HUMAN AUTOANTIBODY RECOGNITION OF DNA

Combinatorial IgG Fab phage display libraries prepared from a systemic lupus erythematosus (SLE) donor and a healthy donor were affinity sel ected against human placental DNA. Human monoclonal antibody Fab fragm ents specific for DNA were isolated from both libraries, although Fabs of the highest affinity were isolated only from the lupus library. Ge nerally, apparent affinities of the Fabs for human placental DNA, puri fied double-stranded DNA, and denatured DNA were approximately equival ent. Surface plasmon resonance indicated Fab binding constants for a d ouble-stranded oligodeoxynucleotide of 0.2-1.3 x 10(8) M(-1) The highe r-affinity Fabs, as ranked by binding to human placental DNA or to the oligonucleotide probe, tested positive in the Crithidia luciliae assa y commonly used in the diagnosis of SLE, and interestingly the genes e ncoding the heavy-chain variable regions of these antibodies displayed evidence of only minimal somatic hypermutation. The heavy chains of t he SLE Fabs were characterized by a predominance of basic residues tow ard the N terminus of complementarity-determining region 3 (CDR3). The crucial role of heavy-chain CDR3 (HCDR3) in high-affinity DNA recogni tion was suggested by the creation of DNA binding in an unrelated anti body by HCDR3 transplantation from SLE antibodies. We propose that hig h-affinity DNA-binding antibodies can arise in SLE without extensive s omatic hypermutation in the variable-region genes because of the expre ssion of inappropriate HCDR3s.