Method: Bone marrow-derived dendritic cell (BMDC) from BALB/c mice was cultured with fluorescent-labeled ovalbumin (OVA). Early endosome antigen 1 (EEA1) and calnexin were detected to identify endosome and endoplasmic reticulum (ER), respectively, by using immunofluorescent staining. To examine whether or not engulfed antigen is exported from endosome to cytoplasm, translocon Sec61 was also detected. Localization of OVA, endosome, ER and Sec61 was examined under confocal laser scanning microscopy. For in vivo study, BALB/c mice were repeatedly immunized with OVA to induce tissue injuries. MG132 was co-immunized with OVA to inhibit proteasomal degradation of antigen. To investigate whether or not antigen peptide-MHC class I complex is transported from endosome to cell surface, mice were immunized with OVA in the presence of primaquine (PQ), an inhibitor of endosomal trafficking to cell surface. We also inhibited Sec61 in vivo by treating with exotoxin A. Proteinuria and histopathology of kidney were assessed. IFNγ-producing CD8 T cell in spleen was detected under flow cytometry.
Result: In BMDC, OVA was first co-localized with an endosomal marker EEA1, and then gradually separated from EEA1. OVA was never co-localized with an ER marker calnexin. Instead, translocon Sec61 did co-localize with OVA, the findings indicated that OVA was exported from endosome to cytoplasm via Sec61. The in vivo treatment with MG132, an inhibitor of proteasomal degradation, inhibited the generation of fully mature CTL and lupus nephritis. Proteinuria and renal damage were not observed in the mice treated with PQ. Full maturation of CD8 T cell to IFNγ-producing CTL was also inhibited. Since PQ inhibits transport from endosome to cell surface, the finding indicates that antigen peptide-MHC class I complex is directly transported from endosome to cell surface for antigen cross-presentation. Further, the treatment with exotoxin A, an inhibitor of Sec61, also inhibited the maturation of effector CTL and the development of lupus nephritis, indicating that export of antigen from endosome to cytoplasm via Sec61 is indispensable for antigen cross-presentation.
Conclusion: Endosomal trafficking, bypassing ER, is required for antigen cross-presentation. Inhibition of this pathway resulted in inhibition of lupus nephritis. Thus, export of antigen from endosome to cytoplasm via Sec61 is essential for antigen cross-presentation and induction of lupus tissue injuries.
Disclosure: K. Tsumiyama, None; M. Takimoto, None; S. Shiozawa, None.