**2.1 AA pathophysiology**

*Human Blood Group Systems and Haemoglobinopathies*

cells and stimulate Treg activity [16].

therapeutic in AA immune-mediated treatment.

**2. Aplastic anemia: general features**

neutrophil count lower than 0.5 × 109

platelet count lower than 20 × 109

above all by its ability to regulate the immune system [11].

A new viable alternative for the treatment of AA has been sought and the use of mesenchymal stem (MSCs) therapy may be a promising therapeutic candidate mainly because of their hypoimmunogenicity and the lack of rejection after transplants and immunomodulatory effects, which may promote decreasing the symptoms of the disease [9, 10]. These benefits are attributed to the paracrine effects,

Actually, is known that MSCs have wide therapeutically potential attributed by paracrine effects and the past decades have seen explosion research directed to understand better these MSCs mechanism and function [12]. One of the main and most important features of MSCs is the low expression of human leukocyte antigen (HLA) class I, with no expression of HLA class II. This feature allows the cell to be characterized as hypoimmunogenic, since it does not stimulate the patient's immune system and can be used safely in transplants [13]. More recently, the studies showed that the main cause of AA is autoimmunity. Through the secretion of bioactive molecules, MSCs have the capacity of regulating immune responses. The mechanism of MSCs may decrease secretion of proinflammatory cytokines such as transforming growth factor (TGF), IFN-γ TNF-α, interleukin (IL)-17 and increase secretion of many soluble mediators, including anti-inflammatory cytokines stimulation that inhibit antigen-presenting cells (APCs) functions, which are capable to decrease proliferation of dendritic cells (DCs) and regulate macrophage activity by polarizing proinflammatory phenotype (M1) to anti-inflammatory phenotype (M2) [14, 15]. Therefore, the decrease of B cells proliferation and antibodies production and adjustment of T cells activities as well as inhibit the proliferation of cytotoxic T

MSCs therapy has gained space due to its vast therapeutic potentials such as immunomodulation mechanisms and main safety as bioproduct. Thus, this chapter will discuss the challenges of allogeneic MSCs as an alternative for an efficient

AA is a disorder characterized by BM hypocellularity, and peripheral blood pancytopenia due to a deficit of HSCs. It affects mostly children, young adults, and adults, over 60 years of age [17]. This condition can be similar to other hematologic disorders, however, in most cases, the AA is caused by reduced HSCs function, an increase in HSCs apoptosis level, consequently, the decreased of HSCs and hemato-

Following the patient diagnosis, AA can be considered as moderate or severe. The patients with pancytopenia may present symptoms of anemia purpura or skin hemorrhage, and in most of the cases there is an infection association, that may worsen the symptoms [20]. Three main criteria are used for the diagnostic:

case must be differentiated from other hematological diseases, as well as from the

Normally the first AA etiology is uncertain and for this reason, the disorder is considered heterogeneous in origin and characterized as idiopathic [23]. AA is associated with exposures to chemical agents (pesticides and benzene), cytotoxic drugs (antineoplastics, antibiotics, non-steroidal anti-inflammatory drugs), active viral infections exposure (Epstein Barr, hepatitis virus, human immunodeficiency virus parvovirus) and radiation exposure [18, 24, 25]. However, these causes considered

cells/L, reticulocyte count lower than 1% and

cells/L [21]. To confirm acquired AA, the clinical

poietic progenitors and lastly, microenvironment fat replacement [18, 19].

signs of malignant cell transformation or myelodysplasia [22].

**150**

Currently, the studies of AA etiology are focused on the immune mechanism of hematopoietic cells destruction. Many researchers [28, 29–31] have demonstrated that the dysfunction of T cells might be a key factor in recent characterization as an autoimmune disease [28]. Most of the acquired AA is the result of an immunemediated process as an imbalance between CD8+ and CD4+ T cells, including Th1, Th2, Treg and Th17 cells, NK, and natural killer T cells (NK T) that leads to apoptosis of BM cells triggered by cytotoxic T cells activation [6, 17].

