Limited clonal relatedness between gut IgA plasma cells and memory B cells after oral immunization.

Understanding how memory B cells are induced and relate to long-lived plasma cells is important for vaccine development. Immunity to oral vaccines has been considered short-lived because of a poor ability to develop IgA B-cell memory. Here we demonstrate that long-lived mucosal IgA memory is readily achieved by oral but not systemic immunization in mouse models with NP hapten conjugated with cholera toxin and transfer of B1-8(high)/GFP(+) NP-specific B cells. Unexpectedly, memory B cells are poorly related to long-lived plasma cells and less affinity-matured. They are α4ß7-integrin(+)CD73(+)PD-L2(+)CD80(+) and at systemic sites mostly IgM(+), while 80% are IgA(+) in Peyer's patches. On reactivation, most memory B cells in Peyer's patches are GL7(-), but expand in germinal centres and acquire higher affinity and more mutations, demonstrating strong clonal selection. CCR9 expression is found only in Peyer's patches and appears critical for gut homing. Thus, gut mucosal memory possesses unique features not seen after systemic immunization.

Pivotal advance: CD45RB glycosylation is specifically regulated during human peripheral B cell differentiation.

A screen of cell surface markers differentially expressed during peripheral B cell differentiation identified that the CD45RB epitope detected by the mAb MEM-55 was highly expressed on CD27(+) memory B cells and absent on CD27(-) naïve B cells. IgG(+)CD27(-) memory and a previously unacknowledged CD27(-) population in blood also expressed high levels of CD45RB(MEM55). Naïve and memory B cells from tonsils followed the pattern observed in blood, and CD38(high) B cells had a bimodal expression pattern when analyzed using flow cytometry. No CD38(high) GC B cells, however, expressed the CD45RB(MEM55) epitope when assayed using immunohistochemistry. Rather, CD38(high)CD45RB(MEM55high) B cells had a distinct cellular phenotype and were localized outside of GCs. CD45RB epitopes, detected by other antibody clones, were expressed at high levels through B cell differentiation, and no changes in splicing of the CD45RB exon were observed during B cell differentiation. Instead, B cells regulated their expression of the CD45RB(MEM55) epitope through site-specific modifications of an O-linked glycochain. CD4(+) T cells differentially spliced CD45 but did not vary the glycosylation of the CD45RB(MEM55) epitope, and CD8(+) cells modified CD45RB(MEM55) expression in a similar manner as B cells. Monocytes expressed the CD45RB exon but not the CD45RB(MEM55) epitope. As CD45 is a highly expressed tyrosine phosphatase that regulates antigen receptor signaling strength in lymphocytes, we conclude that regulated O-linked glycosylation of CD45RB can be used to follow B cell differentiation and that this regulation may be involved in fine-tuning antigen signaling in the cell.