The investigation focused on the Y402H polymorphism in CFH, a gene whose protein regulates the alternative complement pathway. Individuals homozygous for the risk-associated C allele have previously been reported to have a five- to sevenfold higher risk of AMD than those with the protective TT genotype. The variant lies in a ligand-binding region of factor H and is thought to reduce the protein’s ability to attach to polyanionic surfaces such as Bruch’s membrane. Reduced regulation can permit persistent complement activation and the generation of inflammatory fragments, including C5a. Those fragments can recruit immune cells and stimulate inflammatory responses, but the work reported here asks how a locally active protease might connect complement dysregulation to remodeling of extracellular matrix in the retina and choroid.

To investigate systemic granzyme B concentrations, the researchers examined people with dry AMD and compared them with a younger, non-AMD reference group. Both age-matched AMD genotype groups had higher mean plasma concentrations than the younger controls: 89.49 pg/mL with SEM 23.65 for the CC group and 64.48 pg/mL with SEM 24.35 for the TT group, compared with 22.3 pg/mL with SEM 7.5 in controls. The difference between the two AMD genotypes was not statistically significant in a Mann–Whitney U test. This result did not identify a systemic marker that distinguished the two genotypes, and the comparison does not establish when or where the genotype-associated tissue changes begin. Small sample numbers, including eight participants in each AMD group, limit the strength and generalizability of the plasma comparison, while the age difference between the AMD and control cohorts prevents treating the systemic increase as a genotype-specific effect.

The clearest human findings came from immunofluorescence analysis of non-AMD donor eyes. Five eyes with the high-risk CC genotype were compared with five age-matched protective TT eyes. Choroidal mast-cell density was significantly higher in the CC group, as was the density of mast cells expressing granzyme B. Granzyme B-positive mast cells averaged 4.2 cells per 0.1 square millimeter in CC eyes, compared with 1.9 cells per 0.1 square millimeter in TT eyes. The difference in degranulating mast-cell density only approached the threshold for statistical significance, at p = 0.0635, so the result should not be described as a demonstrated difference in degranulation across the groups. Nevertheless, high-magnification images showed extracellular granzyme B and tryptase granules around a degranulating cell in one CC eye, while dual labeling indicated that 87% of resident choroidal mast cells coexpressed the two proteins. A subset of granzyme B-positive cells, averaging 24.2% when the groups were combined, was tryptase-negative, consistent with other immune sources being present. The tissue evidence therefore supports mast cells as a major local source of granzyme B in this experimental context.

Vitronectin supplied a possible functional link between extracellular granzyme B and complement regulation. An in vitro cleavage assay demonstrated that active granzyme B could generate specific vitronectin fragments after incubation with the protein. In human donor tissue, protective TT eyes displayed strong vitronectin immunoreactivity along Bruch’s membrane and throughout the choriocapillaris and Sattler’s layer, whereas the inner choroid of high-risk CC eyes showed significantly less staining, with a reported p value of 0.0144. CC eyes also more often contained vitronectin concentrated in sub-retinal pigment epithelial deposits, while TT eyes had fewer and smaller deposits consistent with age-related change. The observations indicate redistribution as well as depletion. They are consistent with the proposed model that proteolysis removes soluble or matrix-associated vitronectin while leaving aggregation-prone material near the retinal pigment epithelium, although the tissue study alone cannot prove that granzyme B directly caused the pattern in donors.

The contribution of granzyme B was tested more directly in aged mice. The researchers compared 13- to 17-month-old granzyme B knockout animals with age-matched wild-type C57BL/6 mice, with four animals in each group. Choroidal vitronectin staining was generally faint in wild-type eyes but robust in knockout eyes, particularly around the choriocapillaris and larger choroidal blood vessels. Quantified signal was significantly higher in knockout tissue, at p = 0.0007. This animal comparison supports a role for endogenous granzyme B in regulating or turning over perivascular vitronectin. It does not reproduce the complete human genetic-risk environment, and the knockout establishes the influence of granzyme B on vitronectin abundance more directly than it establishes an AMD phenotype or a treatment effect. The study also does not determine whether elevated granzyme B alone is sufficient to produce the human pattern.

