Human adipose tissue–derived mesenchymal stromal cells and their phagocytic capacity

Abstract Mesenchymal stromal cells (MSCs) have evidenced considerable therapeutic potential in numerous clinical fields, especially in tissue regeneration. The immunological characteristics of this cell population include the expression of Toll‐like receptors and mannose receptors, among others. The study objective was to determine whether MSCs have phagocytic capacity against different target particles. We isolated and characterized three human adipose tissue MSC (HAT‐MSC) lines from three patients and analysed their phagocytic capacity by flow cytometry, using fluorescent latex beads, and by transmission electron microscopy, using Escherichia coli, Staphylococcus aureus and Candida albicans as biological materials and latex beads as non‐biological material. The results demonstrate that HAT‐MSCs can phagocyte particles of different nature and size. The percentage of phagocytic cells ranged between 33.8% and 56.2% (mean of 44.37% ± 11.253) according to the cell line, and a high phagocytic index was observed. The high phagocytic capacity observed in MSCs, which have known regenerative potential, may offer an advance in the approach to certain local and systemic infections.

MSCs have multiple clinical applications, mainly in regenerative medicine, through their capacity to migrate to the focus of damaged tissue and induce its repair or replacement; this capacity derives from their pluripotentiality and their secretion of bioactive substances that can act at both local and systemic level. [16][17][18] MSCs have also been attributed with potent immunomodulatory properties that may be useful to control inflammation and tissue damage. 19 They express certain Toll-like receptors (TLRs), mannose receptors (MRs) and scavenger receptors (SRs), indicating a possible role against infection, although this has yet to be confirmed. [20][21][22][23] Also, MSCs seem to have the capacity to induce the phagocytic activity of other cells, such as polymorphonuclear cells or alveolar macrophages by releasing. 24,25 Some other authors have even suggested the use of MSCs in sepsis, through its action at different levels, such as its ability to locate to damage tissue, secrete paracrine signals to systemic and local inflammation, decrease apoptosis, promote neoangiogenesis, stimulate immune cells and show antimicrobial activity. 26 Later, Khan et al. reported that MSCs are new phagocytic cells with a high potential for immunotherapy in the treatment of tuberculosis. 23 The isolation, culture, identification, differentiation, function and regenerative capacity of MSCs have been widely studied, prompting considerable interest in their potential therapeutic usefulness in different diseases. In fact, they are among the most frequently used cell types in regenerative medicine, with numerous clinical applications. 23,[27][28][29] Human adipose tissue MSCs (HAT-MSCs) are found in the stromal vascular fraction of subcutaneous adipose tissue. When the cells of this vascular fraction are cultivated in the required culture medium, both adipose stem cells and stromal vascular fraction cells can be obtained. 30,31 HAT-MSCs have been used for years in regenerative plastic surgery. 31,32 In the same way, these cells have also been used for the treatment of anal fistulas, diabetic foot, alopecia and certain defects in soft tissues such as healing processes, among others. 31,[33][34][35][36] Thus, it has also been possible to demonstrate the regenerative capacity of HAT-MSCs in combination with platelet-rich plasma in soft tissue regeneration and wound healing. 32,37 On the contrary, the anti-inflammatory and immunomodulatory potential of this cell population has been pointed out as a possible cell therapy in infection caused by the SARS-CoV-2 virus. 38 With this background, the objective of this study was to determine the phagocytic capacity of MSCs by means of two study techniques (flow cytometry and transmission electron microscopy), using target particles of different size and origin, such as latex beads and microorganisms. This will contribute to the knowledge of the physiology of this cell population.    (Table S1). At the same passage, cells were differentiated to osteogenic and adipogenic lineage, following the protocol of Zajdel et al. 41 and Munir et al. 42 respectively.

| Establishment of human adipose tissue MSCs
For the experiments, cells from each donor were used at passages 3-4.

| Study of phagocytic capacity
The phagocytic capacity of HAT-MSCs was analysed by flow cytometry and transmission electron microscopy.  43 and then cultured at 37°C in 5% CO 2 atmosphere. Cells were then cultured and treated following the protocol described by Ruiz et al. 44

| Statistical analysis
R software (version 2.9.2, Auckland, New Zealand) was used for data analyses of phagocytosis by flow cytometry. Phagocytic capacity was compared using the Student's t test. p ≤ 0.05 was considered statistically significant in all tests. At least three experiments were performed in all assays and for each culture. Data were expressed as means ± standard deviation (SD).

| RE SULTS
Flow cytometry showed that a high percentage of the HAT cells isolated and characterized as MSCs had phagocytic capacity ranging from 33.8% to 56.2% (44.37% ± 11.253), according to the cell line.
The mean channel fluorescence was also elevated in all three lines, ranging between 4314 and 4973 (4619 ± 0.332), indicating a high phagocytic index (Figure 1; Table 1).
Transmission electron microscopy confirmed the results ob-

CO N FLI C T O F I NTE R E S T
The authors declare no competing or financial interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data available on request from the authors.

Concepción Ruiz
https://orcid.org/0000-0003-4332-6812 F I G U R E 4 Phagocytosis of latex particles by HAT-MSCs incubated for 24 h at 37°C and analysed by transmission electron microscopy