A Structural and Functional Interrogation of the Ligand-Independent Signaling Properties of the IgE BCR

Abstract The US has recently experienced an alarming increase in the prevalence of allergic diseases. Allergic disease is characterized by an Immunoglobulin E (IgE) mediated immune response to common environmental antigens, with pathological manifestations ranging from rhinitis to life-threatening anaphylaxis. Under normal physiological conditions, production of IgE is tightly regulated by mechanisms affecting IgE- producing lymphocytes. A critical aspect of this regulation involves signaling through the B cell antigen receptor (BCR), which consists of a variable antigen-binding, membrane-bound immunoglobulin (mIg) and an invariant Ig⍺/Igβ signaling module. Unlike the canonical IgM BCR, the IgE BCR carries out robust, antigen-independent signaling. Autonomous activity of the IgE BCR induces IgE B cells towards developmental outcomes that restrict affinity maturation and militate against the emergence of IgE lymphocytes in long-lived or memory compartments. The aberrant production of allergen-reactive IgE can induce fatal anaphylaxis. Despite the potentially severe consequences of IgE dysregulation for human health, available data on ligand-independent IgE BCR signaling comes exclusively from murine models. Limited data suggests a conserved role for autonomous IgE BCR signaling pathways in human IgE B cells. Differences in IgE biology between humans and mice, however, highlight a need for further investigation in human model systems. Importantly, humans express two isoforms of membrane-bound IgE (mIgE), of which one is entirely absent in mouse. Human mIgE isoforms are differentiated by the length of their respective extracellular membrane-proximal domain (EMPD) primary sequences. The EMPD plays a key role in BCR assembly, suggesting a function for divergent EMPDs in modulating BCR signaling properties. I hypothesize that human mIgE isoforms give rise to IgE BCRs with distinct ligand-independent signaling properties. To address this hypothesis, I will perform a series of cell- based and molecular assays to characterize the ligand-independent signaling properties of human IgE BCR isoforms (Aim 1). In parallel, I will use a single particle analysis cryo-EM pipeline to obtain high-resolution structures of both human IgE BCR isoforms (Aim 2). Insights arising from this research will contribute to an understanding of how the IgE BCR regulates IgE responses in human health and disease, with implications for the development of strategies for managing conditions associated with IgE dysregulation. In parallel, this work will afford an exceptional training opportunity, supporting the development of expertise in structural biology, mass spectroscopic proteomic analysis, and immunobiology.