T1_99 ligand incuces the genera ion of CD20+plasmablasts and plasma cells from CD27+memory B-cells

Plasma cells (PCs) have a heterogeneous phenotype in humans. While bone marrow PCs are CD2O CD138+, tonsil PCs are CD2O+CD138+/ and peripheral plasmablasts (PBs) are CD2O CD138. In vitro, PCs are mainly generated by the activation of CD27+ memory B-cells through transient stimulation of CD40, and their phenotype appears similar to that of bone marrow PCs. While CD20 expression is lost at the plasmablastic stage, CD138 expression appears only at the PC stage. Thus, the CD2O+CD138+ phenotype of tonsil PCs does not represent an intermediate stage in the differentiation of memory Bcells into PCs. Because it has been previously shown that TLR9 activation was more able than CD40 stimulation to induce the differentiation of IgM+ CD27+ B-cells, we wondered whether TLR9 or CD40 stimulation would induce the same phenotype of PCs. Thus, we compared the differentiation of CD27+ B-cells isolated from either the tonsils or peripheral blood and stimulated with either CD4015expressing fibroblasts or a TLR9 ligand, CpG oligodeoxynucleotide (CpG ODN). We observed that CpG ODN mainly induced CD27+ Bcell differentiation into CD2O+CD38+CD138 PBs and CD2O+CD38+CD138 +/- PCs, which appear similar to tonsil PCs. Removal of CpG ODN during differentiation induced a decrease in the CD20+ plasmablastic population, and, conversely, stimulation of CD4015induced preplasmablasts with CpG ODN increased the population of CD2O+CD38+ PBs. Analysis of Ig secretion showed that CpG ODN induced increased IgM secretion compared to CD4OL. PCs from patients with multiple myeloma, the malignant counterpart of bone marrow PCs, rarely express CD20. We show that CpG ODN did not induce or increase CD20 in nine IgG or IgA myeloma cell lines. These data strongly suggest that CpG ODN mainly targets CD27+ IgM+ B-cells.