Inhibitory Fcγ Receptor Deletion Enhances CD8 T Cell Stemness, Increasing Anti-PD-1 Therapy Responsiveness in Glioblastoma

Background and Objectives of the Study

Glioblastoma (GBM) is an aggressive central nervous system tumor with poor prognosis, and the median survival of patients is only 14.6 months. Despite the significant efficacy of immune checkpoint blockade (ICB) in other cancers, its effectiveness in treating GBM has been highly limited. This poor efficacy is primarily due to the unique tumor microenvironment (TME) of GBM, which is protected by the blood-brain barrier, leading to reduced antigen presentation capacity and limited functionality of cytotoxic T lymphocytes (CTLs). Even when T cells successfully infiltrate the tumor, they often become “exhausted” due to various immunosuppressive factors.

Recent studies have revealed the presence of progenitor-exhausted T cells (Tpex) among exhausted T cells, suggesting that T cells with stem-like characteristics may play a significant role in enhancing the efficacy of ICB. In this context, the present study focuses on a specific inhibitory receptor, FcγRIIB, expressed on CD8 T cells, aiming to enhance the stemness of CD8 T cells by deleting this receptor to improve the effectiveness of anti-PD-1 therapy against GBM. The objective of the study is to explore whether deletion of FcγRIIB can enhance the anti-tumor response of CD8 T cells in a mouse GBM model, thereby improving the responsiveness and efficacy of anti-PD-1 therapy.

Methods

The study utilized FcγRIIB-deficient (FcγRIIB−/−) mice and a murine GL261 glioblastoma model. The main steps included the following:

1. Model Construction and Anti-PD-1 Treatment: The GL261 glioblastoma cells were injected intracranially to establish a GBM model in mice, and some mice were treated with anti-PD-1 monotherapy or anti-PD-1 therapy combined with FcγRIIB deletion to observe survival rates and clinical symptoms.

2. Analysis of Immune Cells: Flow cytometry and single-cell RNA sequencing (scRNA-seq) were used to analyze the subsets of tumor-infiltrating CD8 T cells, focusing on the potential appearance of T cell subsets with high stemness in FcγRIIB−/− mice.

3. Effector Cell Function Testing: Effector abilities and proliferative characteristics of CD8 T cells following anti-PD-1 treatment were assessed by evaluating cytokine expression levels, cellular proliferation markers, and in vitro killing assays to confirm the role of FcγRIIB deletion in enhancing anti-tumor responses.

4. Adoptive Transfer and Lymph Node Cell Source Analysis: OT-I cell adoptive transfer experiments were performed to investigate the key role of tumor-specific memory T cells from tumor-draining lymph nodes (TDLNs) in enhancing the response to anti-PD-1 therapy.

Main Findings

1. FcγRIIB Deletion Improves Survival Under Anti-PD-1 Treatment: The study found that anti-PD-1 monotherapy had limited efficacy in the mouse GBM model, but when combined with FcγRIIB deletion, it significantly improved the long-term survival rate and extended median survival time. Furthermore, some long-term surviving mice showed no recurrence when rechallenged with the same tumor cells, indicating durable tumor-specific memory in FcγRIIB−/− mice following anti-PD-1 therapy.

2. Tumor-Specific Memory T Cells Exhibited High Stemness Characteristics: Analysis showed that tumor-infiltrating CD8 T cells, particularly tumor-specific memory T cells (TTSMs), in FcγRIIB−/− mice exhibited high levels of stemness (e.g., TCF1 expression). These cells primarily resided in TDLNs, providing a stable source of antigens while avoiding the immunosuppressive effects of the TME.

3. FcγRIIB Deletion Enhanced CD8 T Cell Function: Flow cytometry and scRNA-seq analysis indicated that CD8 T cells in FcγRIIB−/− mice displayed superior effector function compared to wild-type mice. This included significantly increased expression levels of cytokines such as IFN-γ and TNF-α. Moreover, these CD8 T cells showed strong proliferative activity, suggesting that FcγRIIB deletion may enhance anti-tumor responses by promoting CD8 T cell proliferation and effector function.

4. TDLNs as the Source of Responsive Tumor-Specific Memory T Cells: Blocking the migration of CD8 T cells from TDLNs to the tumor using an S1P1 inhibitor in vivo significantly reduced the efficacy of anti-PD-1 therapy in FcγRIIB−/− mice, suggesting that TTSMs in TDLNs are critical for enhancing the anti-PD-1 response.

5. FcγRIIB Expression Detected in Peripheral Blood of GBM Patients: Additionally, high expression of FcγRIIB was detected in the peripheral blood mononuclear cells of GBM patients, suggesting that this receptor might be associated with the exhaustion phenotype of CD8 T cells, further supporting the potential role of FcγRIIB as a therapeutic target in GBM.

Significance and Applications

This study reveals the potential to enhance anti-tumor responses by inhibiting or deleting FcγRIIB, offering new insights into improving ICB efficacy in immunosuppressive tumors such as GBM. The results indicate that FcγRIIB plays an essential role in regulating the stemness of CD8 T cells, affecting their functionality. Therefore, deleting FcγRIIB could help activate and sustain tumor-specific T cells, thereby enhancing the effectiveness of anti-PD-1 therapy. Specifically, for tumor types like GBM, which are traditionally resistant to ICB, the use of FcγRIIB deletion combined with anti-PD-1 therapy has promising potential.

In terms of clinical applications, this study provides a new direction for personalized immunotherapy in GBM patients. Targeting FcγRIIB may enable ICB therapies to achieve more sustained and robust anti-tumor effects. In addition, protecting and activating tumor-specific memory T cells within lymph nodes also provides a novel theoretical basis for optimizing ICB therapies. In the future, combining FcγRIIB regulation strategies in tumor immunotherapy may also be applied to other ICB-resistant cancers, improving patient survival rates and quality of life.