Crystallographic characterization of the fab fragment of a monoclonal anti-ss-DNA antibody

Female F1 hybrids from the cross of NZB† and NZW inbred mice spontaneously develop an autoimmune syndrome similar to SLE (Howie and Helyer, 1968; Andrews iet al., 1978; Tan, 1982). The F1 progeny produce autoantibodies specific for nuclear components such as oligonucleotides and nucleoprotein complexes. Hybridoma technology has made it possible to generate plasmacytomas secreting monoclonal antibodies reactive with a variety of these nuclear antigens. As an example, splenocytes from F1 mice exhibiting this pathology were fused with nonsecreting murine myeloma cells to produce a cell line (BV 04–01) secreting a monoclonal IgG2b (κ) antibody (Ballard et al., 1985; Ballard and Voss, submitted). The BV 04–01 antibody bound thermally-denatured (single-stranded) DNA from calf thymus or plasmids in preference to the duplex forms. The protein exhibited a base specificity for pyrimidines, with greater affinity for thymine than uracil. Using 5'-phosphorylated oligodeoxythymidilic acids. Ballard and Voss (1985) found that binding was detectable when the epitope size exceeded 5 bases. The oligonucleotide binding was characterized by an inverse temperature dependence between 2° and 40°C (ΔH ~ −7.5 Kcal/mole). The influence of ionic strength on DNA binding by the BV04–01 antibody was determined from titrations with NaCl increasing in concentration from 0.15 to 1.5 M (Ballard and Voss, 1985). Binding decreased less than 2-fold over this range, indicating that electrostatic interactions with the phosphate moieties played a relatively minor role in the stabilization of the DNA-protein complex (see Riggs et al., 1970; Lee et al., 1982). The Fab region of the BV04–01 antibody proved to be an excellent candidate for structural studies by X-ray diffraction methods. After cleavage of the antibody with papain, the Fab fragments were purified, crystallized and subjected to preliminary X-ray analysis. Characterization of these fragments is the subject of the present report.