBI by m6A/YTHDF2 drives extrinsic radioresistance.

Activation of TGF-ß signaling serves as an extrinsic resistance mechanism that limits the potential for radiotherapy. Bone morphogenetic protein and activin membrane-bound inhibitor (BAMBI) antagonizes TGF-ß signaling and is implicated in cancer progression. However, the molecular mechanisms of BAMBI regulation in immune cells and its impact on antitumor immunity after radiation have not been established. Here, we show that ionizing radiation (IR) specifically reduces BAMBI expression in immunosuppressive myeloid-derived suppressor cells (MDSCs) in both murine models and humans. Mechanistically, YTH N6-methyladenosine RNA-binding protein F2 (YTHDF2) directly binds and degrades Bambi transcripts in an N6-methyladenosine-dependent (m6A-dependent) manner, and this relies on NF-κB signaling. BAMBI suppresses the tumor-infiltrating capacity and suppression function of MDSCs via inhibiting TGF-ß signaling. Adeno-associated viral delivery of Bambi (AAV-Bambi) to the tumor microenvironment boosts the antitumor effects of radiotherapy and radioimmunotherapy combinations. Intriguingly, combination of AAV-Bambi and IR not only improves local tumor control, but also suppresses distant metastasis, further supporting its clinical translation potential. Our findings uncover a surprising role of BAMBI in myeloid cells, unveiling a potential therapeutic strategy for overcoming extrinsic radioresistance.

Progression of cancer from indolent to aggressive despite antigen retention and increased expression of interferon-gamma inducible genes.

Many cancers escape host immunity without losing tumor-specific rejection antigens or MHC class I expression. This study tracks the evolution of one such cancer that developed in a mouse following exposure to ultraviolet light. The primary autochthonous tumor was not highly malignant and was rejected when transplanted into naïve immunocompetent mice. Neoplastic cells isolated from the primary tumor were susceptible to the growth-inhibitory effects of IFNγ in vitro, but expressed very low levels of MHC I antigen and were resistant to tumor-specific T cells unless they were first exposed to IFNγ. Serial passage of the primary tumor cells in vivo led to a highly aggressive variant that caused fast-growing tumors in normal mice. In vitro, the variant tumor cells showed increased resistance to the growth-inhibitory effects of IFNγ but expressed high levels of immunoproteasomes and MHC I molecules and were susceptible to tumor-specific T cells even without prior exposure to IFNγ.

Reprogramming of Neutrophils as Non-canonical Antigen Presenting Cells by Radiotherapy-Radiodynamic Therapy to Facilitate Immune-Mediated Tumor Regression.

Ineffective antigen cross-presentation in the tumor microenvironment compromises the generation of antitumor immune responses. Radiotherapy-radiodynamic therapy (RT-RDT) with nanoscale metal-organic frameworks (nMOFs) induces robust adaptive immune responses despite modest activation of canonical antigen presenting dendritic cells. Here, using transplantable and autochthonous murine tumor models, we demonstrate that RT-RDT induces antitumor immune responses via early neutrophil infiltration and reprogramming. Intravenous or intratumoral injection of nMOFs recruited peripheral CD11b+Ly6G+CD11c- neutrophils into tumors. The activation of nMOFs by low-dose X-rays significantly increased the population of CD11b+Ly6G+CD11c+ hybrid neutrophils with upregulated expression of the co-stimulatory molecules CD80 and CD86 as well as major histocompatibility complex class II molecules. Thus, nMOF-enabled RT-RDT reshapes a favorable tumor microenvironment for antitumor immune responses by reprogramming tumor-infiltrating neutrophils to function as non-canonical antigen presenting cells for effective cross-presentation of tumor antigens.

CDK1 is up-regulated by temozolomide in an NF-κB dependent manner in glioblastoma.

