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I am so excited that our paper exploring how metabolic dysfunction contributes to T-cell exhaustion is out today in @NatImmunol. A thread (1/X): https://rdcu.be/b5AAg
Loss of T-cell function has been a hallmark of tumorigenesis for some time – sufficient to make it as one of the “new generation” of cancer hallmarks. https://www.cell.com/fulltext/S0092-8674(11)00127-9
The features of dysfunctional intratumoral T-cells are fairly well established – loss of proliferation, loss of effector function, activation of exhaustion-associated transcription factors, and global chromatin remodeling ( @EJohnWherry, @SchietingerLab and others)
How exactly this whole-scale change in cell state occurs, however, has remained a mystery. Very clearly, however, it is not signaling through PD-1, as anti-PD-1 only minimally impacts the transcriptome and has virtually no effect on the chromatin landscape of exhausted T-cells.
Starting in about 2015, evidence started to emerge that loss of proliferative capacity was an underappreciated aspect of T-cell dysfunction. Patients who responded to checkpoint blockade saw a rapid burst in T-cell proliferation, while non-responders did not.
Well, that was music to our ears in metabolism world, where Craig, amongst others, had shown that the “Warburg phenomenon” – taking up lots of glucose and excreting it as lactate – was not accidental, but rather an active response to a drive to engage in anabolic synthesis.
What people generally forget is that Craig actually showed this first in T-cells when he proved that the reason T cells need CD28 is to activate this metabolic shift. https://www.sciencedirect.com/science/article/pii/S1074761302003230
In the past 6 or 7 years, the Thompson lab had shifted to showing that metabolites do more than supply ATP or macromolecules for proliferation – they impact cellular identity by serving as substrates or co-factors for enzymatic modifications of chromatin. https://rupress.org/jcb/article/217/7/2247/39126
So: could it be possible that the rewiring of exhausted T-cells was a response to a change in metabolism? Perhaps – many others had shown that the metabolism of exhausted T-cells did indeed differ from effector or memory T-cells. https://www.sciencedirect.com/science/article/pii/S1074761316302825
What these studies did not do is show whether a change in metabolism could actually drive the phenotypic hallmarks of exhaustion. And that’s for a very good reason – its quite difficult to even observe, and certainly alter, the metabolism of individual cell types within a mouse.
So we asked a pretty dumb question – could we mimic the process of intratumoral T-cell exhaustion by reproducing chronic tumor antigen encounter in a dish?
Indeed we could – and this offered the opportunity to perform in-depth metabolic assays that would be difficult, if not impossible, to perform in vivo. So we started by asking the most fundamental metabolic questions – what did the cells eat, and how did they use what they ate?
To our surprise, we found that, even prior to the development of exhaustion, chronic antigenic stimulation made T-cells almost 100% glycolytic. We were surprised because our own lab had shown that activation of glycolytic flux was essential for the proliferation of T-cells.
These cells, however, became highly glycolytic – but stopped proliferating. We were sufficiently weirded out by this to appeal to people much smarter than ourselves.
Luckily, @dannykwells, @Satpathology, and @KatieEYost confirmed that this same phenomenon was occurring in bona fide T-cells extracted from patient tumors – the most exhausted cells were the most glycolytic.
So if the cells could engage in aerobic glycolysis, what couldn’t they do? We quickly learned that exhausted cells defaulted to glycolysis because of their inability to oxidize carbon substrates within the mitochondria – known more commonly as oxidative phosphorylation.
It turns out that persistent TCR stimulation revs up the demand on a T-cell – particularly the demand to produce lots of protein (like, for example, cytotoxic granules). That production is expensive...
...and in an attempt to meet that bioenergetic demand, T-cells try to burn as much fuel as possible in their mitochondria – so much so that they end up overwhelming the capacity of their electron transport chain. The resultant electron leakage generates free radicals.
Unfortunately, those free radicals damage mitochondrial TCA cycle enzymes and the ETC itself – making the mitochondria more likely to produce free radicals, and leaving exhausted T-cells dependent on glycolysis – and available glucose – to survive.
So – why does this produce the hallmarks of T-cell exhaustion? By spending the available ATP on translation, exhausted T-cells give up the capacity to produce NTPs for DNA replication – which is why exhausted T-cells can no longer proliferate.
What was shocking to us was that not only did accumulation of free radicals block proliferation – it was actually sufficient to activate TOX, the most well-characterized driver of the T-cell exhaustion program. https://rdcu.be/b5AAM
Similarly shocking – neutralizing free radical damage could reverse the hallmarks of T-cell exhaustion – despite the persistent presence of antigen!
When we looked at what chronically stimulated T-cells in whom redox stress had been neutralized looked like, they still bore the fingerprints of exhaustion – but maintained a so-called “progenitor state”, marked by TCF-1 expression and retained proliferative capacity.
As a result, when transferred into a mouse, these "progenitor" exhausted T-cells could more efficiently reject tumors. https://www.nature.com/articles/s41590-019-0312-6
What is most exciting to us about this work is that it establishes altered metabolism as an upstream regulator of the T-cell exhaustion program – rather than merely a bystander or a consequence. In other words – change metabolism, and you can change the phenotype.
So many questions have emerged from this work: how does persistent antigen create the metabolic insufficiencies that we observe? How are metabolic liabilities interpreted on the epigenetic level in T-cells? How do fluctuations in nutrient availability affect this process?
We will be tackling these and many other questions relating to how metabolic patterns affect immune cell fate decisions in the brand-new @VardhanaLab, starting at @sloan_kettering this fall! Please contact me if you are interested in joining our team!
And thanks to all our collaborators, @NatImmunol for a straightforward and lovely review process, and @parkerici, @BWFUND, and @theNCI for supporting this work.
