IMMUNOBIOLOGY CD45RO enriches for activated, highly mutated human germinal center B cells

To date, there is no consensus regarding the influence of different CD45 isoforms during peripheral B-cell development. Examining correlations between surface CD45RO expression and various physiologic processes ongoing during the germinal center (GC) reaction, we hypothesized that GC B cells, like T cells, that up-regulate surface RO should progressively acquire phenotypes commonly associated with activated, differentiating lymphocytes. GC B cells (IgD CD38 ) were subdivided into 3 surface CD45RO fractions: RO , RO / , and RO . We show here that the average number of mutations per IgVH transcript increased in direct correlation with surface RO levels. Conjunctional use of RO and CD69 further delineated low/moderately and highly mutated fractions. Activation-induced cytidine deaminase (AID) mRNA was slightly reduced among RO GC B cells, suggesting that higher mutation averages are unlikely due to elevated somatic mutation activity. Instead, RO GC B cells were negative for Annexin V, comprised mostly (93%) of CD77 centrocytes, and were enriched for CD69 cells. Collectively, RO GC B cells occupy what seems to be a specialized niche comprised mostly of centrocytes that may be in transition between activation states. These findings are among the first to sort GC B cells into populations enriched for live mutated cells solely using a single extracellular marker. (Blood. 2007;110:3917-3925)