Decoding Clonal Hematopoiesis: Emerging Themes and Novel Mechanistic Insights.

Clonal hematopoiesis (CH), the relative expansion of mutant clones, is derived from hematopoietic stem cells (HSCs) with acquired somatic or cytogenetic alterations that improve cellular fitness. Individuals with CH have a higher risk for hematological and non-hematological diseases, such as cardiovascular disease, and have an overall higher mortality rate. Originally thought to be restricted to a small fraction of elderly people, recent advances in single-cell sequencing and bioinformatics have revealed that CH with multiple expanded mutant clones is universal in the elderly population. Just a few years ago, phylogenetic reconstruction across the human lifespan and novel sensitive sequencing techniques showed that CH can start earlier in life, decades before it was thought possible. These studies also suggest that environmental factors acting through aberrant inflammation might be a common theme promoting clonal expansion and disease progression. However, numerous aspects of this phenomenon remain to be elucidated and the precise mechanisms, context-specific drivers, and pathways of clonal expansion remain to be established. Here, we review our current understanding of the cellular mechanisms driving CH and specifically focus on how pro-inflammatory factors affect normal and mutant HSC fates to promote clonal selection.

A comprehensive analysis of cell-autonomous and non-cell-autonomous regulation of myeloid leukemic cells: The prospect of developing novel niche-targeting therapies.

Leukemic cells (LCs) arise from the hematopoietic stem/and progenitor cells (HSCs/HSPCs) and utilize cues from the bone marrow microenvironment (BMM) for their regulation in the same way as their normal HSC counterparts. Mesenchymal stromal cells (MSCs), a vital component of the BMM promote leukemogenesis by creating a protective and immune-tolerant microenvironment that can support the survival of LCs, helping them escape chemotherapy, thereby resulting in the relapse of leukemia. Conversely, MSCs also induce apoptosis in the LCs and inhibit their proliferation by interfering with their self-renewal potential. This review discusses the work done so far on cell-autonomous (intrinsic) and MSCs-mediated non-cell-autonomous (extrinsic) regulation of myeloid leukemia with a special focus on the need to investigate the extrinsic regulation of myeloid leukemia to understand the contrasting role of MSCs in leukemogenesis. These mechanisms could be exploited to formulate novel therapeutic strategies that specifically target the leukemic microenvironment.

The Intercellular Communication Between Mesenchymal Stromal Cells and Hematopoietic Stem Cells Critically Depends on NF-κB Signalling in the Mesenchymal Stromal Cells.

Mesenchymal stromal cells (MSCs) regulate the fate of the hematopoietic stem cells (HSCs) through both cell-cell interactions and paracrine mechanisms involving multiple signalling pathways. We have previously shown that co-culturing of HSCs with CoCl2-treated MSCs expands functional HSCs. While performing these experiments, we had observed that the growth of CoCl2-treated MSCs was significantly stunted. Here, we show that CoCl2-treated MSCs possess activated NF-κB signalling pathway, and its pharmacological inhibition significantly relieves their growth arrest. Most interestingly, we found that pharmacological inhibition of NF-κB pathway in both control and CoCl2-treated MSCs completely blocks their intercellular communication with the co-cultured hematopoietic stem and progenitor cells (HSPCs), resulting in an extremely poor output of hematopoietic cells. Mechanistically, we show that this is due to the down-regulation of adhesion molecules and various HSC-supportive factors in the MSCs. This loss of physical interaction with HSPCs could be partially restored by treating the MSCs with calcium ionophore or calmodulin, suggesting that NF-κB regulates intracellular calcium flux in the MSCs. Importantly, the HSPCs co-cultured with NF-κB-inhibited-MSCs were in a quiescent state, which could be rescued by re-culturing them with untreated MSCs. Our data underscore a critical requirement of NF-κB signalling in the MSCs in intercellular communication between HSCs and MSCs for effective hematopoiesis to occur ex vivo. Our data raises a cautionary note against excessive use of anti-inflammatory drugs targeting NF-κB.

Extracellular Vesicles Isolated from Mesenchymal Stromal Cells Primed with Hypoxia: Novel Strategy in Regenerative Medicine.

Mesenchymal stromal cells (MSCs) regulate other cell types through a strong paracrine component called the secretome, comprising several bioactive entities. The composition of the MSCs' secretome is dependent upon the microenvironment in which they thrive, and hence, it could be altered by pre-conditioning the MSCs during in vitro culture. The primary aim of this review is to discuss various strategies that are being used for the pre-conditioning of MSCs, also known as "priming of MSCs", in the context of improving their therapeutic potential. Several studies have underscored the importance of extracellular vesicles (EVs) derived from primed MSCs in improving their efficacy for the treatment of various diseases. We have previously shown that co-culturing hematopoietic stem cells (HSCs) with hypoxia-primed MSCs improves their engraftment potential. Now the question we pose is, would priming of MSCs with hypoxia favorably alter their secretome? and would this altered secretome work as effectively as the cell to cell contact did? Here we review the current strategies of using the secretome, specifically the EVs (microvesicles and exosomes), collected from the primed MSCs with the intention of expanding HSCs ex vivo. We speculate that effective priming of MSCs in vitro could modulate the molecular profile of their secretome, which could eventually be used as a cell-free biologic in clinical settings.