Surface phenotype and function of tonsillar germinal center and mantle zone B cell subsets

Human peripheral lymphoid tissues contain distinct B cell populations that differ in their buoyant density, cell surface phenotype, and responsiveness to proliferative signals. Two major B cells subpopulations from reactive tonsils or lymph nodes have phenotypes that appear to correspond to mantle zone and germinal center B cells. These subpopulations were distinguished using two-color immunofluorescence to measure surface IgM, IgD, DR, DQ, and Bp35 (B1) in pairwise combinations. The two populations were separated on density gradients, and their proliferative responses to activation signals was studied. The dense B cells proliferated in response to anti-mu on beads or anti-Bp35 antibodies but not to T cell-derived growth factors. The dense B cells are almost all IgM+IgD+Bp35dull (mantle zone phenotype). In contract, more buoyant B cells proliferated in response to T cell factors alone but not to anti-mu on beads or anti-Bp35. These cells are IgD-, express little or no IgM and higher levels of Bp35 (germinal center phenotype). An additional minor subpopulation of dense Bp35dull IgD- B cells was detected in tonsils, suggesting that IgD may be lost prior to B cell entry into germinal centers. B cells in peripheral blood and spleen have surface phenotypes distinct from each other and from the tonsil subpopulations. In particular, the levels of IgM, Bp35, and DR all vary among different lymphoid locations. The mean surface density of DR is low in peripheral blood, intermediate in the spleen, and highest in reactive tonsils and lymph nodes. While Bp35 surface level does correlate with the stage of B cell activation, the surface level of DR, DQ, or DP does not since the surface density of DR, DQ, and DP are similar on both mantle zone and germinal center B cells. Both tonsillar populations express a wide range of class II antigen densities and display coordinate expression of DR and DQ antigen levels.