If granzyme B reduces functional vitronectin, complement inhibition should decline and the terminal membrane attack complex should accumulate. The human tissue analysis produced a corresponding pattern. Protective TT eyes had minimal, patchy staining for C5b-9, the membrane attack complex also known as MAC, whereas CC eyes showed prominent deposits around the choriocapillaris and at the interface between Bruch’s membrane and the retinal pigment epithelium. Quantitative analysis confirmed higher MAC immunoreactivity in the high-risk group, with a reported p value of 0.0079. These findings extend previous observations of increased choroidal MAC in high-risk CFH genotypes. Together, the lower vitronectin signal and higher MAC deposition support impaired local complement control, although the cross-sectional donor data cannot establish the temporal order of the two events in an individual eye. The findings also do not show that MAC deposition caused the observed mast-cell infiltration or vitronectin redistribution.

Cell experiments offered a potential explanation for how complement signaling initiates the pathway. Exposure of ARPE-19 retinal pigment epithelial cells to 5 micrograms per milliliter of C5a for 24 hours significantly increased expression of GZMB, TNFα and IL8 messenger RNA, with reported p values of 0.024, 0.0007 and 0.007, respectively. ELISA measurements showed that granzyme B protein remained at similar levels within cell lysates but increased significantly in culture supernatants after stimulation, with a reported p value of 0.0006. The result distinguished increased secretion from increased intracellular storage. C5a-treated cells also showed disruption of the continuous hexagonal pattern of the tight-junction protein ZO-1 after 48 hours, with intracellular accumulation of the protein and fewer cells retaining an intact barrier; the difference in cells displaying intact staining had a reported p value of 0.0037. These findings provide direct experimental evidence that C5a can stimulate granzyme B release and stress RPE cultures. As a simplified culture system, however, ARPE-19 cells do not reproduce the full immune, vascular and extracellular environment of a human eye.

Bulk RNA sequencing of ARPE-19 cells exposed to 100 nanomolar granzyme B for 18 hours identified 1,464 differentially expressed genes. A volcano plot displayed a twofold-change threshold, while a separate greater-than-1.5-fold criterion was applied to the selected epithelial, vascular, barrier, immune, stress and trophic genes shown in the detailed analysis. RPE identity markers RBP1, TJP1 and BEST1 remained stable across control and stimulated groups, but functional enrichment pointed to tissue morphogenesis, epithelial differentiation and vasculature development. Some structural and cell-fate genes increased, including GJA1, PPL, BMP4 and ID3, while CCN2, VEGFA and EDN1 declined. Damage-sensing and stress-associated genes, including TLR3, IL-33 and CYP1B1, increased, while several broad inflammatory or survival cytokines were reduced. The pattern is compatible with attempted barrier compensation alongside diminished vascular and trophic support, but transcriptional signatures alone do not demonstrate retinal atrophy or predict its development in patients.

Collectively, the experiments support a model in which the CFH Y402H risk genotype is linked to chronic complement activation, increased C5a signaling, granzyme B release by RPE cells and infiltrating mast cells, and extracellular remodeling. Cleaved vitronectin may then provide less inhibition of membrane attack complex assembly while contributing to sub-RPE deposits, creating conditions associated with early tissue changes. The study also notes that prior work found granzyme B can cleave C5 to generate active C5a, raising the possibility of a self-reinforcing loop, although that specific feedback mechanism was not directly tested here. Postmortem tissue, limited donor numbers, unequal control ages, the inability to assess biological-sex effects and the reduced fidelity of cell culture and mouse models constrain interpretation. The work identifies extracellular granzyme B as a mechanistic link and a potential therapeutic target, but it does not demonstrate that inhibiting it would prevent AMD or protect vision in patients.

Subject of Research: Granzyme B, vitronectin cleavage and complement dysregulation in early age-related macular degeneration

Article Title: Factor H polymorphism elevates granzyme B and impairs complement inhibition in human and mouse models

Article References: Chakravarthy, H., Xi, J., Yoo, H.-S., Buss, A., Li, B., Bell, R., Haegert, A., Le Bihan, S., Granville, D. J., & Matsubara, J. A. (2026). Factor H polymorphism elevates granzyme B and impairs complement inhibition in human and mouse models. iScience, 29(11), Article 117957. https://doi.org/10.1016/j.isci.2026.117957

