Both types of memory B cells consistently upregulate the orphan receptor EBI-2 (T. Kaji and T. Takemori, unpublished), allowing them

to migrate into the outer B cell follicle [11]. However, it remains uncertain whether GC-independent memory B cells develop at the border of T- and B-cell zones or in the follicle. Although T-cell CXCR5 is needed for optimal GC responses, CXCR5-deficient PARP inhibitor T cells are able to access follicles and induce GCs, albeit smaller in size compared with wild-type T cells [36, 40]. Likewise, a small number of GC B cells were generated in the spleen of mice in the absence of TFH cells at day 7 after immunization [2], raising the possibility that non-TFH cells may also access follicles and help B cells to respond at an early stage of the immune response. TFH cells secrete IL-21 [41]. IL-21 signaling profoundly affects GC function by promoting the proliferation of GC B cells and their differentiation into memory B cells. Accordingly, in mice deficient for IL-21, memory B cells exhibit lower levels

of somatic mutations in rearranged Ig V region genes compared with memory B cells from wild-type controls [8]. There is no specific cell surface marker known for memory B cells, although PD-L1, PD-L2, CD35, CD80, and ecto-5′-nucleotidase CD73 have CX-5461 been reported to be expressed on memory B cells in the spleen in contrast to naïve B cells [26] or naïve and GC B cells [42]. Along these lines, we have confirmed that the levels

of PD-L2 and CD80 expression are significantly increased in both GC-independent and -dependent memory B cells compared with those in naïve and GC B cells [2] (Fig. 1). However, as previously reported [9], CD73 is expressed on GC B cells and a subset of memory B cells in wild type mice as the immune response progresses. On GC-independent memory B cells, CD73 is expressed at a low level. In our study, approximately 80% of CD73+ memory B cells in wild-type mice carried somatically mutated Ig V region gene segments [2]. Thus, CD73 expression may preferentially mark somatically mutated memory cells. Although we observed costimulatory MHC class II, CD40, and CD80 molecules to be almost why equally expressed on both day 7 and day 40 GC-independent and -dependent memory B cells, the cell surface expression level of PD-L2 increased from day 7 to day 40 after immunization on both types of cells [2]. Thus, GC-independent and -dependent memory cells express several common surface markers at comparable levels, except for CD73. The memory B-cell population consists of clones that have proliferated in response to an antigen and then remain in a resting state for a long period of time [23]. Their survival is independent of T-cell help and of continuous contact with cognate antigen [43, 44]. It has been suggested that memory B cells localize in spleen and other secondary lymphoid organs [26], and also circulate in blood [6].