Fc receptors are found on the surface of a variety of cells and specifically bind to the Fc region of immunoglobulins (Igs). Four different subclasses of Fc receptors for IgG (FcγRs) have been identified in mice, including the activating receptors FcγRI, FcγRIII, and FcγRIV as well as the inhibitory receptor FcγRIIB (78). All FcγRs belong to the Ig superfamily. The activating receptors share a common γ-chain that comprises an immunoreceptor tyrosin-based activating motif (ITAM) (79) and express an individual ligand-binding α-chain, whereas the inhibitory FcγRIIB is a single chain receptor containing an immunoreceptor tyrosin-based inhibitory motif (as illustrated in Figure 2).

Besides microglial, endothelial, and mesangial cells as well as osteoclasts (78), FcγRs are particularly expressed by leukocytes. The majority of immune cells co-express activating and inhibitory FcγRs and the ratio of these receptors expressed by individual cells set a threshold for cell activation. Apart from distinct FcγRs, four different subclasses of IgG, in mice IgG1, IgG2a, IgG2b, and IgG3, with varying affinities toward the FcγRs are known. FcγRI shows high affinity and specificity for the different IgG isotypes, in contrast to FcγRIIB and FcγRIII that have lower binding capacities but recognize a broader spectrum (80). FcγRIV, in turn, seems to be the most important receptor for effector function of IgG2a and IgG2b (81). Moreover, it needs to be considered that the low-affinity receptors FcγRIIB, FcγRIII, and FcγRIV can only interact with multimeric IgG, which is present in immune complexes (ICs). This prevents unspecific binding, whereas FcγRI is saturated with monomeric serum IgG, but also requires ICs for activation (82). These kinetics suggest that the low-affinity receptors can regulate immunity more effective since the high-affinity binding to monomeric IgG of FcγRI hampers interaction with ICs (78). Apart from FcγRs IgG also binds to the neonatal Fc receptor (FcRn), which is expressed by vascular endothelium. FcRn prevents catabolism of IgG and is important for IgG half-life (83).

Triggered by the crosslinking of ICs with the α-chain of the activating FcγRs, the ITAM becomes phosphorylated, leading to the activation of members of the family of spleen tyrosin kinases (SYK) (78, 84). Subsequently, SYK-dependent phosphoinositides increase the activity of phospholipase Cγ (PLCγ) and Bruton’s tyrosine kinase (BTK) (78, 84) leading to an increase of intracellular calcium levels. Upon activation of FcγRIIB, in turn, the SH2-containing inositol polyphosphate 5-phosphatase (SHIP) hydrolyzes phosphoinositides (85). This events lead to a reduction of the activity of kinases, such as PLCγ and BTK, and, therefore, diminish the increase of intracellular calcium (78, 84). Hereby, activating as wells as inhibitory receptors affect the same signaling pathway and FcγRIIB exhibits important negative regulatory function in this manner. The loss or impairment of this inhibitory receptor can lead to autoimmunity as well as a prolonged immune response (86). The balance of activating and inhibitory receptors can be altered by the surrounding cytokine environment. The diverse role of Fc receptors in autoimmunity is elucidated by the fact that Fc receptor common γ-chain (FcRγ) knockout mice are resistant to the induction of autoimmune disease or show a milder course of disease, whereas FcγRIIB knockout mice often show worse outcome compared to wild-type mice (87–89). Mechanistically, it could be shown in nephrotoxic nephritis that DCs from FcγRIIB knockout animals show a more pronounced expression profile of cytokines associated with T cell activation that is absent in FcRγ-deficient mice (90). Furthermore, autoimmune-prone mouse-strains show a reduced expression of FcγRIIB, which is due to a promoter polymorphism in the Fcgr2 region (91). However, it needs to be considered that the FcRγ-chain is not only expressed by activating FcγRs but is also associated with a variety of other receptors, such as the T cell receptor (TCR)-CD3 (cluster of differentiation) complex, FcαR, FcεR, Nkp46, and IL3 (79, 92), when using the FcRγ knockout animals.

