Bullous pemphigoid (BP), the most common autoimmune blistering disease, is induced by
autoantibodies to
type XVII collagen (COL17). Previous studies demonstrated that COL17 harbors several
epitopes targeted by autoreactive T and B cells and that the target
epitopes change sequentially during the disease course. To elucidate the details of the humoral immune response to COL17, we used an active BP mouse model in which BP is induced by the adoptive transfer of spleen cells from wild-type mice immunized with human COL17-expressing
skin grafting to immunodeficient COL17-humanized (Rag-2-/-, mouse Col17-/-, human COL17+) mice. By immunoblot analysis,
antibodies to the NC16A domain and other extracellular domains (ECDs) of COL17 were detected earlier than
antibodies to intracellular domains (ICDs) in the active BP model. Time course analysis by
enzyme-linked
immunosorbent assay demonstrated a delayed peak of
antibodies to ICD
epitopes in active BP model. The blockade of CD40-CD40
ligand interaction soon after the adoptive transfer suppressed the production of
antibodies to the non-collagenous 16A (NC16A) domain but not to an ICD
epitope, suggesting the sequential activation from T and B cells against the ECD
epitopes including the NC16A domain to those against ICD
epitopes in vivo. Both wild-type mice immunized with a fragment of the NC16A domain and the recipients of those spleen cells produced
IgG antibodies to ICD and ECD
epitopes, showing intramolecular
epitope spreading from the NC16A domain to other
epitopes of COL17. Furthermore, we found that a portion of the active BP model mice show intermolecular
epitope spreading from human COL17 to murine BP230. The appearance of
antibodies to ICD
epitopes of COL17 or of
antibodies to murine BP230 did not correlate with the skin changes in the mice, suggesting that those
antibodies have low pathogenicity. These results suggest that the immune response to the ECD
epitopes of COL17, especially to the NC16A domain, triggers intramolecular, and intermolecular
epitope spreading to ICD
epitopes of COL17 and to murine BP230. These novel findings provide insight into the mechanism of
epitope spreading in organ-specific, antibody-mediated autoimmune disorders.