A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes' role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies' role in neural circuit function, complex behaviors, and disease

Structural characterization of anti-inflammatory Immunoglobulin G Fc proteins

Immunoglobulin G (IgG) is a central mediator of host defense due to its ability to recognize and eliminate pathogens. The recognition and effector responses are encoded on distinct regions of IgGs. The diversity of the antigen recognition Fab domains accounts for IgG’s ability to bind with high specificity to essentially any antigen. Recent studies have indicated that the Fc effector domain also displays considerable heterogeneity, accounting for its complex effector functions of inflammation, modulation and immune suppression. Therapeutic anti-tumor antibodies, for example, require the pro-inflammatory properties of the IgG Fc to eliminate tumor cells, while the anti-inflammatory activity of Intravenous Immunoglobulin G (IVIG) requires specific Fc glycans for activity. In particular, the anti-inflammatory activity of IVIG is ascribed to a small population of IgGs in which the Asn297-linked complex N-glycans attached to each Fc C_H2 domain include terminal α2,6-linked sialic acids. We used chemoenzymatic glycoengineering to prepare fully di-sialylated IgG Fc and solved its crystal structure. Comparison of the structures of asialylated Fc, sialylated Fc, and F241A Fc, a mutant that displays increased glycan sialylation, suggests that increased conformational flexibility of the C_H2 domain is associated with the switch from pro- to anti-inflammatory activity of the Fc

Human IgG/FcγR Interactions Are Modulated by Streptococcal IgG Glycan Hydrolysis

BACKGROUND: The human pathogen Streptococcus pyogenes produces an endoglycosidase, EndoS that hydrolyzes the chitobiose core of the asparagine-linked glycan on the heavy chain of human IgG. IgG-binding to Fc gamma receptors (FcgammaR) on leukocytes triggers effector functions including phagocytosis, oxidative burst and the release of inflammatory mediators. The interactions between FcgammaR and the Fc domain of IgG depend on the IgG glycosylation state. METHODOLOGY/PRINCIPAL FINDINGS: Here we show for the first time that EndoS hydrolyzes the heavy chain glycan of all four human IgG subclasses (IgG1-4), in purified form and in a plasma environment. An inactive form of EndoS, obtained by site-directed mutagenesis, binds IgG with high affinity, in contrast to wild type EndoS that only transiently interacts with IgG, as shown by Slot-blotting and surface plasmon resonance technology. Furthermore, EndoS hydrolysis of the IgG glycan influences the binding of IgG to immobilized soluble FcgammaR and to an erythroleukemic cell line, K562, expressing FcgammaRIIa. Incubation of whole blood with EndoS results in a dramatic decrease of IgG binding to activated monocytes as analyzed by flow cytometry. Moreover, the IgG bound to K562 cells dissociates when cells are treated with EndoS. Likewise, IgG bound to immobilized FcgammaRIIa and subsequently treated with EndoS, dissociates from the receptor as analyzed by surface plasmon resonance and Western blot. CONCLUSIONS/SIGNIFICANCE: We provide novel information about bacterial enzymatic modulation of the IgG/FcgammaR interaction that emphasizes the importance of glycosylation for antibody effector functions. Moreover, EndoS could be used as a biochemical tool for specific IgG N-glycan hydrolysis and IgG purification/detection, or as a potential immunosuppressing agent for treatment of antibody-mediated pathological processes

Activating Fc γ receptors contribute to the antitumor activities of immunoregulatory receptor-targeting antibodies

Fc γ receptor (FcγR) coengagement can facilitate antibody-mediated receptor activation in target cells. In particular, agonistic antibodies that target tumor necrosis factor receptor (TNFR) family members have shown dependence on expression of the inhibitory FcγR, FcγRIIB. It remains unclear if engagement of FcγRIIB also extends to the activities of antibodies targeting immunoregulatory TNFRs expressed by T cells. We have explored the requirement for activating and inhibitory FcγRs for the antitumor effects of antibodies targeting the TNFR glucocorticoid-induced TNFR-related protein (GITR; TNFRSF18; CD357) expressed on activated and regulatory T cells (T reg cells). We found that although FcγRIIB was dispensable for the in vivo efficacy of anti-GITR antibodies, in contrast, activating FcγRs were essential. Surprisingly, the dependence on activating FcγRs extended to an antibody targeting the non-TNFR receptor CTLA-4 (CD152) that acts as a negative regulator of T cell immunity. We define a common mechanism that correlated with tumor efficacy, whereby antibodies that coengaged activating FcγRs expressed by tumor-associated leukocytes facilitated the selective elimination of intratumoral T cell populations, particularly T reg cells. These findings may have broad implications for antibody engineering efforts aimed at enhancing the therapeutic activity of immunomodulatory antibodies

