Specificity of autoantibodies to histone H1 in SLE: relationship to DNA-binding domains

Protein disulfide isomerases are antibody targets during immune-mediated tumor destruction

The identification of cancer antigens that contribute to transformation and are linked with immune-mediated tumor destruction is an important goal for immunotherapy. Toward this end, we screened a murine renal cell carcinoma cDNA expression library with sera from mice vaccinated with irradiated tumor cells engineered to secrete granulocyte macrophage colony-stimulating factor (GM-CSF). Multiple nonmutated, overexpressed proteins that function in tumor cell migration, protein/nucleic acid homeostasis, metabolism, and stress responses were detected. Among these, the most frequently recognized clone was protein disulfide isomerase (PDI). High titer antibodies to human PDI were similarly induced in an acute myeloid leukemia patient who achieved a complete response after vaccination with irradiated, autologous GM-CSF-secreting tumor cells in the setting of nonmyeloablative allogeneic bone marrow transplantation. Moreover, ERp5, a closely related disulfide isomerase involved in major histocompatibility complex (MHC) class I chain-related protein A (MICA) shedding, also evoked potent humoral reactions in diverse solid and hematologic malignancy patients who responded to GM-CSF-secreting tumor cell vaccines or antibody blockade of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). Together, these findings reveal the unexpected immunogenicity of PDIs and raise the possibility that these gene products might serve as targets for therapeutic monoclonal antibodies.

Rabbit anti-chromatin antibodies recognize similar epitopes on a histone H1 molecule as lupus autoantibodies

Autoantibodies in systemic lupus erythematosus (SLE) are believed to be produced through an antigen-driven mechanism, i.e., autosensitization with autoantigens. If this is the case, it is conceivable that antigenic determinants reactive with SLE autoantibodies and induced antibodies raised by immunization are quite similar. To examine this issue, rabbits were immunized with purified histone H1 or chromatin. Although immunization with purified histone H1 did not produce detectable amounts of anti-histone H1 antibodies, immunization with chromatin mounted relatively good antibody responses against not only all histone subunits but also ssDNA. Epitopes on a histone H1 molecule reactive with SLE autoantibodies and rabbit antisera were determined using deletion mutants of a histone H1 molecule. At least two epitopes reactive with induced antibodies were found on a histone H1 and these were closely related to or the same as major autoepitopes in SLE. These data might indicate that B cells producing anti-histone H1 autoantibodies are quite similar to those induced by immunization and provide additional evidence that autosensitization with an autoantigen might be operative and a particulate antigen chromatin could be one of the immunogens for the production of anti-nuclear antibodies of various kinds in SLE.

Relationship between autoepitope and DNA-binding site on a histone H1 molecule

Autoepitope and DNA-binding domain on a histone H1 molecule were compared using truncated histone H1 peptides as antigens. At least two epitopes (epitope A, N-terminal side; epitope B, C-terminal side) were found both of which were composed of approximately 20 amino acids. IgM from all 17 anti-histone H1-positive SLE sera reacted with epitope A. IgG from 12 sera reacted with epitope A and IgG from 4 sera reacted with epitope B. In one case, no IgG anti-histone H1 reactivities were found while IgM from the same patient reacted with epitope A. Epitope A had the ability to bind DNA. The reactivities against histone H1 of affinity-purified antiepitope A autoantibodies were inhibited by DNA. These data suggest that some anti-histone H1 antibodies are directed against a histone H1 DNA-binding site, raising the possibility that an idiotype/anti-idiotype network, at least in part, is involved in the generation of anti-histone H1 autoantibodies.