Plasmablasts and germinal center founder cells developing in vivo

A range of cellular interactions and cytokines influence immunoglobulin class switch recombination (CSR), which occurs both in extrafollicular antibody responses and germinal centers. This study addresses where CSR occurs during extrafollicular responses, allowing identification of the cellular interactions and cytokine-producing cells that influence CSR in vivo. The response in mice of B cells specific for 4-hydroxy-3-nitrophenyl-acetyl (NP) to NP-Ficoll was analyzed. Immunohistology and flow cytometry confirm responding NP-binding cells move to the outer T zone of the spleen and start dividing there on the second day. During the third day they move either to follicles as germinal center founding cells, or extrafollicular foci as plasmablasts. Analysis of expression of AID, Bcl-6 and Blimp-1 mRNA in single NP-binding cells indicates a proportion of T zone B blasts express AID transiently without Bcl-6 and these cells are shown to undergo CSR. By contrast Blimp-1+ cells emerging on the third day and were universally AID- and hence unable to initiate CSR. Bcl-6 mRNA without protein, expressed by 40% of naïve NP-binding cells, is rapidly downregulated. Bcl-6 and AID are coexpressed by germinal centre founding cells only on moving to follicles during the third day after immunization. Keywords: TaqMan Low Density Array C57BL/6 mice with one knockin of NP-specific immunoglobulin heavy chain from quasimonoclonal (QM) mice were immunized with NP-Ficoll. This induces germinal center founder cells and plasmablasts within 3 days. B cells were FACS-sorted in 24 hour intervals according to NP-binding, and B220 CD138 expression levels.