Human Fc receptors have a similar nomenclature and signaling pathways, but possess different expression patterns and binding affinities. So far, six different FcγRs have been identified. The high-affinity receptor FcγRI and the low-affinity receptors FcγRIIA, FcγRIIC, and FcγRIIIA are activating receptors, whereas FcγRIIB is inhibitory and the function of FcγRIIIB is yet unknown (93). The different IgG isotypes in humans are named IgG1, IgG2, IgG3, and IgG4, which bind to the different FcγRs in a concentration-dependent manner (94). Functionally, in humans IgG1 and IgG3 are the most pro-inflammatory IgG subclasses, whereas in mice IgG2a and IgG2b show a high inflammatory activity (78).

Taken together, humans and mice possess a similar repertoire of different FcγRs regarding their function and share common pathways. However, an important characteristic is the high rate of FcγR polymorphisms in humans (95), which affect binding affinities for IgG (94) and are associated with different kinds of autoimmune disease as well as immunological function (96).

Intravenous immunoglobulins are therapeutic preparations of polyclonal IgG, which are extracted from the plasma of thousands of donors. The different IgG subclasses are similarly distributed in IVIg preparations like in the blood of healthy individuals. Different mechanisms of action for IVIg in diseases models have been proposed. The Fc-mediated effects are most likely of major significance (97), which is supported by the fact that infusion of IgG preparations only containing the Fc fragment can protect mice as wells as humans from disease (98, 99). First of all IVIg can compete for activating FcγRs and, therefore, limit the access of activating ICs (100). In a similar manner, IVIg can inhibit complement deposition (101). The inhibitory receptor FcγRIIB seems to be essential for the protective effects, since protection by IVIg is lost in FcγRIIB-deficient mice and FcγRIIB is upregulated following IVIg treatment (88, 99). Mechanistically, FcγRIIB signaling can suppress B cell-mediated T cell-dependent immune responses (102). Specific intracellular adhesion molecule grabbing none-integrin receptor 1(SIGN-R1) and its human homolog dendritic cell-specific intracellular adhesion molecule-3 grabbing non-integrin, which is among others expressed by macrophages (103), have also been proposed to be important in IVIg function mediated by FcγRIIB. These two receptors can directly recognize specific sialic acid rich isoforms of IgG, which leads to an up to tenfold increase of effectiveness of IVIg (104) and induces a non-inflammatory phenotype in a specialized subset of macrophages. Conversely, macrophage colony-stimulating factor 1 (M-CSF-1)-deficient mice also loose IVIg protection, which might be due to a lack of M-CSF-1-dependent regulatory SIGN-R1 expressing macrophages (105). Furthermore, inhibition of BTK, a kinase downstream of FcγRs, whose activity is reduced upon FcγRIIB activation, leads to reduced mature IL-1β in the context of inflammation (25), as well as preventing IL-1β, IL-17, IFNγ, and TNFα production upon FcγR stimulation (106).

Another proposed functional pathway is the ability of IVIg to neutralize anti-ideotypic antibodies in a F(ab)² fragment-dependent manner and, therefore, protect from disease (107). Other F(ab)² fragment-dependent mechanisms include blockade of specific receptors, such as the Fas receptor via anti-Fas antibodies (108), the binding of cytokines such as IL-5 as wells as granulocyte macrophage colony-stimulating factor (109) and the inhibition of TCR-mediated T cell proliferation (110). In addition, IVIg do not only hemper T cell proliferation but can also influence the differentiation of T helper cells into Th17 cells via interference with the retinoic acid-related orphan receptor C (111) and can induce a shift toward a Th2 phenotype in childhood ITP and women with recurrent spontaneous abortion (112, 113). Furthermore, there is evidence that IVIgs increase the expression of IL-10 as wells as TGF-β in T regulatory cells (114) via specific epitopes in the Fc region of IgG (115). Interestingly, IVIg were also able to bind to peripheral blood T cells, which do not express FcγRs, through yet unknown receptors (116) and can reduce the production of inflammatory cytokines, such as IL-2, IL-3, IL-4, IL-5, IFNγ, and TNF in in vitro setting (117).