The alkylating agent, temozolomide (TMZ), is the most commonly used chemotherapeutic for the treatment of glioblastoma (GBM). The anti-glioma effect of TMZ involves a complex response that includes G2-M cell cycle arrest and cyclin-dependent kinase 1 (CDK1) activation. While CDK1 phosphorylation is a well-described consequence of TMZ treatment, we find that TMZ also robustly induces CDK1 expression. Analysis of this pathway demonstrates that CDK1 is regulated by NF-κB via a putative κB-site in its proximal promoter. CDK1 was induced in a manner dependent on mature p50 and the atypical inhibitor κB protein, BCL-3. Treatment with TMZ induced binding of NF-κB to the κB-site as assessed by gel shift analysis and chromatin immunoprecipitation. Examination of a CDK1 promoter-reporter demonstrated the functional relevance of the κB-site and underlined the requirement of p50 and BCL-3 for activation. Targeted knockdown of CDK1 or chemical inhibition with the selective CDK1 inhibitor, RO-3306, potentiated the cytotoxic effect of TMZ. These results identify CDK1 as an NF-κB target gene regulated by p50 and BCL-3 and suggest that targeting CDK1 may be a strategy to improve the efficacy of TMZ against GBM.

Radiotherapy and immunotherapy converge on elimination of tumor-promoting erythroid progenitor cells through adaptive immunity.

Tumor-induced CD45-Ter119+CD71+ erythroid progenitor cells, termed "Ter cells," promote tumor progression by secreting artemin (ARTN), a neurotrophic peptide that activates REarranged during Transfection (RET) signaling. We demonstrate that both local tumor ionizing radiation (IR) and anti-programmed death ligand 1 (PD-L1) treatment decreased tumor-induced Ter cell abundance in the mouse spleen and ARTN secretion outside the irradiation field in an interferon- and CD8+ T cell-dependent manner. Recombinant erythropoietin promoted resistance to radiotherapy or anti-PD-L1 therapies by restoring Ter cell numbers and serum ARTN concentration. Blockade of ARTN or potential ARTN signaling partners, or depletion of Ter cells augmented the antitumor effects of both IR and anti-PD-L1 therapies in mice. Analysis of samples from patients who received radioimmunotherapy demonstrated that IR-mediated reduction of Ter cells, ARTN, and GFRα3, an ARTN signaling partner, were each associated with tumor regression. Patients with melanoma who received immunotherapy exhibited favorable outcomes associated with decreased expression of GFRα3. These findings demonstrate an out-of-field, or "abscopal," effect mediated by adaptive immunity, which is induced during local tumor irradiation. This effect, in turn, governs the therapeutic effects of radiation and immunotherapy. Therefore, our results identify multiple targets to potentially improve outcomes after radiotherapy and immunotherapy.

In vivo depletion of DC impairs the anti-tumor effect of agonistic anti-CD137 mAb.

Anti-CD137 mAb are capable of inducing tumor rejection in several syngeneic murine tumor models and are undergoing clinical trials for cancer. The anti-tumor effect involves co-stimulation of tumor-specific CD8(+) T cells. Whether antigen cross-presenting DC are required for the efficacy of anti-CD137 mAb treatment has never been examined. Here we show that the administration of anti-CD137 mAb eradicates EG7-OVA tumors by a strictly CD8beta(+) T-cell-dependent mechanism that correlates with increased CTL activity. Ex vivo analyses to determine the identity of the draining lymph node cell type responsible for tumor antigen cross-presentation revealed that CD11c(+) cells, most likely DC, are the main players in this tumor model. A minute number of tumor cells, revealed by the presence of OVA cDNA, reach tumor-draining lymph nodes. Direct antigen presentation by tumor cells themselves also participates in anti-OVA CTL induction. Using CD11c diphtheria toxin receptor-green fluorescent protein-->C57BL/6 BM chimeric mice, which allow for sustained ablation of DC with diphtheria toxin, we confirmed the involvement of DC in tumor antigen cross-presentation in CTL induction against OVA(257-264) epitope and in the antitumor efficacy induced by anti-CD137 mAb.

