ABSTRACT
Mimicking endochondral bone formation is a promising strategy for bone regeneration. To become a successful therapy, the cell source is a crucial translational aspect. Typically, autologous cells are used. The use of non-autologous mesenchymal stromal cells (MSCs) represents an interesting alternative. Nevertheless, non-autologous, differentiated MSCs may trigger an undesired immune response, hampering bone regeneration. The aim of this study was to unravel the influence of the immune response on endochondral bone regeneration, when using xenogeneic (human) or allogeneic (Dark Agouti) MSCs. To this end, chondrogenically differentiated MSCs embedded in a collagen carrier were implanted in critical size femoral defects of immunocompetent Brown Norway rats. Control groups were included with syngeneic/autologous (Brown Norway) MSCs or a cell-free carrier. The amount of neo-bone formation was proportional to the degree of host-donor relatedness, as no full bridging of the defect was observed in the xenogeneic group whereas 2/8 and 7/7 bridges occurred in the allogeneic and the syngeneic group, respectively. One week post-implantation, the xenogeneic grafts were invaded by pro-inflammatory macrophages, T lymphocytes, which persisted after 12 weeks, and anti-human antibodies were developed. The immune response toward the allogeneic graft was comparable to the one evoked by the syngeneic implants, aside from an increased production of alloantibodies, which might be responsible for the more heterogeneous bone formation. Our results demonstrate for the first time the feasibility of using non-autologous MSC-derived chondrocytes to elicit endochondral bone regeneration in vivo. Nevertheless, the pronounced immune response and the limited bone formation observed in the xenogeneic group undermine the clinical relevance of this group. On the contrary, although further research on how to achieve robust bone formation with allogeneic cells is needed, they may represent an alternative to autologous transplantation.
ABSTRACT
In engineering of tissue analogues, upscaling to clinically-relevant sized constructs remains a significant challenge. The successful integration of a vascular network throughout the engineered tissue is anticipated to overcome the lack of nutrient and oxygen supply to residing cells. This work aimed at developing a multiscale bone-tissue-specific vascularisation strategy. Engineering pre-vascularised bone leads to biological and fabrication dilemmas. To fabricate channels endowed with an endothelium and suitable for osteogenesis, rather stiff materials are preferable, while capillarisation requires soft matrices. To overcome this challenge, gelatine-methacryloyl hydrogels were tailored by changing the degree of functionalisation to allow for cell spreading within the hydrogel, while still enabling endothelialisation on the hydrogel surface. An additional challenge was the combination of the multiple required cell-types within one biomaterial, sharing the same culture medium. Consequently, a new medium composition was investigated that simultaneously allowed for endothelialisation, capillarisation and osteogenesis. Integrated multipotent mesenchymal stromal cells, which give rise to pericyte-like and osteogenic cells, and endothelial-colony-forming cells (ECFCs) which form capillaries and endothelium, were used. Based on the aforementioned optimisation, a construct of 8 × 8 × 3 mm, with a central channel of 600 µm in diameter, was engineered. In this construct, ECFCs covered the channel with endothelium and osteogenic cells resided in the hydrogel, adjacent to self-assembled capillary-like networks. This study showed the promise of engineering complex tissue constructs by means of human primary cells, paving the way for scaling-up and finally overcoming the challenge of engineering vascularised tissues.
