Marginal zone B cell lymphomas with extensive plasmacytic differentiation are neoplasms of precursor plasma cells

Occasional marginal zone lymphomas have extensive plasmacytic differentiation (P-MZL). We present an argument based on our data and previous published observations that P-MZL represent neoplasms of precursor plasma cells. Five P-MZL were analyzed by flow cytometry and the phenotype was compared with conventional MZL, plasma cell myeloma (PCM), reactive plasmacytoses (RP), and normal marrow plasma cells. The clonal cells in four of five P-MZL were CD19+, CD20(dim/⁻), CD38(bright), CD45++, CD56⁻, CD138⁻, and surface light chain(dim/⁻). One case was CD56+, CD138+, and CD19⁻. A separate clonal mature B cell population [CD20+, CD79b+] was not detected in any of these tumors. Hierarchical clustering demonstrated similarity between the neoplastic cells in four of the five P-MZL with plasma cells in RP comprised of precursor plasma cells. Analysis of normal bone marrow plasma cells revealed a CD45++CD38(bright)CD19+CD138⁻/+ precursor plasma cell population similar to the plasma cells in RP and P-MZL and a very small population of mature CD45+/⁻CD38(bright)CD138+CD19⁻ plasma cellsresembling the neoplastic cells in PCM. The findings suggest that P-MZL represents a malignancy of a plasma cell stage of differentiation distinct from the plasma cell stage corresponding to PCM. We propose the term precursor plasma cell neoplasm for these tumors.