A subfraction of B220(+) cells in murine bone marrow and spleen does not belong to the B cell lineage but has dendritic cell characteristics.

Although CD45R/B220 is commonly used as a pan-B cell marker in the mouse, not all B220(+) cells belong to the B cell lineage. Here we report the characterization of a subpopulation of B220(+)CD19(-) cells in murine bone marrow, which failed to express markers that are present in early CD19(--) B cell precursors. Instead, these cells expressed low levels of MHC class II and CD11c, which are typically found on dendritic cells (DC). Moreover, these B220(+)CD19(-)CD11c(+) cells expressed Gr-1, indicating that they are related to the recently identified murine plasmacytoid DC or their progenitors. Therefore, we evaluated surface marker expression of the B220(+)CD19(-)CD11c(+) cells in lymphoid tissues of C57BL/6 mice, recombinase activating gene-1 deficient mice, lacking mature B and T lymphocytes, and mice with a targeted disruption of the Ig H chain mu membrane exon (mu MT), lacking mature B lymphocytes. When comparing bone marrow and spleen, we found that the surface profiles of B220(+)CD19(-)CD11c(+) cells were remarkably similar, indicating that they are in a comparable maturation or activation stage in the two lymphoid compartments. In addition, the almost complete absence of peripheral B220(+) B-lineage cells in mu MT mice allowed the anatomical localization of the B220(+)CD19(-)CD11c(+) cells to the red pulp and the T cell areas in the spleen. Taken together, our findings indicate that the mouse bone marrow contains a recirculating population of B220(+)CD19(-) CD11c(+) plasmacytoid DC, the development of which is largely independent of the presence of mature T and B cells.