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117957

Keywords: age-related macular degeneration, CFH Y402H, complement factor H, granzyme B, vitronectin, membrane attack complex, retinal pigment epithelium, choroidal mast cells, extracellular matrix, C5a, Factor, polymorphism

The biology at the heart of the study begins at the neuromuscular junction, the microscopic relay where motor neurons hand their signals to muscle fibers. In the most common form of myasthenia gravis, autoantibodies bind the acetylcholine receptor clustered on the muscle side of this synapse. Those antibodies do more than simply block the receptor; many of them recruit the complement system, an ancient arm of immune defense designed to punch holes in the membranes of invading microbes. When complement is triggered at the synapse, it assembles the membrane attack complex, a pore-forming structure that tears into the postsynaptic membrane. The result is destruction of the synaptic folds, loss of acetylcholine receptors, inflammatory infiltration of macrophages into the endplate region, and ultimately a failure of neuromuscular transmission that patients experience as drooping eyelids, double vision, and life-threatening weakness of the muscles that control breathing and swallowing.

Complement activation follows a strictly ordered cascade, and C3 sits at its pivotal center. All major activation pathways converge on this protein, which, once cleaved, seeds opsonization of target tissue, amplifies the response through amplification loops, and drives the terminal assembly of the membrane attack complex. This convergence point has long made C3 an attractive therapeutic target, and drugs that inhibit it are already reshaping the treatment landscape of several complement-mediated disorders. What the new study adds is a direct test, in a rigorous animal model, of whether near-total elimination of circulating C3, achieved not with a blocking antibody but by silencing the gene that produces the protein in the liver, can prevent the structural and functional damage of myasthenia gravis during its acute phase.

The experimental approach relied on small interfering RNA, short synthetic RNA molecules that hijack the cell’s own RNA-induced silencing complex to degrade a specific messenger RNA before it can be translated into protein. To make sure the siRNA reached its destination, the researchers equipped it with a triantennary N-acetylgalactosamine ligand, a chemical handle that binds with high affinity to the asialoglycoprotein receptor abundantly displayed on hepatocytes. This GalNAc-targeting technology, which has already transformed the delivery of other liver-directed RNA therapies, allowed the compound to be administered by simple subcutaneous injection rather than intravenous infusion. Female Lewis rats received the C3-siRNA at different dosages before the researchers induced disease by passive transfer, injecting a monoclonal antibody directed against the acetylcholine receptor, a model known as passive transfer myasthenia gravis that reproduces the acute complement-mediated attack on the synapse without requiring the animals to mount their own autoimmune response.

The degree of target engagement was remarkable. Two administrations of the C3-siRNA at 30 milligrams per kilogram, spaced one week apart, reduced plasma C3 levels by more than 98 percent, pushing them below the detection limit of the assay. Against this near-total depletion of the central complement component, the researchers then challenged the animals with the pathogenic anti-receptor antibody and tracked the consequences with an unusually comprehensive battery of readouts: clinical scoring of weakness, direct measurements of muscle strength, electromyographic assessment of transmission failure at the junction, quantification of total and functional acetylcholine receptor levels in muscle, histological analysis of macrophage infiltration and sectional necrosis in the motor endplate region, and immunostaining for complement activation products deposited at the synapse.

The protective effect was evident across essentially every parameter measured. Treated animals showed significantly reduced deposition of the membrane attack complex at the neuromuscular junction, indicating that the terminal complement attack had been blunted. The characteristic tissue damage of the disease was correspondingly diminished: sectional necrosis within the endplate region was reduced, and the influx of macrophages that normally follows complement-mediated injury was attenuated. Skeletal muscle C3 messenger RNA, which is upregulated locally during the disease process, rose less in the treated animals. Importantly, the therapy partially preserved functional acetylcholine receptor levels, meaning that the receptors remaining at the synapse were better able to do their job of transmitting neuronal signals to the muscle. The structural preservation translated into function: muscle strength assessments and electromyography showed that the development of muscle weakness was effectively attenuated in the treated rats compared with controls.

