In several conditions such as chronic viral infections and primary immunodeficiency, a unique B cell subset has been found expanded. This B cell subset lacks, or expresses low, surface levels of the complement receptor 2 (CD21) and has therefore been termed CD21–/low B cells. However, little is known about the CD21–/low B cell subset in peripheral blood from healthy donors. We show that CD21–/low cells represent approximately 5% of B cells in peripheral blood from adults but are barely detectable in cord blood, after excluding transitional B cells. The CD21–/low subset can be divided into CD38– 24+ and CD38– 24low cells, where most of the CD38– 24+ are CD27+ IgM+ IgD+ and the CD38– 24low are switched CD27–. Expression levels of additional markers, e.g. CD95 and CD62L, are similar to those on classical memory B cells. The majority of CD21–/low cells lack expression of the ABCB1 transporter, which is a marker restricted to naïve B cells. We also investigated the CD21–/low B cells response to various single stimuli. In order to do that, we determined the change in CD69 expression, an activation marker, after 3 hours stimulation. Both CD21+ naïve and memory B cells responded to all single stimuli by up-regulation of CD69 and to a similar extent. The CD21–/low subsets did not respond to the same extent and the lowest response was observed for the CD24low subset. Stimulation with a combination of BCR, Toll-like receptor (TLR) 7/8 and interleukin (IL)2 induces proliferation and differentiation of the CD21–/low B cells comparable to CD21+CD27+ memory B cells. Their response excluding BCR agonist is not on par with that of classical memory B cells, although clearly above that of naïve B cells. In conclusion, as in disease and tonsils of healthy individuals, the CD21–/low B cell subset in peripheral blood from healthy adults is a functionally and phenotypically heterogeneous population composed mainly of memory B cells.
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