Fluorescence-activated cell sorting of human T and B lymphocytes: I. Direct evidence that lymphocytes with a high density of membrane-bound immunoglobulin are precursors of plasmacytes

Human peripheral lymphocytes bearing either a high or a low amount of membrane-bound immunoglobulin were studied. Cells were “tagged” with fluorescein-labeled antiimmunoglobulin reagents and separated by means of a new electronic instrument, a fluorescence-activated cell sorter (FACS), into populations with either > 10⁵ or < 5 × 10³ immunoglobulin molecules per cell. Fractions of high purities were obtained. (>80% and >99.9%, respectively). In vitro, different functional properties were observed: lymphocytes with high densities of membrane-Ig gave a late proliferative response after stimulation with Pokeweed mitogen (PWM). A considerable proportion of stimulated cells developed into mature plasmacytes as detected by cytoplasmic staining. Those lymphocytes with a low density or complete absence of membrane-Ig could be stimulated by both Phytohemagglutinin (PHA) and Pokeweed mitogen, but no differentiation into plasmacytes occurred. The functions are similar to those of bone marrow-derived (B) and thymus-derived (T) lymphocytes in mice. Thus, the designation as B lymphocytes for human lymphocytes with a large quantity of membrane-bound immunoglobulin seems justified.