MEMORY B-CELLS FROM HUMAN TONSILS COLONIZE MUCOSAL EPITHELIUM AND DIRECTLY PRESENT ANTIGEN TO T-CELLS BY RAPID UP-REGULATION OF B7-1 AND B7-2

Human memory B cells that carry mutated IgV region genes were isolated from tonsils by negative selection of IgD(+) naive a cells and CD38(+) germinal center B cells and plasma cells. They were mainly found within the intraepithelial areas, but not in the B cell follicles of human tonsils. Memory a cells but not naive B cells have the capacity to present antigen directly to T cells, owing to the constitutive expression of the accessory molecules B7-1/CD80 and B7-2/CD86. Signals through antigen receptors and CD40 antigen result in these two molecules being further up-regulated more rapidly and strongly on memory a cells than on naive B cells. The unique anatomical localization of memory B cells beneath the surface of mucosa, together with their strong APC capacity, may explain the well-known prompt and robust secondary antibody responses.