Neonatal Fc receptor in human immunity: Function and role in therapeutic intervention

Abstract

The humoral immune response provides specific, long-lived protection against invading pathogens, via immunoglobulin production and other memory functions. IgG, the most abundant immunoglobulin isotype, has the longest half-life and protects against bacterial and viral infections. The neonatal Fc receptor (FcRn) transports IgG across barriers, for example, the placenta, enhancing fetal humoral immunity to levels similar to their mothers’. Importantly, FcRn, by protecting IgG from intracellular degradation, results in an approximately 21-day circulating IgG half-life and high plasma levels; similarly, FcRn recycles albumin and is the portal of entry for enteric cytopathic human orphan (echo) virus infection. Dysregulated immune responses may lead to antibodies against self-antigens (autoantibodies), resulting in organ-specific or systemic autoimmune diseases. Autoantibody-mediated diseases have been treated by nonspecific immunoglobulin-lowering/modulating therapies, including immunoadsorption, plasma exchange, and high-dose intravenous immunoglobulin. However, targeting FcRn with specific inhibitors results in reduction in only IgG levels. The effectiveness of FcRn inhibitors in autoimmune diseases, including myasthenia gravis and immune thrombocytopenia, provides further evidence that IgG is a primary driver in these autoantibody-mediated diseases. We describe the role of FcRn in human biology, including insights that clinical testing of FcRn inhibitors have provided into FcRn biology and autoimmune disease mechanisms, allowing fact-based speculation on their therapeutic potential. The humoral immune response provides specific, long-lived protection against invading pathogens, via immunoglobulin production and other memory functions. IgG, the most abundant immunoglobulin isotype, has the longest half-life and protects against bacterial and viral infections. The neonatal Fc receptor (FcRn) transports IgG across barriers, for example, the placenta, enhancing fetal humoral immunity to levels similar to their mothers’. Importantly, FcRn, by protecting IgG from intracellular degradation, results in an approximately 21-day circulating IgG half-life and high plasma levels; similarly, FcRn recycles albumin and is the portal of entry for enteric cytopathic human orphan (echo) virus infection. Dysregulated immune responses may lead to antibodies against self-antigens (autoantibodies), resulting in organ-specific or systemic autoimmune diseases. Autoantibody-mediated diseases have been treated by nonspecific immunoglobulin-lowering/modulating therapies, including immunoadsorption, plasma exchange, and high-dose intravenous immunoglobulin. However, targeting FcRn with specific inhibitors results in reduction in only IgG levels. The effectiveness of FcRn inhibitors in autoimmune diseases, including myasthenia gravis and immune thrombocytopenia, provides further evidence that IgG is a primary driver in these autoantibody-mediated diseases. We describe the role of FcRn in human biology, including insights that clinical testing of FcRn inhibitors have provided into FcRn biology and autoimmune disease mechanisms, allowing fact-based speculation on their therapeutic potential. The humoral (antibody) immune response offers a targeted, specific, long-lived protection against many invading pathogens. It is an adaptive response wherein high-affinity antibodies are generated to pathogenic antigens. Antibodies (also termed physicochemically as immunoglobulins) provide protection in 3 ways: neutralization (binding pathogens and preventing their entry into cells), opsonization (facilitating pathogen uptake by phagocytic cells), and complement activation (facilitating opsonization and direct pathogen killing or lysis through activation of complement proteins).1Janeway C.A. Travers P. Walport M. Shlomchik M.J. The humoral immune response.in: Immunobiology: the immune system in health and disease. 5th ed. Garland Science, New York2001Google Scholar Immunoglobulins exist in 5 isotypes, IgM, IgD, IgG, IgA, and IgE, which vary in structure and function. IgG is the most abundant circulating immunoglobulin isotype, has the longest serum half-life, and is fundamental for systemic immunity and protection against infection. IgG provides a link between innate immune effector mechanisms and specific humoral recognition of antigen.2Vidarsson G. Dekkers G. Rispens T. IgG subclasses and allotypes: from structure to effector functions.Front Immunol. 2014; 5: 520Crossref PubMed Scopus (678) Google Scholar IgG itself can be subdivided into 4 subclasses, IgG1, IgG2, IgG3, and IgG4, which are observed in decreasing abundance and have subtle but significant differences in structure and function (reviewed by Vidarsson et al2Vidarsson G. Dekkers G. Rispens T. IgG subclasses and allotypes: from structure to effector functions.Front Immunol. 2014; 5: 520Crossref PubMed Scopus (678) Google Scholar). The neonatal fragment crystallizable (Fc) receptor (FcRn) is a multifunctional atypical Fc-gamma receptor (FcγR). FcRn was initially identified as responsible for IgG transport from maternal to fetal across the placenta, and for IgG M. The neonatal Fc receptor a Immunol. PubMed Scopus Google Scholar of FcRn have been including albumin transport of IgG and albumin across a of barriers, responses to and as a receptor for enteric cytopathic human orphan (echo) M. 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