Naïve and memory B Cells respond differentially to T-dependent signalling but display an equal potential for differentiation toward the centroblast-restricted CD77/globotriaosylceramide phenotype

Resting (CD38(low)) tonsillar B cells differentiate to express the centroblast-restricted CD77/ globotriaosylceramide antigen on high-level engagement of CD154. As the CD38(low) population comprises both naive and memory subsets, we wished to compare the propensity of each to develop this germinal center phenotype; particularly as the capacity of memory B cells to re-enter a follicular reaction remains unclear. Resting B lymphocytes were therefore separated into CD27(-)IgA(-)IgG(-) and IgD(-) fractions to generate subsets enriched for naive and memory cells, respectively. Following stimulation via BCR and/or CD40 - surrogate signals for B cells engaged in T-dependent signaling - differences between the two subsets were seen in the kinetics and/or magnitude of responses such as entry into DNA synthesis, induction of the costimulatory molecules CD80 and CD86; up-regulation of CD23, and changes in BCL-6 mRNA expression. Nevertheless, naive and memory cells revealed a nigh identical capacity for acquiring CD77: both appeared equally sensitive in this regard, with high-level CD40 engagement via cell-bound CD154 being required for both subsets to achieve the hallmark centroblast phenotype. These findings suggest that, provided with the opportunity to encounter cell membrane CD154 in abundance, both naive and memory B cells display the potential to be diverted towards a germinal center pathway of differentiation.