Soluble factors secreted by glioblastoma cell lines facilitate recruitment, survival, and expansion of regulatory T cells: implications for immunotherapy

Immune-checkpoint blockade and active immunotherapy for glioma

Cancer immunotherapy has made tremendous progress, including promising results in patients with malignant gliomas. Nonetheless, the immunological microenvironment of the brain and tumors arising therein is still believed to be suboptimal for sufficient antitumor immune responses for a variety of reasons, including the operation of “immune-checkpoint” mechanisms. While these mechanisms prevent autoimmunity in physiological conditions, malignant tumors, including brain tumors, actively employ these mechanisms to evade from immunological attacks. Development of agents designed to unblock these checkpoint steps is currently one of the most active areas of cancer research. In this review, we summarize recent progresses in the field of brain tumor immunology with particular foci in the area of immune-checkpoint mechanisms and development of active immunotherapy strategies. In the last decade, a number of specific monoclonal antibodies designed to block immune-checkpoint mechanisms have been developed and show efficacy in other cancers, such as melanoma. On the other hand, active immunotherapy approaches, such as vaccines, have shown encouraging outcomes. We believe that development of effective immunotherapy approaches should ultimately integrate those checkpoint-blockade agents to enhance the efficacy of therapeutic approaches. With these agents available, it is going to be quite an exciting time in the field. The eventual success of immunotherapies for brain tumors will be dependent upon not only an in-depth understanding of immunology behind the brain and brain tumors, but also collaboration and teamwork for the development of novel trials that address multiple layers of immunological challenges in gliomas.

Multifaceted oncolytic virus therapy for glioblastoma in an immunocompetent cancer stem cell model

Glioblastoma (World Health Organization grade IV) is an aggressive adult brain tumor that is inevitably fatal despite surgery, radiation, and chemotherapy. Treatment failures are attributed to combinations of cellular heterogeneity, including a subpopulation of often-resistant cancer stem cells, aberrant vasculature, and noteworthy immune suppression. Current preclinical models and treatment strategies do not incorporate or address all these features satisfactorily. Herein, we describe a murine glioblastoma stem cell (GSC) model that recapitulates tumor heterogeneity, invasiveness, vascularity, and immunosuppressive microenvironment in syngeneic immunocompetent mice and should prove useful for a range of therapeutic studies. Using this model, we tested a genetically engineered oncolytic herpes simplex virus that is armed with an immunomodulatory cytokine, interleukin 12 (G47-mIL12). G47Δ-mIL12 infects and replicates similarly to its unarmed oncolytic herpes simplex virus counterpart in mouse 005 GSCs in vitro, whereas in vivo, it significantly enhances survival in syngeneic mice bearing intracerebral 005 tumors. Mechanistically, G47-mIL12 targets not only GSCs but also increases IFN-γ release, inhibits angiogenesis, and reduces the number of regulatory T cells in the tumor. The increased efficacy is dependent upon T cells, but not natural killer cells. Taken together, our findings demonstrate that G47Δ-mIL12 provides a multifaceted approach to targeting GSCs, tumor microenvironment, and the immune system, with resultant therapeutic benefit in a stringent glioblastoma model.

Targeting Tregs in Malignant Brain Cancer: Overcoming IDO

One of the hallmark features of glioblastoma multiforme (GBM), the most common adult primary brain tumor with a very dismal prognosis, is the accumulation of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Regulatory T cells (Tregs) segregate into two primary categories: thymus-derived natural Tregs (nTregs) that develop from the interaction between immature T cells and thymic epithelial stromal cells, and inducible Tregs (iTregs) that arise from the conversion of CD4⁺FoxP3⁻ T cells into FoxP3 expressing cells. Normally, these Treg subsets complement one another’s actions by maintaining tolerance of self-antigens, thereby suppressing autoimmunity, while also enabling effective immune responses toward non-self-antigens, thus promoting infectious protection. However, Tregs have also been shown to be associated with the promotion of pathological outcomes, including cancer. In the setting of GBM, nTregs appear to be primary players that contribute to immunotherapeutic failure, ultimately leading to tumor progression. Several attempts have been made to therapeutically target these cells with variable levels of success. The blood brain barrier-crossing chemotherapeutics, temozolomide, and cyclophosphamide (CTX), vaccination against the Treg transcriptional regulator, FoxP3, as well as mAbs against Treg-associated cell surface molecules CD25, CTLA-4, and GITR are all different therapeutic approaches under investigation. Contributing to the poor success of past approaches is the expression of indoleamine 2,3-dioxygenase 1 (IDO), a tryptophan catabolizing enzyme overexpressed in GBM, and critically involved in regulating tumor-infiltrating Treg levels. Herein, we review the current literature on Tregs in brain cancer, providing a detailed phenotype, causative mechanisms involved in their pathogenesis, and strategies that have been used to target this population, therapeutically.

