**4.1 Immuno-modulating capacity of MSCs**

MSCs act on different types of cells of the immune system by releasing more than 200 bioregulatory substances with antifibrotic, antiapoptotic, antimicrobial, chemoattraction, stem cell support, hematopoietic, angiogenesis, mitogenesis and neuroprotector properties [7]. In addition, MSCs have two fundamental effects on the immune system, which are an immune-enhancing and anti-inflammatory response [3]. These cells interact with T cells, B cells, natural killer (NK) cells, dendritic cells (DCs) macrophages, monocytes, and neutrophils, exerting immunoregulatory action on the innate and adaptive immune response [17].

**Figure 2.** *MSCs role in the wound healing process.*

The immunoregulatory potential of MSCs depends on several factors, such as their tissue of origin, MSC dose, administration time, MSC activation, and their contact with immune system cells.

#### **4.2 Mechanism of action of MSCs under the innate immune response**

The innate immune response is the body's first line of defense against any external action produced by pathogenic agents such as bacteria, fungi, and virus. It is a fast-acting and nonspecific response to those pathogens. This defense process causes tissue inflammation through the activation of immune system cells such as neutrophils, macrophages, monocytes, natural killer and dendritic cells, and the release of enzymes that form the complement system [19].

MSCs secrete prostaglandin E2 (PGE2), transforming growth factors (TGF-B), and indolamine2,3-dioxygenase that can modulate NK, inhibiting their proliferation, cytokine release, and cytotoxicity. This mechanism can also be exerted through cell-to-cell contact. In addition, MSCs also act on monocytes and macrophages. PGE2, TGF-B, hepatic growth factor, interleukin 6 released by MSCs, reprogram macrophages with a pro-inflammatory M1 phenotype to an anti-inflammatory M2 phenotype with increased production of interleukin-10, and decreased production of tumor necrosis factor and gamma interferon [20–22].

The release of these same soluble factors (PGE2, TGF-B, and interleukin 6) acts on monocytes by inhibiting their differentiation into dendritic cells. Dendritic cells are antigen-presenting cells, when their maturation is inhibited, the correct expression of presenting and co-stimulatory molecules does not occur, which results in a lack of response on the part of T cells [3, 23]. Furthermore, MSCs have the capacity to inhibit the infiltration of monocytes, macrophages, and neutrophils into sites of inflammation, dependent on the tumor necrosis factor stimulated gene 6 protein (TSG6). Similarly, MSCs can also enhance the infiltration of the cells into tumors in a chemokine-dependent manner. In this case, MSCs can promote tumor progression, metastasis, and treatment resistance. For instance, the stimulation of chemokine production may stimulate the capacity of MSCs to attract macrophages, monocytes, and neutrophils. Conversely, an inflammation stage might activate the expression of indoleamine-2,3-dioxygenase (IDO) produced by MSCs that can cause immunosuppressive consequences on myeloid cell migration. For these reasons, it is hard to predict whether the immunomodulatory response of MSCs is expected to be negative or positive, because of MSCs complex innate immune cell interactions [23].

#### **4.3 Mechanism of action of MSCs under the adaptive immune response**

The adaptive immune response develops a defense mechanism specific for each pathogen. Therefore, a memory effect is created for each antigen after the first contact, in order to develop a faster and more effective response the next time the organism is in contact with the same antigen [3].

The immune system acts through two pathways, the cellular immune response composed of T lymphocytes that directly attack the pathogens that invade the organism and the humoral immunity response composed of antibodies against pathogenic antigens produced by B lymphocytes [22].

One of the major mechanisms of action of MSCs is their ability to regulate T cells through cell-to-cell interaction or secretion of inflammatory components. In this

environment, MSCs can change from T-helper 1 (Th1) phenotype (proinflammatory) into a T-helper 2 (Th2) phenotype (anti-inflammatory) [3]. There is some contradiction about the effects of MSCs on B cells, although there is clear evidence that MSCs have close interaction with these cells. Thus, MSCs are capable of inhibiting B cell proliferation through cell-to-cell contact and with the arrest in the cell cycle. MSCs can regulate immune responses, but their immunomodulatory capacity is not yet fully understood.

This anti-inflammatory and immunomodulatory capacity of MSCs is very promising for the treatment and recovery of skin tissue [14].
