Human IgE does not bind to human FcRn

The neonatal Fc receptor (FcRn) is known to mediate placental transfer of IgG from mother to unborn. IgE is widely known for triggering immune responses to environmental antigens. Recent evidence suggests FcRn-mediated transplacental passage of IgE during pregnancy. However, direct interaction of FcRn and IgE was not investigated. Here, we compared binding of human IgE and IgG variants to recombinant soluble human FcRn with β2-microglobulin (sFcRn) in surface plasmon resonance (SPR) at pH 7.4 and pH 6.0. No interaction was found between human IgE and human sFcRn. These results imply that FcRn can only transport IgE indirectly, and thereby possibly transfer allergenic sensitivity from mother to fetus.

Neonatal Fc Receptor–Targeted Therapies in Neurology

Autoantibodies are increasingly recognized for their pathogenic potential in a growing number of neurological diseases. While myasthenia gravis represents the prototypic antibody (Ab)-mediated neurological disease, many more disorders characterized by Abs targeting neuronal or glial antigens have been identified over the past two decades. Depletion of humoral immune components including immunoglobulin G (IgG) through plasma exchange or immunoadsorption is a successful therapeutic strategy in most of these disease conditions. The neonatal Fc receptor (FcRn), primarily expressed by endothelial and myeloid cells, facilitates IgG recycling and extends the half-life of IgG molecules. FcRn blockade prevents binding of endogenous IgG to FcRn, which forces these antibodies into lysosomal degradation, leading to IgG depletion. Enhancing the degradation of endogenous IgG by FcRn-targeted therapies proved to be a powerful therapeutic approach in patients with generalized MG and is currently being tested in clinical trials for several other neurological diseases including autoimmune encephalopathies, neuromyelitis optica spectrum disorders, and inflammatory neuropathies. This review illustrates mechanisms of FcRn-targeted therapies and appraises their potential to treat neurological diseases.

Dynamics of intracellular neonatal Fc receptor-ligand interactions in primary macrophages using biophysical fluorescence techniques

The neonatal Fc receptor (FcRn) is responsible for the recycling of endocytosed albumin and IgG and contributes to their long plasma half-life. We recently identified a FcRn-dependent, recycling pathway from macropinosomes in macrophages (Toh et al, 2019), however, little is known about the dynamics of intracellular FcRn-ligand interactions to promote recycling. Here we demonstrate a multiplexed biophysical fluorescent microscopy approach to resolve the spatiotemporal dynamics of albumin-FcRn interactions in living bone marrow-derived macrophages (BMDMs). We used the phasor approach to fluorescence lifetime imaging microscopy (FLIM) of Förster resonance energy transfer (FRET) to detect the interaction of a FcRn-mCherry fusion protein with endocytosed Alexa Fluor 488-labelled human serum albumin (HSA-AF488) in BMDMs, and Raster Image Correlation Spectroscopy (RICS) analysis of single fluorescent-labelled albumin molecules to monitor the diffusion kinetics of internalised albumin. Our data identified a major fraction of immobile HSA-AF488 molecules in endosomal structures of human FcRn-positive mouse macrophages and an increase in FLIM- FRET following endocytosis, including detection of FRET in tubular-like structures. A non-binding mutant of albumin showed minimum FLIM-FRET and high mobility. These data reveal the kinetics of FcRn-ligand binding within endosomal structures for recruitment into transport carriers for recycling. These approaches have wide applicability for analyses of intracellular ligand-receptor interactions.