Perhaps the most scientifically provocative observation came from what the treatment did not achieve. Despite the near-total elimination of C3 from the plasma, low levels of C3 activation products and membrane attack complex were still detectable specifically at the neuromuscular junction. This residual deposition is more than a technical footnote. It demonstrates that even a vanishingly small amount of circulating C3 is sufficient to sustain some degree of complement activation at the inflamed synapse, where local amplification mechanisms and the intense concentration of activating antibodies can make the most of scarce substrate. The authors interpret this as evidence of the powerful pathogenic potency of C3 in acetylcholine receptor-positive myasthenia gravis: the protein is so effective a driver of tissue injury that even trace quantities can leave a molecular fingerprint of damage at the junction.

That observation carries practical implications for how complement-targeted therapies might be deployed in patients. If near-complete suppression of plasma C3 leaves residual terminal pathway activity at the neuromuscular junction, then the therapeutic goal in myasthenia gravis may need to be either maximally deep C3 suppression, combination approaches that pair hepatic silencing with additional inhibition at the tissue level, or careful biomarker monitoring of complement deposition at the synapse itself rather than relying solely on systemic measures. At the same time, the study shows that the degree of protection achieved with hepatic silencing alone was already substantial, preserving receptor function and blunting weakness in the acute disease phase, which supports the idea that C3 silencing could serve as an adjunctive therapy layered onto existing immunosuppressive regimens rather than replacing them outright.

The work also illustrates how the tools of molecular medicine are being repurposed for organ-specific immunology. Rather than designing a protein drug to neutralize C3 in the bloodstream, the team used a GalNAc-conjugated siRNA to instruct the liver itself to stop making the protein, achieving durable, deep, and convenient subcutaneously delivered target reduction. This strategy builds on the broader success of RNA interference therapeutics in hepatic targets and suggests that complement proteins synthesized primarily by the liver are well suited to this modality. For a disease like myasthenia gravis, where chronic immunosuppression carries significant long-term burdens and where existing complement inhibitors require frequent intravenous administration, a long-acting subcutaneous gene-silencing approach could meaningfully change the treatment calculus, although the authors are careful to frame the current results as preclinical support for a potential adjunctive strategy rather than a validated therapy.

As with any animal study, the path from Lewis rats to patients runs through the usual gauntlet of safety, dosing, and clinical trial design, and the passive transfer model captures the acute effector phase of the disease rather than the full chronic autoimmune process that unfolds in patients. Nevertheless, the study delivers a clear and technically rigorous message: C3 is a dominant and remarkably potent driver of the complement-mediated destruction of the neuromuscular junction in antibody-positive myasthenia gravis, silencing its hepatic production protects receptor levels, synaptic architecture, and muscle function, and even the faint residual complement activity that survives near-total depletion underscores how aggressively this pathway must be restrained. For a disease in which the immune system’s own microbial-killing machinery turns against the synapse, teaching the liver to hold that machinery back may prove to be one of the more consequential therapeutic ideas to emerge from complement research in recent years.

Subject of Research: Complement C3 gene silencing as a therapy for complement-mediated neuromuscular junction damage in myasthenia gravis

Article Title: C3 complement silencing prevents acetylcholine receptor loss and reduces complement activation in the rat passive transfer myasthenia gravis model

Article References: Schöttler, A. K., Mané-Damas, M., Arets, B., Peng, S., Molenaar, P. C., De Baets, M. H., Claessen, S. M. H., Barbour, T., Richardson, E., Eyerman, D. J., Scheibler, L., Losen, M., & Martinez-Martinez, P. (2026). C3 complement silencing prevents acetylcholine receptor loss and reduces complement activation in the rat passive transfer myasthenia gravis model. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-08832-5

Image Credits: AI Generated

DOI: 10.1186/s12967-026-08832-5

Keywords: myasthenia gravis, complement C3, neuromuscular junction, small interfering RNA, GalNAc delivery, membrane attack complex, acetylcholine receptor, passive transfer model, complement inhibition, gene silencing, autoimmune disease, Lewis rats

The research, led by Ying Zhou, Xue Zhang, Tiancong Qi, and Zi-Han Wang working with senior authors Lu-Jun Yu and Qin-Fang Chen at Sun Yat-Sen University and their collaborators, began with a deliberately simple strategy. The team used the central autophagy protein ATG8e as bait in a yeast two-hybrid screen, fishing an Arabidopsis cDNA library for any protein capable of physically binding to it. ATG8 sits at the heart of autophagosome formation, the process by which a double-membrane vesicle engulfs cellular cargo before delivering it to the vacuole for destruction. Proteins that dock onto ATG8 typically do so through a short peptide signature known as the ATG8-interacting motif, or AIM, defined by the consensus sequence of an aromatic residue followed by two variable positions and a bulky hydrophobic one. Among the fifteen candidate interactors that emerged from the screen, one stood out: MEMBRANE ATTACK COMPLEX/PERFORIN-LIKE 2, or MACP2, a member of a protein family better known for its role in immunity and programmed cell death.

