**3.2 Apoptosis in the regulation of immune mechanisms involved in the pathogenesis of T1DM**

Both humoral and cellular immunity factors are involved in the development of T1DM. Islet cell autoantigens are recognized by autoantibodies and autoreactive effector T lymphocytes resistant to apoptosis, which are involved in the destruction of b-cells through the release of a triad of pro-inflammatory cytokines TNF-α, IL-1b, IFN-g, cytotoxic enzymes (perforin, granzyme B) and other compounds, including free radicals [7]. Experimental studies on mice of the NOD (nonobese diabetic) line on the creation of autoimmune diabetes, close to human T1DM, showed that the central role in the pathogenesis of this disease belongs to autoreactive T-lymphocytes, specific for pancreatic islet b-cells [1, 5].

Autoreactive clones of T cells that react with the islets of Langerhans have been described both in animals and in humans [5, 7]. From the peripheral blood of children with newly diagnosed T1DM, a clone of T-lymphocytes (CD4Th1 + cells) was isolated, which recognizes glutamate decarboxylase, an epitope of the B-chain of the insulin molecule, and other autoantigens of the islets of Langerhans. It has been shown that by transplanting autoreactive T lymphocytes of a sick animal, it is possible to induce T1DM in a healthy syngeneic animal. Strong evidence for the involvement of T cells in the immunopathogenic mechanisms of T1DM is that monoclonal antibodies to the

CD3 antigen can interrupt early T1DM in NOD mice and restore their tolerance to b-cell autoantigens [38, 39]. Most often, diabetogenic clones of T-lymphocytes consist of CD4+ cells, but there are clones formed by CD8+ cells. Some authors believe that CD8+-lymphocytes without the presence of CD4+ cells are not able to lead to the destruction of b-cells [38–40].

In recent years, the cellular mechanisms of death of b-cells of the islets of Langerhans are considered leading [5, 7]. The direct effect of immunocompetent cells on target cells (including b-cells), leading to the destruction of the latter, is referred to as cellular cytotoxicity. Two main pathways for the realization of cellular cytotoxicity have been described, involving perforin- and Fas-dependent mechanisms. It has recently been established that in some cases such destruction is accompanied by the development of apoptosis [1, 7]. A specific feature of the development of apoptosis in this case is the primary damage to the membrane of target cells, which is atypical for this process, with the penetration of granzyme proteins into them. It is believed that it is the latter that include the mechanism of programmed cell death [7]. If CD8+CTLs realize their cytotoxicity by activating both mechanisms, then natural killer cells use exclusively the perforin-dependent pathway, while CD4+ lymphocytes activate Fasdependent mechanisms and are restricted by the major histocompatibility complex of the second class [1, 2]. Here it is necessary to mention the role of cytokines in the immune-mediated destruction of b-cells. Together with IL-1 and TNF-a, IFN-g is able to increase the expression of molecules of the major histocompatibility complex of the second class on the cells of the pancreatic islets. This leads to recognizing them as alien. The death of pancreatic islet cells occurs as a result of apoptosis, direct cytotoxic action of TNF-a and cytotoxic T lymphocytes (CD8+CTL), as well as by antibodydependent cytotoxicity [6]. According to a number of researchers [41, 42], the initial step in the development of T1DM is the presentation by macrophages or dendritic cells of specific autoantigens of b-cells to T helpers, which is carried out in association with molecules of the major histocompatibility complex of the second class.

In accordance with modern concepts, type 1 diabetes mellitus is considered as an autoimmune insulitis, in the pathogenesis of which, in addition to autoantibodies, the role of cellular immunity reactions is undeniable [2]. Autoreactive lymphocytes migrate from the bloodstream to the target organ (pancreas) and penetrate into the islets of Langerhans, forming inflammatory infiltrates—insulitis. This is evidenced by the results of histological studies that reveal lymphocytic infiltration of pancreatic islets, formed mainly by CD8+ and CD4+ Т cells [1, 5, 7].

Activated macrophages secrete IL-12, which stimulates CD4+ cells that secrete IFN-g and IL-2. IFN-g activates "resting" macrophages, which in turn produce IL-1b and TN-Fa, which is accompanied by a sharp rise in the level of free radicals in b-cells [7]. IL-2 causes the migration of peripheral CD8+ lymphocytes to the islets of Langerhans, probably due to the induction of the expression of specific homing receptors. Naive cytotoxic T cells that carry specific receptors for b-cell autoantigens differentiate into effector cytotoxic CD8+ lymphocytes after recognizing a specific b-cell peptide associated with molecules of the major histocompatibility complex of the first class, which occurs in the presence of CD4+ lymphocytes. Then CD8+ lymphocytes start the process of destruction of b-cells due to the secretion of perforin and granzyme B. CD4+ lymphocytes expressing the Fas-ligand destroy b-cells by the mechanism of Fas-mediated apoptosis, as well as indirectly—due to the secretion of cytokines IFN-g and TNF-a. In this way, macrophages, CD4+ and CD8+ cells are thought to act synergistically to destroy b-cells, leading to the onset of autoimmune diabetes [41, 42].
