CD19⁻CD138⁻ Pre-PCs are a feature of the normal late B-cell development. (A) Flow cytometric analysis of the normal late B-cell development in PB (n = 15 normal donors); a representative donor sample is shown. After gating out non–B-cell lineage cells, analysis of CD19⁺ cells, as expected, shows a polyclonal pattern of cytoplasmic Ig LC expression (ie, expression of κ and λ chains). PBLs are identified as CD38^hiCD27^hiCD138⁻ and PCs as CD38^hiCD27^hiCD138⁺ cells. For identification of Pre-PCs, gating on lineage-CD19⁻ cells reveals a small cytoplasmic Ig LC⁺ population, which on sequential gating is found to be enriched in CD38^hiCD27^hi cells (48% in the case shown); these include CD138⁺ PCs and CD138⁻ Pre-PCs. Both CD19⁺ PBLs/PCs and CD19⁻ Pre-PCs/PCs are CD319⁺. (B) Histograms showing cell size as assessed by FSC-A and expression levels of CD19, cytoplasmic Ig LC, CD319, CD20, and HLA-DR in the above 4 populations as well as in naive and memory B cells. Numbers next to histograms represent median intensity fluorescence values. (C) Identification of PBLs, CD19⁺ PCs, Pre-PCs, and CD19⁻ PCs in BM from healthy donors; a representative of 5 samples is shown. Using a strategy similar to that described for PB, all 4 cell types were found to be CD38^hiCD27^hi. (D) Histograms showing cell size and expression levels of CD19, cytoplasmic Ig LC, CD319, CD20, and HLA-DR of BM B lineage populations.