Immune Surveillance and the Role of Stem-Like T Cells and Lymph Nodes
Tumors are capable of evading immune responses by shaping a locally immunosuppressive microenvironment. Within this setting, infiltrating CD8+ T cells lose their effector functions. One primary mechanism involves upregulating immune checkpoint receptors—PD-1 and CTLA-4—on T cells. Engagement of these receptors with ligands on tumor cells dampens T cell activity and weakens anti-tumor immunity.
Immune checkpoint blockade has become a foundational approach in cancer immunotherapy, yet only 15–30% of patients respond to treatment, implying additional mechanisms suppressing anti-tumor immune responses.
A distinct subset of stem-like CD8+ T cells is a key factor in therapeutic response. These cells express moderate levels of PD-1 and retain the ability to self-renew. They express genes such as Tcf7, Ccr7, and IL7R, which are critical for T cell development, maintenance, migration, and survival. Stem-like T cells serve as a reservoir that responds to checkpoint blockade by generating new effector T cells. The presence of these cells correlates with durable responses to checkpoint inhibitors in cancers such as melanoma. Conversely, depletion of stem-like T cells impairs the efficacy of immunotherapy.
This stem-like T cell population is maintained in lymph nodes, where T cells preserve their proliferative capacity. Within the tumor microenvironment, however, they rapidly become exhausted due to chronic antigen stimulation.
Interferon-Gamma: A Double-Edged Sword in Anti-Tumor Immunity
Interferon-gamma (IFN-γ) is a critical cytokine activating immune responses against tumors. IFN-γ–induced genes are widely used as biomarkers to predict chemotherapy, radiotherapy, and immunotherapy responses. IFN-γ can inhibit tumor cell proliferation and angiogenesis. However, IFN-γ may also contribute to immune suppression and tumor progression. As a result, therapies based on recombinant IFN-γ have shown limited success in clinical oncology.
Researchers at the University of Oxford have demonstrated that IFN-γ can directly impair the anti-tumor activity of intratumoral T cells, independent of immune checkpoint signaling. In patients with metastatic melanoma, high expression of the IFNγR2 receptor subunit negatively correlates with CD8+ T cell clonal expansion and immunotherapy efficacy. These findings suggest that downregulation of the IFN-γ receptor may be an intrinsic mechanism through which CD8+ T cells optimize their anti-tumor function.
A mouse melanoma model confirmed that deleting the IFN-γ receptor, specifically in CD8+ T cells, improved tumor control. Receptor ablation enhanced T cell infiltration into the tumor core. In contrast, continuous IFN-γ signaling suppressed the self-renewal of stem-like CD8+ T cells, reducing clonal diversity and diminishing the effectiveness of the T cell response against tumors.
Despite immunotherapy, IFN-γ receptor deletion enhanced anti-tumor immunity in the melanoma model. Survival outcomes improved significantly when these modified T cells were adoptively transferred into tumors.
Conclusion
The role of IFN-γ in anti-tumor immunity appears more complex than previously understood. On one hand, IFN-γ enhances T cell infiltration and tumor recognition. On the other hand, it suppresses the self-renewal of stem-like T cells, which are crucial for maintaining a robust and sustained immune response during immunotherapy.
Recently developed partial IFN-γ receptor agonists may offer a way to selectively promote the cytokine’s immunostimulatory functions while limiting its immunosuppressive ones. This approach opens new possibilities for boosting anti-tumor immunity by modulating the IFN-γ signaling pathway.
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