Role of tonsillar IgD+CD27+ memory B cells in humoral immunity against pneumococcal infection.

IgD+CD27+ memory B cells are a major compartment of circulating memory B cells. However, the characteristics of these cells in the tonsils have been unclear. In this study, IgD+CD27+ memory B cells residing in the tonsillar marginal zone were found to exhibit similar characteristics as IgD+CD27+ memory B cells in the periphery, namely large morphology, expression of surface molecules, and hypermutated Ig variable region genes. Furthermore, these IgD+CD27+ memory B cells predominantly produced IgM, including IgM specific against pneumococcal polysaccharides. Taken together, these results provide convincing evidence that tonsillar IgD+CD27+ memory B cells impart protective humoral immunity against pneumococcal infection by producing high-affinity IgM.