Primary Bone Marrow-Derived MSC Subsets Have Distinct Wnt-Signatures Compared to Conventionally Cultured MSC and Differ in Their Capacity to Support Hematopoiesis in Vitro,

Abstract 3414

Mesenchymal stromal cells (MSC) are of promising therapeutic use to suppress immunogenic responses following transplantation, and to support expansion of hematopoietic stem- and progenitors cells (HSPC) from small transplants derived for instance from cord blood. Culture-expanded MSC produce a wide variety and quantity of Wnt-proteins and the crucial role of Wnt-signaling in the hematopoietic stem cell niche is well established. However, studies addressing Wnt-signaling in MSC have (i) only focused on culture-expanded MSC and (ii) did not discriminate between phenotypically distinct subpopulations which are present in bulk cultures of expanded MSC. Recently we identified three new subpopulations of MSC in human bone marrow (BM) based on expression of CD271 and CD146: CD271^brightCD146⁻, CD271^brightCD146⁺, CD271⁻CD146⁺. These fractions co-express the “classical” MSC markers CD90 and CD105 and lack expression of CD45 and CD34 (Maijenburg et al, Blood 2010, 116, 2590). We and others demonstrated that the adult BM-derived CD271^brightCD146⁻ and CD271^brightCD146⁺ cells contain all colony forming units-fibroblasts (Maijenburg et al, Blood 2010, 116, 2590; Tormin et al, Blood 2010, 116, 2594). To investigate how these primary subsets functionally compare to conventional, culture-expanded MSC, we investigated their Wnt-signature and hematopoietic support capacity. To this end, we sorted CD271^brightCD146⁻ and CD271^brightCD146⁺ cells from human adult BM (n=3) and compared their Wnt-signatures obtained by Wnt-PCR array to the profiles from cultured MSC from the same donors.

Fifteen genes were consistently differentially expressed in the two sorted uncultured subsets compared to their conventionally cultured counterparts. Expression of CCND1, WISP1 and WNT5B was strongly increased, and WNT5A was only detected in the conventionally cultured MSC. In contrast, WNT3A was exclusively expressed by sorted primary CD271^brightCD146⁻ and CD271^brightCD146⁺ cells, that also expressed higher levels of JUN, LEF1 and WIF1.

The differences in Wnt (target)-gene expression between CD271^brightCD146⁻ and CD271^brightCD146⁺ cells were more subtle. The Wnt-receptors LRP6 and FZD7 were significantly higher expressed in CD271^brightCD146⁺ cells, and a trend towards increased expression in the same subset was observed for CTNNB1, WNT11 and MYC.

When the sorted subsets were cultured for 14 days (one passage), the differences in Wnt-related gene expression between the subsets was lost and the expanded sorted cells acquired an almost similar Wnt-signature as the MSC cultured from BM mononuclear cells from the same donors. The cultured subsets lost the expression of Wnt3a and gained the expression of Wnt5a, similar to the unsorted MSC cultured from the same donors in parallel. Despite the loss of a distinct Wnt-signature, co-culture experiments combining the sorted MSC subsets with human HSPC revealed that CD271^brightCD146⁺ cells have a significantly increased capacity to support HSPC in short-term co-cultures (2 weeks) compared to CD271^brightCD146⁻ cells (p<0.021, n=3), which was analyzed in hematopoietic colony assays following co-culture. In contrast, a trend towards better long-term hematopoietic support (co-culture for 6 weeks) was observed on CD271^brightCD146⁻ cells.

In conclusion, we demonstrate for the first time that primary sorted uncultured MSC subsets have a distinct Wnt-signature compared to cultured unsorted MSC and display differences in hematopoietic support. As it was recently shown that CD271^brightCD146⁻ and CD271^brightCD146⁺ MSC localize to separate niches in vivo (Tormin et al, Blood 2011), our data indicate that the two MSC subsets are not necessarily distinct cell types and that the different Wnt-signature may be a reflection of these distinct microenvironments. Cell culturing for only one passage dramatically changed the Wnt-signature of the sorted MSC subsets, indicating that Wnt-signaling in in vitro expanded MSC does not resemble the Wnt-signature in their tissue resident counterparts in vivo.