**3.4 RAGE and antigen presenting cells**

Professional antigen presenting cells (APCs) include dendritic cells (DCs), macrophages and B cells. These cell populations have been extensively studied in T1D due to their tolerogenic and immunogenic properties and importance in therapy development [63].

DCs are categorized into conventional (cDCs), plasmacytoid (pDCs), monocytederived (moDCs) and Langerhans cells (LCs). They are important regulators of immune tolerance, can select specific T cell subsets for anergy or deletion and therefore play pivotal roles in the pathogenesis of T1DM. Their immunomodulatory abilities have been widely explored and are targets for various therapeutic strategies in development including antigen specific immunotherapy [64]. Within the pancreata of NOD mice, DCs and macrophages can be detected as early as 3 weeks of age [65, 66]. Interestingly, studies of chronic high AGE feeding of healthy rats also show the appearance of an islet specific infiltrate that is comprised primarily of macrophages [21]. Other studies suggested that plasmacytoid DCs (pDCs) are recruited to the pancreata of NOD mice where they promote diabetogenic T cell activity and initiate T1DM [56]. Many other studies highlighted the importance of DC function in disease initiation and progression (reviewed here [67, 68]). Their maturation status dictates the level and type of response exerted. The immature or tolerogenic DCs usually exhibit low levels of MHC class II and costimulatory molecules expression such as CD80 and CD86, reduced ability to stimulate T cells and produce proinflammatory cytokines, however their phagocytic and antigen processing and presentation capacity is not affected [68]. Several strategies explored immunomodulatory potential of tolerogenic DCs and maintaining them in their immature state for treatment and prevention of autoimmune disorders including T1DM (reviewed in [64, 68, 69]. DCs will continue to be in the T1DM therapy development spotlight due to their proven ability to induce T cell anergy or deletion while promoting and increasing regulatory T and B cell populations.

The exposure of immature DCs from human PBMCs to a high glucose environment and AGEs results in the upregulation of costimulatory markers, increases

production of reactive oxygen species (ROS) and proinflammatory cytokines such as IL-12 and IL-6, decrease in regulatory cytokines such IL-10 and enhances expression of AGEs scavenger receptors SR-A and CD36 and RAGE [70, 71]. In another study by Ge et al., AGE-BSA treatment increased DC expression of both SR-A and RAGE but pre-treatment of DCs with RAGE neutralizing antibody halted maturation by impairing upregulation of costimulatory markers and expression of IL-12 [72]. Another study described the effects of AGEs-stimulation in the presence of the antioxidant resveratrol in immature DCs derived from healthy donor PBMCs. Pre-treatment of DCs with resveratrol prior to AGE exposure prevented their maturation, MAPK and NK-κB activation and production of inflammatory cytokines and reduced RAGE expression [73]. This once more highlights that delineating the AGE-RAGE axis in inflammatory or aging processes may have future benefit in understanding T1DM pathogenesis.

Failure of peripheral and central tolerance not only results in autoreactive T cells but also autoreactive B cells and their migration to pancreas and pancreatic lymph nodes. B cells can present autoantigens to islet specific CD4+ and CD8+ effector T cells, which in turn causes destruction of pancreatic β-cells. B cells are necessary for the development of autoimmune diabetes in NOD mice in particular autoantibody production and B cell depletion prevents development of the disease [74–76]. In newly diagnosed individuals with T1DM, B cell depletion using anti-CD20 monoclonal antibody rituximab showed initial preservation of β-cell function and the need for exogenous insulin was reduced for up to 1 year post treatment. However, following a two-year follow up period, initial improvements in C-peptide (a marker of beta cell insulin secretion/function) were diminished and the clinical trial was terminated [77, 78]. Nonetheless, despite disappointing therapeutic results, B cell importance in T1DM progression is well appreciated and several improvements to the B cell depletion therapeutic approaches have since been proposed [79].

In murine models of antibody mediated autoimmune disorders such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) the absence of RAGE tended to cause reduction in germinal center B cells along with decreased anti-dsDNA autoantibody titers and increases in follicular B cells [80]. Germinal centers play important role in B cell maturation, clonal expansion, and class-switching as well an antibody production? [81]. Though these modest changes in B cells in RAGE−/− animals may not be sufficient to prevent autoimmunity, they are indicative that RAGE signaling is an important contributor to these processes [80]. Another study explored the effects of RAGE/HMGB1 interactions on activation of autoreactive B cells. In that particular study, it was concluded that HMGB1 binding promotes activation of autoreactive B cells through TLR9 rather than RAGE [82].

Macrophages act alongside DCs for antigen presentation [83, 84]. Though human pancreatic islets have macrophages present, it is unclear whether there are changes between resident and infiltrating populations in T1DM [85]. The phenotype of macrophages can in general be divided into M1/pro-inflammatory cytotoxic and M2/antiinflammatory, alternatively activated cells. However, the local microenvironment plays important role in the process of monocyte to macrophage differentiation. Human macrophages represent less than half of the APC population seen in T1DM and form a mixture of both M1 and M2 phenotypes [86]. In NOD mice APCs are represented almost entirely by macrophages and they have an intricate relationship with lymphocytes, although this may be due to limitations in assessing APC phenotypes in mice. Some studied observed that the absence of resident macrophages arrested T1DM development in NOD mice [87]. Depletion of islet-resident APCs caused complete elimination
