IgM‐ and IgD‐bearing peripheral blood lymphocytes differentiate to IgM but not IgG or IgA immunoglobulin‐secreting cells

Human peripheral blood mononuclear cells were depleted of surface IgM+ or IgD+ cells and assayed for mitogen-induced differentiation to immunoglobulin-secreting cells (ISC) of IgM, IgG and IgA classes. Stimulatory agents included T cell-dependent pokeweed mitogen, B cell mitogen Staphylococcus aureus bacteria strain Cowan I, and a combination of the two which gives uniform, high levels of ISC from all normal donors. Depletion of either IgM- or IgD-bearing B lymphocytes resulted in loss of cells bearing the other Ig class and blocked most of the mitogenic reactivity to anti-IgM and anti-IgD. Proliferative responses to Cowan I in these depleted populations were about 20% that of unfractionated mononuclear cells. Depletion of T cells increased the mitogenic response to Cowan I and to the two antibody preparations, showing that they are T-independent mitogens. Depletion of IgD+ cells caused partial loss of mitogen-induced IgM ISC (22%-60% of unseparated controls) but no loss of IgG or IgA ISC. Depletion of IgM-bearing cells caused complete loss of IgM ISC, but no loss of IgG or IgA ISC. We previously demonstrated that anti-IgM ISC, but no loss of IgG or IgA ISC. We previously demonstrated that anti-IgM antibody blocked mitogen induction of Ig secretion of these three classes in spleen cells, but only IgM secretion in blood mononuclear cells. Together, the results suggest that the majority of cells in normal blood responding to mitogens to mature to IgG or IgA production belong to IgM-, IgD- B cell subjects, in contrast to precursors of secreting cells for these isotypes in the spleen. Thus, these blood precursors appear to be more mature than the corresponding spleen cells.