Mesenchymal stem cells (MSCs) have stimulated great interest because of their immunomodulatory and anti-inflammatory properties, and they hold great promise in regenerative medicine and immunotherapy.

To enhance the efficacy of MSCs as a cellular therapy, it is crucial to determine the mechanisms through which they exert their immunomodulatory effects. While the precise pathways governing the immunomodulatory actions of MSCs remain elusive, accumulating evidence points towards their ability to suppress immune responses via direct cell-to-cell interactions and the secretion of immune regulatory molecules.

Here are some of the key mechanisms through which MSCs exert their immunomodulatory effects:

1. Suppression of T-cell Proliferation:
   • MSCs can inhibit the proliferation and activation of T cells, which are central players in adaptive immune responses. They can modulate both CD4+ and CD8+ T cell responses, thereby dampening the immune reaction.
2. Regulation of B Cells:
   • MSCs can influence B cell function, including proliferation, differentiation, and antibody secretion. They can suppress the formation of plasma cells and the production of antibodies.
3. Induction of Regulatory T Cells (Tregs):
   • MSCs promote the generation and expansion of regulatory T cells (Tregs), which are crucial for maintaining immune tolerance and suppressing excessive immune responses.
4. Modulation of Dendritic Cells (DCs):
   • MSCs can alter the maturation and function of dendritic cells, which are antigen-presenting cells critical for initiating adaptive immune responses. MSCs can induce a tolerogenic phenotype in DCs, reducing their ability to stimulate T cell activation.
5. Secretion of Anti-inflammatory Molecules:
   • MSCs produce and release various anti-inflammatory molecules, such as prostaglandin E2 (PGE2), transforming growth factor-beta (TGF-β), indoleamine 2,3-dioxygenase (IDO), interleukin-10 (IL-10), and hepatocyte growth factor (HGF). These molecules can suppress inflammation and modulate immune cell function.
6. Interactions with Innate Immune Cells:
   • MSCs can interact with innate immune cells, including macrophages and natural killer (NK) cells, influencing their activation and function. MSCs can promote the polarisation of macrophages toward an anti-inflammatory M2 phenotype.

Humanised Mouse Models

Utilising mouse models in preclinical studies represents a critical step in the evaluation and optimisation of MSCs immunomodulatory effects prior to clinical translation. Mouse models offer a valuable platform for investigating the safety, efficacy, and mechanisms underlying MSC-based interventions in a controlled laboratory setting. These models enable researchers to simulate various disease states and immune responses, providing insights into the potential therapeutic applications of MSCs across a spectrum of disorders, including autoimmune diseases, inflammatory conditions, and tissue injuries.

Humanised mice are increasingly used as models to study immune-mediated disease, as they simulate human immunobiology more closely than conventional murine models. These mice are immunocompromised mice in which the immune system has been reconstituted with human immune cells. These humanised mice serve as a tool to develop a better understanding of the mode of action of MSCs in mitigating the immune response in an in vivo environment that closely resembles human immunobiology. Here are some common types of humanised mouse models:

1. Human Peripheral Blood Monocytes Cells (PBMCs) Engrafted Mice:
   • These mice are generated by injecting human peripheral blood monocytes cells (PBMCs) into immunocompromised mice. The PBMCs engraft and populate the mouse peripheral blood, providing a short-term model to study human immune responses.
2. Human CD34+ Hematopoietic Stem Cells (CD34+ HSC) Mice:
   • CD34+ HSC mice are generated by engrafting immunocompromised mice with human CD34+ HSCs typically derived from umbilical cord blood. This results in the development of a human immune system within the mouse, including T cells, B cells, and myeloid cells.
3. Bone Marrow, Liver, Thymus (BLT) Humanised Mice:
   • BLT humanised mice are generated by transplanting human foetal liver and thymus tissues under the kidney capsule and injecting human CD34+ hematopoietic stem cells into the mouse. This model supports the development of a more complete human immune system, including T cell education in a human thymic environment.

Table 1: Comparison between CD34+, PBMC and BLT generated humanised mice.

Humanised Mice as Models to Study the Immunomodulatory Effects of MSCs

Humanised mice serve as invaluable models for investigating the immunomodulatory effects of MSCs before their clinical application. Given that MSCs used in clinical settings often originate from allogeneic sources, they have the potential to trigger an immune response in recipients. Hence, it is crucial to assess the immunogenicity and safety profile of MSCs in preclinical mouse models, with humanised mice proving to be particularly beneficial for this purpose.

In a safety assessment study by Lee et al., a humanised mouse model was established by intravenously injecting CD34+ cells into immunocompromised NOD/SCID IL2γnull (NSG) mice to examine the immunological safety of allogeneic human MSCs. The findings indicated that allogeneic umbilical cord-derived MSCs exhibit low immunogenicity both in vitro and in vivo, thereby deemed “immunologically safe” for allogeneic clinical applications.

Another study by Roemeling-van Rhijn et al. explored the use of MSCs as alternative immunosuppressive therapies for allograft rejection. They evaluated the immunomodulatory efficacy of bone marrow-derived MSCs (BM-MSCs) and adipose tissue-derived MSCs (AT-MSCs) in a PBMC humanised servere combined immunodeficiency (SCID) mouse model transplanted with allogeneic human skin grafts. The results demonstrated that both BM-MSCs and AT-MSCs effectively suppressed alloreactivity towards the skin graft, highlighting the potential of humanised SCID-PBMC models for evaluating MSC immunosuppressive efficacy in allograft rejection scenarios.

In the context of the treatment of Graft-versus-Host Disease (GvHD), which is a severe complication following allogeneic hematopoietic stem cell transplantation (HSCT), NSG-PBMC humanised mice serve as pertinent models for aGvHD. In a study discussed, human MSC cell therapy significantly prolonged the survival of NSG mice with aGvHD while reducing target organ pathology. This underscores the utility of NSG-PBMC GvHD mice as suitable models for elucidating the mechanisms underlying MSC immunosuppression and assessing the potential of MSCs as cellular-based therapies for GvHD.

The utilisation of humanised mouse models represents a pivotal advancement in the evaluation of MSC immunomodulatory effects before its clinical translation. The studies discussed underscore the importance of assessing the immunogenicity and safety profile of MSCs, particularly in the context of allogeneic transplantation, where the risk of immune rejection is heightened. Such assessments facilitate the examination of MSCs’ immunomodulatory properties and their potential therapeutic utility in addressing conditions like allograft rejection and GvHD. The findings from these studies provide valuable insights into the efficacy, safety, and mechanisms underlying MSC-based interventions, thus guiding the development of novel cellular therapies with enhanced clinical efficacy and reduced adverse effects. Moving forward, continued research in humanised mouse models will play a pivotal role in advancing our understanding of MSC immunobiology and facilitating the translation of MSC-based therapies from bench to bedside, ultimately offering new avenues for the treatment of debilitating diseases and improving patient outcomes.

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