Phenotypic and molecular characterization of human peripheral blood B- cell subsets with special reference to N-region addition and Jκ-usage in Vκ-Jκ-joints and κ/λ-ratios in naive versus memory B-cell subsets to identify traces of receptor editing processes

Abstract

We identified a population of IgM+IgD+ B-cells in the peripheral blood (PB) of humans that express somatically mutated V-region genes like classical class switched or IgM-only memory B-cells and comprise around 15% of PB B- cells in adults. Mutated IgM+IgD+ cells differ from unmutated naive IgM+IgD+ cells in that they express the CD27 cell surface antigen. In addition, a very small subset of IgD- only B-cells was identified in the PB that carried rearranged VH-genes with an extremely high load of somatic mutations (up to 60 mutations per gene). A common characteristic of the four somatically mutated subsets, which altogether comprise 40% of PB B- lymphocytes in adults, is the surface expression of CD27. This antigen may thus represent a general marker of memory B-cells in the human. Somatically mutated and unmutated PB B-cell subsets were analyzed for N-region addition and Jκ-usage in VκJκ-joints, and in addition for the respective κ/λ- ratios: N-nucleotides could be identified in a large fraction of Vκ-regions of all B-cell subsets, indicating that N-region insertion already occurs in the pre-germinal center (GC) phase of B-cell development. Both the Jκ-usage in expressed VκJκ-joints and the κ/λ-ratio from somatically mutated B- cells do not differ substantially from those of the unmutated cells, so that in terms of these parameters, a contribution of secondary VκJκ- rearrangements in shaping the memory B-cell repertoire is not detectable.