Human myeloma: Several subsets of circulating lymphocytes express plasma cell‐associated antigens

Peripheral blood lymphocytes from 9 monoclonal gammopathies of undetermined significance (MGUS) and 27 multiple myelomas (MM) were studied with a panel of monoclonal antibodies (MoAb) that recognize B and T lymphocytes and plasma cells. No difference in the percentage of B lymphocytes, identified by B1 and B4 MoAb, was observed in MGUS and MM patients versus normal controls. However, high percentages of circulating lymphocytes expressing plasma cell‐associated antigens were detected in MM (HAN‐PC1 + = 29.4 ± 20.4%; TEC‐T10+ = 27.8 ± 19.2%) whereas they were in the normal range in MGUS (HAN‐PC1 + = 8.8 ± 5.8% p = 0.006; TEC‐T10+ = 5.7 ± 4.7% p<0.001). Almost identical results were obtained using PCA‐1 MoAb in 17 of these patients. TEC‐T10+ and PCA‐1 + lymphocytes were sorted and re‐analyzed with phycoerythrin conjugated MoAb in 3 healthy subjects, 2 MGUS, and 4 MM patients. In normal subjects and in MGUS the majority of PCA‐1 + cells belonged to the B lineage (Leu 2‐, Leu3‐, Leu 15‐, HLA‐Dr +), whereas the majority of TEC‐T10+ cells are represented by activated T cells and NK cells (Leu 15 +). In MM an abnormal expansion of T lymphocytes was chiefly responsible for the high values of lymphocytes expressing plasma cell‐associated antigens. Moreover, in MM a clinical evaluation showed a correlation between the presence of these lymphocytes and an aggressive disease. Indeed, they can be considered a useful prognostic marker.