In vitro, classical complement activation differs by disease severity and between SARS-CoV-2 antigens

Antibodies specific for the spike glycoprotein (S) and nucleocapsid (N) SARS-CoV-2 proteins are typically present during severe COVID-19, and induced to S after vaccination. The binding of viral antigens by antibody can initiate the classical complement pathway. Since complement could play pathological or protective roles at distinct times during SARS-CoV-2 infection we determined levels of antibody-dependent complement activation along the complement cascade. Here, we used an ELISA assay to assess complement protein binding (C1q) and the deposition of C4b, C3b, and C5b to S and N antigens in the presence of anti-SARS-CoV-2 antibodies from different test groups: non-infected, single and double vaccinees, non-hospitalised convalescent (NHC) COVID-19 patients and convalescent hospitalised (ITU-CONV) COVID-19 patients. C1q binding correlates strongly with antibody responses, especially IgG1 levels. However, detection of downstream complement components, C4b, C3b and C5b shows some variability associated with the antigen and subjects studied. In the ITU-CONV, detection of C3b-C5b to S was observed consistently, but this was not the case in the NHC group. This is in contrast to responses to N, where median levels of complement deposition did not differ between the NHC and ITU-CONV groups. Moreover, for S but not N, downstream complement components were only detected in sera with higher IgG1 levels. Therefore, the classical pathway is activated by antibodies to multiple SARS-CoV-2 antigens, but the downstream effects of this activation may differ depending on the specific antigen targeted and the disease status of the subject.

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Continual Low-Level Activation of the Classical Complement Pathway

Journal of Experimental Medicine ◽

10.1084/jem.194.6.747 ◽

2001 ◽

Vol 194 (6) ◽

pp. 747-756 ◽

Cited By ~ 43

Author(s):

Anthony P. Manderson ◽

Matthew C. Pickering ◽

Marina Botto ◽

Mark J. Walport ◽

Christopher R. Parish

Keyword(s):

Complement Activation ◽

Alternative Pathway ◽

Blood Stasis ◽

Classical Pathway ◽

Complement Pathway ◽

Complement Components ◽

Classical Complement Pathway ◽

Alternative Complement ◽

Classical Complement

There is evidence that the classical complement pathway may be activated via a “C1-tickover” mechanism, analogous to the C3-tickover of the alternative pathway. We have quantitated and characterized this pathway of complement activation. Analysis of freshly collected mouse and human plasma revealed that spontaneous C3 activation rapidly occurred with the generation of C3 fragments in the plasma. By the use of complement- and Ig-deficient mice it was found that C1q, C4, C2, and plasma Ig were all required for this spontaneous C3 activation, with the alternative complement pathway further amplifying C3 fragment generation. Study of plasma from a human with C1q deficiency before and after therapeutic C1q infusion confirmed the existence of a similar pathway for complement activation in humans. Elevated levels of plasma C3 were detected in mice deficient in complement components required for activation of either the classical or alternative complement pathways, supporting the hypothesis that there is continuous complement activation and C3 consumption through both these pathways in vivo. Blood stasis was found to stimulate C3 activation by classical pathway tick-over. This antigen-independent mechanism for classical pathway activation may augment activation of the complement system at sites of inflammation and infarction.

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Classical Pathway of Complement Activation in ITP: Inhibition By TNT003, a Novel C1s Inhibitor

Blood ◽

10.1182/blood.v124.21.1453.1453 ◽

2014 ◽

Vol 124 (21) ◽

pp. 1453-1453

Author(s):

Ellinor I.B. Peerschke ◽

Sandip Panicker ◽

Alexa M. Sughroue ◽

James B. Bussel

Keyword(s):