The MACPF family is famous in animal biology. Members of this family, including the complement membrane attack complex and perforin, oligomerize into ring-shaped pores that punch holes in target membranes, a mechanism central to immune defense and development. Plants encode a small set of MACPF proteins, and in Arabidopsis four of them carry the conserved pore-forming domain: CAD1, NSL1, MACP1, and MACP2. Previous work had shown that MACP2 regulates programmed cell death and influences resistance to bacterial pathogens and susceptibility to necrotrophic fungi, and its transcript levels rise in response to a range of biotic and abiotic stresses. But its involvement in autophagy had never been suspected. The new study confirmed the physical interaction between MACP2 and ATG8e using two independent methods: co-immunoprecipitation in Arabidopsis mesophyll protoplasts, where tagged ATG8e pulled down tagged MACP2 from cell extracts, and bimolecular fluorescence complementation, in which the two proteins reconstituted a fluorescent signal at the plasma membrane only when co-expressed. Crucially, when the researchers mutated two conserved residues within the AIM, changing phenylalanine 134 and leucine 137 to alanines, the interaction vanished entirely, proving that MACP2 docks onto ATG8 through the canonical autophagy receptor motif.

With the interaction established, the team turned to genetics to ask what MACP2 actually does in a living plant. They compared wild-type Arabidopsis with macp2 knockout mutants and with lines overexpressing the protein, subjecting all genotypes to carbon starvation by transferring them to continuous darkness and to nitrogen starvation by growing them on nitrogen-free medium. The results were striking and, at first glance, counterintuitive. Plants lacking MACP2 were more tolerant of starvation, retaining greener leaves and higher chlorophyll content than wild type, while plants overproducing MACP2 senesced prematurely and wilted faster under the same conditions. Under normal growth the differences were subtle, appearing only as the plants aged, with overexpression lines yellowing by five to six weeks while the knockouts stayed green longer than their wild-type counterparts. This pattern, in which too much of a protein mimics the hypersensitivity of autophagy-defective mutants, suggested that MACP2 acts as a positive regulator of autophagy induction: without it, the starvation signal is blunted, and with too much of it, the signal becomes destructive.

The signal in question, the study reveals, is hydrogen peroxide. Reactive oxygen species have long been recognized as double-edged messengers in plant cells: at low concentrations they act as second messengers that coordinate growth and stress responses, but in excess they oxidize proteins, lipids, and DNA beyond repair. Outside the cell, in the apoplast, hydrogen peroxide is generated primarily by plasma membrane NADPH oxidases of the RBOH family, particularly RBOHD and RBOHF. Earlier work had hinted that these apoplastic reactive oxygen species are required for autophagosome formation during hypoxia, but the molecular channel connecting extracellular peroxide to the intracellular autophagy machinery had never been identified. Using an Amplex Red fluorescence assay, the researchers measured total hydrogen peroxide levels across their genotypes and found that MACP2-overexpressing plants accumulated significantly more peroxide during senescence and starvation, while macp2 knockouts accumulated significantly less. To visualize peroxide influx directly, they deployed HyPer, a genetically encoded fluorescent biosensor specific for hydrogen peroxide, and showed that the peroxide-dependent fluorescence ratio in root cells rose sharply after carbon starvation in wild-type seedlings but remained substantially lower in macp2 mutants both before and after starvation.

