Requirement of the Fc receptor common gamma-chain for gamma delta T cell-mediated promotion of murine experimental autoimmune encephalomyelitis

Therapeutic Potential of Intravenous Immunoglobulin in Acute Brain Injury

Acute ischemic and traumatic injury of the central nervous system (CNS) is known to induce a cascade of inflammatory events that lead to secondary tissue damage. In particular, the sterile inflammatory response in stroke has been intensively investigated in the last decade, and numerous experimental studies demonstrated the neuroprotective potential of a targeted modulation of the immune system. Among the investigated immunomodulatory agents, intravenous immunoglobulin (IVIg) stand out due to their beneficial therapeutic potential in experimental stroke as well as several other experimental models of acute brain injuries, which are characterized by a rapidly evolving sterile inflammatory response, e.g., trauma, subarachnoid hemorrhage. IVIg are therapeutic preparations of polyclonal immunoglobulin G, extracted from the plasma of thousands of donors. In clinical practice, IVIg are the treatment of choice for diverse autoimmune diseases and various mechanisms of action have been proposed. Only recently, several experimental studies implicated a therapeutic potential of IVIg even in models of acute CNS injury, and suggested that the immune system as well as neuronal cells can directly be targeted by IVIg. This review gives further insight into the role of secondary inflammation in acute brain injury with an emphasis on stroke and investigates the therapeutic potential of IVIg.

Regulation of FcRγ function by site-specific serine phosphorylation

The common FcRγ, an immunoreceptor tyrosine-based activation motif (ITAM)- containing adaptor protein, associates with multiple leukocyte receptor complexes and mediates signal transduction through the ITAM in the cytoplasmic domain. The presence of multiple serine and threonine residues within this motif suggests the potential for serine/threonine phosphorylation in modulating signaling events. Single-site mutational analysis of these residues in RBL-2H3 cells indicates that each may contribute to net FcRγ-mediated signaling, and mass spectrometry of WT human FcRγ from receptor-stimulated cells shows consistent preferential phosphorylation of the serine residue at position 51. Immunoblot analysis, mass spectrometry, and mutational analyses showed that phosphorylation of serine 51 in the 7-residue spacer between the 2 YxxL sequences regulates FcRγ signaling by inhibiting tyrosine phosphorylation at the membrane proximal Y47 position of the ITAM, but not phosphorylation at position Y58. This inhibition results in reduced Syk recruitment and activation. With in vitro kinase assays, PKC-δ and PKA show preferential phosphorylation of S51. Serine/threonine phosphorylation of the FcRγ ITAM, which functions as an integrator of multiple signaling elements, may explain in part the contribution of variants in PKC-δ and other PKC isoforms to some autoimmune phenotypes.

Anti-CD48 Monoclonal Antibody Attenuates Experimental Autoimmune Encephalomyelitis by Limiting the Number of Pathogenic CD4+ T Cells

CD48 (SLAMF2) is an adhesion and costimulatory molecule constitutively expressed on hematopoietic cells. Polymorphisms in CD48 have been linked to susceptibility to multiple sclerosis (MS), and altered expression of the structurally related protein CD58 (LFA-3) is associated with disease remission in MS. We examined CD48 expression and function in experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. We found that a subpopulation of CD4⁺ T cells highly upregulated CD48 expression during EAE and were enriched for pathogenic CD4⁺ T cells. These CD48⁺⁺CD4⁺ T cells were predominantly CD44⁺ and Ki67⁺, included producers of IL-17A, GM-CSF, and IFN-γ, and were most of the CD4⁺ T cells in the CNS. Administration of anti-CD48 mAb during EAE attenuated clinical disease, limited accumulation of lymphocytes in the CNS, and reduced the number of pathogenic cytokine-secreting CD4⁺ T cells in the spleen at early time points. These therapeutic effects required CD48 expression on CD4⁺ T cells but not on APCs. Additionally, the effects of anti-CD48 were partially dependent on FcγRs, as anti-CD48 did not ameliorate EAE or reduce the number of cytokine-producing effector CD4⁺ T cells in Fcεr1γ^-/- mice or in wild-type mice receiving anti-CD16/CD32 mAb. Our data suggest that anti-CD48 mAb exerts its therapeutic effects by both limiting CD4⁺ T cell proliferation and preferentially eliminating pathogenic CD48⁺⁺CD4⁺ T cells during EAE. Our findings indicate that high CD48 expression is a feature of pathogenic CD4⁺ T cells during EAE and point to CD48 as a potential target for immunotherapy.