Taken together IVIg can act through a variety of different pathways. Apart from Fc and F(ab)² fragment-dependent effects, IVIgs can either indirectly or even directly address cells that do not express FcγRs. The individual roles and mechanisms need to be explored individually in the different autoimmune diseases, also considering that there are probably joint effects.

Intravenous immunoglobulins are established as a first-line therapy in a variety of neurological disease. They are used in the treatment of CIDP, Guillain–Barré syndrome (GBS), myasthenia gravis, and inflammatory myopathies as well as in autoimmune encephalitis and neuromyelitis optica (136, 137). CIDP, for example, is a heterogeneous autoimmune-mediated inflammatory demyelinating disease of peripheral nerves and several clinical trials showed that IVIg are beneficial (138). The pathophysiology of disease remains unknown; but in a subset of patients, it appears to be mediated by IgG-autoantibodies against myelin. IgG isolated from this group of patients was able to induce disease in rats (91), suggesting that IgG can play an important role in the development of CIDP. Furthermore, the NODmouse strain, which has a promoter polymorphism in the Fcgr2 region (139), can develop spontaneous autoimmune peripheral polyneuropathy under certain circumstances (140, 141). Conversely, FcγRIIB expression is impaired in B cells of patients with CIDP but is upregulated following IVIg treatment (142). Likewise, for GBS, another disease of the peripheral nervous system (PNS), IVIg represent an established therapeutic regime. In approximately 50% of patients with the GBS, autoantibodies against gangliosides can be found (143). One of the proposed mechanisms of IVIg action in GBS as wells as CIDP is the presence of anti-ideotypic antibodies that are able to bind and neutralize pathogenic autoantibodies (144). Another exemplary IgG-mediated neurological disorder is myasthenia gravis, in which autoantibodies against the acetylcholine receptor are produced in a T helper cell-dependent manner. Mechanistically, IVIg protection in myasthenia gravis is also most likely promoted by anti-ideotypic antibodies.

Apart from the efficacy to treat PNS affecting diseases, data for IVIg on diseases of the CNS is more controversial. Although beneficial effects and potential therapeutic pathways have been observed in the mouse model of MS (145) and AD, for example, translation into the human system remains difficult so far. A major limiting factor in the context of diseases of the CNS is the BBB, which controls access of IVIg to the brain. In the healthy brain, IgG is present in small amounts and is able to cross the BBB in a controlled manner through yet unknown mechanism. The clearance from the CNS is mediated by the FcRn (146), which is expressed by brain endothelial cells (147, 148). In the context of inflammation, when the BBB is disrupted, IVIg are able to enter the brain in a less restricted manner. Accordingly, following administration of IVIg in a model of experimental stroke, increased intracerebral IgG was observed in the ischemic brain (147, 148). IVIg is also able to cross the intact BBB in a saturation-dependent process and were found to co-localize with neurons as well as endothelial cells (149). Interestingly, administration of IVIg reduced the amount of endogenous IgG in this model, suggesting a competition for brain access. A positive impact of IVIg on the integrity of the BBB has also been reported, since IVIg treatment was able to prevent BBB-breakdown in sepsis (150).

