Introduction: The tumor microenvironment of head and neck squamous cell carcinoma (HNSCC) is a metabolically complex matrix in which cancer cells interact with metabolically dysregulated stromal support cells and infiltrating immune cells. It is poorly understood how the metabolic environment in tumors affects immune cell infiltration and function. Metformin affects cell metabolism by inhibiting mitochondrial oxidative phosphorylation complex I, activating of AMP-activated protein kinase (AMPK), and inhibiting the mammalian target of rapamycin (mTOR). Here we evaluated effects of Metformin on immune cell infiltrates in HNSCC primary tumors and metastatic lymph nodes.
Methods: A group of 37 primary tumor specimens from patients treated with metformin in the neoadjuvant setting were compared to 52 matched control specimens from treatment-naïve patients. Among the 37 Metformin-treated patients, 7 patients had nodal metastases that were studied. These nodal specimens were compared to 23 specimens from treatment-naïve patients. Immune profiling of the TME and lymph nodes was carried out by immunohistochemical detection of FOXP3 (Forkhead Box P3) expressed in regulatory T cells, and CD8 expressed by effector T cells. Digital image analysis was performed using Aperio software to quantify staining intensity and co-localization.
Results: Metformin pretreatment was associated with fewer intratumoral FOXP3+ cells when compared to that of primary control specimens (p<0.0001), but did not significantly affect FOXP3+ cells in metastatic lymph nodes. By contrast, Metformin treatment did not significantly affect CD8+ cells in either primary or lymph node specimens.
Conclusion: Metformin treatment reduces expression of FOXP3 by tumor infiltrating T cells. This observation is consistent with the hypothesis that Metformin may favorably impact HNSCC immune responses by reducing regulatory T cells expressing FOXP3. Whether this effect is due to reprogramming of tumor and/or immune cell metabolism requires further investigation.