Autoantigen components recognizable by scleroderma sera are exported via ectocytosis of fibroblasts

Previously, we have demonstrated that ectocytosis, a unique cell trafficking process to export a specific subset of cellular proteins in the form of membrane vesicles, can be triggered from human skin fibroblasts cultured in a three-dimensional collagen lattice upon stress relaxation. The same culturing system was employed in the present study using fibroblasts isolated from patients with systemic sclerosis (SSc). To see whether any putative intracellular autoantigens causing SSc might escape out of cells by way of ectocytosis, the same stress-relaxation method was used to induce a synchronized ectocytosis among cultured cells. Membrane vesicles released by scleroderma fibroblasts were subsequently isolated, resolved on SDS-PAGE and immunoblotted with sera from 89 patients with various autoimmune diseases and 11 normal volunteers. Three major polypeptides with apparent mol. wts of 12-14, 32-34 and 70-80 kDa are prominent bands on both SDS-PAGE and immunoblots. The 32-34 kDa polypeptide has been further identified as a member of the annexin protein family, while the 70-80 kDa protein has been shown to be topoisomerase I, as judged by its reactivity to patients' sera and a rabbit polyclonal antibody, and as also judged by a functional assay. In conclusion, our results suggest that ectocytosis might be one of the potential pathways for cells to export intracellular antigens and subsequently cause autoimmune reactions.