Phenotypic Characterization of the CD45+ and CD45− Plasma Cell Compartments in Monoclonal Gammopathies.

Background: Multiple myeloma cells are heterogenous in terms of the expression of CD45 and previous studies have suggested important clinical and biological implications of the CD45 expression. It has been shown that expression of CD45 on myeloma cells has prognostic value and that the CD45 positive cells contain the proliferative fraction of the myeloma cells. We have also demonstrated that the CD45+ population has a higher apoptotic rate compared to CD45− cells. Given the hypothesis that CD45+ cells represent myeloma cells earlier in their development and are more proliferative and more dependent on the marrow microenvironment we studied differences between the two populations in patients with different stages of monoclonal gammopathies and in normal plasma cells.

Methods: Fresh bone marrow aspirates were subjected to ACK lysis for removal of RBCs subjected to multicolor flow cytometry to study the expression of different surface antigens on the plasma cells. Plasma cells were identified based on their characteristic CD38/45 expression. The surface expression of CD19 (B cell marker), CD56 (NCAM), CD87, (involved in cellular adhesion, cell motility and angiogenesis), CD126 (IL-6 receptor alpha chain), sVEGF (surface VEGF; surrogate measure of VEGF receptor expression), and CD71 (proliferation marker) were studied and compared between the CD45 + and CD45− cells by gating separately on these populations.

Results: Patients with active myeloma (76), smoldering myeloma (32), MGUS/amyloid (44), and normal patients (23) were studied. Not all markers were performed on all patients. Among the patient samples, the expression of CD19, CD87, CD71, CD126, and sVEGF were higher among the CD45+ cells compared to CD45− cells (Table: % of cells with expression). There was no significant difference between the two in terms of the expression of CD56. Among the normal plasma cells, the expression of CD19, CD87, CD126 and sVEGF was greater on the CD45+ cells compared to CD45− plasma cells and CD56 was not different. When the patient cells were compared to normal cells, the expression of CD19 was higher among the normal CD45+ plasma cells and CD56, CD87 and CD126 were less. The expression sVEGF was comparable

Conclusion: In this study we demonstrate differences between CD45+ and CD45− plasma cells in patients with monoclonal gammopathies. These findings are important for understanding the disease biology and provide an insight into the functional differences seen between these two populations. The increased expression of CD19 in the CD45+ cells supports the notion that these cells are likely to be at an earlier developmental stage. The comparable CD56 expression is not in accordance with previous studies and needs to be examined in a larger group of patients. Increased expression of sVEGF and CD126 as well as the adhesion molecules is in accordance with increased responsiveness and dependence of the CD45+ cells to VEGF and IL-6 and the increased proliferative rates. These findings indirectly support the hypothesis that CD45 negative cells are less dependent on the marrow microenvironment and the cytokine milieu that it provides.

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