For MS, which is suggested to be primarily a T cell-mediated disease, there is experimental evidence from the experimental autoimmune encephalitis (EAE) model that treatment with IVIg is beneficial. It could be shown that IVIg decreased the production of inflammatory cytokines such as IFNγ and TNF (144, 151) on one side and led to an expansion of peripheral T regulatory cells and subsequent suppression of conventional T helper cells in an Fc-fragment independent manner on the other side (152). Apart from the impact on the cells of the adaptive immunity, there is also evidence for a direct involvement of FcγRs. It could be shown that FcRγ-deficient mice develop milder EAE (89), although this effect was attributed to γδ T cells that use this chain in other receptors than the FcγR. In addition, the FcRγ can be detected on OPCs in remyelinating plaques in MS as well as on microglia in inactive plaques (153). Despite the beneficial action in the experimental setting, the effect of treatment in humans is controversial. Some small studies were able to show protection, whereas a large multicentre, randomized, double-blind, placebo-controlled trial failed to reproduce the previous results (154).

Only little is known about the role of FcγRs in the context of postischemic inflammation, but various experimental studies emphasize a promising therapeutic effect of IVIgs in the in vivo stroke model as well as in in vitro settings by reducing infarct size and improving neurological outcome. The direct mechanisms remain unknown so far. First of all, it could be shown in the MCAO model that the common γ-chain knockout, which lacks all activating FcγRs, is protected from stroke (155). Compared to WT mice, the common γ-chain-deficient mice showed significant reduction of infarct volume at 24, 72 h, and 14 days after stroke as well as an improved neurological outcome after being subjected to 60 min of transient focal ischemia. Mechanistically, it was assumed that FcγRs are important for the activation of microglia and induction of the inducible nitric oxide synthase (iNOS). Hence, microglia of the common γ-chain KO mice expressed fewer ionized calcium-binding adapter molecule 1, a protein that is upregulated in microglia upon activation, as well as less iNOS in immunohistochemistry and on protein level.

In addition to the effects mediated by inhibition of the activating FcγRs, it was recently shown that IVIg treatment likewise protects the brain from ischemia-induced reperfusion injury and the subsequent inflammatory response. Administration of IVIg, 3 h post reperfusion significantly reduced the amount of infiltrating leukocytes 24 h after MCAO, which were identified as CD45 high cells in flow cytometry (156). In line with recent preclinical studies showing a beneficial effect of the α4-integrin-inhibitor natalizumab in experimental stroke, the reduction of infiltrating leukocytes by IVIg could be mediated in an α4-integrin-inhibitor-dependent manner as it has been shown in the EAE model (157). Of note, this study concluded that IVIg treatment is even detrimental in stroke, since they found more leukocytes occupying pial vessels, which is thought to be due to platelet-mediated pro-adhesive effects. Still, it needs to be considered that they did not examine the actual number of infiltrating cells.

Furthermore, IVIg treatment of primary neuronal cultures subjected to oxygen and glucose deprivation, an in vitro model of ischemic stroke, inhibited upregulation of TLR2, TLR4, and TLR8 (158). These findings could also be reproduced in vivo where IVIg administration after transient MCAO significantly reduced ischemia-induced upregulation of TLR2, TLR4, and TLR8. The authors also observed an IVIg-dependent suppression of HMGB1-mediated TLR activation, which is released by dying cells as a danger signal in the context of ischemic stroke. In addition, it was found that IVIg attenuated the ischemia-induced increase of complement factor C3b, which is known to contribute to ischemic injury (159) and among others upregulates the intracellular adhesion molecule 1 (ICAM-1) in vivo and in vitro. In line with these findings, IVIg also specifically affect endothelial cells and diminish the upregulation of VCAM-1 and ICAM-1 in in vitro settings (160).

