Epitope recognition in histone H1 by SLE autoantibodies in the presence of a DNA-ligand

The autoimmune response to chromatin antigens in systemic lupus erythematosus: autoantibodies against histone H1 are a highly specific marker for SLE associated with increased disease activity

This study investigates specificity, sensitivity and concomitant presence of antibodies against histone H1 (H1), nucleosomes (NUC), chromatin (CHR) and dsDNA in patients with systemic lupus erythematosus (SLE), analyses their association with SLE disease activity and characterizes the immunodominant epitope reactivity of anti-H1 antibodies and its relation to SLE disease activity. In a cross-sectional study 394 sera of patients with various rheumatic diseases and healthy subjects were analysed by ELISA for antibodies against H1, NUC, CHR and dsDNA. In addition, a longitudinal analysis was performed that included 121 sequential serum samples derived from 16 SLE patients to assess the relation of these antibodies as well as antibodies to histone H2B to SLE disease activity. To assess epitope reactivity of anti-H1 antibodies overlapping synthetic peptides covering the entire H1 sequence were used. Anti-H1 antibodies yielded a sensitivity of approximately 45% and a specificity of over 98% for SLE, which was comparable to that found for anti-dsDNA antibodies. Anti-CHR and anti-NUC antibodies were of similar sensitivity but slightly (anti-CHR) or considerably (anti-NUC) less specific for SLE (95 and 85%, respectively). The sequential analysis revealed a strong correlation of anti-H1 antibodies with SLE disease activity that was better than the correlation of anti-dsDNA and anti-NUC antibodies, while only weak correlation was found for anti-CHR and anti-H2B antibodies. The immunodominant epitope for anti-HI was localised between amino acids 204 and 218 (pp204-218) and immune reactivity to this epitope also correlated with disease activity. Anti-H1 is a highly specific marker for SLE with a diagnostic value comparable to anti-dsDNA. A positive testing for anti-H1 indicates increased disease activity, as does the appearance of antibodies to its immunodominant epitope pp204-218.

Mapping of linear epitopes of human histone H1 recognized by rabbit anti-H1/H5 antisera and antibodies from autoimmune patients

Seventeen synthetic peptides of 15-16 residues, covering the complete sequence of the major human H1b variant, were tested for their capacity to bind serum IgG antibodies from 128 patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and Sjögren's syndrome (pSS). One peptide (residues 111-127) of the human H1 degree variant and six synthetic and natural fragments of H5 were also tested. Results were compared to those obtained with antibodies from 11 rabbits immunized against chicken H1 and H5, and calf H1. The activity of peptides was tested in direct ELISA and in inhibition assays with free peptides in solution. A major epitope recognized by antibodies from SLE, RA and pSS patients as well as by rabbit antibodies was identified in the C-terminus of H1b (residues 204-218). Other peptides in the globular (residues 79-94) and C-terminal domains of H1b and peptide 111-127 of H1 degree were also recognized, albeit at a lower level and frequency, and some of them contain sequence homologies with peptide 204-218. Patients' antibodies and rabbit antisera were tested with complete H1 proteins from HeLa cells, calf thymus and chicken erythrocytes and with chicken H5. Less than 25% of autoimmune sera contained IgG antibodies reacting with H1/H5 in a direct ELISA. In dot-immunoassay, antigenic activity with intact H1/H5 proteins was detected in a larger number of sera. Using antibodies raised in rabbits against peptides 1-16 and 204-218 of H1b, we found no reaction with H1 immobilized on a solid-phase. In contrast, peptides 144-159, 170-185 and 204-218, which contain identical structural domains, compete with H1 in solution indicating that any of these three regions are accessible at the surface of free H1 and may be involved in the induction of specific antibodies in autoimmune patients.

Folding of terminal histone-H1 peptides in the presence of the oligonucleotide 5'-(AT)6-3'

Peptides H1(1-16) H1(204-218) of human histone H1, comprising the terminal parts of the N- and C-domain, and H1(120-210), comprising the entire C-domain of calf thymus H1, were studied using CD spectroscopy in the presence of trifluoroethanol (TFE) and the oligonucleotide 5'-(AT)6-3'. TFE induces a strong negative ellipticity at 220 nm, showing that the H1 fragments are capable of helical folding. The CD spectrum of free (AT)6 shows strong negative and positive absorptions in the 200-300 nm region resembling the psi-spectrum of DNA. Free (AT)6 showed no helix-coil transition and remained single stranded at room temperature. Combinations of the H1 peptides with increasing concentrations of (AT)6 in low-ionic-strength phosphate buffer developed a strong negative ellipticity at 235 nm. This ellipticity increased with rising (AT)6 concentration and diminished when the (AT)6 concentration exceeded the 1:1 molar ratio in H1(1-16) and H1(204-218) and the 2:1 molar ratio in H1(120-210). The 235 nm ellipticity is attributed to a complex of the H1 peptide with (AT)6 in which the protein is helical. Interaction between histone peptide and (AT)6 is also indicated by UV-absorption spectra which show that the 260 nm absorption is decreased and the 280 nm absorption is increased as compared to free (AT)6. The free peptides show no absorption in this window. The altered 260 and 280 nm absorption suggests that the single-stranded (AT)6 assumes a left-handed pitch and this is confirmed by the displacement of the 270 nm positive ellipticity of free (AT)6 towards 260 nm. Implications of a left-handed linker DNA for chromatin function are discussed.