Capacity of myeloma plasma cells to present antigen to T cells and induce T-cell stimulation. Different numbers of CD38⁺⁺ plasma cells (▪), CD38⁺ cells (□), BMMCs (•), or monocytes (○) from one patient were cultured in the presence of PPD with autologous purified T cells. PPD-induced cell proliferation (A) or IFN-γ–secreting cells (B) were measured. Means ± SD were determined on triplicate values from two experiments.