Background/Purpose:  We found that systemic lupus erythematosus (SLE) was induced experimentally by repeatedly immunizing the mice normally not prone to autoimmune diseases by any exogenous antigen so far examined (Tsumiyama K et al. PLoS One 4(12):e8382, 2009). We have then proposed a novel ‘self-organized criticality theory’ that explains the cause of SLE. Systemic autoimmunity, or SLE, necessarily takes place when host’s immune system is overstimulated by repeated exposure to antigen to levels that surpass the immune system’s stability limit, i.e., self-organized criticality. The autoreactive lymphocyte clones, which we name autoantibody-inducing CD4⁺ T cells (aiCD4⁺ T cells) are newly generated via de novo T cell receptor (TCR) revision from thymus-passed non-autoreactive clones at peripheral lymphoid organs. They not only stimulated B cells to generate varieties of autoantibodies but also helped final differentiation of CD8⁺ T cell into cytotoxic T lymphocyte (CTL) via antigen cross-presentation to induce tissue injuries identical to SLE. Here we show the essential role of a translocon Sec61 for antigen cross-presentstion and lupus tissue injuries with regard to self-organized criticality theory.

Methods:  Bone marrow-derived dendritic cell (BMDC) of BALB/c mice was cultured with fluorescent-labeled ovalbumin (OVA). Early endosome antigen 1 (EEA1) and calnexin were detected by using immunofluorescent staining to identify endosome and endoplasmic reticulum (ER), respectively. A translocon Sec61 was also detected to examine whether or not engulfed antigen is exported from endosome to cytoplasm through Sec61. For in vivo study, BALB/c mice were repeatedly immunized with OVA to induce SLE. The CD11c⁺ DC isolated form spleen (spDC) of the mice immunized 12x with OVA was cultured with fluorescent-labeled OVA to examine localization of engulfed antigen. Localization of OVA, EEA1, calnexin and Sec61 in BMDC and spDC was examined under confocal laser microscopy. Further, exotoxin A was co-immunized with OVA to inhibit Sec61 in vivo in BALB/c mice. Renal lesion and the generation of CTL were assessed by proteinuria and the number of IFNγ-producing CD8⁺ T cell.

Results:  In BMDC, OVA was co-localized with an endosomal marker EEA1 until 15 min of culture, and subsequently separated from EEA1. OVA did not co-localize with an ER marker calnexin. Instead, OVA was co-localized with a translocon Sec61. The same result was obtained using spDC, suggesting that OVA could be exported from endosome directly to cytoplasm via Sec61. After repeated immunization with OVA, we found that the amount of Sec61 localized in endosome was gradually and significantly increased compared with control. While we showed previously that antigen cross-presentation is a pre-requisite for lupus tissue injury inducing nephritis, we did found that treatment of BALB/c mice with a Sec61 inhibitor exotoxin A inhibited both the generation of CTL and lupus nephritis.

Conclusion: Direct export of antigen from endosome to cytoplasm via Sec61 is essential not only for antigen cross-presentation but also development of lupus nephritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sec61-is-indispensable-for-antigen-cross-presentation-and-the-development-of-lupus-nephritis-a-novel-self-organized-criticality-theory-explaining-the-cause-of-systemic-lupus-erythem/