Peripheral blood CD27+ IgG+B cells rapidly proliferate and differentiate into immunoglobulin-secreting cells after exposure to low CD154 interaction

In vitro CD40 stimulation of human B cells isolated from lymphoid organs is dominated by memory B cells undergoing faster proliferation and higher differentiation than naive B cells. In contrast, we previously reported that blood memory B cells mainly differentiate into immunoglobulin-secreting cells in response to CD40 stimulation. However, variations in CD40-CD154 interaction are now recognized to influence B-cell fate. In this study, we have compared the in vitro response of blood CD27(-) and CD27(-) IgG(-) to CD27(+) and CD27(+) IgG(+) B cells following low-density exposure to CD154 in the presence of a mixture of interleukin-2 (IL-2), IL-4 and IL-10. The evolution of these cell populations was monitored during initiation and following long-term stimulation. Over a 5-day period, CD27(+) B cells underwent differentiation into immunoglobulin-secreting cells more readily than CD27(-) cells, and CD27(+) IgG(+) B cells gave rise to a near homogeneous population of CD19(+) CD27(++) CD38(+) IgG(lo) cells capable of high immunoglobulin G (IgG) secretion. During the same period, CD27(-) IgG(-) B cells partially became CD19(++) CD27(-) CD38(-) IgG(++) cells but showed no IgG secretion. Long-term stimulation revealed that CD27(+) IgG(+) B cells retained a high expansion capacity and could maintain their momentum towards differentiation over naive B cells. In addition, long-term stimulation was driving CD27(-) IgG(-) and total CD19(+) B cells to evolve into similar CD27(+) and CD27(-) subsets, suggesting naive homeostatic proliferation. Overall, these results tend to reconcile memory B cells from blood and lymphoid organs regarding their preferential differentiation capacity compared to naive cells, and further suggest that circulating memory IgG(+) cells may be intrinsically prone to rapid activation upon appropriate stimulation.