**4. The implication of ER stress in autoimmune responses**

#### **4.1. ER stress and innate immune response**

The importance of innate immunity was highlighted in the pathophysiology of type 1 diabe‐ tes (54-57). Type 1 diabetes was initially considered a T-cell-mediated autoimmune disease (58), in which T-cell was believed as the major immune cell causing β cell destruction while the involvement of innate immune response has been ignored for a long time. However, re‐ cent studies suggest a critical role of innate immune responses in the development of type 1 diabetes (54;55). As the first line of defense mechanism, innate immunity is implicated in the initiation as well as the progression of autoimmune responses against pancreatic β cell.

Innate immune response is regulated by elements of the UPR pathway (59). For example, Cyclic-AMP-responsive-element-binding protein H(CREBH), an ER stress-associated tran‐ scription factor, regulates the expression of serum amyloid P-component and C-reactive protein, the two critical factors implicated in innate immune responses. Like ATF6, CREBH is an ER-membrane-bound protein. In response to ER stress, CREBH release an N-terminal fragment and transit to nucleus to regulate the expression of target genes. Innate immune response, in turn, regulates the expression of CREBH through inflammatory cytokines such as IL-1β and IL-6 (60). The development of dendritic cells, the major innate immune cells, is also regulated by ER stress response (61). High levels of mRNA splicing for XBP-1 are found in dendritic cell, and mice deficient in XBP-1 show defective differentiation of dendritic cell. Both conventional (CD11b+ CD11c+

) and plasmacytoid dendritic cells (B220+ CD11c+

release of PERK from BiP. Once released from BiP, PERK becomes oligomerized and auto‐ phosphorylated. As a result, PERK inactivates eukaryotic initiation factor 2α (eIF2α) by the phosphorylation of Ser51 to inhibit mRNA translation and protein load on ER (34;40). In ad‐ dition, phosphorylated eIF2α also promotes the expression of stress-induced genes includ‐ ing the transcription factors ATF4 and CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP) (41). Deficiency of PERK results in an abnormally elevated

*IRE1/XBP-1 axis:* IRE1 is another axis of signal involved in UPR. There are 2 isoforms of IRE1: IRE1α and IRE1β. IRE1α is expressed in most cells and tissues, while IRE1β is restrict‐ ed in intestinal epithelial cells (42;43). Once disassociated with BiP, IRE1 becomes activated. Activated IRE1 possesses endoribonuclease activity and cleaves 26 nucleotides from the mRNA encoding X-box binding protein-1 (XBP-1), resulting in the increased production of XBP-1 (44). XBP-1 is a transcriptional factor belonging to basic leucine zipper transcription factorfamily. It heterodimerizes with NF-Y and enhances gene transcription by binding to the ER stress enhancer and unfolded protein response element in the promoters of targeted genes involved in ER expansion, protein maturation, folding and export from the ER, and degradation of mis-folded proteins (44-49). In addition, IRE1α also mediates the degradation

*ATF6 axis:* The third axis of ER stress signal is mediated by ATF6. Unlike PERK and IRE1 which oligomerize upon UPR, ATF6 translocates into the Golgi apparatus after released from BiP. The transmembrane domain is then cleaved in the Golgi apparatus (51). The 50 kDa cleaved ATF6 is relocated into the nucleus where it binds to the ER stress response ele‐ ment CCAAT(N)9CCACG to regulate the expression of targeted genes. For example, once released from the ER membrane, ATF6 enhances the transcription of XBP-1 mRNA which is further regulated by IRE1 (44). In addition, ATF6 also increases the expression of the two

The importance of innate immunity was highlighted in the pathophysiology of type 1 diabe‐ tes (54-57). Type 1 diabetes was initially considered a T-cell-mediated autoimmune disease (58), in which T-cell was believed as the major immune cell causing β cell destruction while the involvement of innate immune response has been ignored for a long time. However, re‐ cent studies suggest a critical role of innate immune responses in the development of type 1 diabetes (54;55). As the first line of defense mechanism, innate immunity is implicated in the initiation as well as the progression of autoimmune responses against pancreatic β cell.

Innate immune response is regulated by elements of the UPR pathway (59). For example, Cyclic-AMP-responsive-element-binding protein H(CREBH), an ER stress-associated tran‐ scription factor, regulates the expression of serum amyloid P-component and C-reactive

major chaperon systems in the ER: calnexin/calreticulin and BiP/GRP94 (44;52;53).

**4. The implication of ER stress in autoimmune responses**

**4.1. ER stress and innate immune response**

protein synthesis in response to the accumulation of unfolded proteins in ER.

of ER-targeted mRNAs, thus decreasing the ER burden (50).

202 Type 1 Diabetes

) are decreased by >50%. Dendritic cells deficient for XBP-1 are vulnerable to ER stress-in‐ duced apoptosis (61). Moreover, the secretion of inflammatory cytokine IL-23 by dendritic cell also involves ER stress response. CHOP, a UPR mediator, can directly bind to the *IL-23* gene and regulate its transcription. ER stress combined with Toll-like receptor (TLR) ago‐ nists was found to markedly increase the mRNA of IL-23 p19 subunit and the secretion of IL-23, while knockdown of CHOP suppressed the induction of IL-23 by ER stress and TLR signaling (62).

The association of ER stress with innate immune response is confirmed in many disease models. Richardson and coworkers reported that innate immune response induced by *P. aeruginosa* infection causes ER stress in *C. elegans*, and loss-of-function mutations of XBP-1 lead to larval lethality (63). In consistent with that, polymorphisms of *XBP-1* gene were found to be associated with Crohn's disease and ulcerative colitis in humans (64), the two autoimmune diseases share similar properties with type 1 diabetes. Lack of XBP-1 in intesti‐ nal epithelial cells may induce Paneth cell dysfunction which further results in impaired mucosal defense to *Listeria monocytogenes* and increased sensitivity to colitis (64).

In addition to IRE1/XBP-1 axis, PERK/eIF2α/ATF4 axis of UPR is also associated with innate response. TLR signaling, the most important innate signaling pathway, can induce selective suppression of the PERK/eIF2α/ATF-4/CHOP axis of UPR pathway (65). The activation of TLR decreases eIF2α-induced ATF4 translation. For instance, pretreatment of LPS, an ago‐ nist for TLR4, attenuated ATF4/CHOP signaling and prevented systemic ER stress-induced apoptosis in macrophages, renal tubule cells, and hepatocytes (65). In contrast, loss of Toll-IL-1R-containing adaptor inducing IFN-β (TRIF), an important adapter for TLR signaling, abrogated the protective effect of LPS on renal dysfunction and hepatosteatosis induced by ER stress, suggesting that TLR signaling suppresses ATF4/CHOP via a TRIF-dependent pathway (65).
