Cellular basis of B cell clonal populations in old mice

Previous studies from this laboratory have shown that >85% of old mice have stable B cell clonal populations detectable by Ig heavy chain complementar y-determining region 3 mRNA size analysis and confirmed by sequence analysi s, B cells from the same clone are frequently detected in several lymphoid compartments of the same mouse. We now report the phenotype of all ten stab le B cell clonal populations detected in five 20-month-old C57BL/6 mice. Th ese clonal B cells appear to develop in the periphery and nine of the ten B cell clonal populations expressed the CD5 cell surface marker. Stable B ce ll expansions may be dominated by cells at two stages of differentiation, S ome B cell populations were detected with DNA as well as RNA and represent large clonal populations of B cells, detectable in several lymphoid compart ments. These populations are found predominantly in B cell populations expr essing CD45R/B220 and the mRNA coding for the membrane-bound form of the Er Ig heavy chain, which suggests a predominance of B lymphocytes in these po pulations. In other cases, smaller clonal populations were detected only in splenic RNA samples, These clonal populations were found predominantly amo ng CD45R/B220(-) B cells and did not express the membrane-bound form of the mu Ig heavy chain. We offer the hypothesis that the B:cell clonal populati ons present in old mice may-be precursors of the two types of B cell neopla sms which are dominated by CD5(+) B cells (B cell chronic lymphocytic leuke mia) or plasma cells (multiple myeloma).