Allogeneic Chondrogenic Mesenchymal Stromal Cells Alter Helper T Cell Subsets in CD4+ Memory T Cells

Implantation of chondrogenically differentiated mesenchymal stromal cells (MSCs) leads to bone formation in vivo through the process of endochondral ossification. The use of allogeneic MSCs for this purpose may be a promising new approach to replace the current gold standard of bone regeneration. However, the success of using allogeneic cells depends on the interaction between the implanted cells and the host's endogenous immune cells. Th17 T cells and other CD4 helper T cell subtypes have been shown to negatively impact chondrogenesis, however, it is unclear how the interaction between these cells affects bone regeneration mediated by these cells. The aim of the current work was to assess the effect of chondrogenic MSC pellets on Th1, Th2, Th17, and regulatory T cells in vitro. Human MSCs were nonchondrogenic (-TGFβ3) and chondrogenically (+TGFβ3) differentiated for 7 or 21 days. Memory T cells (sorted from the CD4 population of peripheral blood mononuclear cells [PBMCs]), as well as total PBMCs were cocultured with allogeneic nonchondrogenic and chondrogenic MSC pellets for 3 days. Seven-day differentiated allogeneic nonchondrogenic and chondrogenic MSC pellets that were cocultured with memory T cells resulted in a significant increase in Th2 and a decrease in Th1 T cells. Furthermore, the co-culture of 21-day differentiated nonchondrogenic and chondrogenic MSC pellets with memory T cells resulted in a significant increase in Th2 and Th17 T cells, as well as a decrease in Th1 and regulatory T cells. Interleukin (IL)-6 was identified as a predominant cytokine involved in this interaction between allogeneic chondrogenically differentiated MSC pellets and memory CD4 T cells, with high levels of IL-6 being secreted in the supernatants of this cocultured condition. The findings of this study highlight the potential of chondrogenically differentiated MSC pellets to alter the ratio of Th1 and Th2 as well as Th17 and regulatory T cell subsets. Additional analysis investigating bone formation by chondrogenically differentiated MSCs in an allogeneic setting may identify a novel role of these T cell subsets in bone regeneration processes mediated by chondrogenically differentiated MSCs. Impact statement Allogeneic mesenchymal stromal cells (MSCs) have the potential to be an off-the-shelf treatment for bone repair. However, the lack of knowledge of the immune cells involved in this process has hampered the progression to the clinic. The current study has shown that allogeneic chondrogenic MSCs have the potential to skew the ratio of specific helper CD4 T cell subsets in vitro. This has now provided insight for future in vivo experiments to investigate the role of these T cell subsets in the early stages of bone regeneration mediated by allogeneic chondrogenic MSCs.