Another immunomodulatory IVIg mechanism includes the suppression of the NLPR1 and NLPR3 inflammasome-mediated neuronal cell death (24, 25). Treatment of primary neuronal cultures with IVIg subjected to simulated ischemia as well as mice subjected to MCAO, reduced levels of inflammasome components such as NLRP1, NLRP3, and apoptosis-associated speck-like protein containing a caspase recruitment domain (ACS). This, in turn, led to a reduction of caspase-1 and mature IL-1β as well as IL-18. Moreover, it was shown that selective inhibition of BTK with ibrutinib, whose activity is reduced after engagement of the inhibitory receptor FcγRIIB, diminishes ischemic injury by decreasing inflammasome NLPR3 activity and, therefore, conversion of pro-IL-1β (25). Conversely, the inhibition of SYK, a kinase upstream of BTK was also able to decrease lesion size in the MCAO model (161). Interestingly, IVIg protection in other autoimmune disease is lost in M-CSF-1-deficient (op/op) mice. Apart from an enormous reduction of microglia proliferation, these M-CSF-1-deficient mice show an enhanced sensitivity to ischemia-induced neuronal injury and cell death (162). M-CSF-1 overexpression, in turn, leads to microglia proliferation, which does not show a difference in phenotype regarding the M1/M2 model, but shows altered immune responses (163). Importantly, M-CSF-1 treatment of mice subjected to MCAO decreases infarct size (162) and the presence of M-CSF-1-dependent macrophages correlates with an increased expression of FcγRIIB (164).

Apart from ameliorating the inflammatory response after stroke the protective capacity of IVIg includes the induction of neuroprotective pathways. For instance, neuronal structure was more intact and had less ischemia-associated alterations on a histopathological level in IVIg-treated rats (165). IVIg treatment of primary neuronal cultures subjected to simulated ischemia for 12 h significantly reduced protein levels of factors involved in neuronal cell death like the phospho-SAPK c-Jun NH²-terminal kinase (p-JNK) and phospho-p65 NFκB and inhibited the loss of the neuronal marker microtubulin associated protein 2 (MAP2) (147, 158). These findings could be confirmed by immunoblots and immunohistochemistry, respectively, in vivo in the transient MCAO model. Furthermore, IVIg could prevent simulated ischemia-induced endothelial disintegration in a brain endothelial cell line and increased the protective protein B-cell lymphoma 2 produced by these endothelial cells as well as by neurons (156). In line with these findings it could be shown that ischemia-induced decrease and reduced phosphorylation of low-density lipoprotein receptor-related protein 1, which is abundantly expressed by neurons, can be inhibited by IVIgs and, subsequently, prevents activation of cell death signaling proteins as NFκB and p-JNK (166).

Apart from the already described pathways, there are multiple other conceivable mechanism how IVIg facilitate protection in the context of ischemic stroke (Figure 3). They could inhibit complement deposition and induce a regulatory phenotype in macrophages in an Fc-fragment manner, as well as targeting the inhibitory receptor FcγRIIB, subsequently supressing T cell-mediated immune responses. Similary, the production of inflammatory cytokines, such as IL-1β and TNFα, could be diminished in FcγRs expressing cells. In addition, the F(ab)² fragment could block receptors such as Fas (167) or inhibit TCR-mediated T cell proliferation. Also indirect effect such as increased IL-10 and TGF-β expression as well as a reduced production of inflammatory cytokine by T cells need to be considered. Ultimately, it remains unsolved wheter IVIg primarily operate via the Fc or F(ab)² fragment, or if indirect mechanisms such as T cell inhibition predominate.

