MEMORY B-LYMPHOCYTES IN HUMAN TONSIL DO NOT EXPRESS SURFACE IGD

To clarify the phenotype of memory B lymphocytes, we have determined t he frequency of somatic hypermutations in purified tonsil B cell popul ations. Our particular interest was the controversial question of whet her any memory B cells express IgD. Ig heavy chain gene rearrangements that used the nonpolymorphic V(H)6 gene were amplified by PCR, cloned , and sequenced. All eight sequences obtained from the surface IgD (sl gD)-negative fraction contained point mutations, with frequency of one mutation in every 24 bases of sequence. In contrast, only 4 of 12 seq uences obtained from the slgD-positive fraction contained point mutati ons, with a mutation frequency of one in 600. This frequency was simil ar to that found for cord blood B cells (one in 550), a population tha t does not contain memory B cells. These results indicate that althoug h memory B cells are present in the slgD-negative fraction, no memory cells can be detected in the slgD-positive fraction of tonsil B lympho cytes.