Apoptosis-associated acetylation on histone H2B is an epitope for lupus autoantibodies

Nucleosomes have been identified as a key autoantigen in systemic lupus erythematosus (SLE). Nucleosomes are present in the circulation due to a disturbed apoptosis and/or an insufficient clearance in SLE. During apoptosis, histones can be modified, thereby making them more immunogenic. Recently, we showed the importance of apoptosis-induced acetylation of histone H4 in the pathogenesis of SLE. The lupus-derived antibody LG11-2 was previously shown to react with the N-terminal tail of histone H2B, which contains amino acid residues that can be modified including phosphorylation of serine 14, known to occur during apoptosis. Here, we evaluate whether apoptosis-induced histone modifications on H2B exist that are targeted by LG11-2 or lupus-derived plasmas.

Immunofluorescence staining and western immunoblot analysis of control, apoptotic and trichostatin A-treated cells/chromatin were performed with monoclonal antibody LG11-2. Reactivity of LG11-2 and plasmas from lupus mice and SLE patients with acetylated and/or phosphorylated H2B peptides was determined in competition ELISA.

LG11-2 showed enhanced reactivity with apoptotic and hyperacetylated H2B compared to normal H2B. This enhanced reactivity was due to the acetylation of lysine 12 in H2B. This modification was also recognized by autoantibodies from pre-diseased lupus mice, but to a lesser extent by plasmas of diseased lupus mice and lupus patients.

The apoptosis-induced acetylation on H2BK12 is a target for autoantibodies in SLE. Since the anti-H2BK12ac reactivity was mainly found in pre-diseased lupus mice, this epitope seems important in the early phase of the anti-chromatin autoimmune response with subsequent epitope spreading to unmodified H2B.