Cancer vaccination may be our best and most benign option for preventing or treating metastatic cancer. However, breakthroughs are hampered by immune suppression in the tumor microenvironment (TME). In this study, we analyzed whether cyclic di-guanylate (c-di-GMP), a ligand for stimulator of interferon genes (STING), could overcome immune suppression and improve vaccination against metastatic breast cancer. Mice with metastatic breast cancer (4T1 model) were therapeutically immunized with an attenuated Listeria monocytogenes (LM)-based vaccine, expressing tumor-associated antigen Mage-b (LM-Mb), followed by multiple low doses of c-di-GMP (0.01 nmol). This resulted in a striking and near elimination of all metastases. Experiments revealed that c-di-GMP targets myeloid-derived suppressor cells (MDSC) and tumor cells. Low doses of c-di-GMP significantly increased the production of IL-12 by MDSCs, in correlation with improved T-cell responses to Mage-b, while high dose of c-di-GMP (range 15–150 nmol) activated caspase-3 in the 4T1 tumor cells and killed the tumor cells directly. Based on these results we tested one administration of high dose c-di-GMP (150 nmol) followed by repeated administrations of low dose c-di-GMP (0.01 nmol) in the 4T1 model, and found equal efficacy compared to the combination of LM-Mb and c-di-GMP. This correlated with a mechanism of improved CD8 T-cell responses to tumor-associated antigens (TAA) Mage-b and Survivin, most likely through cross-presentation of these TAAs from c-di-GMP-killed 4T1 tumor cells, and through c-di-GMP-activated TAA-specific T cells. Our results demonstrate that activation of STING-dependent pathways by c-di-GMP is highly attractive for cancer immunotherapy.

Most patients with cancer do not develop durable antitumor responses after programmed cell death protein 1 (PD-1) or programmed cell death ligand 1(PD-L1) checkpoint inhibition monotherapy because of an ephemeral reversal of T cell dysfunction and failure to promote long-lasting immunological T cell memory. Activating costimulatory pathways to induce stronger T cell activation may improve the efficacy of checkpoint inhibition and lead to durable antitumor responses. We performed single-cell RNA sequencing of more than 2000 tumor-infiltrating CD8 + T cells in mice receiving both PD-1 and GITR (glucocorticoid-induced tumor necrosis factor receptor-related protein) antibodies and found that this combination synergistically enhanced the effector function of expanded CD8 + T cells by restoring the balance of key homeostatic regulators CD226 and T cell immunoreceptor with Ig and ITIM domains (TIGIT), leading to a robust survival benefit. Combination therapy decreased CD8 + T cell dysfunction and induced a highly proliferative precursor effector memory T cell phenotype in a CD226-dependent manner.PD-1 inhibition rescued CD226 activity by preventing PD-1-Src homology region 2 (SHP2) dephosphophorylation of the CD226 intracellular domain, whereas GITR agonism decreased TIGIT expression. Unmasking the molecular pathways driving durable antitumor responses will be essential to the development of rational approaches to optimizing cancer immunotherapy.

T cell activation is initiated upon binding of the T cell receptor (TCR)/CD3 complex to peptide–major histocompatibility complexes (“signal 1”); activation is enhanced by engagement of a second “costimulatory” receptor, such as the CD28 receptor on T cells binding to its cognate ligand(s) on the target cell (“signal 2”). CD3-based bispecific antibodies act by replacing conventional signal 1, linking T cells to tumor cells by binding a tumor-specific antigen (TSA) with one arm of the bispecific and bridging to TCR/CD3 with the other. Although some of these so-called TSAxCD3 bispecifics have demonstrated promising antitumor efficacy in patients with cancer, their activity remains to be optimized. Here, we introduce a class of bispecific antibodies that mimic signal 2 by bridging TSA to the costimulatory CD28 receptor on T cells. We term these TSAxCD28 bispecifics and describe two such bispecific antibodies: one specific for ovarian and the other for prostate cancer antigens. Unlike CD28 superagonists, which broadly activate T cells and resulted in profound toxicity in early clinical trials, these TSAxCD28 bispecifics show limited activity and no toxicity when used alone in genetically humanized immunocompetent mouse models or in primates. However, when combined with TSAxCD3 bispecifics, they enhance the artificial synapse between a T cell and its target cell, potentiate T cell activation, and markedly improve antitumor activity of CD3 bispecifics in a variety of xenogeneic and syngeneic tumor models. Combining this class of CD28-costimulatory bispecific antibodies with the emerging class of TSAxCD3 bispecifics may provide well-tolerated, off-the-shelf antibody therapies with robust antitumor efficacy.

A central mechanism of tumour progression and metastasis involves the generation of an immunosuppressive ‘macroenvironment' mediated in part through tumour-secreted factors. Here we demonstrate that upregulation of the Inhibitor of Differentiation 1 (Id1), in response to tumour-derived factors, such as TGFβ, is responsible for the switch from dendritic cell (DC) differentiation to myeloid-derived suppressor cell expansion during tumour progression. Genetic inactivation of Id1 largely corrects the myeloid imbalance, whereas Id1 overexpression in the absence of tumour-derived factors re-creates it. Id1 overexpression leads to systemic immunosuppression by downregulation of key molecules involved in DC differentiation and suppression of CD8 T-cell proliferation, thus promoting primary tumour growth and metastatic progression. Furthermore, advanced melanoma patients have increased plasma TGFβ levels and express higher levels of ID1 in myeloid peripheral blood cells. This study reveals a critical role for Id1 in suppressing the anti-tumour immune response during tumour progression and metastasis.

Success of cancer vaccination is strongly hampered by immune suppression in the tumor microenvironment (TME). Interleukin (IL)-6 is particularly and highly produced by triple-negative breast cancer (TNBC) cells, and has been considered as an important contributor to immune suppression in the TME. Therefore, we hypothesized that IL-6 reduction may improve efficacy of vaccination against TNBC cancer through improved T-cell responses. To prove this hypothesis, we investigated the effect of curcumin, an inhibitor of IL-6 production, on vaccination of a highly attenuated Listeria monocytogenes (Listeriaat), encoding tumor-associated antigens (TAA) Mage-b in a TNBC model 4T1. Two therapeutic vaccination strategies with Listeriaat-Mage-b and curcumin were tested. The first immunization strategy involved all Listeriaat-Mage-b vaccinations and curcumin after tumor development. As curcumin has been consumed all over the world, the second immunization strategy involved curcumin before and all therapeutic vaccinations with Listeriaat-Mage-b after tumor development. Here, we demonstrate that curcumin significantly improves therapeutic efficacy of Listeriaat-Mage-b with both immunization strategies particularly against metastases in a TNBC model (4T1). The combination therapy was slightly but significantly more effective against the metastases when curcumin was administered before compared to after tumor development. With curcumin before tumor development in the combination therapy, the production of IL-6 was significantly decreased and IL-12 increased by myeloid-derived suppressor cells (MDSC), in correlation with improved CD4 and CD8 T-cell responses in blood. Our study suggests that curcumin improves the efficacy of Listeriaat-Mage-b vaccine against metastases in TNBC model 4T1 through reversal of tumor-induced immune suppression.This study is focused on improving cancer vaccination by reducing immune suppression. Here we demonstrate that curcumin improves vaccine efficacy of Listeria-Mage-b by converting myeloid-derived suppressor cells into an immune stimulating phenotype, that is, through reducing IL-6 and increasing IL-12 production, in correlation with improved T cell responses and a dramatic reduction in the number of metastases. The novel results of this study may be a platform for improvement of other cancer vaccines by curcumin.