The picture that emerges is one of MACP2 acting as a gated doorway for stress chemistry. The authors propose that in response to senescence or nutrient deprivation, MACP2 proteins assemble into homo- or hetero-polymeric pores at the plasma membrane, structurally analogous to the membrane attack complexes of animal immunity, and that these pores allow extracellular hydrogen peroxide, produced by RBOH oxidases, to flood into the cytoplasm. There, the peroxide functions as a signal that triggers the initiation of autophagosome biogenesis. Supporting this model, supplying the antioxidant glutathione to starving seedlings completely rescued the starvation hypersensitivity of MACP2-overexpressing lines and partially rescued the atg5-1 autophagy mutant, demonstrating that the damaging phenotypes are chemically driven by peroxide rather than by some unrelated consequence of MACP2 abundance. The salicylic acid pathway was also implicated: overexpression lines accumulated elevated levels of salicylic acid and its glycoside SAG, showed upregulation of senescence-associated and defense genes including SAG13, SAG29, PR1, and WRKY53, and lost their accelerated senescence when crossed into eds1 or pad4 salicylic acid signaling mutants. Salicylic acid and reactive oxygen species, the authors note, appear to form a mutually reinforcing amplification loop that drives both autophagy induction and senescence.

Perhaps the most elegant evidence for MACP2’s role came from epistasis experiments with the autophagy mutant atg5-1. The atg5-1 mutant, defective in a core autophagy conjugation factor, displays the classic autophagy-defective phenotype: early leaf yellowing, runaway peroxide accumulation, and extreme sensitivity to carbon and nitrogen starvation. Remarkably, when the researchers crossed macp2 into the atg5-1 background, the double mutants were substantially rescued, showing delayed senescence, higher chlorophyll content, greater starvation tolerance, and dramatically lower hydrogen peroxide levels than atg5-1 alone. In other words, removing the peroxide doorway suppressed the damage caused by removing the recycling machinery itself, placing MACP2 genetically upstream of ATG5 as a regulator of the peroxide signal that both induces autophagy and, when unchecked, kills the cell. Consistent with this, direct measurements of autophagosome formation using an eGFP-ATG8e reporter showed that macp2 mutants formed fewer starvation-induced autophagic puncta and released less free eGFP, a readout of autophagic flux, while overexpression lines formed more. Treating seedlings with exogenous hydrogen peroxide or the salicylic acid agonist BTH likewise induced fewer puncta in macp2 and more in overexpressors, confirming that MACP2 channels the peroxide signal into autophagosome biogenesis.

The story acquires a final twist in the regulation of MACP2 itself. Under normal conditions, a MACP2-YFP fusion localizes to the plasma membrane, but after twelve hours of carbon or nitrogen starvation the fluorescent signal redistributes into cytoplasmic vesicles that colocalize with the autophagosome marker mCherry-ATG8f. Prolonged darkness causes MACP2 protein abundance to collapse after roughly 24 to 48 hours, and this degradation is blocked by concanamycin A, an inhibitor of vacuolar degradation, but not by the proteasome inhibitor MG132, indicating that MACP2 is dismantled by autophagy rather than by the 26S proteasome. Protein stability assays in protoplasts from atg5-1 and rpn10-1 mutants, the latter lacking a dual ATG8 and ubiquitin receptor, confirmed that MACP2 degradation requires the autophagy machinery and likely involves RPN10 acting as a selective autophagy receptor. The researchers also found that MACP2 lacking a functional AIM degrades more slowly and that its stability no longer depends on ATG8e, tying the feedback degradation directly to the ATG8 interaction. This creates a self-limiting circuit: early in starvation, MACP2 pores admit peroxide, peroxide induces autophagy, and autophagy preserves cellular homeostasis; but as peroxide continues to accumulate, MACP2 itself is loaded onto autophagosomes and destroyed, throttling the peroxide influx and protecting the cell from self-destruction during extended deprivation.

The implications of this work extend well beyond a single weed species. It identifies, for the first time, a molecular conduit through which extracellular reactive oxygen species gain access to the autophagy induction machinery in plants, and it adds MACP2 to a growing list of membrane proteins, including the aquaporins PIP1;4 and PIP2;1, that govern hydrogen peroxide trafficking across plant membranes. It also resonates with recent findings in animal cells, where the pore-forming protein gasdermin D mediates the release of mitochondrial reactive oxygen species, suggesting that pore-mediated peroxide transport may be a broadly conserved principle of cellular stress signaling. For agriculture, the prospect of tuning MACP2 or its relatives to adjust the sensitivity of crops to nutrient stress, darkness, or pathogen attack is an obvious long-term goal, though the authors caution that the precise architecture of the proposed MACP2 pore and the exact mechanism by which RPN10 targets MACP2 for degradation remain to be worked out. What is already clear is that the plant’s decision to consume itself in hard times is not a spontaneous collapse but a signal-driven choice, and that the gatekeeper of that signal is a protein borrowed from the ancient arsenal of membrane attack.