Besides experimental evidence for positive IVIg effects in postischemic inflammation, there are other models of acute CNS injury in which IVIg have been shown to be beneficial. Similar to inflammatory processes following ischemic stroke, neuroinflammation exacerbates the tissue damage in acute spinal cord trauma (168). Accordingly, several hallmarks of sterile inflammation have been observed following trauma: (i) microglia activation; (ii) upregulation of proinflammatory cytokines, including IL-1β, TNFα, and IL-6 as well as ROS and MMP-9; and (iii) infiltration of neutrophils, monocytes, and lymphocytes. The importance of the local inflammation is underlined by studies showing that immunosuppressive treatment approaches with steroids have favorable effect on functional outcome. Nevertheless, the disadvantage of unspecific immunosuppressive agents is demonstrated by studies showing an increased risk of infection following steroid treatment in models of traumatic spinal cord injury. Consequently, more specific immunomodulatory treatment strategies are needed. The feasibility of immunmodulatory treatments in acute spinal cord injury was recently demonstrated by Gok et al. (169), describing that the treatment with IgG had significant beneficial effects on motor function in a rat model. Furthermore, electron microscopy revealed a significant decrease of intraneuronal vacuoles, as an indicator for more preserved neuronal ultrastructure. Another study on IgG in acute spinal cord injury also showed reduced scar formation and tissue preservation on a histopathological level (170). In addition, the authors detected a reduction in proinflammatory cytokines, such as TNFα, IL-1β, and IL-6 as well as MMP9, and showed that the IgG is able to enter the injured spinal cord while it mainly co-localized to astrocytes. Furthermore, protective IVIg effects were associated with reduced numbers of infiltrating neutrophils as wells as a diminished MPO activity. Overall, the reduction of inflammation showed improved functional recovery assessed by neurobehavioral test as well as significantly enhanced conduction velocity in electrophysiological measurements.

Sterile inflammation is also a well-known feature after traumatic brain injury (171). As a consequence of the initial trauma a tremendous release of DAMPs can be observed, which induced production of proinflammatory cytokines and infiltration of various immune cells. Incidentally, it was found that IgG significantly improved motor test scores compared to saline and reduced MPO activity, as it was used as an additional vehicle control in a study investigating the effect of ICAM-1 blockage in traumatic brain injury (172). Furthermore, it could be shown that IVIg treatment stabilized the BBB and reduced edema formation (173). These effects were accompanied by reduced amounts of IL-6 upon IVIg administration as well as less infiltrating macrophages and increased neuronal density. Moreover, endothelial protection by IVIg was detected in a model of subarachnoidal hemorrhage (174).

Intravenous immunoglobulin have also been investigated in other models of ischemia-reperfusion injury and a protective effect on the evolving inflammatory cascades has been observed. In mesenteric ischemia, a condition that can be associated with hypovolaemic shock, sepsis, and cardiac arrest (175), pretreatment with IVIg reduced complement-mediated tissue damage (176). IVIg also significantly reduced mucosal injury on a histopathological level and diminished C3 deposition in the intestinal mucosa. Interestingly, the number of infiltrating leukocytes was not altered by IVIg administration. Similarly, in a model of liver ischemia, sinusoidal congestion and cytoplasmatic vacuolation were diminished in IVIg-treated mice (177). These effects were associated with a reduced mortality in the IVIg-treated group. Taken together, there are multiple pieces of evidence from experimental studies that IVIg have significant protective effects on acute injuries of the CNS and other organs, in which sterile inflammation is part of the pathology.

Reflecting the available preclinical data on IVIg in acute brain injury, it seems promising to also use IVIg in a clinical setting. However, possible rheologic disadvantages need to be considered. It does not appear intuitively that an agent, which could possibly deteriorate perfusion due to the high viscosity, is suitable to treat diseases with reduced blood flow and impairment of microcirculation. Indeed, IVIg-related thrombosis has been described in the literature sporadically (178). Nevertheless, the positive impact of IVIg seems to predominate these negative effects. Overall, these drawbacks might be overcome in future studies, if specific therapeutic effects can be attributed to single IVIg fractions, thereby allowing to reduce the necessary dosage.

Considering that peripheral immunosuppression and an increased risk of urinary tract as well as upper airway infections is a common epiphenomenon after stroke IVIg treatment shows another promising potential. Apart from reducing the inflammatory reactions after stroke, IVIg administration could also compensate for the transient decrease of IgG after stroke (41) and possibly reduce the risk of infections that deteriorates the outcome of stroke patients. In line with this assumption, it could be shown that IVIg can even enhance microbial-specific immune responses in preterm infants (179) and do not increase mortality in sepsis (180). One clinical trial exploring the effect of IVIg in human stroke was already initiated (clinicaltrials.gov NCT01628055), but had to be stopped due to difficulties in patient recruitment.