Final Report Summary - PAS (Persistence of allergic sensitization)
Allergies reflect an inappropriate activation of the immune system in response to environmental antigens that are usually not harmful. The allergic reaction is mainly driven by antibodies of the IgE type which bind to high affinity receptors on basophils and mast cells. Cross-linking of receptor-bound IgE then leads to release of pro-inflammatory mediators from these cells. The production of IgE can therefore be regarded as the bottle neck of allergic disorders. Allergen-specific IgE can be detected late after the primary response and it remains unclear how allergic sensitization persists over time.
This project was dedicated to gain new insights in the development and regulation of IgE-mediated immune responses. It is well established that class switch recombination to IgE in B cells is induced by IL-4 or IL-13 and requires the activation of the STAT6 signaling pathway in B cells. We looked now at the early events of the B cell response after allergic sensitization or helminth infection. Using conditional IL-4/IL-13-deficient mice we unexpectedly found that selective deletion of IL-4 and IL-13 in CD4 T cells led to impaired germinal center (GC) formation. To determine whether GC formation requires IL-4/IL-13-responsive B cells we analyzed mixed bone marrow chimeras where half of the B cells were wild-type and the other half STAT6-deficient. Both B cell populations could be distinguished by congenic markers. GC B cells were dominated by wild-type B cells indicating that STAT6-dependent genes in B cells are required for GC formation during allergic reactions. This suggests that IL-4/IL-13 from Th2 cells not only regulates the balance between Th1- or Th2-associated antibody isotypes but also play a key role in orchestration of GC formation. We performed microarray analyses and identified STAT6-regulated genes in B cells that might be involved in GC formation. Candidate genes will now be expressed by retroviral vectors to constitute STAT6-deficient B cells and then determine their contribution to the GC response.
With regard to the effector phase of allergic responses we generated constitutively basophil-deficient mice and studied different models of allergic inflammation. Ovalbumin-induced allergic inflammation of the lung and systemic IgE- or IgG1-mediated anaphylactic responses appeared normal in basophil-deficient mice. However, IgE-mediated chronic allergic inflammation of the skin was absolutely dependent on basophils. Basophils were also required for protective immunity against helminths during secondary infection and this effect required release of IL-4/IL-13 from basophils. We propose that long-lived plasma cells or memory B cells are generated during the primary infection and IgE antibodies from these cells keep basophils sensitized so that they can quickly respond during secondary infection.
Another focus of our studies was the analysis of the IgE repertoire and the regulation of memory IgE responses. We used next generation sequencing to reveal a striking overlap between the IgE and IgG1 repertoires of helminth-infected or alum/OVA-immunized mice. Using competitive adoptive transfers with B cells from naïve and immune mice we could show that memory IgE responses can be detected. But this memory IgE response developed mainly by sequential switching from IgG1+ B cells and required T cell-derived IL-4/IL-13. Unexpectedly, mice in which the extracellular part of IgG1 had been replaced by IgE sequences showed impaired memory IgE responses. This indicates that the memory IgE response is actually conserved at the level of IgG1+ B cells. Our results may lead to the development of new therapeutic strategies to treat chronic allergic disorders by targeting allergen-specific IgG1+ B cells.
This project was dedicated to gain new insights in the development and regulation of IgE-mediated immune responses. It is well established that class switch recombination to IgE in B cells is induced by IL-4 or IL-13 and requires the activation of the STAT6 signaling pathway in B cells. We looked now at the early events of the B cell response after allergic sensitization or helminth infection. Using conditional IL-4/IL-13-deficient mice we unexpectedly found that selective deletion of IL-4 and IL-13 in CD4 T cells led to impaired germinal center (GC) formation. To determine whether GC formation requires IL-4/IL-13-responsive B cells we analyzed mixed bone marrow chimeras where half of the B cells were wild-type and the other half STAT6-deficient. Both B cell populations could be distinguished by congenic markers. GC B cells were dominated by wild-type B cells indicating that STAT6-dependent genes in B cells are required for GC formation during allergic reactions. This suggests that IL-4/IL-13 from Th2 cells not only regulates the balance between Th1- or Th2-associated antibody isotypes but also play a key role in orchestration of GC formation. We performed microarray analyses and identified STAT6-regulated genes in B cells that might be involved in GC formation. Candidate genes will now be expressed by retroviral vectors to constitute STAT6-deficient B cells and then determine their contribution to the GC response.
With regard to the effector phase of allergic responses we generated constitutively basophil-deficient mice and studied different models of allergic inflammation. Ovalbumin-induced allergic inflammation of the lung and systemic IgE- or IgG1-mediated anaphylactic responses appeared normal in basophil-deficient mice. However, IgE-mediated chronic allergic inflammation of the skin was absolutely dependent on basophils. Basophils were also required for protective immunity against helminths during secondary infection and this effect required release of IL-4/IL-13 from basophils. We propose that long-lived plasma cells or memory B cells are generated during the primary infection and IgE antibodies from these cells keep basophils sensitized so that they can quickly respond during secondary infection.
Another focus of our studies was the analysis of the IgE repertoire and the regulation of memory IgE responses. We used next generation sequencing to reveal a striking overlap between the IgE and IgG1 repertoires of helminth-infected or alum/OVA-immunized mice. Using competitive adoptive transfers with B cells from naïve and immune mice we could show that memory IgE responses can be detected. But this memory IgE response developed mainly by sequential switching from IgG1+ B cells and required T cell-derived IL-4/IL-13. Unexpectedly, mice in which the extracellular part of IgG1 had been replaced by IgE sequences showed impaired memory IgE responses. This indicates that the memory IgE response is actually conserved at the level of IgG1+ B cells. Our results may lead to the development of new therapeutic strategies to treat chronic allergic disorders by targeting allergen-specific IgG1+ B cells.