Evidence that High Affinity IgE Can Develop in the Germinal Center in the Absence of an IgG1-switched Intermediate

Abstract

High affinity allergen-specific IgE is essential for the severe allergic anaphylaxis response. High affinity antibodies (Abs) are formed by successive rounds of selection of Ag-specific B cells in the germinal center (GC), however several studies have shown that IgE+ GC B cells are impaired in their ability to undergo selection in the GC. A pathway, known as the “indirect switching pathway” for IgE, has been described whereby Ag-specific B cells initially switch to the IgG1 isotype and undergo affinity selection in the GC, with a secondary switch to the IgE isotype after affinity selection. In previous work, using a food allergy model in mice, we investigated how high affinity IgE develops in the GC but we did not test the indirect switching model. Here we analyzed the importance of the indirect switching pathway by constructing IgG1-cre Bcl6-fl/fl mice. In these mice, once B cells switch to IgG1, they delete Bcl6 and thus cannot enter or persist in the GC. When we tested IgG1-cre Bcl6-fl/fl mice with our food allergy model, we found that as expected, IgG1 Abs had decreased affinity, but unexpectedly the affinity of IgE for allergen was unchanged. IgG1-cre Bcl6-fl/fl mice underwent anaphylaxis in response to allergen, consistent with the formation of high affinity IgE. Thus, in a food allergy response, high affinity IgE can be efficiently formed in the absence of indirect switching to IgG1, either by direct selection of IgE+ GC B cells or indirect selection of IgM+ GC B cells that later switch to IgE.