Role of the mammalian target of rapamycin pathway in lentiviral vector transduction of hematopoietic stem cells

The Akt-mTOR network at the interface of hematopoietic stem cell homeostasis

Hematopoietic stem cells (HSCs) are immature blood cells that exhibit multilineage differentiation capacity. Homeostasis is critical for HSC potential and lifelong hematopoiesis, and HSC homeostasis is tightly governed by both intrinsic molecular networks and microenvironmental signals. The evolutionarily conserved serine/threonine protein kinase B (PKB, also referred to as Akt)-mammalian target of rapamycin (mTOR) pathway is universal to nearly all multicellular organisms and plays an integral role in most cellular processes. Emerging evidence has revealed a central role of the Akt-mTOR network in HSC homeostasis, because it responds to multiple intracellular and extracellular signals and regulates various downstream targets, eventually affecting several cellular processes, including the cell cycle, mitochondrial metabolism, and protein synthesis. Dysregulated Akt-mTOR signaling greatly affects HSC self-renewal, maintenance, differentiation, survival, autophagy, and aging, as well as transformation of HSCs to leukemia stem cells. Here, we review recent works and provide an advanced understanding of how the Akt-mTOR network regulates HSC homeostasis, thus offering insights into future clinical applications.

Gene therapies for transfusion dependent β-thalassemia: Current status and critical criteria for success

Thalassemia is one of the most prevalent monogenic diseases usually caused by quantitative defects in the production of β-globin leading to severe anemia. Technological advances in genome sequencing, stem cell selection, viral vector development, transduction and gene editing strategies now allow for efficient exvivo genetic manipulation of human stem cells that can lead to production of hemoglobin, leading to a meaningful clinical benefit in thalassemia patients. In this review, the status of the gene-therapy approaches available for transfusion dependent thalassemia are discussed, along with the critical criteria that affect efficacy and lessons that have been learned from the early phase clinical trials. Salient steps necessary for the clinical development, manufacturing, and regulatory approvals of gene therapies for thalassemia are also highlighted, so that the potential of these therapies can be realized. It is highly anticipated that gene therapies will soon become a treatment option for patients lacking compatible donors for hematopoietic stem cell transplant and will offer an alternative for definitive treatment of β-thalassemia.

Thiosemicarbazone-based lead optimization to discover high-efficiency and low-toxicity anti-gastric cancer agents

In this paper, a series of thiosemicarbazone derivatives containing different aromatic heterocyclic groups were synthesized and the tridentate donor system of the lead compound was optimized. Most of the target compounds showed improved antiproliferative activity against MGC803 cells. SAR studies revealed that compound 5d displayed significant advantages in inhibition effect with an IC₅₀ value of 0.031 μM, and better selectivity between cancer and normal cells than 3-AP and DpC (about 15- and 5-fold improved respectively). Besides, compound 5d showed selective antiproliferative activity in not only other cancer cells but also different gastric cancer cell lines. In-depth mechanism studies showed that compound 5d could induce mitochondria-related apoptosis which might be related to the elevation of intracellular ROS level, and cause cell cycle arrest at S phase. Moreover, 5d could evidently suppress the cell migration and invasion by blocking the EMT (epithelial-mesenchymal transition) process. Consequently, our studies provided a lead optimization strategy of thiosemicarbazone derivatives which would contribute to discover high-efficiency and low-toxicity agents for the treatment of gastric cancer.

Vectofusin-1 Improves Transduction of Primary Human Cells with Diverse Retroviral and Lentiviral Pseudotypes, Enabling Robust, Automated Closed-System Manufacturing

Cell and gene therapies are finally becoming viable patient treatment options, with both T cell- and hematopoietic stem cell (HSC)-based therapies being approved to market in Europe. However, these therapies, which involve the use of viral vector to modify the target cells, are expensive and there is an urgent need to reduce manufacturing costs. One major cost factor is the viral vector production itself, therefore improving the gene modification efficiency could significantly reduce the amount of vector required per patient. This study describes the use of a transduction enhancing peptide, Vectofusin-1®, to improve the transduction efficiency of primary target cells using lentiviral and gammaretroviral vectors (LV and RV) pseudotyped with a variety of envelope proteins. Using Vectofusin-1 in combination with LV pseudotyped with viral glycoproteins derived from baboon endogenous retrovirus, feline endogenous virus (RD114), and measles virus (MV), a strongly improved transduction of HSCs, B cells and T cells, even when cultivated under low stimulation conditions, could be observed. The formation of Vectofusin-1 complexes with MV-LV retargeted to CD20 did not alter the selectivity in mixed cell culture populations, emphasizing the precision of this targeting technology. Functional, ErbB2-specific chimeric antigen receptor-expressing T cells could be generated using a gibbon ape leukemia virus (GALV)-pseudotyped RV. Using a variety of viral vectors and target cells, Vectofusin-1 performed in a comparable manner to the traditionally used surface-bound recombinant fibronectin. As Vectofusin-1 is a soluble peptide, it was possible to easily transfer the T cell transduction method to an automated closed manufacturing platform, where proof of concept studies demonstrated efficient genetic modification of T cells with GALV-RV and RD114-RV and the subsequent expansion of mainly central memory T cells to a clinically relevant dose.

mTOR inhibitors lower an intrinsic barrier to virus infection mediated by IFITM3

Rapamycin and its derivatives are specific inhibitors of mammalian target of rapamycin (mTOR) kinase and, as a result, are well-established immunosuppressants and antitumorigenic agents. Additionally, this class of drug promotes gene delivery by facilitating lentiviral vector entry into cells, revealing its potential to improve gene therapy efforts. However, the precise mechanism was unknown. Here, we report that mTOR inhibitor treatment results in down-regulation of the IFN-induced transmembrane (IFITM) proteins. IFITM proteins, especially IFITM3, are potent inhibitors of virus-cell fusion and are broadly active against a range of pathogenic viruses. We found that the effect of rapamycin treatment on lentiviral transduction is diminished upon IFITM silencing or knockout in primary and transformed cells, and the extent of transduction enhancement depends on basal expression of IFITM proteins, with a major contribution from IFITM3. The effect of rapamycin treatment on IFITM3 manifests at the level of protein, but not mRNA, and is selective, as many other endosome-associated transmembrane proteins are unaffected. Rapamycin-mediated degradation of IFITM3 requires endosomal trafficking, ubiquitination, endosomal sorting complex required for transport (ESCRT) machinery, and lysosomal acidification. Since IFITM proteins exhibit broad antiviral activity, we show that mTOR inhibition also promotes infection by another IFITM-sensitive virus, Influenza A virus, but not infection by Sendai virus, which is IFITM-resistant. Our results identify the molecular basis by which mTOR inhibitors enhance virus entry into cells and reveal a previously unrecognized immunosuppressive feature of these clinically important drugs. In addition, this study uncovers a functional convergence between the mTOR pathway and IFITM proteins at endolysosomal membranes.