Factors Defining Human Adipose Stem/Stromal Cell Immunomodulation in Vitro

Human adipose tissue-derived stem/stromal cells (hASCs) are adult multipotent mesenchymal stem/stromal cells with immunomodulatory capacities. Here, we present up-to-date knowledge on the impact of different experimental and donor-related factors on hASC immunoregulatory functions in vitro. The experimental determinants include the immunological status of hASCs relative to target immune cells, contact vs. contactless interaction, and oxygen tension. Factors such as the ratio of hASCs to immune cells, the cellular context, the immune cell activation status, and coculture duration are also discussed. Conditioning of hASCs with different approaches before interaction with immune cells, hASC culture in xenogenic or xenofree culture medium, hASC culture in two-dimension vs. three-dimension with biomaterials, and the hASC passage number are among the experimental parameters that greatly may impact the hASC immunosuppressive potential in vitro, thus, they are also considered. Moreover, the influence of donor-related characteristics such as age, sex, and health status on hASC immunomodulation in vitro is reviewed. By analysis of the literature studies, most of the indicated determinants have been investigated in broad non-standardized ranges, so the results are not univocal. Clear conclusions cannot be drawn for the fine-tuned scenarios of many important factors to set a standard hASC immunopotency assay. Such variability needs to be carefully considered in further standardized research. Importantly, field experts' opinions may help to make it clearer.

Interaction of allogeneic adipose tissue-derived stromal cells and unstimulated immune cells in vitro: the impact of cell-to-cell contact and hypoxia in the local milieu

Multipotent mesenchymal stem cells (MSCs) are an attractive tool for cell therapy and regenerative medicine. Being applied in vivo, allogeneic MSCs are faced with both activated and unstimulated immune cells. The effects of MSCs on activated immune cells are well described and are mainly suppressive. Less is known about the interaction of MSCs with unstimulated immune cells. We evaluated the contribution of tissue-related O2 level ("physiological" hypoxia-5% O2) and cell-to-cell contact to the interaction between allogeneic adipose tissue-derived MSCs (ASCs) and unstimulated peripheral blood mononuclear cells (PBMCs). Under both O2 levels, ASCs affected the immune response by elevating the proportion of CD69+ T cells and modifying the functional activity of unstimulated PBMCs, providing a significant reduction of ROS level and activation of lysosome compartment. "Physiological" hypoxia partially attenuated the ASC modulation of PBMC function, reducing CD69+ cell activation and more significantly supressing ROS. In direct co-culture, the ASC effects were more pronounced. PBMC viability was preferentially maintained, and the lymphocyte subset ratio was altered in favour of B cells. Our findings demonstrate that allogeneic ASCs do not enhance the activation of unstimulated immune cells and can provide supportive functions. The "hypoxic" phenotype of ASCs may be more "desirable" for the interaction with allogeneic immune cells that may be required in cell therapy protocols.

Human Bone Marrow Mesenchymal Stem/Stromal Cells Preserve Their Immunomodulatory and Chemotactic Properties When Expanded in a Human Plasma Derived Xeno-Free Medium

Due to their immunomodulatory and chemotactic properties, hMSC are being explored to treat immune-related diseases. For their use in human therapies, it is necessary to culture hMSC in xeno-free conditions. In this study, the impact that a xeno-free medium based on a human plasma derivate has on these properties was analysed. Bone marrow-derived hMSC preserved their immunosuppressive and immunostimulatory properties, as observed with in vitro assays with hMSC cocultured with mixed leukocyte reactions or with mitogen-stimulated leukocytes. Moreover, hMSC expanded in xeno-free medium were recruited by macrophages in both migration and invasion assays, which indicates that the cells maintained their chemotactic properties. These data suggest that xeno-free expanded hMSC preserved their immunomodulatory and chemotactic properties, indicating that the described xeno-free medium composition is a potential candidate to culture and expand hMSC for human cell therapies.

