Identifying T helper cells involved in B cell immunoglobulin class-switching to IgE and comparing T helper cells of systemic or mucosal origin

IgE antibodies are key players in allergic diseases. The production of IgE requires B and T cell interactions and the cytokine IL-4. The main goal of this thesis is to identify the CD4+ T cell populations that mediate B cell class switching to IgE. For this work we used mice with defined antigen specific T and B cell populations, the TBmc mice, to facilitate the analysis of the differentiation of T helper cells and B cell class switching and antibody affinity maturation. IL-4-GFP reporter mice were also used to identify, purify and functionally study different populations of IL-4 producing T helper cells. All mice were in BALB/c background, which is genetically susceptible to mount type 2 responses. Our results were validated using a different immunization system and wild type BALB/c mice with polyclonal T and B cell populations. We identified two populations of IL-4-producing cells in the activated/memory compartment of spleen and lymph nodes of immunized mice, a Tfh population, and a Th2-like population. Using in vitro T-B helper assays, we found that the Th2-like cells displayed a higher helper activity for B cell class-switching to IgE than Tfh cells. Activated/memory and naïve CD4+ cells that did not express IL-4 did not support class switching to IgE in vitro. Th2-like cells displayed the highest helper activity for class switching to IgE when cultured with naïve, IgM memory and different types of IgG1 memory B cells, indicating that they mediate direct and sequential switching to IgE. In adoptive transfer experiments in mice, Th2-like cells were also more effective than Tfh cells to mediate B cell class switching to IgE. Th2-like cells had a survival, proliferative and migratory advantage over Tfh cells, and were also able to differentiate into Tfh cells under favorable conditions in vivo. These observations suggest that Th2-like cells are especially important for the production of IgE in memory responses. Similar populations of IL-4 producing Th2-like and Tfh cells were identified in Peyer’s Patches, but unlike the T helper cells from spleen and lymph nodes, the mucosal populations were associated with class switching to IgA and IgG1, and not to IgE. We found that the ability of systemic and mucosal T helper cells to mediate production of IgE or IgA respectively depends on the tissue microenvironment, and thus their different genetic programs are not hard-wired but exhibit considerable plasticity. Identifying the T helper cells for IgE holds potential clinical benefit in that inhibitors of the particular T cell subsets could be used to prevent an allergic reaction from occurring.​Doctor of Philosophy (SBS