Expression of heavy chain genes is restricted in Peyer’s patch germinal center B cells

We have studied Peyer’s patches (PP) of conventional BALB/c mice to determine heavy chain (Igh) gene expression in PP germinal centers (GC). Using fluorescence activated cell sorting (FACS) analysis of surface immunoglobulin (sIg), we found that PP GC B cells expressed only slgM or slgA but not detectable slgG1, sIgG3, or slgE. In situ hybridization using isotype specific riboprobes on tissue sections or cytospots of purified GC B cells detected IgM and IgA message production within the GC B cell subset at heterogeneous levels between that expressed in small resting B cells and plasmablasts. Using Southern and Northern analyses, we further examined whether GC B cells delete their genomic DNA following a switch to a non-IgM/lgD isotype, and which mRNAα and mRNAμ species they express. Our results indicate that PP GC B cells appear to delete their DNA for Cμ and Cγ1 upon switching from IgM to IgA and that they express both secretory and membrane message for μ and α chains. From these studies we suggest that PP GC B cells are restricted in the Igh gene usage.