The Biological Function and Dynamics of the IgE Response in vivo: A Reflection of the Necessity of the IgE Antigen Receptor

The biological function of IgE and therewith-connected necessity of this special immunoglobulin isotype is not known so far. In certain parasitic diseases IgE titres are elevated up to three orders of magnitude, leading to a speculation about a specific function of IgE during parasitic infections. The value of this notion is however debated because it was shown that high titres of IgE might be even detrimental during infestations with worms. What remains, is the general accepted pathological function of IgE as key molecule of the allergic response. Modern therapeutic approaches suggest, among others, systemic targeting of total IgE, using anti IgE antibodies. However, by systemically targeting IgE, we deliberately resign on an antibody isotype whose biological function is not known at all. Therefore studying the in vivo function of the IgE immunoglobulin warrants an in depth study of this relevant biological question. Studying the regulation of IgE synthesis, one gets the impression that under normal conditions our organism tries everything to keep IgE the least abundant antibody class in the serum. Compared to other Ig isotypes, total IgE serum levels in mouse and man are very low not only under steady state conditions, but also after immunisation. However, if the decision: "force IgE expression" occurred, the organism sets value on highest quality of the IgE molecules produced. The biological mechanism of this dangerous migration is unknown, but obviously several independent mechanisms are responsible for this fact: First: Interestingly, the half life of IgE was calculated as maximal 10-12h, which contrasts significantly from other immunoglobulin classes that form a substantial component of serum proteins. Second: CD23, the low affinity receptor for IgE (Fc?RII) acts as a buffer in negative feedback regulation of IgE synthesis. CD23 knock-out mice over express IgE, whereas transgenic mice over expressing CD23 are deficient in IgE. Third: mIgE knock-out mice indicate the necessity of the IgE receptor for an IgE-mediated immune response. Two major conclusions were achieved from the study of the two knock out mice: the transmembrane domain of mIgE is indispensable for T-cell dependent IgE secretion and the cytoplasmic domain not only determines the absolute amount of IgE produced, but also influences the quality of the immunoglobulins. Fourth: in vitro data of our laboratories indicate an influence of an inefficient processing of the mRNA transcripts for the membrane form of IgE. The observation correlates well with the weak expression level of mIgE protein after class switch to IgE and the extremely low serum levels of IgE and represents yet another evolutionary mechanism to restrain serum IgE levels. Nevertheless, our knowledge about the biological function of the mIgE receptor and the regulation of the expression of mIgE itself is at best limited. In the last years we analyzed the process of affinity maturation and memory induction by examining the immune response in answer to a ph-Ox based immunisation strategy and clearly showed that there is no doubt that the IgE immune response maturese, but the consecutive switch is not the major operational mechanism in the murine system. Furthermore we isolated two candidate proteins (HPK1 and HAX1) that directly interact with the cytoplasmic tail of IgE and present the possibility of an IgE specific signal transduction. Additionally we constructed transgenic mice that might help us to understand the regulation of IgE expression. In the present project proposal we want to carry on our investigations on the regulation of IgE expression, by analysing the biological function of HPK1 and HAX1 during an IgE mediated immune response, but would like to expand this study by asking questions concerning the biological relevance of the IgE molecule in general and the development of IgE producing plasma and memory cells, their migration and their survival capacities in particular. Understanding the biology of plasma cells may help us understand human multiple myeloma, a disease that involves malignant plasma cells, but may also help to interfere antibody production of class E (IgE) in answer to harmless foreign antigen, like pollen or food antigens, leading to allergic disorders, like allergic rhinitis (hay fever) or allergic asthma, with clinical manifestations ranging from sneezing to a potentially life-threatening anaphylactic shock. Knowledge about the nature and longevity of plasma cells, and in turn, possibilities to affect this longevity would be crucial to provide a rational basis for designing effective treatments for these diseases.

Our knowledge about the biological function of the mIgE receptor and the regulation of the expression of mIgE itself is at best limited. In previous projects and publications we analyzed the process of affinity maturation and memory induction of the IgE isotype and isolated two candidate proteins (HPK1 and HAX1) that directly interact with the cytoplasmic tail of IgE. Additionally we constructed transgenic mice that helped us to understand the regulation of IgE expression. In the expired project we continued our investigations on the biological function of HPK1 and HAX1 and studied the development of IgE producing plasma and memory cells. In Hpk1-/- mice we were able to observe a downregulation of IgM receptor levels leading to formal enrichment of the anergic transitional 3 peripheral B-cell population. Furthermore, we could prove the interaction between HPK1 and HS1 and now hypothesize that HPK1 has direct or indirect impact on HS-1 activation, which may also be the scientific link to Hax1, which has been first identified as HS1 associated protein. Hax1-/- mice are characterized by a severely diminished cellularity of lymphoid tissues accompanied by a significant reduction of B- and T-lymphocytes. The reason for that observation might be that the correct bone marrow stromal niche for the maintenance and development of hematopoietic stem cells is not provided in Hax1-/- mice. The defective lymphocyte migration and trafficking could thus be explained by a cytoskeleton caused dysfunction, affecting lymphopoiesis at several stages from a very early phase on. Previously we published the limited humoral IgE response as result of a limited expression of IgE as a membrane- bound, antigen-receptor-type molecule, finally restricting the recruitment of IgE producing cells in the immune response. In the expired project, a further IgE limiting molecular mechanism could be presented. By means of gene targeting techniques we could show that IgE-antibody-secreting-cells migrate less efficiently towards the chemokine CXCL12 gradient guiding plasmablasts to plasma cell niches, than IgG1-antibody-secreting- cells. Thus, IgE plasmablasts have an intrinsic, lower chance to contribute to the long-lived plasma cell pool and thus to humoral immunologic memory than IgG1 plasmablasts. Understanding the biology of plasma cells may help us understand human multiple myeloma, a disease that involves malignant plasma cells, but may also help to interfere antibody production of IgE in answer to harmless foreign antigen, like pollen or food antigens, leading to allergic disorders, like allergic rhinitis (hay fever) or allergic asthma, with clinical manifestations ranging from sneezing to a potentially life-threatening anaphylactic shock. Knowledge about the nature and longevity of plasma cells, and in turn, possibilities to affect this longevity would be crucial to provide a rational basis for designing effective treatments for these diseases.