Radiotherapy and Immunotherapy for Cancer: From "Systemic" to "Multisite".

In the era of cancer immunotherapy, there is significant interest in combining conventional cancer therapies, such as radiotherapy, with drugs that stimulate the immune system. The observation that ionizing radiation applied to murine tumors delays the growth of distant tumors ("abscopal effect") and that this effect is potentiated by immunostimulatory drugs, led to clinical trials in which often only one lesion is irradiated in combination with immunotherapy drugs. The results of these initial clinical trials combining radio therapy and immunotherapy show that a meaningful abscopal effect is still infrequent. Recent preclinical data suggest that preexistent intratumoral T cells can survive radiation and contribute to its therapeutic effect. In this review, we discuss possible mechanisms underlying the preclinical/clinical discrepancies regarding the abscopal effect, and we propose the irradiation of multiple or all tumor sites in combination with systemic immunotherapy as a possible avenue to increase the efficacy of radio-immunotherapy.

Therapeutic antitumor efficacy of anti-CD137 agonistic monoclonal antibody in mouse models of myeloma.

PURPOSE: Eradication of post-treatment residual myeloma cells is needed to prevent relapses, and immunostimulatory monoclonal antibodies (mAb) such as anti-CD137, CTLA-4, CD40, etc., which enhance the immune response against malignancies, represent a means of achieving this purpose. This study explores anti-CD137 mAbs for multiple myeloma treatment in preclinical models of the disease because they safely augment tumor immunity and are in clinical trials for other cancers. EXPERIMENTAL DESIGN: The antitumor effect of anti-CD137 mAb on mouse plasmacytomas derived from HOPC and NS0 cell lines was studied and compared with that of anti-CTLA-4, anti-CD40, and anti-ICAM-2 mAbs. The antitumor effect of anti-CD137 mAb was also examined in a mouse syngeneic disseminated myeloma (5TGM1) model, which more closely resembles human multiple myeloma. Depletions of specific cell populations and gene-targeted mice were used to unravel the requirements for tumor rejection. RESULTS: Agonistic mAb against CD137 and blocking anti-CTLA-4 mAb showed activity against i.p. HOPC tumors, resulting in extended survival of mice that also became immune to rechallenge. Anti-CD137 mAbs induced complete eradications of established s.c. NS0-derived tumors that were dependent on IFN-gamma, natural killer cells, and CD8(+) T lymphocytes. Natural killer cells accumulated in tumor draining lymph nodes and showed increased IFN-gamma production. Antitumor efficacy of anti-CD137 mAb was preserved in CD28-deficient mice despite the fact that CD28 signaling increases the expression of CD137 on CD8(+) T cells. Importantly, anti-CD137 mAb treatment significantly decreased systemic tumor burden in the disseminated 5TGM1 model. CONCLUSIONS: The immune-mediated antitumor activity of anti-CD137 mAb in mouse models holds promise for myeloma treatment in humans.

Tumor-reprogrammed resident T cells resist radiation to control tumors.

Successful combinations of radiotherapy and immunotherapy depend on the presence of live T cells within the tumor; however, radiotherapy is believed to damage T cells. Here, based on longitudinal in vivo imaging and functional analysis, we report that a large proportion of T cells survive clinically relevant doses of radiation and show increased motility, and higher production of interferon gamma, compared with T cells from unirradiated tumors. Irradiated intratumoral T cells can mediate tumor control without newly-infiltrating T cells. Transcriptomic analysis suggests T cell reprogramming in the tumor microenvironment and similarities with tissue-resident memory T cells, which are more radio-resistant than circulating/lymphoid tissue T cells. TGFß is a key upstream regulator of T cell reprogramming and contributes to intratumoral Tcell radio-resistance. These findings have implications for the design of radio-immunotherapy trials in that local irradiation is not inherently immunosuppressive, and irradiation of multiple tumors might optimize systemic effects of radiotherapy.

