Characterization and VH Sequences of Human Monoclonal Anti-F(ab′)2 Autoantibodies from Normals and Sjögrenʹs Syndrome Patients

To investigate the genetic background of anti-F(ab′)2 autoantibodies and the mechanism behind their production we have analyzed 10 human monoclonal antibodies directed against IgG F(ab′)2 and IgG Fab. They were all derived from peripheral blood by the EBV/hybridoma technique. Eight were from three healthy individuals and two from two patients with primary Sjögrenʹs syndrome (pSS). They react with epitopes on distinct regions of IgG, including epitopes present on or near the hinge of IgG, epitopes on the Fdγ, and an antigenic determinant(s) present on λ light chains. These determinants are either exposed on the intact IgG molecule or revealed following pepsin or papain digestion. The VH germline gene repertoire used is diverse and with considerable overlap with that used by rheumatoid factors (RF). The two IgG antibodies from normals are extensively mutated (13 and 24 mutations/VH), but with a replacement to silent mutation ratio in the CDRH1 + 2 of only 3.7. The IgM antibodies from normals are also heavily mutated (mean 10 mutations/VH). This suggests that anti-F(ab′)2 from normals are generated by an antigen-driven somatic hypermutation mechanism. In contrast, the two IgM antibodies from pSS are virtually unmutated in both VH and VL. Together with published data of pSS RF and anti-Ro 52-kDa sequences (1–3), this suggests that there is an expanded population of naı̈ve B cells with autoantibody specificities in the peripheral blood of pSS patients.