Phagocytosis of Keratin Filament Aggregates Following Opsonization With IgG-Anti-Keratin Filament Autoantibodies

Abstract

IgG-anti-keratin intermediate filament autoantibodies occur in low titers in all normal human sera. These same antibodies are present in high titers in the sera of patients who have diseases in which cells containing keratin intermediate filaments (KIF) have been damaged, such as systemic lupus erythematosus, graft-versus-host disease, and cutaneous tumors. Since some human autoantibodies are thought to function in part as opsonins promoting the removal of insoluble cellular proteins after tissue injury, we investigated the influence of IgG-anti-KIF autoantibodies on the phagocytosis of insoluble KIF aggregates by human monocytes and polymorphonuclear neutrophils (PMN). Keratin intermediate filaments assembled in vitro were reconstituted into dense spherical KIF aggregates 0.3-2.5 microns in diameter by dialysis against phosphate-buffered saline. Immunoelectron microscopy revealed that, as expected, human IgG-anti-KIF autoantibodies bound to the KIF aggregates. Human monocytes or PMN were incubated either with nonopsonized KIF aggregates or with KIF aggregates that had been reacted with IgG-anti-KIF autoantibodies. The uptake of KIF aggregates was visualized by indirect immunofluorescence, immunoperoxidase staining, and immunoelectron microscopy. Monocytes rapidly and efficiently bound and phagocytosed KIF aggregates that had been coated with IgG-anti-KIF autoantibodies. Nonopsonized KIF aggregates, in contrast, were taken up much less efficiently. Differences were most marked at 4 degrees C for 60 min with phagocytosis of opsonized KIF aggregates by 23 +/- 8% of monocytes in contrast to phagocytosis by only 0.2 +/- 3% monocytes when nonopsonized KIF aggregates were used. Similar results occurred at 37 degrees C for 5 min with phagocytosis by 38 +/- 28% vs 1.8 +/- 0.4% of monocytes of opsonized and nonopsonized KIF aggregates, respectively. A high percentage of PMN also phagocytosed opsonized KIF aggregates, whereas nonopsonized KIF aggregates were ingested less avidly. These data indicate that the opsonization of extracellular KIF aggregates by IgG-anti-KIF autoantibodies plays an important role in promoting the phagocytosis of KIF aggregates. The subsequent phagocytosis represents a rapid and very effective mechanism for the removal of insoluble KIF following keratinocyte cell death.