Harnessing MSC Osteoprogenitor Subpopulations to Augment Hematopoietic Transplantations

Adjuvant MSC infusions could improve transplantation outcomes via suppression of graft-vs-host reactions and enhancement of HSPC engraftment. However, bone marrow (BM) MSCs are a heterogeneous cell population, particularly when culture expanded to obtain therapeutically relevant cell numbers. A systematic study of expanded subpopulations is hindered by the lack of unique surface markers. In lieu of immunophenotype, we hypothesized and verified that biophysical properties of MSCs can be a useful strategy for isolating unique subpopulations. Here, we will describe a clinically relevant biophysical strategy for deriving different MSC subpopulations and discuss their potential utility in the context of hematopoietic transplantation. Cell diameter, membrane stiffness and nuclear membrane fluctuations are collectively predictive of at least two MSC subpopulations (multipotent and osteoprogenitors). Both MSC subpopulations are distinct in their transcriptome (different signatures of VEGF, IL8, ANG1, etc) and differentiation potential (osteogenic and adipogenic) but no surface marker reliably distinguishes them apart. In functional assays, osteoprogenitor vs multipotent MSCs show stronger capacities for CD34+CD38−HSPC expansion (Day 10: 42.7 ± 6.21 vs 29.1 ± 8.27, n = 5, respectively), but no significant differences (n = 3 biological repeats) in their ability to suppress CD8+T-cell proliferation in mixed lymphocyte reactions. When used as a rescue therapy for lethal irradiation, osteoprogenitors improved survival rates by 30-40% compared to multipotent MSCs. Part of the recovery process was prompted by MSC secretome mediated stimulation of BM tissue repair, including increased cell proliferation, reduced inflammation and the restoration of niche environments. These paracrine effects may be beneficial in fostering favorable post-myeloablative BM environments for donor HSPCs to home, engraft and survive. In murine transplantation models, we observed a higher rate of early engraftment following adjuvant therapy with osteoprogenitors. Week 4 CD45 chimerism in peripheral blood was 2- to 3- fold higher (∼11% vs ∼4%) in the osteoprogenitor treated group compared to the other MSC groups. These results demonstrate heterogeneity in expanded MSCs populations that would lead to inconsistencies in their overall properties. Newer biophysical technologies have the potential for isolating useful subpopulations defined by key quality attributes for specific forms of MSC-therapy; these capabilities may advance MSC-based therapy for regenerative medicine. Here, we demonstrate the augmentation of HSPC transplants in murine models using a biophysically isolated osteogenic MSC subpopulation (osteoprogenitors) which led to the fastest engraftment rates; this may be due to their superior abilities for promoting BM tissue regeneration as well as hematopoietic expansion.