A B220(-), CD19(-) population of B cells in the peripheral blood of quasimonoclonal mice

We describe a new population of non-naive a cells in the peripheral blood o f quasimonoclonal (QM) mice. Surface Ig of switched isotypes is expressed, but not B220 nor CD19, These cells are larger and denser than naive a 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 a cell blasts, was not present in this population. We sorted the switched, idioty pe-positive, B220(-) cells from the peripheral blood of QM mice and sequenc ed Ig H chain and lambda 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 i n the B220(-), CD19(-) population must have been in an immune response and we suggest that it includes the memory a cell subset.