A major subpopulation of Fc receptor-bearing lymphocytes revealed by rosette formation in dextran media: studies with pig, sheep and rat lymphocytes.

Blood lymphocytes from young pigs which formed 9.1 +/-0.7% Fc rosettes (mean +/- S.E., range 4.1-16.0), with rabbit and pig immunoglobulin-coated indicator cells used in optimum conditions in phosphate-buffered saline (PBS), formed 32.6 +/- 1.8% (16.9-44.3) in the presence of 4% dextran (DFc). The proportion of DFc+/Fc+ lymphocytes varied from 2.2 to 5.9 (3.8 +/- 0.3). Compared with the PBS test, in dextran rosettes are formed with more lymphocytes and with red cells coated with less immunoglobulin. Ficoll at 14% gave similar enhancement. Dextran enhancement of Fc rosettes was also observed with sheep PBL, but not with rat thoracic duct lymphocytes. The FC portion of IgG is responsible for both Fc and DFc rosettes. Thus most Fc and DFc rosettes, formed with serum antibody-coated bovine RBC, are revealed by IgG coated but not F(ab')2-coated BRBC, and show dose-dependent inhibition in the presence of free pig IgG (complete at 5-10 mg/nl) but not IgM (up to 2 mg/ml). Separation of lymphocytes on nylon wool suggests that DFc rosettes are formed by B cells and some T cells. Over half of the weak Fc rosette-forming lymphocytes (DFc-Fc) elute in the non-adherent fraction, which contains few B cells, and therefore are a subpopulation of T cells.