CD28 expression and CD28-mediated interaction of tumor plasma cells with BM DCs (effect on proteasome subunit expression by tumor plasma cells). Representative plots (A) and percentages (B-C) of tumor plasma cells expressing CD28 (from 20 MGUS and 20 MM patients). Mann-Whitney U test. (D) Representative histograms for the expression of selected proteasome subunits by tumor plasma cells after incubation with BM mDCs in different conditions. (E) Effect of CD28 blockade on proteasome subunit expression in tumor plasma cells. Expression levels in positive cells are reported in MESF units. Wilcoxon signed-rank test. (F-G) Representative histograms (F) and levels (MESF units) (G) of expression of proteasome subunits by CD28^– tumor plasma cells after incubation with BM mDCs in different conditions. P values are shown only for statistically significant comparisons.