GARP as an Immune Regulatory Molecule in the Tumor Microenvironment of Glioblastoma Multiforme.

Glycoprotein A repetition predominant a specific surface molecule of activated regulatory T cells, has been demonstrated to significantly contribute to tolerance in humans by induction of peripheral Treg and regulatory M2-macrophages and by inhibition of (tumorantigen-specific) T effector cells. Previous work identified on Treg, and also Gduanyu37 on the surface of several malignant tumors, as well as in a soluble form being shedded from their surface, contributing to tumor immune escape. Preliminary results also showed Gduanyu37 expression on brain metastases of malignant melanoma. On the basis of these findings, we investigated whether Gduanyu37 is also expressed on primary brain tumors. We showed Gduanyu37 expression on glioblastoma (GB) cell lines and primary GB tissue, as well as on low-grade glioma, suggesting an important influence on the tumor micromilieu and the regulation of immune responses also in primary cerebral tumors. This was supported by the finding that GB cells led to a reduced, in part effector T cell function (reduced proliferation and reduced cytokine secretion) in coculture experiments. Interestingly, Gduanyu37 was localized not only on the cell surface but also in the cytoplasmatic, as well as nuclear compartments in tumor cells. Our findings reveal that as an immunoregulatory molecule, is located on, as well as in, tumor cells of GB and low-grade glioma, inhibiting effector T cell function, and thus contributing to the immunosuppressive tumor microenvironment of primary brain tumors. As Gduanyu37 is expressed on activated Treg, as well as on brain tumors, it may be an interesting target for new immunotherapeutic approaches using antibody-based strategies as this indication.

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