Nikolaos Patsoukis

During activation, T cells undergo metabolic reprogramming, which imprints distinct functional fates. We determined that on PD-1 ligation, activated T cells are unable to engage in glycolysis or amino acid metabolism but have an increased rate of fatty acid β-oxidation (FAO). PD-1 promotes FAO of endogenous lipids by increasing expression of CPT1A, and inducing lipolysis as indicated by elevation of the lipase ATGL, the lipolysis marker glycerol and release of fatty acids. Conversely, CTLA-4 inhibits glycolysis without augmenting FAO, suggesting that CTLA-4 sustains the metabolic profile of non-activated cells. Because T cells utilize glycolysis during differentiation to effectors, our findings reveal a metabolic mechanism responsible for PD-1-mediated blockade of T-effector cell differentiation. The enhancement of FAO provides a mechanistic explanation for the longevity of T cells receiving PD-1 signals in patients with chronic infections and cancer, and for their capacity to be reinvigorated by PD-1 blockade.

The receptor programmed death 1 (PD-1) inhibits T cell proliferation and plays a critical role in suppressing self-reactive T cells, and it also compromises antiviral and antitumor responses. To determine how PD-1 signaling inhibits T cell proliferation, we used human CD4(+) T cells to examine the effects of PD-1 signaling on the molecular control of the cell cycle. The ubiquitin ligase SCF(Skp2) degrades p27(kip1), an inhibitor of cyclin-dependent kinases (Cdks), and PD-1 blocked cell cycle progression through the G(1) phase by suppressing transcription of SKP2, which encodes a component of this ubiquitin ligase. Thus, in T cells stimulated through PD-1, Cdks were not activated, and two critical Cdk substrates were not phosphorylated. Activation of PD-1 inhibited phosphorylation of the retinoblastoma gene product, which suppressed expression of E2F target genes. PD-1 also inhibited phosphorylation of the transcription factor Smad3, which increased its activity. These events induced additional inhibitory checkpoints in the cell cycle by increasing the abundance of the G(1) phase inhibitor p15(INK4) and repressing the Cdk-activating phosphatase Cdc25A. PD-1 suppressed SKP2 transcription by inhibiting phosphoinositide 3-kinase-Akt and Ras-mitogen-activated and extracellular signal-regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signaling. Exposure of cells to the proliferation-promoting cytokine interleukin-2 restored activation of MEK-ERK signaling, but not Akt signaling, and only partially restored SKP2 expression. Thus, PD-1 blocks cell cycle progression and proliferation of T lymphocytes by affecting multiple regulators of the cell cycle.

Programmed Death-1 (PD-1; CD279) is an inhibitory receptor induced in activated T cells. PD-1 engagement by its ligands, PD-L1 and PD-L2, maintains peripheral tolerance but also compromises anti-tumor immunity. Blocking antibodies against PD-1 or its ligands have revolutionized cancer immunotherapy. However, only a fraction of patients develop durable antitumor responses. Clinical outcomes have reached a plateau without substantial advances by combinatorial approaches. Thus, great interest has recently emerged to investigate, in depth, the mechanisms by which the PD-1 pathway transmits inhibitory signals with the goal to identify molecular targets for improvement of the therapeutic success. These efforts have revealed unpredictable dimensions of the pathway and uncovered novel mechanisms involved in PD-1 and PD-L1 regulation and function. Here, we provide an overview of the recent advances on the mechanistic aspects of the PD-1 pathway and discuss the implications of these new discoveries and the gaps that remain to be filled.

mice, accumulation of GMP and MDSC was prevented, whereas systemic output of effector myeloid cells was increased. Myeloid cell-specific PD-1 ablation induced an increase of T effector memory cells with improved functionality and mediated antitumor protection despite preserved PD-1 expression in T cells. In PD-1-deficient myeloid progenitors, growth factors driving emergency myelopoiesis induced increased metabolic intermediates of glycolysis, pentose phosphate pathway, and TCA cycle but, most prominently, elevated cholesterol. Because cholesterol is required for differentiation of inflammatory macrophages and DC and promotes antigen-presenting function, our findings indicate that metabolic reprogramming of emergency myelopoiesis and differentiation of effector myeloid cells might be a key mechanism of antitumor immunity mediated by PD-1 blockade.

Sclerotium-forming filamentous fungi are of great agricultural and biological interest because they can be viewed as models of simple metamorphosis. They differentiate by asexually producing sclerotia but the processes involved in sclerotial metamorphosis were poorly understood. In 1997, it was shown for the first time that the sclerotial differentiation state in Sclerotium rolfsii concurred with increasing levels of lipid peroxides. This finding prompted the development of a theory supporting that sclerotial metamorphosis is induced by oxidative stress. Growth factors that reduce or increase oxidative stress are expected to inhibit or promote sclerotium metamorphosis, respectively. This theory has been verified by a series of published data on the effect of certain hydroxyl radical scavengers on sclerotial metamorphosis, on the identification and quantification of certain endogenous antioxidants (such as ascorbic acid, β-carotene) in relation to the fungal undifferentiated and differentiated states, and on their inhibiting effect on sclerotial metamorphosis as growth nutrients. In 2004-2005, we developed assays for the measurement of certain redox markers of oxidative stress, such as the thiol redox state, the small-sized fragmented DNA, and the superoxide radical. These new advances allowed us to initiate studies on the exact role of glutathione, hydrogen peroxide, and superoxide radical on sclerotial metamorphosis. The emerging data, combined with similar data from other better-studied fungi, allowed us to make some preliminary postulations on the ROS-dependent biochemical signal transduction pathways in sclerotiogenic filamentous fungi.