Subject of Research: Role of the Arabidopsis pore-forming protein MACP2 in linking extracellular reactive oxygen species to autophagy induction during nutrient starvation

Article Title: Arabidopsis MACP2 contributes to autophagy induction by modulating starvation-induced reactive oxygen species homeostasis

Article References: Zhou, Y., Zhang, X., Qi, T., Wang, Z.-H., Wang, Y., Wang, L.-N., Zeng, Y.-L., He, H., Jiang, L., Xie, D., Xiao, S., Yu, L.-J., & Chen, Q.-F. (2025). Arabidopsis MACP2 contributes to autophagy induction by modulating starvation-induced reactive oxygen species homeostasis. Advanced Biotechnology, 3(3), Article 25. https://doi.org/10.1007/s44307-025-00078-4

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00078-4

Keywords: autophagy, MACP2, Arabidopsis, reactive oxygen species, hydrogen peroxide, ATG8, nutrient starvation, leaf senescence, membrane attack complex, salicylic acid, plant stress, RPN10

At its core, the complement cascade is a proteolytic amplification machine. It can be triggered through three converging routes: the classical pathway, initiated when the C1 complex binds antibody-antigen complexes or other target structures; the lectin pathway, ignited when pattern-recognition molecules such as mannose-binding lectin and ficolins detect carbohydrate motifs on pathogens or damaged cells; and the alternative pathway, which samples surfaces continuously through spontaneous hydrolysis and is stabilized by properdin. All three routes converge on C3, whose cleavage generates the opsonin C3b that tags targets for phagocytosis and the anaphylatoxin C3a that recruits and activates inflammatory cells. Further cleavage of C5 releases the potent anaphylatoxin C5a and assembles the C5b-9 membrane attack complex, a pore-forming structure that can lyse susceptible cells. In host defence, this is a marvel of efficiency. In the nervous system, where many cell types are exquisitely vulnerable to lysis and inflammation, the same machinery can become an instrument of destruction.

What makes complement particularly fascinating in a neurological context is that it is not merely a circulating defence system but an active participant in normal brain physiology. Seminal experimental work has shown that classical pathway components, including C1q and C3, physically tag synapses during postnatal development, allowing microglia to prune weak or inactive connections in an activity-dependent manner. This complement-dependent synaptic sculpting is essential for wiring the developing brain, and related mechanisms continue to influence adult neural circuit plasticity, including microglia-mediated elimination of synapses linked to forgetting. The complement system also contributes to the clearance of apoptotic cells and cellular debris, processes fundamental to maintaining tissue health. These physiological roles carry a sobering implication: any therapeutic strategy that blanketly suppresses complement risks interfering with brain development, synaptic maintenance, and debris clearance, a concern that hangs over much of the current drug pipeline.

In autoimmune neurological disease, aberrant complement activation is now documented with striking consistency. In myasthenia gravis, autoantibodies against the acetylcholine receptor recruit complement to the neuromuscular junction, where the membrane attack complex destroys the postsynaptic membrane, a mechanism established experimentally decades ago and confirmed ultrastructurally by localization of the lytic component C9 at the motor end-plate. Complement activation profiles correlate with disease severity, and eculizumab, a monoclonal antibody that blocks cleavage of C5, demonstrated efficacy in refractory generalized disease in the pivotal REGAIN trial, earning regulatory approval and opening the floodgates for a broader class of anti-C5 agents, including the longer-acting ravulizumab and the subcutaneous peptide zilucoplan, alongside newer C5-targeting candidates such as gefurulimab now in phase 3 testing.

