Human IgM⁺CD27⁺ B cells: Memory B cells or "memory" B cells?

Memory B cells are generated in germinal centers (GC) and contribute to serological immunity by rapidly differentiating into plasma cells. Human memory B cells can be identified by the expression of CD27. These cells exhibit more rapid responses than naive (CD27^-) B cells following stimulation in vitro, consistent with the heightened kinetics of secondary responses in vivo. CD27⁺ B cells express mutated Ig V region genes; however a significant proportion continue to express IgM, suggesting the existence of IgM⁺ memory B cells. The observation that mutated IgM⁺ CD27⁺ B cells are generated in humans who cannot form GC led to the conclusions that these cells are generated independently of GC and thus are not memory cells and that they mediate responses to T cell-independent Ag. Although some studies support the idea that IgM⁺ CD27⁺ B cells participate in T cell-independent responses, many others do not. In this review we will provide alternate interpretations of the biology of IgM⁺ CD27⁺ B cells and propose that they are indeed memory cells.