*1.6. Stem Cell Targeted Therapy*

Stem cell therapy has recently gained further attention and momentum as a promising approach to curing T1D through transplantation of (differentiated) stem cell-derived beta cells that are capable of producing insulin in vivo [145]. This form of regenerative medicine influenced therapy relies on the transplantation of autologous stem cell grafts which can act in immunomodulation or assist in insulin production [146]. The aim of this form of therapy is to provide longitudinal resolve to patients suffering from T1D. This also contributes to the mission of precision medicine by providing a long-term solution to patient complications and removing the reliance on expensive, short term therapies such as insulin injections, which can be difficult to obtain and manage across the stratified sociocultural spectrum in the United States and globally [147]. The value added by clinical translation of stem cell therapy for T1D supersedes previous treatments in the temporal dimension due to its ability to provide functioning beta cells to the patient longitudinally, thus providing for a prolonged period of insulin production during which the patient may not need to rely on other drugs or therapies. Very quickly stem cell therapy can then alter the social landscape of treatment for T1D and similar autoimmune diseases through multiple dimensions by, providing relief to the patient both physiologically and financially. Due to the complexity of autoimmune diseases and their turbulent nature, it is the hope of many scientists that stem cell therapy can provide a long-term resolution to such diseases. It is evident that stem cell therapy has the potential to introduce a great deal of paradigm shifts in the current approach to treatment of autoimmune disease such as T1D.

Currently, there are various approaches to stem cell-based transplantation and therapy for T1D that are being explored in the clinic [148–150]. These approaches often involve immunosuppression of the patient to tolerable levels and subsequent transplantation of the autologous stem cells in the patient to avoid immune rejection and further patient autoreactivity [151]. One such study observed the role of autologous nonmyeloablative hematopoietic stem cell transplantation (AHST). Following this treatment, all but 1 of the 15 patients of various gender and age 14 to 31 were able to remain insulin independent for at least 6 months. The study also showed increased C-peptide levels and decreased anti-GAD antibodies, which is a clinically used biomarker for the diagnosis of T1D [148]. This clinical study is evidence of the longitudinal improvement in patient symptoms and physiological complications that result from T1D. Furthermore, it highlights the impact of a combined approach in which immunosuppression followed by AHST is considered the standard. Another study performing autologous stem cell transplantation for the treatment of T1D used mesenchymal stromal stem cells derived from patient umbilical cord, which were transplanted for treatment of T1D [150]. In this study, improvement in C-peptide levels and reduction in insulin dependence was observed for all patients who underwent treatment. This study highlights another type of stem cell, in this case mesenchymal stromal cells, that can be clinically used in transplantation models for diabetic patients. However, it is evident that these models may require imaging tools and modalities that

can allow for the visualization of such parameters as transplant density, biodistribution, viability, and immunogenicity [152–154]. This can permit both short term and longitudinal monitoring of the transplant in the patient and allow for timely intervention, as is the case in many post-transplant graft loss incidents [155]. It can also allow for the visualization of certain molecular (bio)markers that can indicate the existence of specific cellular states or functions [156–158].

Presently, the main modalities used for molecular imaging of stem cells and stem cell transplants are magnetic resonance imaging (MRI) or magnetic particle imaging (MPI) [159,160]. Each of these modalities rely on the utilization of superparamagnetic iron oxide nanoparticles (SPIONs) for targeted molecular imaging of extracellular and intracellular markers. These SPIONs have previously been altered for targeting of immune cells, islet cells, and stem cells amongst many other cell targets [161–163]. They have also been explored for monitoring cell transplants longitudinally [164]. This is particularly useful in the context of providing tools for monitoring of stem cell transplants for the treatment of T1D because of the possibility for post-transplant rejection, due to host immune rejection, issues with cell transplant procedure, or cell viability post-transplant in the patient. The use of nanoparticles to monitoring stem cell transplants during treatment of autoimmune diseases such as T1D opens new doors to paradigms of theranostics and precision medicine, as autologous stem cells are typically used for transplantation and therapeutic purposes. The dynamic array of moieties in the domain of radionuclides, small nucleic acids, and antibodies that can conjugate to nanoparticles provide a platform for targeting specific cells from a heterogenous distribution and performing various forms of combined therapy and imaging (theranostics) [165]. In the context of guiding and improving therapeutic outcomes, nanoparticle-based tracking of stem cells has provided a far more effective and reliable method in vivo when compared to conventional methods such as labeling cells with organic dye or directly labeling them with fluorescent probes, because of their optimal magnetic and optical properties. Although there are a wide variety of structural platforms and nanomaterials with which to engineer theranostic nanoparticles such as silicon and quantum dots, the most biocompatible and clinically used tool for contrast enhancement and targeted therapy are SPIONs. Several studies have highlighted the ability of SPIONs to image and track stem cells post-transplantation in both mice and humans. One such study labeled MSCs with SPIONs and encapsulated these in collagen-based microcapsules for monitoring of the cells post-transplantation [166]. Although this study focused on use of SPIONs for monitoring MSC transplantation for the treatment of myocardial infarctions, SPIONs can also be used for the labeling of differentiated beta cells derived from iPSCs for monitoring of transplant thereof in vivo [167]. Wang et al. have shown imaging of endogenous beta-cell mass through targeting of the glucagon like peptide 1 receptor (GLP-1R) [168]. This was done through conjugation of the exendin 4 to magnetic nanoparticles and subsequent injection of this probe in mice. The group was able to show specific accumulation of the probe in GLP-1R expressing endogenous beta cells and indicated the correlation between reduced signal interference with decreasing beta cell mass over time. This approach can be extrapolated to instances of imaging GLP-1R-expressing, stem cell-derived beta cell transplantations. This also provides a mechanism for the direct targeting of endogenous and transplanted beta cells, regardless of origin, to deliver interventional nanodrugs and therapeutic molecules in vivo. Additionally, these SPIONs have enabled the use of an emerging imaging modality of MPI. Prior studies have performed MPI of human islet cells labeled with dextran-coated SPIONs and transplanted under the left kidney capsule of mice [164]. However, the limitations of MPI result mainly from its inability to decipher viable vs. non-viable cell transplants, especially after a brief period of time where dead cells and their nanoparticles can undergo degradation and generate false positive signals that do not originate from live cells [169]. Despite these limitations, nanoparticles are gaining popularity in their use for monitoring such cell transplants and are continuously being explored as a platform.
