The Immune Consequences of a Ketogenic Diet in GBM and its Therapeutic Implications
Glioblastoma (GBM) is one of the most complex, deadly, and immune-resistant cancers. Recent investigations have identified a ketogenic diet (KD) as a nutritional intervention in GBM. KD is a high-fat diet with a low carbohydrate intake. It is postulated that a KD dramatically shifts nutritional bioavailability in both tumors and its microenvironment. Since immune cells also rely upon similar nutrients for performing their functions, we hypothesized that a KD might also influence their anti-tumor activity. Consistent with previous publications, utilizing genetically engineered murine GBM tumor cells orthotopically implanted in immune-competent mice, we demonstrated improved survival in mice fed a KD when compared to mice fed a standard diet (p=0.043). To begin to understand the immune consequences of a KD in GBM, we immunophenotyped these tumors. Of the immune cells analyzed, we discovered that KD played an important role in influencing macrophage polarization, which recent investigations suggest play a critical role in inducing a potent immune suppression in GBM. Specifically, anti-tumor activity was observed in mice fed a KD, there was a paradoxical 50% increase in immune suppressive M2 macrophages (CD45+CD11b+F4/80+CD206+) coupled with a decrease in pro-inflammatory M1 macrophages (CD45+CD11b+F4/80+CD80hi). To extend these findings, we recapitulated KD in vitro using a modified cell culture media. Consistent with in vivo studies, murine macrophages cultured in these conditions skewed polarization towards the M2 phenotype with immune suppressive properties and we went on to mechanistically link these findings to activation of transcription factor PPARγ. Although anti-tumor activity was observed in mice fed a KD, we hypothesized this parallel increase in M2 macrophage polarization tempered its potential therapeutic benefit. Colony-stimulating factor 1 (CSF-1) plays a central role in macrophage differentiation and CSF-1 receptor inhibition is actively being investigated as a strategy to skew their polarization towards an M1 anti-tumor phenotype. To test this hypothesis, we performed investigations combining KD with the brain penetrant, clinically relevant CSF-1R inhibitor BLZ945. Consistent with our hypothesis, this combination demonstrated a striking improvement in survival in comparison to KD or BLZ945 alone (p=0.0004) with 50% of mice achieving long term survival (>50 days). Correlative studies confirmed the capacity of BLZ945 to normalize KD-induced increases in M2s and the combination induced an increase of iNOS+ M1s, which are responsible for performing pro-inflammatory functions in tumors. Collectively, although anti-tumor activity was observed with a KD, parallel increases in M2 macrophage tempered its therapeutic benefit. Combinatorial strategies using agents designed to modulate macrophage polarization represent a rational approach to improve the anti-tumor activity of a KD in GBM.
Kesarwani P, Kant S, Zhao Y, Miller CR, Chinnaiyan P. The immune consequences of a ketogenic diet in GBM and its therapeutic implications. Cancer Res. 2022 Jun 15;82(12 Suppl):632. doi:10.1158/1538-7445.AM2022-632.