The gamma chain subunit of Fc receptors is required for alpha-synuclein-induced pro-inflammatory signaling in microglia

Background

The protein alpha-synuclein (α-SYN), which is found in the Lewy bodies of dopamine-producing (DA) neurons in the substantia nigra (SN), has an important role in the pathogenesis of Parkinson’s disease (PD). Previous studies have shown that neuroinflammation plays a key role in PD pathogenesis. In an AAV-synuclein mouse model of PD, we have found that over-abundance of α-SYN triggers the expression of NF-κB p65, and leads to microglial activation and DA neurodegeneration. We also have observed that Fcγ receptors (FcγR), proteins present on the surface of microglia that bind immunoglobulin G (IgG) and other ligands, are key modulators of α-SYN-induced neurodegeneration.

Methods

In order to study the role of FcγRs in the interactions of α-SYN and microglia, we treated the primary microglial cultures from wild-type (WT) and FcγR^−/− mice with aggregated human α-SYN in vitro.

Results

Using immunocytochemistry, we found that α-SYN was taken up by both WT and FcγR^−/− microglia, however, their patterns of internalization were different, with aggregation in autophagosomes in WT cells and more diffuse localization in FcγR^−/− microglia. In WT microglia, α-SYN induced the nuclear accumulation of NF-κB p65 protein and downstream chemokine expression while in FcγR^−/− mouse microglia, α-SYN failed to trigger the enhancement of nuclear NF-κB p65, and the pro-inflammatory signaling was reduced.

Conclusions

Our results suggest that α-SYN can interact directly with microglia and can be internalized and trafficked to autophagosomes. FcγRs mediate this interaction, and in the absence of the gamma chain, there is altered intracellular trafficking and attenuation of pro-inflammatory NF-κB signaling. Therefore, blocking either FcγR signaling or downstream NF-κB activation may be viable therapeutic strategies in PD.

IgG glycan hydrolysis by EndoS inhibits experimental autoimmune encephalomyelitis

Studies in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis, have shown that B cells markedly influence the course of the disease, although whether their effects are protective or pathological is a matter of debate. EndoS hydrolysis of the IgG glycan has profound effects on IgG effector functions, such as complement activation and Fc receptor binding, suggesting that the enzyme could be used as an immunomodulatory therapeutic agent against IgG-mediated diseases. We demonstrate here that EndoS has a protective effect in myelin oligodendrocyte glycoprotein peptide amino acid 35–55 (MOG_35-55)-induced EAE, a chronic neuroinflammatory demyelinating disorder of the central nervous system (CNS) in which humoral immune responses are thought to play only a minor role. EndoS treatment in chronic MOG_35-55-EAE did not impair encephalitogenic T cell priming and recruitment into the CNS of mice, consistent with a primary role of EndoS in controlling IgG effector functions. In contrast, reduced EAE severity coincided with poor serum complement activation and deposition within the spinal cord, suggesting that EndoS treatment impairs B cell effector function. These results identify EndoS as a potential therapeutic agent against antibody-mediated CNS autoimmune disorders.

