Allogeneic lymphocyte-dependent antibodies (LDA). Specificity of Fc receptors.

Cellular receptors for the Fc fragment of IgG have been studied by the erythrocyte antibody (EA) rosette technique using either human red blood cells coated with anti-D IgG alloantibodies or pigeon red blood cells coated with aggregated IgG. Human lymphocytes were shown to form 4 to 35% of anti-D EA rosettes and 15% of E-aggregated IgG rosettes. Anti-D EA rosette values depended closely upon the amount and the source of anti-D antibodies, Ripley's type antisera giving the highest percentage of rosettes. Cell filtration through a nylon-wool column, a procedure known to remove B cells, caused a complete depletion of E-aggregated IgG rosettes and significant but only partial decrease of anti-D EA rosettes. A low but significant percentage of double rosettes were formed when anti-D-coated erythrocytes were mixed with aggregated IgG coated red blood cells or sheep red blood cells. Removal of EA-rosette-forming cells by passage on a Ficoll-Hypaque gradient K cell activity (assessed by lymphocyte-dependent antibody cytotoxicity using anti-HLA antibody-coated target cells) even when forming rosettes at low levels of erythrocyte sensitization with anti-D serum. Conversely, B cell markers (erythrocyte antibody complement rosettes or surface Ig) were only decreased at high levels of erythrocyte sensitization and remained unaffected after depletion of EA-rosette-forming cells formed at low levels of erythrocyte sensitization. These data suggest the existence of several populations of Fc receptor-bearing cells in human peripheral blood which may be differentiated according to the degree of erythrocyte IgG sensitization. Rosettes made with low concentrations of anti-D serum are mainly formed by non-B cells endowed with K cell activity, whereas those formed with high concentration of anti-D serum include B cells, monocytes, and K cells.