The absence of CD56 (NCAM) on malignant plasma cells is a hallmark of plasma cell leukemia and of a special subset of multiple myeloma

In this study, we show that malignant plasma cells from patients with eithe r primary (n=12) or secondary (n=15) plasma cell leukemia (PCL) do not expr ess CD56 at all, neither in the bone marrow nor the peripheral blood in 81% of cases. On the other hand, multiple myeloma (MM) at diagnosis overexpres s it in 63 of 94 (67%) cases (P = 0.0001). In three secondary PCL evaluated serially, CD56 was also tacking at diagnosis showing that CD56 is not down regulated at the end stage of the disease but rather not upregulated in thi s subset of patients. This last concept is strengthened by the observation that 29% of MM patients lacking CD56 or weakly expressing it at diagnosis p resent a detectable leukemic phase vs 11% only in CD56(+) MM (P = 0.06). Fo rty percent of all the CD56(-/weak) malignant plasma cell disorders present or develop a leukemic phase vs only 15% of CD56(+) cases (P < 0.008). CD56 (-/weak) MM subset is also associated with a significantly less aggressive osteolytic potential (P=0.012). We conclude that the lack or weak expressio n of CD56 is a characteristic feature of PCL but also delineates a special subset of MM at diagnosis mainly characterized by a lower osteolytic potent ial and a trend for malignant plasma cells to circulate in the peripheral b lood more overtly.