The expression of acidic ribosomal phosphoproteins on the surface membrane of different tissues in autoimmune and normal mice which are the target molecules for anti-double-stranded DNA antibodies

Affinity-purified polyclonal anti-double-stranded DNA (anti-dsDNA) antibodies from patients with systemic lupus erythematosus (SLE) exert a cytostatic effect on cultured rat glomerular mesangial cells (MC). The cognate antigens expressed on the surface of MC have been proved to be acidic ribosomal phosphoproteins (P proteins) in our previous study. The mesangial cytostatic effect of anti-dsDNA antibodies is attributed to the cross-reactivity of the antibodies with membrane-expressed P proteins, but not to the effect of minute amounts of anti-ribosomal P proteins antibodies contained in the anti-dsDNA preparations. Immunofluorescence staining of the native cells demonstrated that anti-dsDNA antibodies bound to the surface of rat mesangial cells, rat brain astrocytes (RBA-1) and mouse fibroblasts (3T3). Anti-dsDNA antibodies also exert potent cytostatic effects on these cells in a dose-dependent manner. In addition, the plasma membranes of different cell lines and tissues from normal and autoimmune mice were isolated and probed by anti-dsDNA antibodies in Western blot analysis. We found the actively proliferating cells such as MC, RBA-1 and 3T3 may express both PO (38,000 MW) and P1 (19,000 MW) on the surface membrane. In addition, the kidney, liver and spleen from either autoimmune MRL-lpr/lpr or BALB/c mice may constantly express PO protein, but the expression of P1 is inconsistent. In contrast, brain and muscle from either mice failed to express P proteins on their surface. Unexpectedly, a high molecular weight substance (larger than 205,000 MW) with unknown nature appears in the membrane of brain and muscle tissues in both mice. Immunoprecipitation of the surface-biotinylated MC-lysate by anti-dsDNA antibodies further confirmed that P1 (19 000 MW) and P2 (17,000 MW) are really expressed on the cell surface. These results suggest that P proteins expressed on the surface of different tissues become the targets for anti-dsDNA antibodies mediating pleomorphic tissue damage in patients with SLE.