Molecular genetic features reflecting the preference for isotype switching to IgA expression by Peyer's patch...

It has proven difficult to evaluate the functional potential of germinal center (GC) B cells, including those from Peyer's patches (PP), by either in vivo or in vitro methods. Thus, rather than assess secreted Ig product as an indicator of functional potential we have instead sought to detect mRNAs related to the various Ig heavy chains in GC B cells from PP by in situ hybridization. We have found that the GCs of PP contain the vast majority of B cells with easily detectable levels of mRNA alpha. These levels are intermediate between those of small resting B cells and plasmablasts. When PP B cells are enriched for cells bearing GC markers, approximately 50% contain mRNA mu and 40% mRNA alpha. Similar enrichment for sIgA+ B cells gave 50% of cells with easily detectable mRNA alpha and few if any positive for mRNA mu. The sizes of these mRNAs were similar to those encoding the membrane and secretory form of mu and alpha chains. No C alpha germ-line transcripts could be detected by Northern analyses using a probe for sequences 5' to the alpha switch regions. Finally, GC and sIgA+ cells from PP also showed the absence of a portion of their genomic DNA for CH genes 5' of C alpha. Thus, it seems likely that most of the GC cells expressing mRNA alpha have undergone conventional VDJ recombination to C alpha at the DNA level in order to switch to the expression of IgA. Our findings reflect the extraordinary preference for switching to IgA by GC cells in PP.