678 The neonatal Fc receptor is elevated in monocyte-derived immune cells in pancreatic cancer

BackgroundPancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related death in the United States with 5-year survival rates below 10%. PDAC is commonly diagnosed after metastasis has occurred and treatment options are limited. Immune checkpoint inhibitor (ICI) monoclonal antibody (mAb) therapy has shown great promise in other cancers, however little efficacy has been observed in patients with PDAC. The protein responsible for recycling IgG based mAb therapeutics like ICIs in the bloodstream, as well as processing peptides for antigen presentation, is the neonatal Fc receptor (FcRn). Little is known about FcRn in cancer, and to our knowledge no characterization of host FcRn, or FcRn extrinsic to tumor cells exists in PDAC patients. We hypothesized that PDAC patients and tumor-bearing animals would have altered FcRn expression by their immune populations compared to their healthy counterparts.MethodsC57BL/6 mice were orthotopically injected with KPC-luc (KrasLSL-G12D, Trp53LSL-R270H, Pdx1-cre) pancreatic tumor cells, and FcRn expression in myeloid-derived splenocytes were analyzed by fluorescence cytometry. Time-of-flight mass cytometry (CyTOF) was utilized to immunophenotype peripheral blood mononuclear cells (PBMCs) of PDAC or non-cancer patients for expression levels of FcRn within these immune populations.ResultsPDAC tumor-bearing mice exhibit altered FcRn expression among myeloid immune cell populations. Mice with pancreatic tumors had elevated expression of FcRn on migratory cDC2 (CD8-CD11b+CD103+CD24++; p = 0.017), monocytic MDSC (CD11b+Ly6G-Ly6C+; p = 0.0023), granulocytic MDSC (CD11b+Ly6G+Ly6C±; p = 0.0542), and cDC2 (CD8-CD11b+CD103-CD24±; p=0.036) cells. PBMCs from non-cancer obese patients (healthy control samples; n=8) and PDAC patients prior to surgical resection (n=13) were subjected to CyTOF analyses. The majority of FcRn expression was concentrated to monocyte (p=0.017), DCs (p=0.017) and MDSC (p=0.012) immune populations. Overall, we observed increased expression of FcRn on myeloid-derived immune populations from patients with PDAC. FcRn expression was elevated in both monocytes and DC populations in PDAC relative to non-cancer PBMCs. Monocytic and granulocytic MDSC from patients with PDAC had significantly elevated FcRn positivity compared to healthy controls (p = 0.034, p = 0.026, respectively).ConclusionsFcRn is upregulated in monocytes, dendritic cells and MDSC immune populations in patients and mice with pancreatic tumors. Future investigations into FcRn function in preclinical models and PDAC patients will hopefully elucidate new mechanisms of ICI resistance and possible alternative approaches for improving immunotherapy efficacy in these patients.Ethics ApprovalAll patients provided voluntary written informed consent (Institutional Review Board protocol: 2010C0051) to participate. The protocols and subsequent amendments were approved by The Ohio State University Institutional Review Board. All animal protocols were approved by the Ohio State University Institutional Animal Care and Use Committee (IACUC) at The Ohio State University (Approved IACUC protocols 2009A0178-R4 and 2017A00000117-R1) and mice were treated in accordance with institutional guidelines for animal care. The Ohio State University Laboratory Animal Shared Resource is an Association for Assessment and Accreditation of Laboratory Animal Care International accredited program that follows Public Health Service policy and guidelines. All other experiments were completed under the research protocols (2014R00000086; 2013R00000056) approved by the Ohio State University Institutional Biosafety Committee.

Neonatal Fc receptor expression in lymphoid and myeloid cells in systemic lupus erythematosus

The neonatal Fc receptor (FcRn) is a ubiquitously expressed protein historically involved in IgG and albumin recycling. Recent data suggest an involvement in the pathophysiology of antibody-mediated autoimmune diseases. Among them, systemic lupus erythematosus (SLE) implies clinical and biological abnormalities of innate and adaptive circulating immune cells, potentially involving newly described functions of FcRn. In this study, FcRn expression was assessed by flow cytometry in peripheral blood leukocytes of 41 SLE patients with either active or inactive disease and 32 healthy donors. FcRn expression in B cells, natural killer cells, and T cells of SLE patients was statistically lower as compared to healthy donors. Conversely, FcRn level was statistically higher in non-classical monocyte subpopulations (CD14+CD16+ monocytes) of SLE patients versus healthy donors providing an interesting perspective to further explore its role in SLE pathophysiology.