Interspecific structural differences in nucleosome as revealed by heteroimmunization in mice with human nucleosome

Although mounting evidence suggests the association of anti-nucleosome (NS) antibodies with lupus nephropathy in humans, the influence of interspecific differences in NS structure on the diagnosis has not been studied fully. Thus, we investigated the interspecific differences in NS structure by immunizing normal BALB/c mice with human nucleosomes (hNS). We purified hNS and mouse nucleosomes (mNS) from individual established cell lines. Purified NS was of high-pressure liquid chromatography grade and contained less than 1% dinucleosome, if any. Immune responses to NS were tested by an enzyme-linked immunosorbent assay. Of 6 mice, 2 responded to both hNS and mNS. However, antibodies produced in individual mice had higher affinity to mNS than to hNS. IgG response to hNS was IgG1 and IgG2b in subclass, whereas that to mNS was restricted to IgG1. Coincident with this response difference, agarose gel electrophoresis showed a mobility difference between hNS and mNS: the former was slower than the latter. In conclusion, immunodifferentiation in vivo in mice of autologous from heterologous NS together with their mobility difference in agarose gel suggest the presence of interspecific differences in NS. In humans, 2 out of 14 randomly tested patients with systemic lupus erythematosus preferred hNS over mNS; the IgG subclass in one was IgG1, and in the other IgG4. Taken together, interspecific differences in NS will provide a new area of study not only in biochemistry but also in immunology and/or autoimmunity.