Immune complexes and late complement proteins trigger activation of Syk tyrosine kinase in human CD4(+) T cells

In systemic lupus erythematosus (SLE), the autoantibodies that form immune complexes (ICs) trigger activation of the complement system. This results in the formation of membrane attack complex (MAC) on cell membrane and the soluble terminal complement complex (TCC). Hyperactive T cell responses are hallmark of SLE pathogenesis. How complement activation influences the T cell responses in SLE is not fully understood. We observed that aggregated human γ-globulin (AHG) bound to a subset of CD4(+) T cells in peripheral blood mononuclear cells and this population increased in the SLE patients. Human naive CD4(+) T cells, when treated with purified ICs and TCC, triggered recruitment of the FcRγ chain with the membrane receptor and co-localized with phosphorylated Syk. These events were also associated with aggregation of membrane rafts. Thus, results presented suggest a role for ICs and complement in the activation of Syk in CD4(+) T cells. Thus, we propose that the shift in signalling from ζ-chain-ZAP70 to FcRγ chain-Syk observed in T cells of SLE patients is triggered by ICs and complement. These results demonstrate a link among ICs, complement activation and phosphorylation of Syk in CD4(+) T cells.

Fcγ receptors are required for NF-κB signaling, microglial activation and dopaminergic neurodegeneration in an AAV-synuclein mouse model of Parkinson's disease

Overexpression of alpha-synuclein (α-SYN), a protein which plays an important role in the pathogenesis of Parkinson's disease (PD), triggers microglial activation and adaptive immune responses, and leads to neurodegeneration of dopaminergic (DA) neurons. We hypothesized a link between the humoral adaptive immune response and microglial activation in α-SYN induced neurodegeneration. To test this hypothesis, we employed adeno-associated virus serotype 2 (AAV2) to selectively over-express human α-SYN in the substantia nigra (SN) of wild-type mice and FcγR-/- mice, which lack high-affinity receptors for IgG. We found that in wild-type mice, α-SYN induced the expression of NF-κB p65 and pro-inflammatory molecules. In FcγR-/- mice, NF-κB activation was blocked and pro-inflammatory signaling was reduced. Microglial activation was examined using immunohistochemistry for gp91PHOX. At four weeks, microglia were strongly activated in wild-type mice, while microglial activation was attenuated in FcγR-/- mice. Dopaminergic neurodegeneration was examined using immunohistochemistry for tyrosine hydroxylase (TH) and unbiased stereology. α-SYN overexpression led to the appearance of dysmorphic neurites, and a loss of DA neurons in the SN in wild-type animals, while FcγR-/- mice did not exhibit neuritic change and were protected from α-SYN-induced neurodegeneration 24 weeks after injection. Our results suggest that the humoral adaptive immune response triggered by excess α-SYN plays a causative role in microglial activation through IgG-FcγR interaction. This involves NF-κB signaling, and leads to DA neurodegeneration. Therefore, blocking either FcγR signaling or specific intracellular signal transduction events downstream of FcγR-IgG interaction, such as NF-κB activation, may be viable therapeutic strategies in PD.

Inhibition of Experimental Autoimmune Encephalomyelitis in Human C-Reactive Protein Transgenic Mice Is FcγRIIB Dependent

We showed earlier that experimental autoimmune encephalomyelitis (EAE) in human C-reactive protein (CRP) transgenic mice (CRPtg) has delayed onset and reduced severity compared to wild-type mice. Since human CRP is known to engage Fc receptors and Fc receptors are known to play a role in EAE in the mouse, we sought to determine if FcγRI, FcγRIIb, or FcγRIII was needed to manifest human CRP-mediated protection of CRPtg. We report here that in CRPtg lacking either of the two activating receptors, FcγRI and FcγRIII, the beneficial effects of human CRP are still observed. In contrast, if CRPtg lack expression of the inhibitory receptor FcγRIIB, then the beneficial effect of human CRP is abrogated. Also, subcutaneous administration of purified human CRP stalled progression of ongoing EAE in wild-type mice, but similar treatment failed to impede EAE progression in mice lacking FcγRIIB. The results reveal that a CRP → FcγRIIB axis is responsible for protection against EAE in the CRPtg model.