STING Promotes Homeostasis via Regulation of Cell Proliferation and Chromosomal Stability.

Given the integral role of stimulator of interferon genes (STING, TMEM173) in the innate immune response, its loss or impairment in cancer is thought to primarily affect antitumor immunity. Here we demonstrate a role for STING in the maintenance of cellular homeostasis through regulation of the cell cycle. Depletion of STING in human and murine cancer cells and tumors resulted in increased proliferation compared with wild-type controls. Microarray analysis revealed genes involved in cell-cycle regulation are differentially expressed in STINGko compared with WT MEFs. STING-mediated regulation of the cell cycle converged on NFκB- and p53-driven activation of p21. The absence of STING led to premature activation of cyclin-dependent kinase 1 (CDK1), early onset to S-phase and mitosis, and increased chromosome instability, which was enhanced by ionizing radiation. These results suggest a pivotal role for STING in maintaining cellular homeostasis and response to genotoxic stress. SIGNIFICANCE: These findings provide clear mechanistic understanding of the role of STING in cell-cycle regulation, which may be exploited in cancer therapy because most normal cells express STING, while many tumor cells do not.See related commentary by Gius and Zhu, p. 1295.

Low surface expression of B7-1 (CD80) is an immunoescape mechanism of colon carcinoma.

Artificially enforced expression of CD80 (B7-1) and CD86 (B7-2) on tumor cells renders them more immunogenic by triggering the CD28 receptor on T cells. The enigma is that such B7s interact with much higher affinity with CTLA-4 (CD152), an inhibitory receptor expressed by activated T cells. We show that unmutated CD80 is spontaneously expressed at low levels by mouse colon carcinoma cell lines and other transplantable tumor cell lines of various tissue origins. Silencing of CD80 by interfering RNA led to loss of tumorigenicity of CT26 colon carcinoma in immunocompetent mice, but not in immunodeficient Rag-/- mice. CT26 tumor cells bind CTLA-4Ig, but much more faintly with a similar CD28Ig chimeric protein, thus providing an explanation for the dominant inhibitory effects on tumor immunity displayed by CD80 at that expression level. Interestingly, CD80-negative tumor cell lines such as MC38 colon carcinoma and B16 melanoma express CD80 at dim levels during in vivo growth in syngeneic mice. Therefore, low CD80 surface expression seems to give an advantage to cancer cells against the immune system. Our findings are similar with the inhibitory role described for the dim CD80 expression on immature dendritic cells, providing an explanation for the low levels of CD80 expression described in various human malignancies.

Safety and Clinical Activity of Pembrolizumab and Multisite Stereotactic Body Radiotherapy in Patients With Advanced Solid Tumors.

Purpose Stereotactic body radiotherapy (SBRT) may stimulate innate and adaptive immunity to augment immunotherapy response. Multisite SBRT is an emerging paradigm for treating metastatic disease. Anti-PD-1-treatment outcomes may be improved with lower disease burden. In this context, we conducted a phase I study to evaluate the safety of pembrolizumab with multisite SBRT in patients with metastatic solid tumors. Patients and Methods Patients progressing on standard treatment received SBRT to two to four metastases. Not all metastases were targeted, and metastases > 65 mL were partially irradiated. SBRT dosing varied by site and ranged from 30 to 50 Gy in three to five fractions with predefined dose de-escalation if excess dose-limiting toxicities were observed. Pembrolizumab was initiated within 7 days after completion of SBRT. Pre- and post-SBRT biopsy specimens were analyzed in a subset of patients to quantify interferon-γ-induced gene expression. Results A total of 79 patients were enrolled; three patients did not receive any treatment and three patients only received SBRT. Patients included in the analysis were treated with SBRT and at least one cycle of pembrolizumab. Most (94.5%) of patients received SBRT to two metastases. Median follow-up for toxicity was 5.5 months (interquartile range, 3.3 to 8.1 months). Six patients experienced dose-limiting toxicities with no radiation dose reductions. In the 68 patients with imaging follow-up, the overall objective response rate was 13.2%. Median overall survival was 9.6 months (95% CI, 6.5 months to undetermined) and median progression-free survival was 3.1 months (95% CI, 2.9 to 3.4 months). Expression of interferon-γ-associated genes from post-SBRT tumor biopsy specimens significantly correlated with nonirradiated tumor response. Conclusion Multisite SBRT followed by pembrolizumab was well tolerated with acceptable toxicity. Additional studies exploring the clinical benefit and predictive biomarkers of combined multisite SBRT and PD-1-directed immunotherapy are warranted.

