Cellular immunologic studies were performed on the leukemic cells of a 59 year old white man with monocytic leukemia. Morphologically, most circulating leukocytes were monocytes. They demonstrated Fc and C3 (third component of complement) receptors, phagocytized latex particles, showed In vitro cytoplasmic spreading and lysed antibody-coated chicken erythrocytes. Phagocytosis of, as well as rosetting with, C3-coated erythrocytes and very rapid cytoplasmic spreading suggested in vivo monocyte "activation." The cells were easily maintained in primary culture for up to 13 weeks, with acquisition of typical macrophage morphology. In addition, nearly all cells demonstrated surface immunoglobulin (Slg) which was (1) trypsin-sensitive and did not regenerate in culture, (2) could be restored after trypsinization by incubation of cells In autologous or normal serum and (3) persisted in culture on 75 per cent of cells for at least seven days. Fluoresceinated monospecific antiserums showed an immunoglobulin G (IgG), kappa pattern on one occasion and an IgG, kappa, lambda pattern on another. Eluates of the cells prepared on three occasions showed only IgG and kappa reactivity using numerous antiserums. We concluded that most Slg was probably Fc-receptor bound with unusually high affinity and with greater representation of IgG (kappa) than IgG (lambda). These studies suggest that apparent "monoclonal" Slg does not necessarily indicate a truly clonal proliferation of B cell origin.
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