Human Immunoglobulin (Ig)M⁺IgD⁺ Peripheral Blood B Cells Expressing the CD27 Cell Surface Antigen Carry Somatically Mutated Variable Region Genes: CD27 as a General Marker for Somatically Mutated (Memory) B Cells

Immunoglobulin (Ig)M⁺IgD⁺ B cells are generally assumed to represent antigen-inexperienced, naive B cells expressing variable (V) region genes without somatic mutations. We report here that human IgM⁺IgD⁺ peripheral blood (PB) B cells expressing the CD27 cell surface antigen carry mutated V genes, in contrast to CD27-negative IgM⁺IgD⁺ B cells. IgM⁺IgD⁺CD27⁺ B cells resemble class-switched and IgM-only memory cells in terms of cell phenotype, and comprise ∼15% of PB B lymphocytes in healthy adults. Moreover, a very small population (<1% of PB B cells) of highly mutated IgD-only B cells was detected, which likely represent the PB counterpart of IgD-only tonsillar germinal center and plasma cells. Overall, the B cell pool in the PB of adults consists of ∼40% mutated memory B cells and 60% unmutated, naive IgD⁺CD27⁻ B cells (including CD5⁺ B cells). In the somatically mutated B cells, V_H region genes carry a two- to threefold higher load of somatic mutation than rearranged V_κ genes. This might be due to an intrinsically lower mutation rate in κ light chain genes compared with heavy chain genes and/or result from κ light chain gene rearrangements in GC B cells. A common feature of the somatically mutated B cell subsets is the expression of the CD27 cell surface antigen which therefore may represent a general marker for memory B cells in humans.