Syndecan-1 expression on malignant cells from the blood and marrow of patients with plasma cell proliferative disorders and B-cell chronic lymphocytic leukemia

Syndecan-1 is a low-affinity receptor for basic fibroblast growth factor (bFGF). In this study, we used flow cytometry to examine expression of syndecan-1 on monoclonal cells from the blood (n = 37) and marrow (n = 81) of patients with plasma cell (PC) proliferative disorders (PCPD) and blood cells from patients (n = 39) with B cell chronic lymphocytic leukemia (B-CLL). The marrow CD38⁺CD45^- and CD38⁺CD45⁺ PC were syndecan-1 positive in all patients with PCPD and there was no difference between patients with monoclonal gammopathy of undetermined significance (MGUS) vs multiple myeloma or cases with vs without bone lesions. In 38% of cases, syndecan-1 expression on the PC was heterogeneous with ≤ 25% of PC syndecan-1 negative. We found similar syndecan-1 expression on blood and marrow PC in the 36 cases with paired samples. CLL cells were syndecan-1 negative in 97% (38/39) of the cases. Syndecan-1 is a useful marker to detect malignant plasma cells in the blood or marrow; however, it is not helpful in distinguishing MGUS from active myeloma. In addition, syndecan-1 is present on the less mature (CD45⁺) PC, and there is heterogeneity of expression within and between patients. The relevance of the bFGF bound to myeloma cells via syndecan-1 remains to be elucidated.