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Review
. 2019 Feb 15:10:193.
doi: 10.3389/fimmu.2019.00193. eCollection 2019.

Role of Radiation Therapy in Modulation of the Tumor Stroma and Microenvironment

Affiliations
Review

Role of Radiation Therapy in Modulation of the Tumor Stroma and Microenvironment

Hari Menon et al. Front Immunol. .

Abstract

In recent decades, there has been substantial growth in our understanding of the immune system and its role in tumor growth and overall survival. A central finding has been the cross-talk between tumor cells and the surrounding environment or stroma. This tumor stroma, comprised of various cells, and extracellular matrix (ECM), has been shown to aid in suppressing host immune responses against tumor cells. Through immunosuppressive cytokine secretion, metabolic alterations, and other mechanisms, the tumor stroma provides a complex network of safeguards for tumor proliferation. With recent advances in more effective, localized treatment, radiation therapy (XRT) has allowed for strategies that can effectively alter and ablate tumor stromal tissue. This includes promoting immunogenic cell death through tumor antigen release to increasing immune cell trafficking, XRT has a unique advantage against the tumoral immune evasion mechanisms that are orchestrated by stromal cells. Current studies are underway to elucidate pathways within the tumor stroma as potential targets for immunotherapy and chemoradiation. This review summarizes the effects of tumor stroma in tumor immune evasion, explains how XRT may help overcome these effects, with potential combinatorial approaches for future treatment modalities.

Keywords: cancer; immunotherapy; radiation therapy (radiotherapy); stroma; tumor microenvironment.

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Figures

Figure 1
Figure 1
Overview of tumor stromal mechanisms of immune evasion. (1) The tumor stroma disrupts normal chemokine pathways. (2) Chemokine dysregulation leads to increased M2 TAM populations. (3) M2 TAMs release VEGF, which inhibits DC maturation. (4) M2 TAMs also release chemokines and cytokines (e.g. TGF-β), which attract Tregs and MDSCs. (5) Stromal macrophages limit CD8+ T-cell infiltration and migration. (6) ICAM and VCAM downregulation lead to decreased CTL penetration. (7) CAFs and the stromal matrix inhibit CTL mobility. (8) Depletion of resources and accumulation of tumor metabolic byproducts leads to blunting of CTL functionality.
Figure 2
Figure 2
Overview of XRT's effects on the tumor stroma. (1) XRT ablates and reprograms the stroma. (2) Increased STING pathway activation leads to upregulation of type I IFNs. (3) TAMs are polarized from the M2 to the M1 phenotype. (4) Radiation increases MHC-I expression on tumor cells. (5) Tumor destruction leads to increased antigen presentation via ICD. (6) Upregulation of VCAM-1 and ICAM-1 expression leads to increased T-cell adhesion within the stroma. (7) Upregulation of expression of chemokines such as CXCL16 leads to T-cell trafficking into the TME. (8) Radiation alters stromal cell metabolism leading to increased reactive oxygen species and subsequent surrounding cell death due to changes in oxygen requirement.

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