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BACKGROUND. The human bone marrow (BM) niche contains a population of mesenchymal stromal cells (MSCs) that provide physical support and regulate hematopoietic stem cell (HSC) homeostasis. [beta]-Thalassemia (BT) is a hereditary disorder characterized by altered hemoglobin beta-chain synthesis amenable to allogeneic HSC transplantation and HSC gene therapy. Iron overload (IO) is a common complication in BT patients affecting several organs. However, data on the BM stromal compartment are scarce. METHODS. MSCs were isolated and characterized from BM aspirates of healthy donors (HDs) and BT patients. The state of IO was assessed and correlated with the presence of primitive MSCs in vitro and in vivo. Hematopoietic supportive capacity of MSCs was evaluated by transwell migration assay and 2D coculture of MSCs with human CD3[4.sup.+] HSCs. In vivo, the ability of MSCs to facilitate HSC engraftment was tested in a xenogenic transplant model, whereas the capacity to sustain human hematopoiesis was evaluated in humanized ossicle models. RESULTS. We report that, despite iron chelation, BT BM contains high levels of iron and ferritin, indicative of iron accumulation in the BM niche. We found a pauperization of the most primitive MSC pool caused by increased ROS production in vitro which impaired MSC stemness properties. We confirmed a reduced frequency of primitive MSCs in vivo in BT patients. We also discovered a weakened antioxidative response and diminished expression of BM niche-associated genes in BT-MSCs. This caused a functional impairment in MSC hematopoietic supportive capacity in vitro and in cotransplantation models. In addition, BT-MSCs failed to form a proper BM niche in humanized ossicle models. CONCLUSION. Our results suggest an impairment in the mesenchymal compartment of BT BM niche and highlight the need for novel strategies to target the niche to reduce IO and oxidative stress before transplantation. FUNDING. This work was supported by the SR-TIGET Core grant from Fondazione Telethon and by Ricerca Corrente., Introduction [beta]-Thalassemia (BT) is one of the most common inherited monogenic disorders in the world, with an overall carrier rate of 1.5% of the world population, and 60,000 BT symptomatic [...]

Cytotoxic T cell (CTL) infiltration of the tumor carries the potential to limit cancer progression, but their exclusion by the immunosuppressive tumor microenvironment hampers the efficiency of immunotherapy. Here, we show that expression of the axon guidance molecule Plexin-A4 ( Plxna4 ) in CTLs, especially in effector/memory CD8 + T cells, is induced upon T-cell activation, sustained in the circulation, but reduced when entering the tumor bed. Therefore, we deleted Plxna4 and observed that Plxna4 -deficient CTLs acquired improved homing capacity to the lymph nodes and to the tumor, as well as increased proliferation, both achieved through enhanced Rac1 activation. Mice with stromal or hematopoietic Plxna4 deletion exhibited enhanced CTL infiltration and impaired tumor growth. In a melanoma model, adoptive transfer of CTLs lacking Plxna4 prolonged survival and improved therapeutic outcome, which was even stronger when combined with anti-programmed cell death protein 1 (PD-1) treatment. PLXNA4 abundance in circulating CTLs was augmented in melanoma patients versus healthy volunteers but decreased after the first cycle of anti-PD-1, alone or in combination with anti-cytotoxic T-Lymphocyte Associated Protein 4 (CTLA-4), in those patients showing complete or partial response to the treatment. Altogether, our data suggest that Plxna4 acts as a "checkpoint," negatively regulating CTL migration and proliferation through cell-autonomous mechanisms independent of the interaction with host-derived Plxna4 ligands, semaphorins. These findings pave the way toward Plxna4-centric immunotherapies and propose Plxna4 detection in circulating CTLs as a potential way to monitor the response to immune checkpoint blockade in patients with metastatic melanoma., (©2021 The Authors; Published by the American Association for Cancer Research.)