Mesenchymal stromal cell (MSC)- based control of angiogenesis and inflammation in cartilage formation

The overall goal of my PhD studies was to analyze how the regulation of angiogenesis, a key factor that plays very important roles in tissue repair, could ultimately influence cartilage formation by mesenchymal stromal cells (MSC) and by differentiated nasal chondrocytes (NC) as a proof of concept.
Due to their multipotency and great self-renewal capacity, MSC is a highly attractive cell source for cartilage formation and regeneration. MSC have immunomodulatory properties and are in direct contact with inflammatory cells (monocytes) in the cartilage repair environment. By directly regulating the inflammatory response of monocytes and by regulating their phenotype, MSC could significantly benefit from their ability to control inflammatory events during cartilage repair processes. Thus, we also assessed whether MSC could modulate the properties and the phenotype of monocytes in order for monocytes to actively aid MSC in cartilage repair.
In chapter 1, we sought a proof of principle to confirm that the blocking of angiogenesis does indeed improve cartilage formation when using genetically-modified nasal chondrocytes (NCs) or antiangiogenic peptides associated with NCs. For this purpose, NCs were genetically-modified to express mouse soluble VEGF receptor-2 (sFlk-1) or were associated with an antiangiogenic peptide in order to have their chondrogenic capacity assessed in vitro and in vivo. Improved cartilage regeneration could be observed after in vivo implantation of NCs in an ectopic nude mouse model. Whereas the anti-angiogenic approaches did not improve chondrogenesis in vitro, frank chondrogenesis occured in vivo only in the constructs generated by NCs expressing sFlk-1 or treated with the peptide. Blood vessel ingrowth was significantly hampered in the anti-angiogenic experimental groups when compared with naïve NCs, which correlated with chondrogenis improvement. Strikingly, the anti-angiogenic effect was even more evident when NC donors with low chondrogenic capacity were used, and no-preculture with chondrogenic soluble factors was performed prior to in vitro implantation of the constructs.
In chapter 2, we investigated how angiogenesis control by genetically-modified bone marrow-derived MSC could augment their chondrogenic potential in vivo. These MSC were genetically modified to release sFlk-1, the soluble version of VEGF receptor 2 (VEGFR2), which acted as a decoy receptor and could sequester VEGF from the immediate surroundings of MSC. Importantly, no external morphogens were supplemented in order for MSC chondrogenic differentiation to occur. Moreover, the in vivo chondrogenic capacity of sFlk-1-releasing MSC was assessed up to 12 weeks by an ectopic nude mouse model, in which collagen sponges seeded with MSC were implanted subcutaneously. Angiogenesis, as analyzed by blood vessel invasion, was markedly reduced in the constructs seeded with sFlk-1-releasing MSC. Frank and stable cartilage formation was only achieved once VEGF was blocked.
In chapter 3, we aimed at investigating whether MSC can instruct monocytes to acquire traits of mesenchymal progenitors or tissue repair macrophages when in direct or indirect contact with the latter. Thus, MSC could instruct monocytes to assist in the tissue-repairing process. MSC and monocytes were cocultured either in 3D collagen sponges or in transwells for a period of five consecutive days. We demonstrated that MSC instruct monocytes into acquiring a hybrid macrophage-mesenchymal phenotype, which varies greatly depending on which kind of contact with MSC the monocytes were exposed to (whether direct cell-to-cell contact or through the release of soluble factors by MSC). However, future studies will be needed to elucidate the mechanism in which MSC could instruct monocytes into differentiating into this hybrid state, and how one could better control this process in order for monocytes to optimally aid MSC in cartilage repair.