IgM anti-Fc_γR autoantibodies trigger neutrophil degranulation

Anti-Fc_γR IgM monoclonal antibodies (mAbs) isolated from lipopolysaccharide-stimulated spleen cells from tightskin (TSK) mice were found to be polyspecific, reacting with a wide variety of molecules, including double-stranded DNA, topoisomerase, RNA polymerase, and different collagen types. Approximately 60% of the polyspecific IgM mAbs have anti-Fc_γR specificity. These anti-Fc_γR mAbs induce the release of hydrolases from both azurophil and specific granules of human neutrophils. 25-45% of the total cellular content (determined in Nonidet P-40 lysates) of neutrophil elastase, 10-25% of β-glucuronidase, and 30-50% of alkaline phosphatase was released after incubation with the mAbs. The degranulation process was accompanied by dramatic morphological changes shown by scanning and transmission electron microscopy. The release of hydrolytic enzymes stimulated by the IgM anti-Fc_γR mAbs was inhibited by preincubation of neutrophils with Fab fragments of either anti-human Fc_γRII (IV.3) or anti-human Fc_γRIII (3G8) mAbs. The binding of the anti-Fc_γR TSK mAbs to human neutrophils was inhibited by Fab fragments of mAb 3G8. However, we found that the TSK anti-Fc_γR mAbs do not bind to human Fc_γRII expressed in either CHO cells or the P388D1 mouse macrophage cell line. Since the enzyme release could be inhibited by Fab fragments of mAb IV.3, we suggest that the signal transduction may require Fc_γRII activation subsequent to crosslinking of the glycan phosphatidyl inositol-anchored Fc_γRIII-1. These data demonstrate for the first time that polyspecific autoantibodies with Fc_γR specificity can trigger neutrophil enzyme release via human Fc_γRIII-1 in vitro and indicate a possible role for such autoantibodies in autoimmune inflammatory processes.