Peyer's patch B cells with memory cell characteristics undergo terminal differentiation within 24 hours in response to interleukin-6.

Culture of Peyer's patch (PP) B cells with interleukin-6 (IL-6) for 7 days results in a six- to eightfold increase in secretion of IgA, while little or no increase in IgM or IgG secretion occurs in these cultures. Further, greater than 80% of IgA is produced within the first 72 h of culture. Using a sensitive enzyme-linked immunospot (ELISPOT) assay, we have shown that culture of PP B cells with IL-6 for 24 h gave increased IgA spot-forming cells (SFC) (4- to 6- fold) even though secreted IgA, as measured by RIA, had only increased 1.6- to 2.0-fold. In addition, significant increases in IgA SFC numbers could be demonstrated as early as 4 h after addition of IL-6. The increase in IgA secretion was not the result of IL-6-induced B-cell proliferation, since culture of B cells with IL-6 resulted in no increase in [3H]thymidine incorporation compared to untreated controls. This was supported by studies with mitomycin C which, when added to B cell cultures, had no effect on the IL-6-induced increase in numbers of IgA SFC. Increased IgA secretion was totally abolished by actinomycin D, an inhibitor of RNA transcription, showing that continued production of alpha mRNA is essential for IL-6-induced IgA secretion. Separation of PP B cells into peanut agglutin (PNA)Hi (germinal center [GC]) and PNALo (non-GC) subpopulations before culture with IL-6 showed that only PNALo B cells transcribe increased levels of alpha mRNA message and secrete high levels of IgA in response to this cytokine. Although the GC are the site of B-cell proliferation and presumably of switching to IgA and contain 70 to 85% of sIgA+ B cells in the PP, these PNAHi B cells do not respond to IL-6. This suggests that memory sIgA+ B cells in PP express IL-6 receptor (IL-6R) and respond to this cytokine with rapid differentiation into plasma cells that secrete IgA.