HSCs and their progenitors play a critical role in the production and maintenance of blood cells. These cells possess significant therapeutic potential, particularly in the context of stem cell transplantation and gene therapy. However, a major challenge in utilising HSCs for clinical applications lies in their limited availability. To address this limitation, substantial research efforts have been directed towards developing methods to expand these cells ex vivo while preserving their essential functions.
This study investigated the role of HOXA and HOXB TFs in the expansion of HSPCs, guided by a revised model of haematopoiesis.
Among the TFs analysed, HOXA11 emerged as the most promising candidate for promoting the expansion of MPPs. Despite a reduction in the overall fitness of cells expressing HOXA11, this TF led to a 76.6 ± 87.0-fold expansion of MPPs.
Notably, HOXA11 expression was associated with an increased expression of key stem cell markers, including CD49f and CD201. The expansion was accompanied by a temporary block in differentiation, seen in the CFU assay, which was successfully reversed using an inducible HOXA11 system. Thus, the TetOn system enabled the precise expansion of MPPs capable of multilineage differentiation. Additionally, this study showed that the effects of HOXA11 on MPP expansion are primarily cell-autonomous and require high levels of expression to achieve this outcome.
A transcriptome analysis of HOXA11 expanded MPPs provided further mechanistic insights into this process. Pathways associated with cellular quiescence and self-renewal were significantly upregulated, consistent with the observed expansion capability. At the same time, pathways promoting differentiation were downregulated, reinforcing the differentiation block observed during expansion. Importantly, the analysis revealed an enhanced expression of genes critical for homing and engraftment, such as CXCR4, ITGB2, and CD44, indicating that HOXA11 expanded MPPs may possess superior therapeutic potential. However, the upregulation of pathways linked to tumorigenesis also raises concerns, underscoring the necessity of further investigation.
Overall, this study positions HOXA11 as a promising candidate for the ex vivo expansion of HSPCs, paving the way for more effective stem cell-based therapies. While the results are compelling, additional in vivo studies will be essential to exclude the risk of oncogenic potential. The reversible nature of the differentiation block, as demonstrated with the TetOn system, suggests that pathways associated with tumorigenesis may be suppressed upon the withdrawal of HOXA11 expression.