ITAM signaling in dendritic cells controls T helper cell priming by regulating MHC class II recycling

Immature dendritic cells (DCs) specialize in antigen capture and maintain a highly dynamic pool of intracellular major histocompatibility complex class II (MHCII) that continuously recycles from peptide loading compartments to the plasma membrane and back again. This process facilitates sampling of environmental antigens for presentation to T helper cells. Here, we show that a signaling pathway mediated by the DC immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptors (DAP12 and FcRγ) and Vav family guanine nucleotide exchange factors controls the half-life of surface peptide-MHCII (pMHCII) complexes and is critical for CD4 T-cell triggering in vitro. Strikingly, mice with disrupted DC ITAMs show defective T helper cell priming in vivo and are protected from experimental autoimmune encephalitis. Mechanistically, we show that deficiency in ITAM signaling results in increased pMHCII internalization, impaired recycling, and an accumulation of ubiquitinated MHCII species that are prematurely degraded in lysosomes. We propose a novel mechanism for control of T helper cell priming.

HIV Coreceptors and Their Roles in Leukocyte Trafficking During Neuroinflammatory Diseases

Due to the increasing resistance of HIV-1 to antiretroviral therapies, there has been much emphasis on the discovery and development of alternative therapeutics for HIV-1-infected individuals. The chemokine receptors CXCR4 (Bleul et al. 1996a; Feng et al. 1996; Nagasawa et al. 1996; Oberlin et al. 1996) and CCR5 (Alkhatib et al. 1996; Deng et al. 1996; Dragic et al. 1996) were identified as target molecules from the time their role as coreceptors for HIV-1 entry into leukocytes was first discovered 10 years ago. Initial studies focused on the use of the chemokine ligands, or altered derivatives, of CXCR4 and CCR5 to prevent the entrance of HIV-1 into immune cells (Schols 2006). While these studies showed some initial promise, there was evidence of significant caveats to their use, including selection of alternative coreceptor utilizing strains (Marechal et al. 1999; Mosier et al. 1999) and the potential to cause inflammatory side effects. These data prompted the development and study of small molecule inhibitors of CXCR4 and CCR5, which have also been used to examine the roles of these molecules in a variety of inflammatory and infectious diseases.

Correlation of specialized CD16(+) gammadelta T cells with disease course and severity in multiple sclerosis

gammadelta T cells may be important innate immune system contributors to the immunopathogenesis of multiple sclerosis (MS), though the mechanisms are not yet fully understood. CD16 is a low affinity Fcgamma receptor, an activation receptor for gammadelta T cells, and a mediator of cytotoxicity. In this study, we found that the percentage of CD16(+) gammadelta T cells is elevated in MS patients compared with healthy controls. The increase is especially pronounced in patients with a progressive course of the disease, and the extent of this elevation shows a positive correlation with the time of disease progression and severity. In vitro cultured gammadelta T cells can be shown to upregulate the expression of CD16 in response to inflammatory cytokines such as IL-2 and -15, that have been shown to be elevated in progressive disease. These results suggest that CD16 expressing gammadelta T cells are somehow involved in the process of disease progression. Understanding more about these cells and their particular function in progressive vs. non-progressive disease could provide important clues to the mechanism of immune-mediated MS disease progression.

Autoantibody-mediated demyelination depends on complement activation but not activatory Fc-receptors

The precise mechanisms leading to CNS inflammation and myelin destruction in both multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) remain the subject of intense debate. In both MS and EAE, autoantibodies (autoAbs) are thought to be involved in tissue destruction through recruiting Fc receptor (FcR)-bearing cells or direct cytotoxic effects through the activation of the complement pathway. Whereas intrathecal immunoglobulin (Ig) production and Ig deposition in inflammatory lesions is a hallmark of MS, mice deficient in B cells and Igs develop severe EAE. Paradoxically, mice of the same genetic background but deficient in FcRgamma are EAE-resistant. We found that the functional expression of FcRgamma on systemic accessory cells, but not CNS-resident cells, appears to be vital for the development of CNS inflammation, independent of antigen-presenting cell function or Ab involvement. On the other hand, we found that the injection of antimyelin oligodendrocyte glycoprotein-Abs drastically worsens disease severity, inflammation, and demyelination. Using FcRgamma(-/-) and C1q(-/-) mice, we could definitively establish that the demyelinating capacity of such autoAb in vivo relies entirely on complement activation and is FcR-independent.