Polymeric human Fc-fusion proteins with modified effector functions

The success of Fc-fusion bio-therapeutics has spurred the development of other Fc-fusion products for treating and/or vaccinating against a range of diseases. We describe a method to modulate their function by converting them into well-defined stable polymers. This strategy resulted in cylindrical hexameric structures revealed by tapping mode atomic force microscopy (AFM). Polymeric Fc-fusions were significantly less immunogenic than their dimeric or monomeric counterparts, a result partly owing to their reduced ability to interact with critical Fc-receptors. However, in the absence of the fusion partner, polymeric IgG1-Fc molecules were capable of binding selectively to FcγRs, with significantly increased affinity owing to their increased valency, suggesting that these reagents may prove of immediate utility in the development of well-defined replacements for intravenous immunoglobulin (IVIG) therapy. Overall, these findings establish an effective IgG Fc-fusion based polymeric platform with which the therapeutic and vaccination applications of Fc-fusion immune-complexes can now be explored

The state of the art: immune-mediated mechanisms of monoclonal antibodies in cancer therapy

A number of antibody products have now become accepted as effective anti-cancer therapies. Despite being mainly designed to act by inhibiting functional tumour antigens, there is increasing evidence that Fc-mediated engagement of the immune system is an important contributor to the efficacy of several of these therapies. The optimisation of this engagement offers the potential not only to augment efficacy against existing targets, but also to exploit non-functional tumour antigens. Antibodies that achieve efficacy wholly or predominantly through Fc-mediated mechanisms, represent rich opportunities for future therapeutics in oncology. This mini review summarises some of the key challenges, which need to be addressed to select the most effective molecules. These include the identification of optimal antibody characteristics and improvement of the drug discovery process, in particular, the relevance and predictive power of existing in vitro and in vivo screening methods. Advances in our understanding of tumour immunobiology and successful application of technologies designed to enhance immune system engagement will further aid this process

CD137 (4-1BB) stimulation leads to metabolic and functional reprogramming of human monocytes/macrophages enhancing their tumoricidal activity

Abstract Immunotherapies have heralded a new era in the cancer treatment. In addition to checkpoint inhibitors, agonistic antibodies against co-stimulatory immune receptors hold the potential to invoke efficient antitumor immunity. Targeting CD137 has gained momentum based on its ability to drive NK- and T-cell-based responses. CD137-engaging mAbs have already entered clinical trials for different types of tumors showing promising results. Despite the efforts to translate CD137-mediated immunotherapy into clinical practice, little remains known regarding the role of CD137 in human monocytes/macrophages. We found CD137 being expressed on monocytes of healthy controls and at even higher levels in patients with multiple myeloma or CLL. CD137HI(GH) monocytes displayed a distinct phenotypic, transcriptomic, and metabolic profile. They possessed an increased phagocytic capacity enabling superior antibody-dependent phagocytosis (ADPC) of multiple myeloma and lymphoma cells that were treated with anti-CD38 or anti-CD20 mAbs. Triggering CD137 promoted both metabolic and tumoricidal activity in an extracellular signal-regulated kinase (ERK)-dependent fashion. In addition, we observed a phenotypic, transcriptomic, and functional skewing towards a M1-like phenotype. Overall, we introduce CD137 as a positive immune checkpoint on human monocytes/macrophages, which can have therapeutic implications especially in view of synergistic effects when combining CD137 agonists with tumor-targeting antibodies

Non-structural protein 1 of avian influenza A viruses differentially inhibit NF-κB promoter activation

<p>Abstract</p> <p>Background</p> <p>Influenza virus infection activates NF-κB and is a general prerequisite for a productive influenza virus infection. On the other hand, non-structural protein 1 (NS1) suppresses this viral activated NF-κB, presumably to prevent expression of NF-κB mediated anti-viral response. NS1 proteins of influenza A viruses are divided into two groups, known as allele A and allele B. The possible functional relevance of this NS1 division to viral pathogenicity is lacking.</p> <p>Findings</p> <p>The ability of NS1 protein from two avian influenza subtypes, H6N8 and H4N6, to inhibit NF-κB promoter activation was assessed. Further, efforts were made to characterize the genetic basis of this inhibition. We found that allele A NS1 proteins of H6N8 and H4N6 are significantly better in preventing dsRNA induced NF-κB promoter activation compared to allele B of corresponding subtypes, in a species independent manner. Furthermore, the ability to suppress NF-κB promoter activation was mapped to the effector domain while the RNA binding domain alone was unable to suppress this activation. Chimeric NS1 proteins containing either RNA binding domain of allele A and effector domain of allele B or vice versa, were equally potent in preventing NF-κB promoter activation compared to their wt. NS1 protein of allele A and B from both subtypes expressed efficiently as detected by Western blotting and predominantly localized in the nucleus in both A549 and MiLu cells as shown by <it>in situ </it>PLA.</p> <p>Conclusions</p> <p>Here, we present another aspect of NS1 protein in inhibiting dsRNA induced NF-κB activation in an allele dependent manner. This suggests a possible correlation with the virus's pathogenic potential.</p