Objectives: In systemic lupus erythematosus (SLE) apoptotic chromatin is present extracellularly, which is most likely the result of disturbed apoptosis and/or insufficient removal. Released chromatin, modified during apoptosis, activates the immune system resulting in the formation of autoantibodies. A study was undertaken to identify apoptosis-induced histone modifications that play a role in SLE.
Methods: The lupus-derived monoclonal antibody BT164, recently established by selection using apoptotic nucleosomes, was analysed by ELISA, western blot analysis and immunofluorescence staining using chromatin, cells, plasma and renal sections. Random peptide phage display and peptide inhibition ELISA were used to identify precisely the epitope of BT164. The reactivity of plasma samples from lupus mice and patients with SLE with the epitope of BT164 was investigated by peptide ELISA.
Results: The epitope of BT164 was mapped in the N-terminal tail of histone H3 (27-KSAPAT-32) and included the apoptosis-induced trimethylation of K27. siRNA-mediated silencing of histone demethylases in cultured cells resulted in hypermethylation of H3K27 and increased nuclear reactivity of BT164. This apoptosis-induced H3K27me3 is a target for autoantibodies in patients and mice with SLE and is present in plasma and in glomerular deposits.
Conclusion: In addition to previously identified acetylation of histone H4, H2A and H2B, this study shows that trimethylation of histone H3 on lysine 27 is induced by apoptosis and associated with autoimmunity in SLE. This finding is important for understanding the autoimmune response in SLE and for the development of translational strategies.