Complement Activation ◽

In Vitro Assay ◽

Classical Pathway ◽

Complement Pathway ◽

Research Support ◽

Vitro Assay ◽

Classical Complement Pathway ◽

Pathway Activation ◽

Classical Complement

Abstract Background: Immune thrombocytopenic purpura (ITP) is an autoimmune disorder in which antiplatelet antibodies mediate accelerated platelet clearance from circulation and also inhibit platelet production, resulting in thrombocytopenia. Activation of the classical pathway (CP) of complement is associated with a variety of immune disorders involving the presence of autoantibodies. The role of the complement system in ITP is poorly understood. Methods: Plasma samples (0.32% sodium citrate) from patients with chronic ITP (n=55) were evaluated for their ability to activate the CP of complement. The 55 patients consisted primarily of adult but also of pediatric patients with ITP, undergoing various treatment regimens. The most common included IVIG, rituximab, and especially thrombopoietic agents (eltrombopag, romiplostim). Almost all patients included in the analysis had chronic ITP, defined as ITP lasting > 12 months. The complement activating capacity (CAC) of patient plasma was evaluated with a previously described in vitro assay ( Peerschke et al., Brit J Haematol, 2009) that measures complement activation on immobilized, fixed heterologous platelets using an ELISA approach with monoclonal antibodies to C1q, C4d, iC3b, and C5b-9. CAC represents assay optical density readings normalized to reference normal plasma pool. A CAC of >1.5 was considered indicative of enhanced complement activation, based on reference ranges established for plasma from healthy volunteers. Patient CAC values were correlated with platelet count. The ability of TNT003 to block in vitro complement activation was assessed relative to an isotype matched control. TNT003 is a mouse monoclonal antibody (IgG2a) that targets the CP-serine protease C1s. Results: A statistically significant (p=0.042) inverse correlation was noted between C4d deposition and platelet count in the 55 ITP patient samples tested. Heightened classical complement pathway activation was demonstrated in 7 of 55 patients (~13%) with ITP as evidenced by increased C4d deposition. 6 of the 7 patients with increased C4d deposition had platelet counts <100k/mcL, and 5 patients had platelet counts <50K/mcL. There was a non-significant trend for higher C4d levels on platelets and lower AIPF (absolute immature platelet fraction, equivalent to platelet reticulocytes). TNT003 (100 mcg/ml) inhibited C4 activation by 44 + 43% in ITP plasma in vitro. Inhibition of downstream complement activation, iC3b and C5b-9 deposition, was 72% + 17 % and 82% + 14% (mean + S.D.), respectively. Similar results were obtained using 10 mM EDTA, a known inhibitor of complement activation. Conclusions: The heterogeneity of patient responses to different treatment modalities in ITP support the concept of different immune mechanisms contributing to thrombocytopenia. Our data demonstrate classical complement pathway activation in a subgroup of patients with ITP, and further present the first evidence of CP complement inhibition by a novel C1s inhibitor in this setting. Failure to completely block C4 activation in ITP plasma in vitro by either of TNT003 or EDTA, suggests the presence of preformed, circulating C4d containing complement complexes in patient plasma. The ability of TNT003 to more completely inhibit C3 activation and C5b-9 assembly downstream of C4 in the in vitro assay system is consistent with direct activation and inhibition of complement at the platelet surface. Thus, TNT003 may mitigate enhanced platelet clearance by RES via inhibition of complement mediated platelet opsonization by C3b and platelet lysis by C5b-9. Further studies are required to evaluate the impact of TNT003 on thrombocytopenia in ITP. Disclosures Peerschke: True North Therapeutics: Research Support Other. Panicker:True North Therapeutics: Employment. Bussel:True North Therapeutics: Research Support Other.

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Extracellular Vesicle-Induced Classical Complement Activation Leads to Retinal Endothelial Cell Damage via MAC Deposition

International Journal of Molecular Sciences ◽

10.3390/ijms21051693 ◽

2020 ◽

Vol 21 (5) ◽

pp. 1693 ◽

Cited By ~ 2

Author(s):

Chao Huang ◽

Kiera P. Fisher ◽

Sandra S. Hammer ◽

Julia V. Busik

Keyword(s):

Extracellular Vesicles ◽

Complement Activation ◽

Cell Damage ◽

Membrane Attack Complex ◽

Extracellular Vesicle ◽

Rat Plasma ◽

Classical Complement Pathway ◽

Endothelium Damage ◽

Classical Complement

Several studies have suggested that there is a link between membrane attack complex (MAC) deposition in the retina and the progression of diabetic retinopathy (DR). Our recent investigation demonstrated that circulating IgG-laden extracellular vesicles contribute to an increase in retinal vascular permeability in DR through activation of the complement system. However, the mechanism through which extracellular vesicle-induced complement activation contributes to retinal vascular cytolytic damage in DR is not well understood. In this study, we demonstrate that IgG-laden extracellular vesicles in rat plasma activate the classical complement pathway, and in vitro Streptozotocin (STZ)-induced rat diabetic plasma results in MAC deposition and cytolytic damage in human retinal endothelial cells (HRECs). Moreover, removal of the plasma extracellular vesicles reduced the MAC deposition and abrogated cytolytic damage seen in HRECs. Together, the results of this study demonstrate that complement activation by IgG-laden extracellular vesicles in plasma could lead to MAC deposition and contribute to endothelium damage and progression of DR.