Tissue-Related Hypoxia Attenuates Proinflammatory Effects of Allogeneic PBMCs on Adipose-Derived Stromal Cells In Vitro

Human adipose tissue-stromal derived cells (ASCs) are considered a perspective tool for regenerative medicine. Depending on the application mode ASC/allogeneic immune cell interaction can occur in the systemic circulation under plenty high concentrations of O2 and in target tissues at lower O2 levels. Here we examined the effects of allogeneic PHA-stimulated peripheral blood mononuclear cells (PBMCs) on ASCs under ambient (20%) oxygen and "physiological" hypoxia (5% O2). As revealed with microarray analysis ASCs under 20% O2 were more affected by activated PBMCs, which was manifested in differential expression of more than 300 genes, whereas under 5% O2 only 140 genes were changed. Altered gene pattern was only partly overlapped at different O2 conditions. Under O2 ASCs retained their proliferative and differentiative capacities, mesenchymal phenotype, and intracellular organelle' state. ASCs were proinflammatory activated on transcription level that was confirmed by their ability to suppress activation and proliferation of mitogen-stimulated PBMCs. ASC/PBMCs interaction resulted in anti-inflammatory shift of paracrine mediators in conditioning medium with significant increase of immunosuppressive LIF level. Our data indicated that under both ambient and tissue-related O2 ASCs possessed immunosuppressive potential and maintained functional activity. Under "physiological" hypoxia ASCs were less susceptible to "priming" by allogeneic mitogen-activated PBMCs.

Factors governing the immunosuppressive effects of multipotent mesenchymal stromal cells in vitro

Multipotent mesenchymal stem/stromal cells (MSCs) are of great interest to researchers because of the unique properties, such as enhanced proliferation, paracrine activity and multilineage differentiation. Their non-immunogenicity, in combination with immunomodulatory properties, opens up the opportunity for the allogeneic application of MSCs. The MSC immunomodulatory capacity is currently being actively studied in vitro using various experimental designs. However, the results are not always univocal. It was found that the outcome of the stromal/immune cell interaction depends on experimental conditions. In this review we considered the impact of different factors, such as the ratio of stromal/immune cells, interaction time, the path of immune cell activation, etc. on the MSC immunomodulation. We also accentuated the importance of local milieu, in particular, oxygen tension, for the realization of MSC immunosuppressive activity.

Proinflammatory interleukins' production by adipose tissue-derived mesenchymal stromal cells: the impact of cell culture conditions and cell-to-cell interaction

The impact of culture conditions and interaction with activated peripheral blood mononuclear cells on the interleukin (IL) gene expression profile and proinflammatory IL-6 and IL-8 production by adipose-derived stromal cells (ASCs) was investigated. A microarray analysis revealed a wide range of IL genes either under standard (20%) or hypoxic (5%) O2 concentrations, some highly up-regulated at hypoxia. IL-6 and IL-8 production was inversely dependent on cell culture density. In early (first-third) passages, IL-6 and IL-8 concentration was higher at 20% O2 and in late (8th-12th) passages under 5% O2. Interaction between ASCs and mononuclear cells in indirect setting was accompanied with a significant decrease of IL-6 and did not result in the elevation of IL-8 concentration. Thereby, the production of proinflammatory interleukins (IL-6 and IL-8) may be affected by the ASC intrinsic features (density in culture, and duration of expansion), as well as by microenvironmental factors, such as hypoxia and the presence of blood-borne cells. These data are important for elucidating ASC paracrine activity regulation in vitro. They would also be on demand for optimisation of the cell therapy protocols, based on the application of ASC biologically active substances. SIGNIFICANCE PARAGRAPH: Ex vivo expansion is widely used for increasing the number of adipose-derived stromal cells (ASCs) and improving of their quality. The present study was designed to elucidate the particular factors influencing the interleukin production in ASCs. The presented data specified the parameters (i.e. cell density, duration of cultivation, hypoxia, etc.) that should be taken in mind when ASCs are intended to be used in protocols implying their paracrine activity. These data would be of considerable interest for researchers and clinicians working in the biomedical science.