The abnormal immunoregulatory cell functions observed in AA can be attributable to abnormal antigen stimulation and some inappropriate T cells activation [28]. Studies demonstrated that patients with AA have a significantly increased proportion of Th1 cells, and showed a reduced fraction of natural killer T cells and regulatory T cells, together with an increased level of TNF-α, a consequent elevation of IL-6, IL-8, and IL17 productions [18]. Additionally, there is also an abnormal production of proinflammatory cytokines including IFN-γ and TGF [4, 5, 28, 32]. The new T cells subset was characterized as Th17 and currently is known that both Th17 cells and the cytokine IL-17, which is secreted by Th17 cells, also is in an association with AA pathogenesis [31]. Studies showed that AA patients who presented an increase in the frequency of Th17 cells had a positive correlation with an increase in the IFN- γ and IL-17 expression. Autoimmunity promotes inflammatory Th17 immune responses that contributed to disease pathophysiology [29].

Otherwise, AA is attributed to inappropriate antigen stimulation and abnormal APCs activation [28], resulting in the priming of T cells specific for hematopoietic cells [33, 34]. APCs exhibit a significant increase in the expression of major histocompatibility class 2 (MHCII), increasing the recognition of CD4+ T cells. In AA, T cells are also stimulated by unknown antigens or abnormal APC activation as DCs and macrophages, which trigger a series of immune responses. Studies have shown that immunoregulatory cell dysfunction leads to a corresponding immune tolerance disorder and renders the body unable to recognize autologous hematopoietic cells [28].

Although the definitive mechanism has not been identified, some genetic factors are the targets of ongoing research, such as the molecular basis of the aberrant immune response and hematopoietic cell deficiency, telomere repair gene mutations in the target cells and unregulated T cell activation pathways and cytokine genes polymorphisms [9, 26, 28]. These changes in the nucleotide sequence and gene regulation are associated with an increased immune response and suggest a genetic basis for aberrant T cells activation in BM failure [35].

## **2.2 AA treatment**

The treatment depends on the severity of the disease, once for moderate cases are based on red blood cell (RBC) transfusions, on platelet transfusions to prevent bleeding, and on supportive care in association with antibiotic aiming to reestablish blood cell volume and prevent secondary infections [17]. However, the pancytopenia of many moderate cases may progress to severe [21]. For severe cases, immunosuppressive therapy is accepted as a first-line treatment option. However, 30–40% of patients with SAA remain pancytopenia following the treatment. Patients with SAA, which are refractory or have a relapse after immunosuppressive treatment, may undergo allogeneic hematopoietic stem cells transplantation (HSCT). However, about one-third of patients do not have a suitable donor for HSCT. Additionally, patients aged >50 years are not eligible for transplant [8].

Furthermore, the immunosuppressive drug treatment has several side effects on patients. On the other hand, the patients often do not respond adequately to the therapies and are not suitable for life treatment (refractory patients) [24]. Therefore, immunosuppressive drugs are considered supporting AA treatment, once it does not promote the cure [20].

#### **2.3 Allogeneic transplantation and alternative methods for AA treatment**

Generally, patients are treated with allogeneic HSCs or whole BM transplantations, which replace since HSCs, hematopoietic precursors, until differentiated bloodstream cells and immune system cells. However, in all types of transplants, the treatment involves a combination of immunosuppressive agents or radiation therapy to prevent and to eliminate residual host BM [24]. The transplantation success varies according to risk factors, such as age and mainly histocompatibility allogeneic HLA-matched sibling donors, which are rare for the majority of patients. Despite being well established for many years, the transplanted patients can trigger late complications, such as the development of graft versus host disease (GVHD) and infections, especially in patients who have received hematopoietic grafts from HLA antigen matched donor [36, 37]. Studies show that the incidence of GVHD after unrelated donor transplantation can achieve ∼14%, and overall survival index was 57% for all 8 HLA-loci matched transplants and 39% for 1-loci mismatched transplant [38]. Thus, for BM and HSCT, the immediate challenge is the extension of stem cell therapies to all patients, regardless of age, with a histocompatible sibling [24].

Since then a new viable alternative for the treatment of AA has been sought and the use of MSCs transplantation becomes of choice. The MSCs therapy may be a promising therapeutic candidate mainly because of their hypoimmunogenicity, the lack of rejection after transplants and immunomodulatory effects, which may promote decreasing the symptoms of the disease [39]. These benefits are attributed to MSCs paracrine effects, above all to their ability to regulate the immune system. MSCs may help for AA treatment, especially for autoimmune type [11].