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Evidence of Classical Complement Pathway Involvement in a Subset of Patients with Warm Autoimmune Hemolytic Anemia

Blood ◽

10.1182/blood-2021-153866 ◽

2021 ◽

Vol 138 (Supplement 1) ◽

pp. 2001-2001

Author(s):

Jeffrey Teigler ◽

Julian Low ◽

Shawn Rose ◽

Ellen Cahir-Mcfarland ◽

Ted Yednock ◽

...

Keyword(s):

Board Of Directors ◽

Classical Pathway ◽

Complement Pathway ◽

Publicly Traded ◽

Inhibitory Antibody ◽

Advisory Committees ◽

Classical Complement Pathway ◽

Complement Deposition ◽

Classical Complement

Abstract Introduction: Autoimmune Hemolytic Anemia (AIHA) is caused by autoantibodies that react with red blood cells (RBCs) resulting in predominantly extravascular hemolysis in an FcR and/or complement-dependent manner. In warm AIHA (wAIHA), autoantibodies are generally of the IgG isotype, while in cold agglutinin disease (CAD) they are predominantly of the IgM isotype. It is well established that the classical complement cascade is critical for the pathogenesis of CAD based on therapeutic clinical studies. Published data also suggest that complement activation plays a role in wAIHA, although it is not clear which patients would most benefit from complement-based therapy. To help address this question, we utilized an assay that measures the ability of autoantibodies in patient sera to induce complement deposition on the surface of donor RBCs (based on Meulenbroek, et al., 2015). Methods: Sera were collected retrospectively from 12 wAIHA patients whose direct antiglobulin tests (DAT) were either IgG+/C3+ or IgG+/C3-. Sera retrospectively collected from two CAD patients were used as positive controls. Individual patient sera were examined in the in vitro complement deposition assay using RBCs from type O+ healthy donors. RBCs and sera were incubated at 37 oC in the presence of either EDTA or an inhibitory antibody against C1q as inhibitors of the classical pathway. RBCs were then stained and processed by flow cytometry to determine the level of C4 deposition. Results: Sera from both CAD patients deposited C4 on the surface of ~70% of healthy human RBCs in vitro. Four out of twelve (33%) sera from wAIHA patients displayed this activity, and all four of these patients were identified as IgG+/C3+ on DAT. Complement deposition ranged from ~10-60% of the RBCs in wAIHA, suggesting heterogeneity in antibody activity for complement deposition in sera from wAIHA patients. Addition of EDTA or an inhibitory antibody against C1q fully blocked deposition of C4 on RBCs by wAIHA sera, indicating dependence of the classical complement pathway. These results indicate differences in the frequency of classical pathway involvement in CAD versus wAIHA and may help identify a subset of wAIHA patients most likely to respond to anti-C1q therapy. Conclusions: The hypothesis of classical complement cascade involvement in wAIHA disease in a subset of patients is supported by our results. Critically, complement deposition on the surface of cells by anti-C1q prevented the deposition of a downstream complement marker, C4. Inhibition of C1q has been shown to block activation of all downstream classical complement components, including C3b and C4b involved in extravascular hemolysis and C5b involved in direct cell lysis. The therapeutic potential of blocking classical complement pathway activity in wAIHA is currently being evaluated in an ongoing Phase 2 interventional trial (NCT04691570) assessing efficacy of an anti-C1q drug candidate in wAIHA patients, focusing on those with evidence of classical complement pathway activity. Disclosures Teigler: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Low: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Rose: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Cahir-Mcfarland: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Yednock: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Kroon: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Keswani: Annexon Inc: Current Employment, Current equity holder in publicly-traded company. Barcellini: Novartis: Honoraria; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Agios: Honoraria, Research Funding; Alexion Pharmaceuticals: Honoraria; Incyte: Membership on an entity's Board of Directors or advisory committees.

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Activation of the Classical Pathway of Complement by Resting and Shear Stress-Stimulated Human Endothelial Cells.

