Ultrastructure of cells binding immunoglobulin-coated erythrocytes in rheumatoid arthritis.

Rosettes obtained by mixing blood mononuclear cells from patients with rheumatoid arthritis and rabbit IgG-coated erythrocytes were examined after micromanipulation. When leucocytes were fractionated by filtration on polyamide fibres, rosettes were all formed by lymphocytes which had an homogeneous morphological appearance. They contained monoribosomes, dense lysosomes and exhibited some degree of membrane activity giving rise to small cytoplasmic projections which firmly bound red cells. On some occasions ingestion of small erythrocyte fragments could be seen. When rosette formation was carried out with cells isolated by centrifugation on Ficoll, a great number of rosette-forming cells appeared to be monocytes. These cells had numerous finger-like projections which deformed and phagocytosed red cells. The detection of monocytes in this system is interpreted as resulting from the presence of surface receptors for IgG molecules. The ultrastructural features of the rosette-forming lymphocytes are discussed with respect to their content in organelles related to protein synthesis and pinocytosis, and also to their membrane activity induced by interaction with IgG-coated erythrocytes.