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

Abstract

Glycoprotein A repetition predominant (GARP), 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 GARP on Treg, and also GARP 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 GARP expression on brain metastases of malignant melanoma. On the basis of these findings, we investigated whether GARP is also expressed on primary brain tumors. We showed GARP 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 GARP-dependent effector T cell function (reduced proliferation and reduced cytokine secretion) in coculture experiments. Interestingly, GARP 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 GARP, 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 GARP 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.

IDH-mutant gliomas harbor fewer regulatory T cells in humans and mice

“…A panel of 29 Treg-associated genes were used in our analysis, based on a set of genes previously described and clinically validated for Treg enrichment 10 and Treg-associated genes curated from literature previously documented to have an immunosuppressive role in malignant glioma . 11 – 14 Our analysis identified 19 Treg-associated genes to be significantly downregulated in IDH-mutant cases (Supplementary Table 1). Genes critical for the function and stability of Tregs ( e.g., FOXP3 and ICOS ), inducible Treg factors ( e.g., IDO1 and TGFB1 ), as well as Treg chemoattractants ( e.g., CCL2 ), were significantly downregulated in IDH-mutant cases when compared to IDH wild-type cases ( Figure 1a ).…”

“…The Treg -associated gene panel was assembled in two distinct ways: (i) Using a gene set previously described and clinically validated for Treg enrichment ; 10 and (ii) using Treg-associated genes from the literature documented to have an immunosuppresive role in malignant glioma . 11 – 14 Data were stratified into two groups based on IDH mutational status; IDH-mutant (n = 413) and IDH wild-type cases (n = 117). Log 2 expression of genes was used to compare the expression levels of 29 Treg -associated genes.…”

Platelet-Derived GARP Induces Peripheral Regulatory T Cells—Potential Impact on T Cell Suppression in Patients with Melanoma-Associated Thrombocytosis

“…GARP (glycoprotein A repetitions predominant) is encoded by leucine-rich repeat containing protein 32 gene (LRRC32) and represents a non-signaling docking receptor for latent TGF-β. The expression of GARP was initially described on the surface of platelets, but it has also been found on activated regulatory T cells (Treg) and on tumor cells, such as melanoma and glioblastoma [35][36][37]. GARP is recognized as playing an important role in the binding and release of TGF-β and hence in peripheral tolerance and progression of cancer.…”

Therapeutic Strategies for Overcoming Immunotherapy Resistance Mediated by Immunosuppressive Factors of the Glioblastoma Microenvironment

“…In the glioma microenvironment, FASL (CD95L) [ 20 ], programmed cell death ligand 1/2 (PD-L1/2) [ 21 , 22 , 23 , 24 , 25 ], galectin-1 [ 26 ], galectin-9 [ 27 ], HVEM [ 28 ], B7-H4 (B7x) [ 29 ], and CD70/gangliosides [ 30 ] expressed on glioma cells act as inhibitory ICMs. Furthermore, AHR and CD39 expressed on TAMs [ 31 ], podoplanin (PDPN) expressed on MDSCs [ 32 ], and glycoprotein A repetitions predominantly (GARP) expressed on Tregs [ 33 ] also cause immunosuppression. As for humoral factors, VEGF [ 34 ], macrophage migration inhibitory factor (MIF) [ 35 ], IL-6 [ 29 ], IL-4/13 [ 36 , 37 ], IL-10, TGFβ [ 38 ], POSTN [ 39 ], colony-stimulating factor-1 (CSF-1) [ 40 ], CCL2 [ 41 ], CXCL12 [ 42 ], and COX-2/PGE2 [ 43 ] directly or indirectly regulate the immunosuppressive tumor microenvironment.…”