CD138 expression is upregulated during maturation of plasma cells in vitro. (A) CD138 expression was determined in double staining with CD38 in unseparated cells at day 0 and after a 4-day culture. The percentage and fluorescence intensity ratio of CD138 expression were defined in the R1-gated cells and are indicated within the dot-plots. (B) After immunomagnetic removal of CD138⁺cells, CD138⁻ cells were cultured for 4 days, and the expression of CD138-PECY5 (FL3-H) (percentage and fluorescence intensity ratio r) was determined in the R2-gated population. CD138⁺ and CD138⁻ cell separation was performed as described in Materials and Methods (RP8).