Blood ◽

10.1182/blood.v106.11.2664.2664 ◽

2005 ◽

Vol 106 (11) ◽

pp. 2664-2664

Author(s):

Wei Yin ◽

Babette Weksler ◽

David Varon ◽

Naphtali Savion ◽

Berhane Ghebrehiwet ◽

...

Keyword(s):

Endothelial Cells ◽

Shear Stress ◽

Complement Activation ◽

Binding Domain ◽

Complement Pathway ◽

Complement Components ◽

Classical Complement Pathway ◽

Pathway Activation ◽

Static Conditions ◽

Classical Complement

Abstract Complement activation is associated with a variety of inflammatory conditions including atherosclerosis, but the mechanism of complement activation in these settings is poorly understood. Endothelial cells (EC) play an important role in vascular pathology and express a variety of complement receptors, including gC1qR/p33, recognizing the globular domain of the complement component C1q. In preliminary studies, purified recombinant gC1qR/p33 was found to support C1q-dependent C4 activation in vitro, comprising 19.5% ± 8.3% (mean ± S.D., n=5) of that produced by aggregated IgG. In contrast, a truncated form of gC1qR/p33, lacking the C1q binding domain, failed to support C4 activation. Additional studies were performed with immortalized bone marrow microvascular EC to investigate classical complement pathway activation and deposition. EC were exposed to anticoagulated (0.32 % sodium citrate) human plasma, diluted (1/10) in 0.01 M HEPES buffered modified Tyrode’s solution, pH 7.5, containing 2 mM Mg Cl2 and 1 mM CaCl2, for 60 min, 37°C. A solid phase ELISA approach was used to detect EC-associated C1q and C4 activation (C4d). Statistically significant deposition of C4d (0.72 ± 0.3. OD units (ODU), n=4)(p=0.04) and C1q (0.57 ± 0.19. ODU, n=4) (p=0.002) was observed on EC that had been immobilized on poly-L-lysine coated microtiter wells. Consistent with classical complement pathway activation, C4d deposition remained at baseline (0.23 ± 0.13, ODU, n=4) in the presence of 10 mM EDTA, but C1q deposition was unaffected. Moreover, no significant C1q or C4d deposition occurred when endothelial cells were exposed to C1q depleted serum. Similar studies were performed using EC grown to confluence on Type I collagen to examine the effect of shear stress (12 dynes/cm2 for 1 hour in a cone-and-plate shearing device), simulating flow conditions in coronary arteries, on classical complement pathway activation and deposition. Compared to static conditions, shear stress resulted in an approximately 50% increase in C1q and C4d deposition on EC. This was accompanied by an approximately 2-fold increase in EC binding of a monoclonal antibody, 60.11, recognizing the N-terminal C1q binding domain of gC1qR/p33. Taken together, these data present evidence for a potential paradigm shift, illustrating immune complex independent classical complement pathway activation by gC1qR/p33, and deposition of activated classical complement components on EC. The generation and deposition of active complement components on EC is likely to contribute directly to vascular inflammation and atherosclerotic changes.

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Adenovirus Activates Complement by Distinctly Different Mechanisms In Vitro and In Vivo: Indirect Complement Activation by Virions In Vivo

Journal of Virology ◽

10.1128/jvi.00082-09 ◽

2009 ◽

Vol 83 (11) ◽

pp. 5648-5658 ◽

Cited By ~ 50

Author(s):

Jie Tian ◽

Zhili Xu ◽

Jeffrey S. Smith ◽

Sean E. Hofherr ◽

Michael A. Barry ◽

...

Keyword(s):

