MSCs are a promising therapy for the treatment of CNS-related autoimmune diseases due to their immunomodulatory and neuroprotective effects. However, the source and availability of MSCs is becoming a crucial issue for their clinical application. BMSCs, the most studied MSCs, have demonstrated the ability to ameliorate both chronic and relapsing-remitting EAE [38–40]. Although BMSCs have demonstrated promising results, the invasive nature of bone marrow biopsies may limit their practicality for wider clinical applications. Adipose tissue has become an appealing cell source for regenerative medicine and tissue engineering, since it contains a large number of ASCs, is easy to obtain in large volumes, and is easily accessible [41–44]. ASCs have been shown to hold many of the same properties as BMSCs, such as the ability to differentiate, inhibit T-cell activation and proliferation, produce anti-inflammatory molecules, and aid in tissue repair through the secretion of cytokines .
Despite the promising potential of ASCs, the need for ex vivo cellular expansion still presents a significant challenge for human applications. The uncultured counterpart of ASCs, the SVF, is a particularly promising candidate for regenerative medicine because the cells can be isolated within hours of obtaining the lipoaspirate and no culture expansion of the cells is required, which would reduce any potential risks associated with growing cells in vitro and remove the need for specialized laboratories. In addition to the clinical risks of ex vivo expansion, the variables used in cell culture, such as percent and source of serum used, type of basal media, media supplements, culture surface substrate, cell seeding density, passage number, and confluency of culture, are undoubtedly giving way to contrasting and confusing results in research.
Adipose tissue comprises one of the largest organs in the body and serves as an important endocrine organ regulating many facets of homeostasis [26, 46, 47]. Comprised of mature adipocytes and other nonadipocyte cells, adipose tissue can be manually disrupted and/or treated with collagenase to isolate the SVF. Although not a fully defined cell population, the SVF includes vascular smooth muscle cells, fibroblasts, mast cells, macrophages, lymphocytes, endothelial cells, pre-adipocytes, and ASCs [48–52].
SVF cells have been used clinically to treat acute and chronic diseases afflicting a range of tissues and organs, including soft tissue defects, breast reconstruction, and autoimmune diseases such as graft-versus-host-disease, rheumatoid arthritis, and Crohn’s disease [27, 30, 41, 46, 53]. To date only one group has demonstrated improved function in MS patients treated with SVF; however, mechanisms behind the improvements were not explored . Similarly, only one study showed that culture-expanded murine ASCs had a significant beneficial effect on chronic EAE by acting simultaneously in the lymphoid organs as well as the inflamed CNS and causing a dramatic change in antigen-specific T cells . This present study is the first to investigate human ASCs and SVF cells in the treatment of EAE. Although SVF cells had similar mean maximum disease scores and time of disease onset to ASCs, the SVF had lower cumulative disease score. We previously compared the ability of murine SVF with ASCs in the same EAE model and showed that the SVF effectively inhibited disease severity and was statistically more effective than ASCs . Unlike the human SVF, which had a disease incidence of 10 out of 12 mice (Table 1), the murine SVF only had 3 out of 12 mice demonstrate clinical signs . EAE mice treated with human SVF had a disease onset of 9.3 ± 1.6 (Table 1) while EAE mice treated with murine SVF had a disease onset of 15 ± 4.5 days post disease induction (unpublished findings). The disparate results between human and mouse SVF may be due to species differences or some undefined mechanism(s). The current results show that uncultured SVF can ameliorate clinical symptoms as well as reduce spinal cord inflammation, demyelination, and axonal damage without the dangers associated with ex vivo cellular expansion.
Furthermore, SVF treatment had a similar effect on the systemic immune response in EAE mice. IFNγ is a cytokine associated with a number of autoinflammatory and autoimmune diseases, due to its role in Th1 cell stimulation, differentiation, and function via STAT1 and STAT4 pathways . These autoreactive T cells play a central role in the direct regulation of T-cell activation and survival during autoimmune inflammation in the pathogenesis of MS and EAE [37, 55]. In this study, IFNγ was reduced comparably between treatment groups. These results point to the ability of SVF cells to play an effector role similar to that of both ASCs and BMSCs, which would occur during the early inflammatory phase of disease, supporting the possibility that uncultured SVF cells could also affect the generation of encephalitogenic effector T cells. Although BMSCs have been shown to reduce levels of IFNγ by direct contact, it is unclear whether ASCs and SVF cells utilize the same mechanism . Although the mechanisms mediating such effects are still only partially understood, it is likely that they involve both direct cell-to-cell contact and paracrine signaling through soluble factors.
In addition to IFNγ, IL-12 is responsible for Th1 cell stimulation, differentiation, and function and plays a central role in the pathology of MS . In this study, IL-12 was reduced in the BMSC-treated group and further reduced in the SVF-treated group. Although this is the first study to show that SVF treatment reduced the levels of IL-12, BMSCs have been shown to reduce levels of IL-12 in a chronic EAE model [56, 57]. Interestingly, murine BMSCs were shown to exert opposing effects on Th1 cells depending on the time of disease onset and the level of effector T-cell activation, suppressing all T cells when administered early during T-cell activation and able to decrease IFNγ and increase IL-17 once T cells become activated . These results indicate that IL-12 may play an important mechanistic role during the increased potency of SVF-based therapy. It is possible that the SVF cells, beyond their ex vivo expanded ASC counterpart, have the ability to further reduce the level of effector T-cell activation, keeping the disease in a more naïve state by reducing IL-12, and therefore Th1 stimulation and differentiation. Whether this is a result of the ASCs being uncultured and retaining more of their in vivo properties, is a result of the SVF being administered in a heterogeneous population, or is a product resulting from the interaction of ASCs with one of the other cell types present remains unclear.
Although BMSCs have been shown to reduce TNFα levels by direct contact in vitro, this study showed that intraperitoneal injection of neither BMSCs, ASCs, or SVF cells affected TNFα levels in an EAE model . This may be due to the BMSCs being injected locally, murine BMSCs utilizing a different mechanism, or the studies being administered at a different time points during T-cell activation and differentiation. This also reiterates the speculation that the timing of stem cell interaction with T cells may drastically change immunomodulatory results and, therefore, disease progression.
These results point to the ability of all three cell types to play an effector role, which would occur during the early inflammatory phase of disease. Although the mechanisms mediating such effects are still only partially understood, it is likely that they involve both direct cell-to-cell contact and paracrine signaling through soluble factors. Further work needs to address whether the treatments utilized similar or unique mechanisms. This work also reiterates the speculation that the timing of cell therapy may drastically change immunomodulatory results and, therefore, disease progression. Further work needs to address cell therapy efficacy and cytokine response during the course of the disease, paying particular attention to cytokines that change quickly after cell treatment, such as T-helper type-17 cells. Although these data demonstrate that BMSCs, ASCs, and the SVF could affect the generation of encephalitogenic effector T cells, whether they can affect viability and function of the encephalitogenic effector T cells in established disease still needs to be determined.