A B220–, CD19– population of B cells in the peripheral blood of quasimonoclonal mice

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Abstract
We describe a new population of non-naive B cells in the peripheral blood of quasimonoclonal (QM) mice. Surface Ig of switched isotypes is expressed, but not B220 nor CD19. These cells are larger and denser than naive B cells but smaller than blasts or plasma cells; they do not stain with syndecan, a marker for plasma cells. Telomerase, which is usually expressed in B cell blasts, was not present in this population. We sorted the switched, idiotype-positive, B220– B cells from the peripheral blood of QM mice and sequenced Ig H chain and λ L chain cDNA. There were many point mutations but no V gene replacements, gene conversions or other type of diversifications. As they express switched isotypes and have mutated their Ig genes, cells in the B220–, CD19– population must have been in an immune response and we suggest that it includes the memory B cell subset.