Gene Therapy ◽

Complement Activation ◽

Direct Interaction ◽

Reticuloendothelial System ◽

In Vitro Studies ◽

Classical Pathway ◽

Complement Protein ◽

The Complement System

ABSTRACT Understanding innate immunity is key to improving the safety of adenovirus (Ad) vectors for systemic gene therapy. Ad has been shown to activate complement in vitro, but activation of complement after Ad injection in vivo has not been directly measured. Using complement protein C3a as a marker of complement activation, we show that types 2 and 5 human Ads cause rapid complement activation after intravenous injection in mice. Unexpectedly, the mechanisms in vivo were different than those in vitro. Antibodies were critical for the activation of complement by Ad in vitro, but antibodies were not required in vivo. The classical pathway was required in vitro, whereas complement activation in vivo involved both classical and nonclassical pathways as well as the reticuloendothelial system. Remarkably, the entry-deficient Ad mutant ts1 was completely unable to activate complement in vivo even though it was fully able to activate complement in vitro. This result demonstrates that the complement system senses intravenously injected Ad primarily by detecting the effects of Ad on cells rather than through direct interaction of complement with virions. Encouragingly, shielding Ad with polyethylene glycol was effective at reducing complement activation both in vitro and in vivo. In summary, intravenously injected Ad rapidly activates complement through multiple pathways, but these pathways are different than those identified by in vitro studies. In vitro studies are poorly predictive of in vivo mechanisms because Ad virions activate complement through indirect mechanisms in vivo.

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Antibody-independent complement activation by myelin via the classical complement pathway.

Journal of Experimental Medicine ◽

10.1084/jem.155.2.587 ◽

1982 ◽

Vol 155 (2) ◽

pp. 587-598 ◽

Cited By ~ 68

Author(s):

J C Cyong ◽

S S Witkin ◽

B Rieger ◽

E Barbarese ◽

R A Good ◽

...

Keyword(s):

Complement Activation ◽

Neurological Disorders ◽

Basic Protein ◽

Classical Pathway ◽

Newborn Mouse ◽

Classical Complement Pathway ◽

Myelin Fraction ◽

Ca Ions ◽

Murine Brain ◽

Classical Complement

Murine or rabbit whole brain homogenates were shown to activate human complement via the classical pathway by an antibody-independent reaction. This activity required Ca++ ions. Anticomplementary activity in fractionated murine brain was found to reside in the myelin fraction and in purified myelin. It was absent, however, both from highly purified myelin basic protein (MBP) and from the MBP-free residue. Because purified MBP is a monomer and this protein exists in brain tissue largely as a dimer, the ability of the cross-linked form of MBP to activate complement was investigated. MBP, dimerized with difluorodinitrobenzene, was highly anticomplementary. The murine brain, inactive when taken from the newborn mouse, was shown to first acquire the capacity to activate complement at 7 d after birth. This finding is consistent with the report that the synthesis of myelin protein has been shown to be initiated in murine brain 8 d after birth. Complement activation by MBP could play an important role in the pathological changes observed in neurological disorders.

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Polyreactive IgM initiates complement activation by PF4/heparin complexes through the classical pathway

Blood ◽

10.1182/blood-2018-03-834598 ◽

2018 ◽

Vol 132 (23) ◽

pp. 2431-2440 ◽

Cited By ~ 11

Author(s):

Sanjay Khandelwal ◽

Joann Ravi ◽

Lubica Rauova ◽

Alexandra Johnson ◽

Grace M. Lee ◽

...

Keyword(s):

B Cells ◽

Complement Activation ◽

Immunoglobulin M ◽

Classical Pathway ◽

Platelet Factor ◽

Classical Complement Pathway ◽

Healthy Donors ◽

Heparin Complexes ◽

Stable Variation

Abstract The mechanisms by which exposure to heparin initiates antibody responses in many, if not most, recipients are poorly understood. We recently demonstrated that antigenic platelet factor 4 (PF4)/heparin complexes activate complement in plasma and bind to B cells. Here, we describe how this process is initiated. We observed wide stable variation in complement activation when PF4/heparin was added to plasma of healthy donors, indicating a responder “phenotype” (high, intermediate, or low). Proteomic analysis of plasma from these healthy donors showed a strong correlation between complement activation and plasma immunoglobulin M (IgM) levels (r = 0.898; P < .005), but not other Ig isotypes. Complement activation response to PF4/heparin in plasma displaying the low donor phenotype was enhanced by adding pooled IgM from healthy donors, but not monoclonal IgM. Depletion of IgM from plasma abrogated C3c generation by PF4/heparin. The complement-activating features of IgM are likely mediated by nonimmune, or natural, IgM, as cord blood and a monoclonal polyreactive IgM generate C3c in the presence of PF4/heparin. IgM facilitates complement and antigen deposition on B cells in vitro and in patients receiving heparin. Anti-C1q antibody prevents IgM-mediated complement activation by PF4/heparin complexes, indicating classical pathway involvement. These studies demonstrate that variability in plasma IgM levels correlates with functional complement responses to PF4/heparin. Polyreactive IgM binds PF4/heparin, triggers activation of the classical complement pathway, and promotes antigen and complement deposition on B cells. These studies provide new insights into the evolution of the heparin-induced thrombocytopenia immune response and may provide a biomarker of risk.