Immunotherapy and immunoescape in colorectal cancer.

Immunotherapy encompasses a variety of interventions and techniques with the common goal of eliciting tumor cell destructive immune responses. Colorectal carcinoma often presents as metastatic disease that impedes curative surgery. Novel strategies such as active immunization with dendritic cells (DCs), gene transfer of cytokines into tumor cells or administration of immunostimulatory monoclonal antibodies (such as anti-CD137 or anti-CTLA-4) have been assessed in preclinical studies and are at an early clinical development stage. Importantly, there is accumulating evidence that chemotherapy and immunotherapy can be combined in the treatment of some cases with colorectal cancer, with synergistic potentiation as a result of antigens cross-presented by dendritic cells and/or elimination of competitor or suppressive T lymphocyte populations (regulatory T-cells). However, genetic and epigenetic unstable carcinoma cells frequently evolve mechanisms of immunoevasion that are the result of either loss of antigen presentation, or an active expression of immunosuppressive substances. Some of these actively immunosuppressive mechanisms are inducible by cytokines that signify the arrival of an effector immune response. For example, induction of 2, 3 indoleamine dioxygenase (IDO) by IFNgamma in colorectal carcinoma cells. Combinational and balanced strategies fostering antigen presentation, T-cell costimulation and interference with immune regulatory mechanisms will probably take the stage in translational research in the treatment of colorectal carcinoma.

Transfer of Allogeneic CD4+ T Cells Rescues CD8+ T Cells in Anti-PD-L1-Resistant Tumors Leading to Tumor Eradication.

Adoptively transferred CD8+ T cells can stabilize the size of solid tumors over long periods of time by exclusively recognizing antigen cross-presented on tumor stroma. However, these tumors eventually escape T-cell-mediated growth control. The aim of this study was to eradicate such persistent cancers. In our model, the SIYRYYGL antigen is expressed by cancer cells that lack the MHC-I molecule Kb needed for direct presentation, but the antigen is picked up and cross-presented by tumor stroma. A single injection of antigen-specific 2C CD8+ T cells caused long-term inhibition of tumor growth, but without further intervention, tumors started to progress after approximately 3 months. Escape was associated with reduced numbers of circulating 2C cells. Tumor-infiltrating 2C cells produced significantly less TNFα and expressed more of the "exhaustion" markers PD-1 and Tim-3 than T cells from lymphoid organs. High-dose local ionizing radiation, depletion of myeloid-derived suppressor cells, infusions of additional 2C cells, and antibodies blocking PD-L1 did not prevent tumor escape. In contrast, adoptive transfer of allogeneic CD4+ T cells restored the numbers of circulating Ag-specific CD8+ T cells and their intratumoral function, resulting in tumor eradication. These CD4+ T cells had no antitumor effects in the absence of CD8+ T cells and recognized the alloantigen cross-presented on tumor stroma. CD4+ T cells might also be effective in cancer patients when PD-1/PD-L1 blockade does not rescue intratumoral CD8+ T-cell function and tumors persist. Cancer Immunol Res; 5(2); 127-36. ©2017 AACR.