The impact of somatic mutation and environmental antigens on the origin of anti-double-stranded - DNA autoantibodies

Abstract

Anti-DNA autoantibodies are characteristic of the human autoimmune disease systemic lupus erythematosus (SLE). To study the impact of somatic mutation and environmental antigens on the molecular origin of autoantibodies in vivo, I studied anti-DNA antibodies isolated from autoimmune mice that develop murine lupus. Such an analysis can address (1) whether the genetic origin of autoantibodies is different from normal antibodies; (2) whether there are germ-line immunoglobulin genes that predispose an individual to the production of autoantibodies; (3) whether the production of anti-DNA antibodies is antigen-driven or is the result of polyclonal B-cell activation; and (4) whether diversification of the immune response to environmental antigens generates autoantibodies.;(NZBxNZW)F1 mice were immunized with phosphorylcholine (PC) because our lab had previously described how a single somatic mutation in vitro converted S107, a protective antibacterial antibody that binds PC, into U4, an anti-DNA autoantibody. Since the anti-PC response is dominated by antibodies encoded by the V{dollar}\sb{lcub}\rm H{rcub}{dollar}S107 family, and because the S107-U4 paradigm predicted that somatic mutation could convert anti-PC antibodies into anti-DNA antibodies, only hybridomas producing antibodies encoded by the V{dollar}\sb{lcub}\rm H{rcub}{dollar}S107 family were studied. We screened for such hybridomas by RNA dot blot hybridization using the pT15 probe which hybridizes to all four members of the family; therefore, hybridomas were selected independently of binding specificity, isotype, or idiotype. The four V{dollar}\sb{lcub}\rm H{rcub}{dollar}S107 germ-line genes from both the NZB and NZW parental strains were cloned and sequenced.;Whereas the anti-PC response in (NZBxNZW)F1 mice is dominated by the V{dollar}\sb{lcub}\rm H{rcub}{dollar}1 gene of the V{dollar}\sb{lcub}\rm H{rcub}{dollar}S107 family, the anti-DNA antibodies encoded by this family use the V{dollar}\sb{lcub}\rm H{rcub}{dollar}11 gene; therefore, it is unclear whether PC immunization has a role in the production of anti-DNA antibodies. The major finding of this study is the IgG{dollar}\sb{lcub}\rm 2a{rcub}{dollar} V{dollar}\sb{lcub}\rm H{rcub}{dollar}11-encoded anti-dsDNA antibodies have undergone extensive somatic diversification from the germ-line gene. Analysis of the mutations indicates that the production of anti-DNA antibodies might be antigen-driven. Furthermore, somatic mutation may produce autoantibodies that are more pathogenic since it can generate anti-DNA antibodies that are more cationic and bind better to dsDNA.