Mesenchymal stem cells: biology and clinical potential in type 1 diabetes therapy

Mechanical Signals As a Non-Invasive Means to Influence Mesenchymal Stem Cell Fate, Promoting Bone and Suppressing the Fat Phenotype

Pluripotent mesenchymal stem cells (MSCs) are considered ideal therapeutic targets in regenerative medicine, as they hold the capacity to differentiate into higher order connective tissues. The potential to harness MSCs for disease treatment and acceleration of repair will ultimately depend on an improved understanding of how physical and/or chemical signals regulate their activity, and the ability of exogenous stimuli to enhance MSC proliferation and define MSC fate. Recent appreciation that bone marrow osteoprogenitors are inversely proportional to adipocyte precursors suggests that their shared progenitor, the MSC, will commit to one lineage at the cost of the other. This interrelationship may contribute to the phenotype of sedentary subjects who have more fat and less bone, while conversely, to the outcome of exercise being less fat and more bone. Mechanical biasing of MSC lineage selection suggests that physical signals may influence the quantity of both fat and bone through developmental, as well as metabolic or adaptive pathways. Considered with the recent finding that low magnitude mechanical signals (LMMS) suppress the development of subcutaneous and visceral fat without elevating energy expenditure, this indicates that MSCs are ideally positioned as mechanosensitive elements central to musculoskeletal adaptation, but that the signals needn't be large to be influential. The biasing of MSC differentiation by mechanical signals represents a unique means by which adiposity can be inhibited while simultaneously promoting a better skeleton, and may provide the basis for a safe, non-invasive, non-pharmacologic strategy to prevent both obesity and osteoporosis, yet uniquely - without targeting the resident fat or bone cell.

Can we learn from viruses how to prevent type 1 diabetes?: the role of viral infections in the pathogenesis of type 1 diabetes and the development of novel combination therapies

We will take a journey from basic pathogenetic mechanisms elicited by viral infections that play a role in the development of type 1 diabetes to clinical interventions, where we will discuss novel combination therapies. The role of viral infections in the development of type 1 diabetes is a rather interesting topic because in experimental models viruses appear capable of both accelerating as well as decelerating the immunological processes leading to type 1 diabetes. Consequently, I will discuss some of the underlying mechanisms for each situation and consider methods to investigate the proposed dichotomy for the involvement of viruses in human type 1 diabetes. Prevention of type 1 diabetes by infection supports the so-called "hygiene hypothesis." Interestingly, viruses invoke mechanisms that need to be exploited by novel combinatorial immune-based interventions, the first one being the elimination of autoaggressive T-cells attacking the beta-cells, ultimately leading to their immediate but temporally limited amelioration. The other is the invigoration of regulatory T-cells (Tregs), which can mediate long-term tolerance to beta-cell proteins in the pancreatic islets and draining lymph nodes. In combination, these two immune elements have the potential to permanently stop type 1 diabetes. It is my belief that only combination therapies will enable the permanent prevention and curing of type 1 diabetes.

Human islet-derived precursor cells can cycle between epithelial clusters and mesenchymal phenotypes

We showed previously that undifferentiated, proliferating human islet-derived precursor cells (hIPCs) are a type of mesenchymal stem/stromal cell (MSC) that can be induced by serum deprivation to form clusters and ultimately differentiate in vitro to endocrine cells. We also demonstrated that partially differentiated hIPC clusters, when implanted under the kidney capsules of mice, continued to differentiate in vivo into hormone-producing cells. However, we noted that not all hIPC preparations yielded insulin-secreting cells in vivo and that in some animals no hormone-expressing cells were found. This suggested that the implanted cells were not always irreversibly committed to further differentiation and may even de-differentiate to a mesenchymal phenotype. In this study, we show that human cells with a mesenchymal phenotype are indeed found in the grafts of mice implanted with hIPCs in epithelial cell clusters (ECCs), which are obtained after 4-day in vitro culture of hIPCs in serum-free medium (SFM); mesenchymal cells were predominant in some grafts. We could mimic the transition of ECCs to de-differentiated mesenchymal cells in vitro by exposure to foetal bovine serum (FBS) or mouse serums, and to a significantly lesser extent to human serum. In a complementary series of experiments, we show that mouse serum and FBS are more effective stimulants of mesenchymal hIPC migration than is human serum. We found that proliferation was not needed for the transition from ECCs to de-differentiated cells because mitomycin-treated hIPCs that could not proliferate underwent a similar transition. Lastly, we show that cells exhibiting a mesenchymal phenotype can be found in grafts of adult human islets in mice. We conclude that epithelial-to-mesenchymal transition (EMT) of cells in hIPC ECCs can occur following implantation in mice. This potential for EMT of human islets or differentiated precursor cells must be considered in strategies for cell replacement therapy for diabetes.

Minimization and withdrawal of steroids in pancreas and islet transplantation

For reducing the corticosteroid (CS)-related side-effects, especially cardiovascular events, CS-sparing protocols have become increasingly common in pancreas transplantation (PT). Lympho-depleting induction antibodies, such as rabbit anti-thymocyte globulin (rATG) or alemtuzumab, have been widely used in successful trials. The results of various CS-sparing protocols combining calcineurin inhibitors (CNI) and mycophenolate or sirolimus, have been mixed for rejection and survival rates. Most of the studies were uncontrolled trials of low-risk patients, therefore the grade of evidence is limited. Large-scale prospective studies with long-term follow up are necessary to assess risks and benefits of CS-sparing regimens in PT before recommending such strategies as standard practice. Islet allo-transplantation for patients with brittle type 1 diabetes mellitus, less invasive and safer procedure than PT, has been attempted since late 1980s, but diabetogenic immunosuppressants at maintenance, mainly CS and high-dose CNI, prevented satisfactory results (10% insulin-independence at 1-year post-transplant). Since 2000, CS-free and CNI-reducing protocols, including more potent induction [daclizumab, OKT3gamma1(ala-ala) anti-CD3 antibody, rATG] and maintenance (sirolimus, mycophenolate) agents, have significantly improved short-term outcomes whereas long-term are still inadequate (from 80% to 20% insulin-independence from 1- to 5-year post-transplant). Main limitations are allo- and autoimmunity, immunosuppression-related islet and systemic toxicity and transplant site unsuitability, which tolerogenic protocols and biotechnological solutions may solve.