Depletion of regulatory T cells in a mouse experimental glioma model through anti-CD25 treatment results in the infiltration of non-immunosuppressive myeloid cells in the brain

The recruitment and activation of regulatory T cells (Tregs) in the micro-environment of malignant brain tumors has detrimental effects on antitumoral immune responses. Hence, local elimination of Tregs within the tumor micro-environment represents a highly valuable tool from both a fundamental and clinical perspective. In the syngeneic experimental GL261 murine glioma model, Tregs were prophylactically eliminated through treatment with PC61, an anti-CD25 mAb. This resulted in specific elimination of CD4+CD25hiFoxp3+ Treg within brain-infiltrating lymphocytes and complete protection against subsequent orthotopic GL261 tumor challenge. Interestingly, PC61-treated mice also showed a pronounced infiltration of CD11b+ myeloid cells in the brain. Phenotypically, these cells could not be considered as Gr-1+ myeloid-derived suppressor cells (MDSC) but were identified as F4/80+ macrophages and granulocytes.

Gliomas promote immunosuppression through induction of B7-H1 expression in tumor-associated macrophages

PURPOSE

Gliomas are known to induce local and systemic immunosuppression, inhibiting T-cell-mediated cytotoxic responses to tumor growth. Tumor-associated macrophages are a significant component of the immune infiltrate in gliomas and may express immunosuppressive surface ligands, such as B7-H1.

EXPERIMENTAL DESIGN

Tumor and peripheral blood samples from patients with glioblastoma (GBM) were analyzed by flow cytometry to evaluate the expression of B7-H1 in circulating and tumor-infiltrating macrophages. Human monocytes from healthy patients were stimulated with conditioned media from glioma cells to evaluate B7-H1 expression. Production of interleukin (IL)-10 by stimulated monocytes was measured by ELISA, and stimulation with IL-10 alone was evaluated for the ability to induce B7-H1 expression. The effect of inhibiting IL-10 and its receptor on glioma-induced B7-H1 expression in monocytes was evaluated.

RESULTS

Circulating monocytes in patients with GBM had significantly increased expression of B7-H1 compared with healthy control patients. Tumor-associated macrophages from matched GBM tissue had even greater B7-H1 expression. Treatment of normal monocytes with glioma-conditioned media could significantly increase B7-H1 expression. Stimulation of monocytes with conditioned media resulted in substantial production of IL-10 and upregulation of the IL-10 receptor. Stimulation of monocytes with IL-10 alone could significantly increase B7-H1 expression, sufficient to induce T-cell apoptosis when cocultured with stimulated monocytes. Inhibition of IL-10 and the IL-10 receptor could knock down the effect of glioma media on B7-H1 by more than 50%.

CONCLUSIONS

Gliomas can upregulate B7-H1 expression in circulating monocytes and tumor-infiltrative macrophages through modulation of autocrine/paracrine IL-10 signaling, resulting in an immunosuppressive phenotype.

Treg infiltration in glioma: a hurdle for antiglioma immunotherapy

Tregs play a crucial role in glioma-mediated immunosuppression; hence, tackling the Treg population in patients with malignant glioma could improve the clinical success rate of antiglioma immunotherapy. Therefore, it is of high importance to elucidate the mechanisms responsible for Treg recruitment and retention within the glioma microenvironment. The current paper demonstrates that, in addition to preferential chemoattraction, glioma-derived soluble factors can also induce preferential Treg proliferation and survival. These data identify new targets for Treg modulating strategies.

IDO expression in brain tumors increases the recruitment of regulatory T cells and negatively impacts survival

PURPOSE

Glioblastoma multiforme (GBM) is an aggressive adult brain tumor with a poor prognosis. One hallmark of GBM is the accumulation of immunosuppressive and tumor-promoting CD4(+)FoxP3(+)GITR(+) regulatory T cells (Tregs). Here, we investigated the role of indoleamine 2,3 dioxygenase (IDO) in brain tumors and the impact on Treg recruitment.

EXPERIMENTAL DESIGN

To determine the clinical relevance of IDO expression in brain tumors, we first correlated patient survival to the level of IDO expression from resected glioma specimens. We also used novel orthotopic and transgenic models of glioma to study how IDO affects Tregs. The impact of tumor-derived and peripheral IDO expression on Treg recruitment, GITR expression, and long-term survival was determined.

RESULTS

Downregulated IDO expression in glioma predicted a significantly better prognosis in patients. Coincidently, both IDO-competent and deficient mice showed a survival advantage bearing IDO-deficient brain tumors, when compared with IDO-competent brain tumors. Moreover, IDO deficiency was associated with a significant decrease in brain-resident Tregs, both in orthotopic and transgenic mouse glioma models. IDO deficiency was also associated with lower GITR expression levels on Tregs. Interestingly, the long-term survival advantage conferred by IDO deficiency was lost in T-cell-deficient mice.

CONCLUSIONS

These clinical and preclinical data confirm that IDO expression increases the recruitment of immunosuppressive Tregs that lead to tumor outgrowth. In contrast, IDO deficiency decreases Treg recruitment and enhances T-cell-mediated tumor rejection. Thus, the data suggest a critical role for IDO-mediated immunosuppression in glioma and support the continued investigation of IDO-Treg interactions in the context of brain tumors.