Assessment of antigen expression by normal and neoplastic plasma cells.

The plots show cells that satisfy both R2 and R3 from Figure 1 and demonstrate the controls used to determine that the gated cells are plasma cells. In addition, they show the antibodies used to discriminate normal (right side of each plot) from neoplastic plasma cells (left side of each plot) and to assess the expression of CD126 and CD130. In this patient with MGUS, the neoplastic plasma cells and the normal plasma cells have distinct CD45 expression. This is the case in approximately half of patients with MGUS. However, in all cases, the CD19 and CD56 expression is sufficient to discriminate normal from neoplastic cells.30,31 (A) and (B) show the negative and positive controls, respectively; both normal and neoplastic cells are CD3⁻ and CD138⁺. The former identifies whether contaminating T cells or nonspecific binding events have been included in the plasma cell gate. The latter determines whether any CD138⁻ nonplasma cells have been included in the plasma cell gate, particularly B-cell progenitors, which are CD38⁺⁺CD19⁺CD56⁻ (as normal plasma cells), but lack CD138 expression. (C) and (D) show CD19 and CD56, respectively. Normal plasma cells are consistently CD19⁺CD56⁻ and appear on the right side of the plots, as they generally show higher CD45 expression than neoplastic cells. Myeloma plasma cells are CD19⁻ with variable CD56 expression in approximately 95% of patients, and are CD19⁺CD56⁺⁺ in a further 4% of patients, and can thus be discriminated from normal plasma cells in more than 99% of patients. In MGUS, multiple populations (ie, CD19⁺CD56⁺, CD19⁻CD56⁻, and CD19⁻CD56⁺) are often detected, although in this case a single neoplastic population (CD19⁻CD56⁺) is present. (E) and (F) show the CD126 and CD130 (IL-6 receptor alpha-chain and gp130, respectively) expression on the gated plasma cells, demonstrating that expression is restricted to the phenotypically neoplastic population.