CLONAL EXPANSIONS OF B-LYMPHOCYTES IN OLD MICE

The effect of age on the diversity of the murine Ig heavy chain repert oire has been studied in unimmunized C57BL/6 mice. We examined the het erogeneity of complementarity-determining region 3 (CDR3) sizes of Ig mRNA of the IgM and IgG isotypes using two V-H families, V(H)J558 and V(H)Q52, which together account for approximately 65% of the Ab repert oire. The broad and bell-shaped profiles representing the diversity of the V(H)J558 family in the spleen of 2- to 6-mo-old C57BL/6 mice beco mes significantly less diverse after 12 mo of age and by 18 mo of age, single CDR3 sizes that dominate the profiles can be observed in the s pleens of >85% of the mice. Readable sequences have been obtained from 40 dominant mRNA CDR3 size species indicating that they represent clo nal populations of B lineage. There are no significant homologies amon g these sequences. Clones of B lymphocytes that express a dominant CDR 3 mRNA species fan also be found in the bone marrow, the mesenteric ly mph nodes, and the thymus of C57BL/6 mice >18 mo of age. Some clones o f B cells can be defected in only one lymphoid compartment; others are found in two or more compartments. The splenic B cell clones in C57BL /6 mice >18 mo of age are stable for at least 2 mo. The CDR3 mRNA spec ies that dominate the splenic repertoire of Ig mRNA-expressing cells i n vivo do not dominate the repertoire of splenic B cells activated in vitro by bacterial LPS, suggesting that they represent a modest popula tion of B cells expressing high levels of Ig mRNA.