In-depth analysis of SARS-CoV-2-specific T cells reveals diverse differentiation hierarchies in vaccinated individuals.

SARS-CoV-2 vaccines pose as the most effective approach for mitigating the COVID-19 pandemic. High-degree efficacy of SARS-CoV-2 vaccines in clinical trials indicates that vaccination invariably induces an adaptive immune response. However, the emergence of breakthrough infections in vaccinated individuals suggests that the breadth and magnitude of vaccine-induced adaptive immune response may vary. We assessed vaccine-induced SARS-CoV-2 T cell response in 21 vaccinated individuals and found that SARS-CoV-2-specific T cells, which were mainly CD4+ T cells, were invariably detected in all individuals but the response was varied. We then investigated differentiation states and cytokine profiles to identify immune features associated with superior recall function and longevity. We identified SARS-CoV-2-specific CD4+ T cells were polyfunctional and produced high levels of IL-2, which could be associated with superior longevity. Based on the breadth and magnitude of vaccine-induced SARS-CoV-2 response, we identified 2 distinct response groups: individuals with high abundance versus low abundance of SARS-CoV-2-specific T cells. The fractions of TNF-α- and IL-2-producing SARS-CoV-2 T cells were the main determinants distinguishing high versus low responders. Last, we identified that the majority of vaccine-induced SARS-CoV-2 T cells were reactive against non-mutated regions of mutant S-protein, suggesting that vaccine-induced SARS-CoV-2 T cells could provide continued protection against emerging variants of concern.

Emvododstat (PTC299) Targets De Novo Pyrimidine Synthesis in Acute Myeloid Leukemia

Background: De novo nucleotide synthesis is a dynamic process that can address the enormous demand for nucleotides and other macromolecules required in acute myeloid leukemia (AML) proliferation. Hence, we hypothesized that targeting de novo nucleotide synthesis would lead to the depletion of the nucleotide pool and pyrimidine starvation in leukemic cells compared to their non-malignant counterparts, impacting proliferative and differentiation pathways. Emvododstat (PTC299) is an inhibitor of dihydroorotate dehydrogenase (DHODH), a rate-limiting enzyme for de novo pyrimidine nucleotide synthesis that is currently in a clinical trial for the treatment of AML. Objectives: The goals of these studies were to demonstrate that emvododstat effects leukemia growth due to the inhibition of de novo pyrimidine nucleotide synthesis. Comprehensive analyses of mitochondrial function, metabolic signaling in PI3K/AKT pathways, apoptotic signatures, and DNA damage responses were evaluated. The clinical relevance of emvododstat efficacy was confirmed in an AML-PDX model. Results: Emvododstat treatment in cytarabine-resistant AML cells and primary AML blasts induced apoptosis, differentiation, and reduced proliferation, with corresponding increases in annexin V- and CD14-positive cells. Indeed, the inhibition of de novo nucleotide synthesis compromises the dynamic metabolic landscape and mitochondrial function, as indicated by decreases in the oxygen consumption rate (OCR) and mitochondrial ROS/membrane potential. These effects can be reversed by the addition of exogenous uridine and orotate. Further immunoblotting and mass cytometry (CyTOF) analyses demonstrated changes in apoptotic and cell signaling proteins (cleaved PARP, cleaved caspase-3) and DNA damage responses (TP53, γH2AX) and PI3/AKT pathway downregulation in response to emvododstat. Finally, in a PDX mouse model of human AML, emvododstat treatment improved survival compared to mice treated with vehicle (median survival 40 days vs 30 days, P=0.0002). This corresponded with a reduction in the bone marrow burden of leukemia and increased expression of differentiation markers in mice treated with PTC299. Conclusion: Inhibition of de novo pyrimidine synthesis triggers differentiation, apoptosis, and/or inhibition of proliferation in AML models. Emvododstat is a novel dihydroorotate dehydrogenase inhibitor being tested in a clinical trial for the treatment of myeloid malignancies and COVID-19.

TAM kinases as regulators of cell death.

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.