Study on the interaction between TGF-β1 protein and GARP and its role in tumor immunity

TGF-β1 protein is a pleiotropic cytokine that plays important roles in various biological processes such as cell proliferation, development, apoptosis, fibrosis, angiogenesis, wound healing, and cancer. This protein is synthesized in a latent form and requires proteolytic cleavage and activation to exert its biological functions. Abnormal expression of TGF-β1 is associated with multiple diseases, including fibrotic disorders, autoimmune diseases, and malignant tumors. Due to its broad and complex biological functions, TGF-β1 is considered a dual-role cytokine that can both suppress tumorigenesis and promote tumor progression.

GARP is a type I transmembrane docking protein encoded by the Lrrc32 gene. It is located in a conserved chromosomal region between human and mouse genomes, and the Lrrc32 gene locus is frequently altered in human cancers. Structurally, GARP consists of three domains: an extracellular domain, a transmembrane domain, and an intracellular domain. The extracellular domain contains two sets of leucine-rich repeats (LRRs) separated by a proline-rich domain and a C-terminal LRR. Two conserved cysteine residues located on the 7th and 12th LRRs form disulfide bonds with the latency-associated peptide (LAP) of latent TGF-β1. In human tissues, GARP is expressed in peripheral blood, placenta, and pancreas, with particularly high expression in activated regulatory T cells, activated B cells, and platelets.

GARP is a key regulator of TGF-β1 expression and activation. By binding to latent TGF-β1, GARP serves as a docking receptor that concentrates latent TGF-β1 on the cell surface and enhances its activity. After cleavage by furin-like proteases, TGF-β1 forms latent TGF-β1, and GARP can increase the efficiency of this protease-dependent cleavage. The endoplasmic reticulum chaperone gp96 ensures proper folding of GARP and its expression on the cell surface. On the cell surface, the GARP-latent TGF-β1 complex interacts with integrins αVβ6 and αVβ8 to release mature TGF-β1 peptides. Mature TGF-β1 then interacts with TGF-β receptors in an autocrine or paracrine manner to activate downstream signaling pathways. In some cases, the GARP-latent TGF-β1 complex can also be released from the cell surface, but the activation mechanism of the soluble complex remains unclear.

The GARP-TGF-β1 complex plays critical roles in embryonic development. During palatogenesis, GARP is specifically expressed in the medial edge epithelial cells of the palatal shelves and is essential for TGF-β3 activation and signaling, which is required for normal palate development. Mice with global Lrrc32 gene deletion die within 24 hours after birth due to palatal fusion defects, a phenotype indistinguishable from that of Tgfb3 gene-deficient mice. Additionally, GARP-TGF-β1 is involved in retinal development and tumorigenesis. In the immune system, GARP-TGF-β1 participates in regulatory T cell conversion, platelet-mediated immune suppression, tumor immune evasion, oral tolerance, and the prevention of graft-versus-host disease, allergies, and lupus.

GARP supports TGF-β1-driven cancer cell growth and dissemination by modulating both innate and adaptive immune systems, facilitating tumor immune evasion. In innate immunity, TGF-β1 suppresses the maturation of natural killer cells and dendritic cells. Tumor-derived TGF-β1 polarizes macrophages into tumor-associated macrophages, which secrete pro-inflammatory cytokines such as IL-6, IL-23, and IL-17 to promote cancer progression. TGF-β1 from tumor-associated macrophages is a major driver of epithelial-to-mesenchymal transition. Tumor-derived TGF-β1 also promotes the formation of cancer-associated fibroblasts, which exert potent pro-tumorigenic effects on epithelial cells through autocrine TGF-β1 secretion. In adaptive immunity, active TGF-β1 impairs anti-tumor immunity by directly inhibiting the clonal expansion and cytotoxicity of CD8-positive cytotoxic T cells. TGF-β1 also induces Foxp3 expression, conferring regulatory and immunosuppressive phenotypes to CD4-positive T cells. In human ovarian cancer ascites, infiltrating FoxP3-positive GARP-positive regulatory T cells are present, and in advanced hepatocellular carcinoma, the frequency of GARP-positive FoxP3-positive regulatory T cells is higher, correlating positively with immunosuppression and more aggressive phenotypes.

Given the significant role of GARP-TGF-β1 in tumor immunity, therapeutic strategies targeting this complex have garnered attention. Monoclonal antibodies blocking the interaction between latent TGF-β1 and GARP have demonstrated therapeutic effects in a syngeneic mouse breast cancer model. Beyond inhibiting GARP-latent TGF-β1 interactions on tumor cells, novel antibody strategies are emerging, such as blocking GARP-latent TGF-β1 complexes on regulatory T cell surfaces to prevent active TGF-β1 secretion. LAP-blocking antibodies reduce tumor growth in melanoma, colorectal cancer, and glioblastoma animal models by decreasing the number of GARP-positive LAP-positive regulatory T cells. Selective inhibition of TGF-β1 production by regulatory T cells can overcome tumor resistance to PD-1/PD-L1 blockade. In one-third of metastatic melanoma cases, evidence of GARP-expressing regulatory T cells and their TGF-β1 production has been observed. Combined GARP-TGF-β1 blockade and PD-1 blockade enhances the effector functions of anti-tumor CD8-positive T cells without increasing immune cell infiltration or depleting intratumoral regulatory T cells.

Nanjing UA-Bio Technology Co., Ltd. (UA-Bio) has independently developed "TGF-β1 Protein, Human" (Catalog No.: UA040085), a high-quality recombinant protein reagent specifically designed for research on cell proliferation, differentiation, immune regulation, and tissue fibrosis. This human transforming growth factor-β1 (TGF-β1) protein, a core member of the TGF-β superfamily, efficiently activates SMAD signaling pathways to regulate cell proliferation, differentiation, apoptosis, and immune responses. It serves as a stable and reliable standardized tool for research in tumor biology, stem cell studies, and fibrotic disease mechanisms.

Nanjing UA-Bio Technology Co., Ltd. is committed to providing cutting-edge, high-quality core reagents and tools for immunology, cell therapy, and innovative drug development. For detailed technical parameters, validation data, or application inquiries regarding "TGF-β1 Protein, Human" (Catalog No.: UA040085), please feel free to contact us.