Human monoclonal IgM DJ binds to ssDNA and human commensal bacteria

In this study we tried to elucidate further the crossreactivity pattern and binding characteristics of human monoclonal IgM DJ which is an anti-DNA antibody and possesses Y7 natural idiotope. Isolated IgM DJ and its enzymatically obtained fragments Fab' and (Fab')2 were tested for binding to more than 26 antigens and nine bacteria in indirect ELISA. Inhibition of binding studies and examination of the stability of antigen-antibody complexes were also done in ELISA assay. IgM DJ bound to single stranded DNA and human lactic acid bacteria, such as L. acidophyllus, B. bifidum and L. plantarum. This binding was shown to be mediated through IgM DJ Fab' fragment. High avidity and low affinity of interactions was estimated from the binding curves of Fab', (Fab')2 fragments and whole IgM. The common epitopic motif on both antigens were negatively charged phosphodiester moieties. Complexes formed with ssDNA and B. bifidum were resistant to washing with high salt. This suggested that electrostatic attraction was not a strong component of the binding. A novel pattern of natural autoantibody reactivity in a human system related to cross-reactivity with DNA and LAB is described. Possible involvement of LAB in induction of natural anti-DNA antibodies is discussed.