Antigenic phenotype and functional characterization of human tonsil B cells

Human tonsil cells were labeled with anti-leu-16, a monoclonal antibody (MoAb) that recognizes the CD20 antigen and that is specific for B cells. Two populations of B cells were identified by flow cytometry on the basis of antigen density. One labeled brightly with anti-Leu-16, and the other labeled at a level comparable to blood B cells. These two populations were characterized with a panel of MoAbs in two- and three- color flow cytometric studies and appeared to correspond to germinal- center and mantle-zone B cells. The pattern of staining of anti-Leu-16 on sections of frozen tonsil supported this characterization. Anti-Leu- 16 labeled germinal center cells more intensely than mantle zone cells and stained a few scattered B cells in the interfollicular zone. The ability of each Leu-16+ population to secrete IgG and IgM in response to mitogens was measured in a particle immunofluorescence assay. Dim Leu-16+ B cells (small, resting B cells and a subpopulation of preactivated cells) secreted IgG and IgM in response to pokeweed mitogen (PWM) but only IgG in response to B cell growth factor (BCGF). Bright Leu-16+ B cells (small to large activated cells and possibly memory cells) did not respond to PWM but secreted IgG in response to BCGF. The functional responses of dim Leu-16+ and bright Leu-16+ B cells were consistent with their identification as mantle-zone and germinal-center B cells. Phenotypic identification and functional studies of mantle-zone and germinal-center B cells may help clarify the differentiation pathway within the germinal center.