CCL5 blockade reduces immunosuppression through control of SREBP-2 activity in glioblastoma

Technical Summary: CCL5 Blockade Reduces Immunosuppression through Control of SREBP-2 Activity in Glioblastoma

Problem Statement
Glioblastoma (GBM) remains a highly aggressive malignancy with limited therapeutic advances, largely due to the immunosuppressive tumor microenvironment (TME). A critical component of this environment is the recruitment and modulation of tumor-associated macrophages (TAMs) and microglia. Chemokine (C-C motif) ligand 5 (CCL5), secreted by tumor cells and immune cells, is known to attract TAMs and support cancer growth; however, the precise molecular mechanisms by which CCL5 drives macrophage-mediated immunosuppression in GBM remain unclear. Specifically, the link between CCL5 signaling, lipid metabolism, and the transcriptional regulation of immunosuppressive markers requires elucidation.

Methodology
The study employed a multi-faceted approach combining in vivo and in vitro models to investigate the CCL5-CCR5-SREBP-2 axis:

• In Vivo Models: An intracranial glioma model was established in C57BL/6J mice using GL261 cells. Mice were treated with temozolomide (TMZ) alone or in combination with a monoclonal anti-CCL5 antibody. Tumor growth and survival were monitored, followed by immunohistochemistry (IHC) and flow cytometry to assess immune cell phenotypes.
• In Vitro Models: Bone marrow-derived macrophages (BMDMs), THP-1 cells, and microglial cell lines (HMC3, BV2) were cultured. Macrophages were polarized using IL-4/IL-13 (M2-like) or LPS/IFN-γ (M1-like). Co-culture systems were established between tumor cells (U251, GL261) and immune cells to simulate the TME.
• Interventions: Treatments included anti-CCL5 antibodies, the SREBP-2 inhibitor Fatostatin A, and CCL5 protein stimulation.
• Analytical Techniques:
  • Molecular Profiling: RT-qPCR and ELISA were used to quantify mRNA and protein levels of immunosuppressive markers (CD206, CSF1R, Arg1, IL-10, TGF-β) and inflammatory markers.
  • Metabolic Assays: Colorimetric assays measured total cholesterol, LDL-C, and HDL-C. SREBP-2 transcriptional activity was assessed via a specific ELISA-based assay.
  • Bioinformatics: Analysis of TCGA (GBM and LGG) and GEO (scRNA-seq) databases was conducted to correlate CCL5 expression with patient survival, immune infiltration, and gene expression profiles.

Key Results

1. Therapeutic Efficacy: The combination of anti-CCL5 antibody and TMZ significantly inhibited tumor growth and prolonged survival in the intracranial mouse model compared to TMZ alone.
2. Modulation of TAM Phenotype: In vivo and in vitro data demonstrated that CCL5 blockade reduced the recruitment of TAMs and specifically decreased the proportion of M2-like macrophages. This was evidenced by the downregulation of M2 markers (CD206, CSF1R, Arg1, Retnla) and immunosuppressive cytokines (IL-10, TGF-β). Notably, CCL5 blockade did not alter the expression of M1-like pro-inflammatory markers (CD80, CD86, Nos2, Il1b) in response to LPS, nor did it prevent M1 polarization, but it did suppress the transition to or maintenance of the M2 phenotype.
3. Mechanistic Link to Lipid Metabolism: CCL5 blockade was found to reduce intracellular cholesterol levels in macrophages and microglia. This reduction correlated with decreased activity of Sterol Regulatory Element-Binding Protein-2 (SREBP-2), a master transcription factor for cholesterol biosynthesis.
4. SREBP-2 as a Mediator: The study established that CCL5 activates SREBP-2, which in turn drives the transcription of immunosuppressive genes. Direct pharmacological inhibition of SREBP-2 using Fatostatin A mimicked the effects of CCL5 blockade, suppressing the expression of CD206, IL-10, and TGF-β. Conversely, CCL5 stimulation upregulated these markers, an effect that was reversed by SREBP-2 inhibition.
5. Bioinformatic Validation: Analysis of clinical data confirmed that high CCL5 expression correlates with poor survival in GBM patients and is positively associated with myeloid cell infiltration and the expression of M2-related genes (MRC1, CSF1R, IL10, TGF).

Significance and Claims
The authors claim that this study elucidates a novel mechanism by which CCL5 blockade reduces immunosuppression in GBM. The central finding is that CCL5 signaling, likely through the CCR5 receptor, activates SREBP-2, thereby promoting cholesterol biosynthesis and the transcriptional program required for M2-like macrophage polarization. By blocking CCL5, the activity of SREBP-2 is suppressed, leading to a remodeling of the TME away from an immunosuppressive state.

The paper posits that targeting the CCL5-CCR5 axis is a potential therapeutic strategy for GBM, not only by reducing TAM recruitment but specifically by inhibiting the metabolic reprogramming (cholesterol synthesis) that sustains the immunosuppressive phenotype of TAMs and microglia. This suggests that combining CCL5 blockade with standard chemotherapy (TMZ) could enhance anti-tumor immunity by disrupting the metabolic drivers of immunosuppression. The study concludes that CCL5 blockade represents a viable approach to reducing immunosuppression in GBM treatment by modulating SREBP-2 activity in myeloid cells.