GBM is the most common primary brain tumor in adults, and the aggressive nature of this tumor contributes to its extremely poor prognosis. Over the years, the heterogeneous and adaptive nature of GBM has been highlighted as a major contributor to the poor efficacy of many treatments including various immunotherapies. The major challenge lies in understanding and manipulating the complex interplay among the different components within the tumor microenvironment (TME). This interplay varies not only by the type of cells interacting but also by their spatial distribution with the TME. This review highlights the various immune and non-immune components of the tumor microenvironment and their consequences f the efficacy of immunotherapies. Understanding the independent and interdependent aspects of the various sub-populations encapsulated by the immune and non-immune components will allow for more targeted therapies. Meanwhile, understanding how the TME creates and responds to different environmental pressures such as hypoxia may allow for other multimodal approaches in the treatment of GBM. Ultimately, a better understanding of the GBM TME will aid in the development and advancement of more effective treatments and in improving patient outcomes.

Immunotherapy has revolutionized cancer treatment but has yet to be translated into brain tumors. Studies in melanoma and sarcoma, amongst other models, have revealed the accumulation of germinal-center-like B cells as a key survival predictor post-PD1 blockade. We seek to leverage B cell immunity to enhance immunotherapy effectiveness in glioblastoma (GBM). In human GBM and murine glioma models, we found that B cells in the tumor microenvironment (TME) are activated, but the expression of co-inhibitory molecules such as CD32 and CD72 blocks downstream effector function. Transcriptomic analysis showed high expression of inhibitory TGFβ receptors on B cells and high levels of TGFβ1 cytokine in the TME. We showed direct inhibition of B cell function through TGFβ signaling that could be prevented with TGFβ receptor blockade. Spatial multiplex immunofluorescence analysis of the TME revealed that tumor and myeloid cells express high levels of TGFβ and are also near B cells, allowing for TGFβ-mediated B cell inhibition. Blocking the TGFβ pathway via transgenic mice with TGFβ receptor knockout on B cells or TGFβ cytokine knockouts in myeloid cells, or generation of a CT2A tumor line with TGFβ cytokine knockdown, all demonstrated a survival benefit and more germinal-center-like B cells. There was also increased T cell proliferation and anti-tumor cytotoxicity. Finally, inhibiting αVβ8 integrin, a required factor that releases active TGFβ, is a translatable approach that also increased B cell proliferation and animal survival. Dual treatment with αVβ8+PD1 blockade showed the most potent survival as well as immunological memory against tumor re-challenge. Analysis of the B and T cell compartments after dual treatment showed synergy, with robust cellular proliferation and functional differentiation of plasmablasts and effector T cells. Collectively, our study highlights the importance of B cells in the TME and a remodeled approach to boost the effects of immunotherapy against GBM.

Immunotherapy has revolutionized cancer treatment but has yet to be translated into brain tumors. Studies in other solid tumors suggest a central role of B-cell immunity in driving immune-checkpoint-blockade efficacy. Using single-cell and single-nuclei transcriptomics of human glioblastoma and melanoma brain metastasis, we found that tumor-associated B-cells have high expression of checkpoint molecules, known to block B-cell-receptor downstream effector function such as plasmablast differentiation and antigen-presentation. We also identified TGFβ-1/TGFβ receptor-2 interaction as a crucial modulator of B-cell suppression. Treatment of glioblastoma patients with pembrolizumab induced expression of B-cell checkpoint molecules and TGFβ-receptor-2. Abrogation of TGFβ using different conditional knockouts expanded germinal-center-like intratumoral B-cells, enhancing immune-checkpoint-blockade efficacy. Finally, blocking αVβ8 integrin (which controls the release of active TGFβ) and PD-1 significantly increased B-cell-dependent animal survival and immunological memory. Our study highlights the importance of intratumoral B-cell immunity and a remodeled approach to boost the effects of immunotherapy against brain tumors.