Mononuclear cells expressing Fc gamma receptors that form Facb rosettes are increased in the peripheral blood of patients with rheumatoid arthritis compared with controls. Healthy individuals with a positive skin response to tuberculin showed a marked increase in numbers of circulating Facb-R+ cells three days after challenge, returning to baseline after seven days. No response was observed in subjects showing a negative skin test. A similar increase in Facb-R+ cell numbers was measured after intramuscular injection of another specific antigen, tetanus toxoid. In addition to this enhancement of Facb-R+ cell numbers, evidence has been obtained that these cells are in an activated state postimmunisation as judged by acquisition of low density and increased expression of class II MHC antigens. Apparently identical changes in Facb-R+ cell numbers and activation may be induced in vitro either by culturing sensitised mononuclear cells with specific antigen for three days or by an overnight incubation of normal cells with gamma-interferon (gamma-IFN). By analogy, therefore, the increased numbers of Facb-R+ cells in patients with rheumatoid arthritis are probably induced by gamma-interferon generated as part of an antigen driven immune response. In this context it is interesting that patients with Felty's syndrome, in whom neutropenia increases susceptibility to infections leading to the possibility of further stimulation of the immune system by micro-organisms, have particularly high levels of circulating Facb-R+ cells.
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