Human peripheral blood B cell subpopulations: surface IgD+ cells respond to pokeweed mitogen with plasma cell differentiation.

Human peripheral blood (PB) mononuclear cells were depleted of T cells and subsequently of monocytes by counterflow centrifugation. The resulting B cell enriched fraction contained less than 3% T cells, 1 +/- 1% monocytes and 71 +/- 6% B cells. Approximately 56% of the B cells carried surface IgM (sIgM) and sIgD, the remaining 44% carried only one isotype (IgG, IgA, IgM or IgD). Using anti-delta or anti-mu-coated Ox red blood cells (OxRBC) the B cell enriched fraction was separated into a sIgD+ or sIgM+ and sIgD- or sIgM- fraction. The sIgD+ as well as the sIgM+ fraction responded (partially) to pokeweed mitogen (PWM) with plasma cell differentiation into cytoplasmic IgM (cIgM) containing cells. Cytoplasmic IgD+ cells were occasionally found in the sIgD+ or sIgM- fraction, but never in the sIgM+ fraction. Cells containing cIgG or cIgA were only found after stimulation of the sIgM- or sIgD- fraction, the latter also produced cIgM+ cells. In a second separation step of the sIgD- and sIgM- fraction with anti-alpha or anti-gamma-coated OxRBC it was shown that the cIgA+ and cIgG+ cells were derived from B cells expressing only sIgA or sIgG respectively. In conclusion, two major B cell subpopulations can be found in PB: a sIgM+D+ fraction, which gives rise to only cIgM+ plasma cells after PWM stimulation and a second population of B cells which express one heavy-chain isotype on their surface. These cells not only differentiate on PWM stimulation, but they also show a strong proliferative response.