After isolation of CD19⁺ B cells of 3 healthy donors, naive (CD27⁻IgG⁻) as well as IgM memory (CD27⁺IgG⁻) cells were sorted by FACS. Purified B cells (CD19⁺ B cells) or B-cell subpopulations (CD27⁻ or CD27⁺IgG⁻ B cells) were then stimulated with IL-2 or SAC plus IL-2. IgG and IgM concentrations in 8-day culture supernatants are given in nanograms per milliliter. While naive B cells produce IgM only in the presence of SAC plus IL-2 and CD4⁺ T cells, IgM secretion by IgM memory B cells is sufficiently activated by either stimulus alone. There was no class switch in vitro detectable in either subpopulation under the applied conditions.

In the presence (+) or absence (None) of CD4⁺ T cells according to the protocol. For details, see “Patients, materials, and methods.”