Introduction
Owing to their inherent hematopoiesis-supportive properties, mesenchymal stromal cells (MSCs) are frequently used for the expansion of hematopoietic stem cells (HSCs) in vitro [1,2]. In vitro expansion is needed for cord blood (CB) samples as the HSCs obtained from one CB unit are not sufficient for transplantation in an adult recipient [3,4]. Likewise, the MSCs are co-infused with the HSCs into the recipients to improve the success of stem cell transplantation [5-8]. However, MSCs are sourced from various hematopoietic and non-hematopoietic sources – the hematopoiesis-supportive properties of MSCs isolated from non-hematopoietic sources might not be at par with those isolated from the hematopoietic tissues. Donors’ age is also an important factor governing the functionality of the MSCs [9,10]. Due to their low number in the tissues, MSCs need a long-term in vitro expansion – a process known to affect their regenerative potency [11]. Considering all these possibilities, priming the MSCs with various agents to boost their regenerative properties has become a focus of several studies [12-16]. However, such manipulation needs to be time efficient and cost-effective .
MSCs are typically characterized by their phenotypic and trilineage differentiation ability [17]; however, these parameters do not reflect their ability to support hematopoiesis. Long-term culture-initiating-cell assay (LTC-IC) is routinely used to determine the functionality of HSCs. Its readout not only helps estimate the number of primitive HSCs present in the sample but also indirectly reflects on the functionality of the stromal cells used as the feeder layers. However, this assay takes about 10-12 weeks to complete, a timeline that may not be suitable for a clinical setup. Hence, there is a need to develop functional assays which can quickly judge the hematopoiesis-supportive ability of the MSCs [18,19].
I have earlier shown that priming bone marrow-derived MSCs (BMSCs) with Transforming Growth Factor β1 (TGFβ1) boosts their hematopoiesis-supportive ability via AKT-eNOS axis [18]. In the present study, I examined whether the BMSCs briefly primed with pharmacological activators of downstream effectors of TGFβ1 pathway could also boost their hematopoiesis-supportive ability. Indeed, I found that short-term treatments of BMSCs with activators of protein kinase C (PKC) and intracellular calcium [Ca2+]i and various fibronectin (FN)- and integrin-specific bioactive peptides boost the potency of BMSCs as evidenced by the formation of a significantly higher number of colonies in semi-solid media and a rapid expansion of CD34+ HSPCs from the BM-derived cells briefly interacted with them. Such an approach comprising priming the BMSCs with pharmacological compounds for a short duration and briefly exposing the HSCs to them can be used in clinical settings to improve the efficacy of stem cell transplantations. This concept would be helpful in other regenerative medicine protocols after identifying suitable pharmacological modulators giving desired effects on the target cells [14,20].
These data demonstrate that it is possible to boost the potency of BMSCs using various small molecule pharmacological agents targeting downstream effectors of the TGFβ1 pathway. Also, the assays used in the present study can be used as quick parameters to judge the functionality of BMSCs to be used in clinical applications, and to identify compounds having salutary effects on the hematopoiesis-supportive ability of BMSCs.