Weekly Digests
‹ Back to September

Enhancing CD8+ T Cell Fatty Acid Catabolism within a Metabolically Challenging Tumor Microenvironment Increases the Efficacy of Melanoma Immunotherapy

September 27, 2017

We know that when oxygen is lacking, T cells increase their glucose uptake and glycolysis, and when glucose is lacking, cells switch to oxidative phosphorylation (OXPHOS), which can utilize many substrates for fuel, but requires oxygen. But what happens to the T cells in the tumor microenvironment (TME), which is often hypoxic and hypoglycemic at the same time?

Zhang et al. utilized a transplantable mouse melanoma model in an attempt to answer this question. The mice were injected with a mixture of two vaccines: one vaccine that induced melanoma-associated antigen (MAA)-specific CD8+ T cells and another that induced irrelevant human papillomavirus protein E7-specific CD8+ T cells. The team found that, contrary to popular belief, CD8+ tumor-infiltrating T cells (TILs) became functionally exhausted regardless of antigen specificity, indicating that chronic antigen stimulation was not required for TIL exhaustion. Using adoptively transferred T cells in a different mouse model, the researchers showed that TIL exhaustion was not an effect unique to their vaccination model.

Building on a hypothesis that metabolic changes caused by the TME led to TIL exhaustion, which could affect both tumor-antigen specific and irrelevant T cells, the team searched for effects of hypoxia. In in vivo studies, MAA- and E7-specific CD8+ TILs were exposed to hypoxia during tumor progression, as indicated by the increased expression of hypoxia-inducible factor 1α (HIF-1α) – the main transcriptional regulator in response to hypoxic conditions. Hypoxia also led to an increase in the inhibitory immune checkpoint receptor LAG-3 (but not PD-1) and impaired TIL function, and these effects could be reversed in vitro and in vivo by shRNA knockdown of HIF-1α. The TILs also upregulated Glut1, the downstream target of HIF-1α that facilitates glucose uptake. However, upregulation of glycolysis in the hypoxic and hypoglycemic TME could be futile, so how would cells adapt to both limitations?

In vitro and in vivo studies showed that under simultaneously hypoxic and hypoglycemic conditions, TILs decreased glycolysis, increased OXPHOS, preserved effector function, and, importantly, increased expression of peroxisome proliferator-activated receptor α (PPAR-α), a key nuclear transcription factor that promotes fatty acid (FA) uptake and catabolism. Interestingly, these changes were not observed in splenic CD8+ T cells, confirming the effect of the TME on T cell metabolism.

The metabolic switch of TILs is facilitated by an enhanced uptake of FAs, which are found in abundance within the tumor interstitial fluid in melanoma mice during tumor progression, as well as the increased expression of the FA β-oxidation (FAO) rate-limiting enzyme Cpt1a, as observed in vaccine-induced CD8+ TILs from late-stage tumors. Importantly, increased levels of PPAR-α, Cpt1a, and FA uptake are observed in CD8+ TILs from melanoma patients, suggesting that the switch toward FA catabolism also occurs in human cancers.

In addition to metabolic changes, PD-1 expression in TILs in the TME also increased over time. Treatment of mice with anti-PD-1 delayed tumor progression but did not affect the metabolism or function of TILs. In a separate in vitro experiment, under simultaneously hypoxic and hypoglycemic conditions, the researchers treated CD8+ T cells with fenofibrate, a PPAR-α agonist, and observed increases in granzyme-B production, FA catabolism, and T cell function, and a concurrent increase in PD-1 expression. These results, which were confirmed in vivo (where fenofibrate significantly delayed tumor progression), suggest that PD-1 expression does not necessarily indicate impaired TIL function. In fact, PD-1 signaling inhibited certain T cell activating pathways, resulting in decreased glycolysis and increased lipolysis and FAO. Thus, PD-1 expression facilitates the metabolic switch of TILs, which then persist employing the FA catabolic pathway regardless of further PD-1 signaling.

When fenofibrate and anti-PD-1 treatments were combined in vivo, they acted synergistically to delay tumor progression and completely prevent tumor growth in 50% of vaccinated mice. These results were promising, but it is important to note that energy production via FA catabolism rather than glycolysis comes with a hefty price tag: FAO requires more oxygen to create an equivalent amount of energy, which leads to increased production of reactive oxygen species within the mitochondria, decreased mitochondrial membrane potential, and deformed cristae and membrane structures, likely resulting in a search for additional energy sources.

Overall, the results of this study suggest that metabolic reprogramming to FA catabolism of CD8+ T cells being prepared for adoptive cell transfer may benefit patients with hypoglycemic tumors, and that this metabolic switch can synergistically combine with PD-1 blockade to reduce tumor progression.

by Anna Scherer


Zhang Y., Kurupati R., Liu L., Zhou X.Y., Zhang G., Hudaihed A., Filisio F., Giles-Davis W., Xu X., Karakousis G.C., Schuchter L.M., Xu W., Amaravadi R., Xiao M., Sadek N., Krepler C., Herlyn M., Freeman G.J., Rabinowitz J.D., Ertl H.C.J. Enhancing CD8+ T Cell Fatty Acid Catabolism within a Metabolically Challenging Tumor Microenvironment Increases the Efficacy of Melanoma Immunotherapy. Cancer Cell. 2017 Sep 11.

In the Spotlight...

Complement in cancer: untangling an intricate relationship

This review by Reis et al. explores the dichotomous role that the activation of the complement system in the tumor microenvironment has and how it can potentially be manipulated. Besides its role as an effector mechanism in antibody-mediated tumor cytolysis, the complement system can potentiate immunotherapy. An imbalanced complement activation can, on the other hand, serve a tumor-promoting role.

Culturing CTLs under Hypoxic Conditions Enhances Their Cytolysis and Improves Their Anti-tumor Function

Compared to cells activated in a normoxic environment, naive CD8+ T cells from OT-I mice activated under hypoxic conditions had similar in vitro survival, marginally slower cell division, and comparable CTLA-4 and PD-1 (but higher TIM3 and LAG3) expression, but contained more granzyme-B per granule and were more cytotoxic. In vivo, both cell types showed similar tumor penetration, but hypoxic CTLs demonstrated improved therapeutic potential.

Customized Viral Immunotherapy for HPV-Associated Cancer

Atherton et al. designed a heterologous vaccination strategy consisting of an adenoviral prime encoding a mutated quadrivalent E6E7 transgene (based on HPV serotypes 16 and 18) that does not disrupt p53 or retinoblastoma, followed by a boost with the oncolytic MG1-Maraba virus containing the same mutant E6E7 transgene; this cured 75% of mice bearing large established TC1 tumors. MG1-E6E7 altered the transcriptional profile of the tumor microenvironment and demonstrated preferential replication within human HPV+ tumor biopsies.

Blood monocytes sample MelanA/MART1 antigen for long-lasting cross-presentation to CD8+ T cells after differentiation into dendritic cells

Healthy human peripheral blood monocytes can internalize and store long peptides and, after 5 days of differentiation to monocyte-derived dendritic cells (MoDC), present the peptide to cognate T cells. Interestingly, circulating monocytes from 2 of 18 uveal melanoma patients with liver metastases were able to activate a MelanA-specific T cell clone after in vitro differentiation into MoDCs. Whether such circulating monocytes alter the tumor-specific immune response is unknown.

Everything New this Week In...