Sequence analysis and fine specificity of two human monoclonal antibodies to histone H1.

Two human IgM lambda monoclonal antibodies (MAb) derived from the splenic lymphocytes of patients with idiopathic thrombocytopenia (Ben) and systemic lupus erythematosus (Wri) were studied. BEN-27 and WRI-170 hybridoma supernatants were screened for binding to ssDNA, dsDNA, poly (ADP-ribose), cardiolipin, histone subclasses and Klebsiella K30 cell wall antigen. Of this panel of antigens, BEN-27 and WRI-170 antibodies reacted only with histone H1. Their fine specificity was defined by direct and inhibition ELISA with synthetic peptides of the major human H1b variant. Antibody WRI-170 was shown to bind to both the N- and C-terminal peptides encompassing residues 1-16 and 204-218 of H1b whereas BEN-27 reacted only with peptide 204-218. To analyse the genetic origin of these autoantibodies, we determined the nucleotide sequence of the heavy (H) and light (L) chain variable regions of these two hybridomas. BEN-27 and WRI-170 MAbs were found to use VH1-DN1-JH4/V lambda 3-J lambda 2 and VH3-DIR2-D21/9-JH1/V lambda 2-J lambda 2 gene segment combinations respectively. Between 70 and 95% homology was demonstrated when the mRNA sequences for BEN-27 and WRI-170 were compared with published VH and V lambda germline sequences. This finding suggests that BEN-27 heavy and light chains and WRI-170 light chain use unidentified VH and V lambda germline gene segments whereas WRI-170 heavy chain derives from a VH gene segment recently identified. It is noteworthy that the CDRs of the two MAbs contain several negatively charged amino acids which are assumed to be of critical importance in antigen binding. Moreover, striking similarities are observed between BEN-27 heavy chain CDR2 and a previously described murine anti-H1 Ab heavy chain CDR2.