Intravenous immunoglobulin (IVIG) preparations comprise pooled IgG antibodies from the serum of thousands of donors and were initially used as an IgG replacement therapy in immunocompromised patients. Since the discovery, more than 30 years ago, that IVIG therapy can ameliorate immune thrombocytopenia, the use of IVIG preparations has been extended to a wide range of autoimmune and inflammatory diseases. Despite the broad efficacy of IVIG therapy, its modes of action remain unclear. In this Review, we cover the recent insights into the molecular and cellular pathways that are involved in IVIG-mediated immunosuppression, with a particular focus on IVIG as a therapy for IgG-dependent autoimmune diseases.

Despite the beneficial therapeutic effects of intravenous immunoglobulin (IVIg) in inflammatory diseases, consistent therapeutic efficacy and potency remain major limitations for patients and physicians using IVIg. These limitations have stimulated a desire to generate therapeutic alternatives that could leverage the broad mechanisms of action of IVIg while improving therapeutic consistency and potency. The identification of the important anti-inflammatory role of fragment crystallizable domain (Fc) sialylation has presented an opportunity to develop more potent Ig therapies. However, translating this concept to potent anti-inflammatory therapeutics has been hampered by the difficulty of generating suitable sialylated products for clinical use. Therefore, we set out to develop the first, to our knowledge, robust and scalable process for generating a well-qualified sialylated IVIg drug candidate with maximum Fc sialylation devoid of unwanted alterations to the IVIg mixture. Here, we describe a controlled enzymatic, scalable process to produce a tetra-Fc-sialylated (s4-IVIg) IVIg drug candidate and its qualification across a wide panel of analytic assays, including physicochemical, pharmacokinetic, biodistribution, and in vivo animal models of inflammation. Our in vivo characterization of this drug candidate revealed consistent, enhanced anti-inflammatory activity up to 10-fold higher than IVIg across different animal models. To our knowledge, this candidate represents the first s4-IVIg suitable for clinical use; it is also a valuable therapeutic alternative with more consistent and potent anti-inflammatory activity.IVIg | sialylation | antibody | inflammation | autoimmune disease

Immunoglobulin G (IgG) antibodies confer protection against pathogenic microorganisms, serve as therapeutics in tumor therapy, and are involved in destruction of healthy tissues during autoimmune diseases. Understanding the molecular pathways and effector cell types involved in antibody-mediated effector functions is a prerequisite to modulate these activities. In this study we used two independent model systems to identify innate immune effector cells required for IgG activity in vivo. We first defined the precise repertoire of receptors for the IgG Fc fragment (FcγR) on innate immune effector cells in the blood and on tissue-resident macrophage populations. Despite expression of relevant activating FcγRs on various phagocyte populations, our data indicate that the majority of these cell types are dispensable for IgG activity in vivo. In contrast, IgG-dependent effector functions were selectively impaired in animals lacking the CX(3)CR1(hi)Ly6C(lo)CD11c(int) monocyte subset, which expressed the full set of FcγRs required for IgG activity.

Intravenous immunoglobulin (IVIg) therapy is widely used to treat a variety of autoimmune diseases including immunothrombocytopenia, chronic inflammatory demyelinating polyneuropathy, and more recently autoimmune skin blistering diseases. Despite this well-documented clinical success, the precise molecular and cellular mechanisms underlying this immunomodulatory activity are discussed controversially. In particular, the clinically relevant therapeutic pathway of IVIg-mediated immune modulation has not been studied in detail. In the present study, we use four independent in vivo model systems of auto-Ab-mediated autoimmune disease to identify a common pathway explaining IVIg activity under therapeutic conditions in vivo. We show that irrespective of the in vivo model system, IVIg activity is strictly dependent on the presence of terminal sialic acid residues and the inhibitory FcγRIIB under preventive as well as therapeutic treatment conditions. In contrast, specific ICAM3 grabbing nonintegrin related 1, previously demonstrated to be essential under preventative treatment conditions, showed a disease-specific impact on IVIg-mediated resolution of established autoimmune disease.

Intravenous immunoglobulin G (IVIg) therapy is widely used to treat autoimmune and inflammatory diseases. Recent evidence suggests that in mice, splenic resident cells might be important for the anti-inflammatory activity of IVIg in a model of serum transfer arthritis. Splenectomized human immunothrombocytopenia (ITP) patients, however, still respond to IVIg therapy. To investigate whether the requirement of the spleen is essential for mouse ITP, we used a passive model of induced ITP and demonstrated that IVIg activity was functional in splenectomized animals. Further analysis showed that the IVIgmediated amelioration of platelet phagocytosis was fully dependent on terminal sialic acid residues in the IVIg preparation and could be blocked with a specific ICAM3 grabbing nonintegrin-related 1 (SIGNR1) specific antibody. These results suggest that, similar to the human system, a spleen-independent but sialic acid-and SIGNR1-dependent pathway is responsible for IVIg-mediated suppression of autoantibody-dependent platelet depletion in mice.Supporting Information available online IntroductionIntravenous immunoglobulin G (IVIg) has been used for more than 30 years to treat autoimmune diseases such as immunothrombocytopenia (ITP), Guillain-Barre Syndrome, Kawasaki's disease, and chronic inflammatory demyelinating polyneuropathy (CIDP) [1, 2]. A variety of mechanisms depending either on the IgG variable (Fab fragment) or the constant crystallizable fragment (Fc fragment) have been suggested to be responsible for the anti-inflammatory activity of this IgG preparation [1, 2]. Data from human patients and mouse models of ITP, rheumaCorrespondence: Dr. Falk Nimmerjahn e-mail: fnimmerj@biologie.uni-erlangen.de toid arthritis, and nephrotoxic nephritis indicate that the IgG Fc fragment is important for IVIg activity in these autoimmune diseases in vivo [3][4][5][6][7]. Furthermore, the anti-inflammatory activity of IVIg was abrogated in mice deficient in the inhibitory Fcγ-receptor IIB (FcγRIIB) in a variety of model systems [3,5,[7][8][9][10][11][12][13][14]. A plausible explanation for the role of FcγRIIB was afforded by studies in mice and human patients with CIDP, which showed that FcγRIIB becomes upregulated on innate immune effector cells thereby raising the threshold for cell activation by immune complexes [3,5,7,15]. Further studies revealed that the sugar moiety attached to the IgG Fc fragment and especially terminal sialic acid residues in the Fc-linked but not the Fab-linked sugar moiety of serum IgG are essential for the anti-inflammatory activity [6,[11][12][13]16]. Removal of terminal sialic acid residues strongly impaired the suppressive activity of IVIg or the isolated Fc fragment, whereas enrichment for 2,6-linked sialic acid residues in the Fc-domain or in vitro hypersialylated IgG enhanced the antiinflammatory activity [6,12,13]. Of note, mouse and human antibodies rich in terminal sialic acid residues have a decreased affinity for classical FcγRs and were shown to gain the capacity to bind to mouse SIGNR1 ...