The selected, high-affinity GC B cells then differentiate into either memory B cells or long-lived PCs, concurrent with downregulation of Bcl6 expression [21]. In accordance with this model, memory B cells and PCs expressing somatically mutated Ig V region genes persist

for long periods of time after termination of the GC response [19, 22]. Memory B cells are long-lived quiescent B cells that exhibit Selleckchem GDC 973 a phenotype distinct from that of other types of B cells, including the ability to elicit a more rapid and robust response upon antigen re-encounter compared to antigen-inexperienced naïve B cells [23]. Whereas naïve B cells express IgM and IgD on the surface, memory B cells have generally undergone CSR and express antibody of other isotypes. Therefore, mouse memory B cells can be isolated as antigen-binding cells expressing class-switched immunoglobulin in combination with high levels of CD38 and low levels of PNA binding surface molecules [24, 25]. PS-341 supplier Using this approach, it became clear that not all IgG memory B cells contain somatic mutations in their Ig V regions [6, 25, 26]. In addition, blockade of inducible costimulator

(ICOS) early in the immune response caused a significant reduction in the frequency of somatically mutated memory and GC B cells but had no effect on the total number of memory B cells [5]. Additionally, under these conditions, the memory B cells generated were largely devoid of somatic mutations. These findings led us to speculate that these unmutated memory cells emerged early from the GC reaction [27] or, alternatively, developed independently of GCs. This latter hypothesis was supported by evidence that unmutated memory B cells can be generated in irradiated mice reconstituted with Bcl6-deficient bone marrow [3]. However, since Bcl6 germline deletion results in an inflammatory disease due to overexpression of Th2 cytokines [17, 18] that may induce

aberrant properties in B cells prior to immunization [28], it remained uncertain whether a GC-independent pathway contributed Baf-A1 cell line significantly to memory cell generation under physiological conditions. Jenkins and colleagues recently reported the generation of antigen-specific B cells with a CD38+/GL-7− memory phenotype in a GC-independent manner at an early stage of the immune response to immunization with PE plus CFA (complete Freund’s adjuvant) [9, 29]. These presumed GC-independent memory B cells could be distinguished from GC-dependent IgG1 memory B cells by the absence of the CD73 surface molecule, whose expression was enriched in mutated memory B cells [2]. However, the functional properties of these cells have not been studied. Taking advantage of a novel mouse strain in which Bcl6 is selectively depleted from B-lineage cells, Kaji et al.