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Heparin-protamine Complexes and C-reactive Protein Induce Activation of the Classical Complement Pathway: Studies in Patients Undergoing Cardiac Surgery and In Vitro

Thrombosis and Haemostasis ◽

10.1055/s-0037-1614002 ◽

2000 ◽

Vol 84 (08) ◽

pp. 237-243 ◽

Cited By ~ 18

Author(s):

Henk Velthuis ◽

Anke Eerenberg-Belmer ◽

Aria Yazdanbakhsh ◽

Eddy de Beaumont ◽

León Eijsman ◽

...

Keyword(s):

Cardiac Surgery ◽

Complement Activation ◽

Complement Pathway ◽

C Reactive Protein ◽

Postoperative Arrhythmia ◽

Classical Complement Pathway ◽

Reactive Protein ◽

Protamine Administration ◽

Classical Complement

SummaryThe administration of protamine to patients undergoing cardiopulmonary bypass (CPB) to neutralize heparin and to reduce the risk of bleeding, induces activation of the classical complement pathway mainly by heparin-protamine complexes. We investigated whether C-reactive protein (CRP) contributes to protamine-induced complement activation.In 24 patients during myocardial revascularization, we measured complement, CRP, and complement-CRP complexes, reflecting CRPmediated complement activation in vivo. We also incubated plasma from healthy volunteers and patients with heparin and protamine in vitro to study CRP-mediated complement activation. During CPB, CRP levels remained unchanged while C3 activation products increased. C4 activation occurred after protamine administration. CRP-complement complexes increased at the end of CPB and upon protamine administration. Incubation of plasma with heparin and protamine in vitro generated complement-CRP complexes, which was blocked by phosphorylcholine and stimulated by exogenous CRP. C4d-CRP complex formation after protamine administration correlated clinically with the incidence of postoperative arrhythmia.Protamine administration during cardiac surgery induces complement activation which in part is CRP-dependent, and correlates with postoperative arrhythmia.

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Identification of Neuronal Pentraxins as Synaptic Binding Partners of C1q and the Involvement of NP1 in Synaptic Pruning in Adult Mice

Frontiers in Immunology ◽

10.3389/fimmu.2020.599771 ◽

2021 ◽

Vol 11 ◽

Author(s):

Réka Á. Kovács ◽

Henrietta Vadászi ◽

Éva Bulyáki ◽

György Török ◽

Vilmos Tóth ◽

...

Keyword(s):

Molecular Mechanisms ◽

Synapse Formation ◽

Potential Interaction ◽

Synaptic Function ◽

Classical Pathway ◽

Microglial Phagocytosis ◽

Classical Complement Pathway ◽

Adult Mice ◽

Synaptic Pruning

Elements of the immune system particularly that of innate immunity, play important roles beyond their traditional tasks in host defense, including manifold roles in the nervous system. Complement-mediated synaptic pruning is essential in the developing and healthy functioning brain and becomes aberrant in neurodegenerative disorders. C1q, component of the classical complement pathway, plays a central role in tagging synapses for elimination; however, the underlying molecular mechanisms and interaction partners are mostly unknown. Neuronal pentraxins (NPs) are involved in synapse formation and plasticity, moreover, NP1 contributes to cell death and neurodegeneration under adverse conditions. Here, we investigated the potential interaction between C1q and NPs, and its role in microglial phagocytosis of synapses in adult mice. We verified in vitro that NPs interact with C1q, as well as activate the complement system. Flow cytometry, immunostaining and co-immunoprecipitation showed that synapse-bound C1q colocalizes and interacts with NPs. High-resolution confocal microscopy revealed that microglia-surrounded C1q-tagged synapses are NP1 positive. We have also observed the synaptic occurrence of C4 suggesting that activation of the classical pathway cannot be ruled out in synaptic plasticity in healthy adult animals. In summary, our results indicate that NPs play a regulatory role in the synaptic function of C1q. Whether this role can be intensified upon pathological conditions, such as in Alzheimer’s disease, is to be disclosed.

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