Mesenchymal Stromal Cells and Macrophages: Synergy for Spinal Cord Repair

Traumatic SCI initially causes mechanical rupture of the spinal cord nervous tissue resulting in immediate loss of function. The ability of damaged spinal cord tissue to self-repair is poor and, consequently, functional impairments are lasting. The primary injury triggers a cascade of cellular and molecular events that lead to secondary damage and injury cavity expansion. Macrophages are one of the immune cells that access the damaged nervous tissue after injury to contribute to repair. But, unlike the inflammatory phases observed in regenerative tissues after damage, macrophages in the injured spinal cord fail to transition from debris clearing, cytotoxic pro-inflammatory phenotypes to anti-inflammatory, reparative, tissue remodeling phenotypes. Secondary damage is exacerbated by the chronic presence of pro-inflammatory macrophages. The preservation of nervous tissue after SCI is key to the success of regenerative and rehabilitative therapeutic approaches and inducing a shift from pro-inflammatory to an anti-inflammatory macrophages in the injury site may result in neuroprotection. Mesenchymal stromal cells (MSC) transplantation into the damaged spinal cord can result in immunomodulation and revascularization, which was associated with neuroprotection in some studies, but the mechanisms by which MSC exert their therapeutic actions remain incompletely understood, and their translational efficacy is currently insufficient. The research described in this thesis evolves around the immunobiology of MSC. MSC are known to communicate with macrophages when they participate in successful tissue repair. The specific aims of this thesis were: (A) to increase our understanding of the interactions between MSC and macrophages; and (B) to investigate strategies that benefit from MSC-macrophage interactions to improve the therapeutic potential of MSC transplants for SCI repair. The strategies used in our second aim were: (1) priming MSC with macrophage-derived inflammation; (2) enhancing immunomodulation using an immunomodulatory biomaterial as MSC transplant matrix; and (3) overexpressing the anti-inflammatory cytokine interleukin-10 (IL10) in transplanted MSC. Chapter 1 provides an overview of the inflammatory events that occur after SCI, the role therein of different macrophage phenotypes, the previously known functions of MSC in SCI repair, and their interactions with macrophages for SCI repair. In Chapter 2, the reciprocal interactions of rat bone marrow-derived MSC and macrophages were investigated in vitro. We show that pro-inflammatory macrophage conditioned medium (CM) activates MSC increasing their secretion of VEGF, IL10, and TNFα, while anti-inflammatory macrophage CM activates MSC increasing their secretion of PDGF, which led to the generation of an interaction model for these cells. In Chapter 3, we used RNAseq and immunoassays to show that pro-inflammatory macrophage CM induces a transcriptomic shift in MSC resulting in upregulation of (neuro)trophic and immunomodulatory factors that are key for tissue repair, and upregulating pro-apoptotic genes and downregulating cell cycle associated genes in MSC. In Chapter 4, we found that acute transplantation of primed MSC after SCI resulted in transient immunomodulation of the injury site macrophages, but not in neuroprotection or functional recovery. In Chapter 5, we found that acute co-transplantation of MSC with immunomodulatory biomaterial NHC into a rat contusive SCI increased the neuroprotective and immunomodulatory effect of MSC. In Chapter 6, we showed that while overexpression of anti-inflammatory cytokine IL10 in MSC has a mild anti-inflammatory effect on macrophages in vitro, transplantation of IL10-overexpressing MSC into a rat contusive SCI did not result in enhanced immunomodulation and induced extended tissue damage compared with controls. Overall, this thesis generated new knowledge about the interaction mechanisms of MSC and macrophages, and provided relevant insight for future applications of MSC transplantation therapies for SCI.