The antibody-mediated demyelinating diseases of the central nervous system tell a similarly compelling story. In neuromyelitis optica spectrum disorder, aquaporin-4-specific IgG1 antibodies bind astrocytic endfeet, fix complement, and generate astrogliotic, necrotic lesions; passive transfer experiments show that immunoglobulin G plus complement reproduces the pathology in animals. Blocking C5 with eculizumab dramatically reduced relapse rates in aquaporin-4-positive patients, and ravulizumab has since replicated these results with a more convenient dosing schedule. The related condition, myelin oligodendrocyte glycoprotein antibody-associated disease, displays its own distinctive complement activation signatures, with recent work showing that activation patterns downstream of C5 cleavage differ between MOGAD and aquaporin-4 antibody-positive neuromyelitis optica, and that complement profiles can predict clinical outcomes. In multiple sclerosis, complement deposition is a prominent feature of lesions, oligodendrocytes are peculiarly susceptible to complement lysis even in the absence of antibody, and growing clinical evidence links circulating complement activation products to structural brain damage, disease severity, and disability progression, including in primary progressive disease. Notably, animal studies reveal that some complement components, such as C5-derived signals, may actually facilitate remyelination, reinforcing the argument for nuanced, pathway-selective modulation rather than indiscriminate shutdown.

In disorders of the peripheral nervous system, complement has moved from suspected accomplice to validated therapeutic target. In Guillain-Barré syndrome, autoantibodies against gangliosides on motor nerve terminals and perisynaptic Schwann cells recruit the membrane attack complex, and complement inhibition protects nerve terminals in murine models. Eculizumab was tested in a large phase 3 trial and, although the primary endpoint was not met, encouraging signals and real-world comparative data for the proximal inhibitor ANX005, which blocks C1q, suggest that early, pathway-targeted intervention may still prove valuable. Chronic inflammatory demyelinating polyneuropathy has emerged as a particularly fertile field: recent complement profiling of sural nerves, together with the demonstration that terminal pathway activation tracks with disease severity, has set the stage for phase 2 studies of the C1s inhibitor riliprubart, with promising early efficacy and safety findings. Multifocal motor neuropathy and IgM anti-MAG antibody neuropathies are likewise being reinterpreted through a complement lens, with the C2-blocking antibody empasiprubart, derived from the ARGX-117 program, advancing through clinical development on the logic that IgM-driven complement activation depends critically on the classical pathway component C2.

Perhaps the most provocative frontier is neurodegeneration. In Alzheimer disease, genetic association studies first implicated complement genes as susceptibility loci, and mechanistic work then showed that C1q and C3 drive early synapse loss in mouse models, with microglia engulfing complement-tagged synapses. Complement C3 deficiency protects aged, plaque-laden mice from neurodegeneration, yet other data indicate that specific complement components exert tissue-protective effects, promoting clearance of amyloid debris and supporting repair. Similarly, in amyotrophic lateral sclerosis, elevations of C5a and the membrane attack complex in patient blood and dysregulation of the cascade in the hSOD1G93A mouse model motivated a randomized trial of ravulizumab, which did not slow disease progression but provided dose-finding and safety data that will inform future, more precisely targeted efforts. These mixed results crystallize the central dilemma of the field: complement is both arsonist and firefighter, and the therapeutic challenge is to extinguish the destructive arm of the cascade while preserving its reparative functions.

The authors of the review emphasize that the existing complement drug arsenal, which includes C5 inhibitors such as eculizumab, ravulizumab, and crovalimab; the C1s inhibitor sutimlimab; the C3-targeting peptide pegcetacoplan; factor B and factor D blockers such as iptacopan and danicopan; C1-esterase inhibitor concentrates; and the C2-blocking antibody empasiprubart, was largely forged in hematology, ophthalmology, and nephrology before being imported into neurology. Expanding these agents to the brain raises distinctive pharmacological hurdles, including the blood-brain barrier, the need for sustained pathway suppression in a compartment with limited drug access, and the infection risks inherent in disabling a key arm of host defence, particularly meningococcal disease in patients receiving terminal pathway inhibitors. Proximal inhibitors that block C1 or C2 confer broader cascade suppression and may better preserve certain distal functions, but they carry their own trade-offs in infection susceptibility and disruption of physiological opsonization and debris clearance.

Looking ahead, Dalakas and Lünemann argue that the future of complement-targeted neurology will depend on systematic, biomarker-guided patient selection. Reliable, disease-specific measures of complement activation, whether from cerebrospinal fluid, blood, or tissue, will be essential to identify which patients are most likely to benefit from proximal versus distal inhibition, and to time intervention before irreversible tissue injury has occurred. As biologics targeting other axes of autoimmunity, from FcRn antagonists and neonatal Fc receptor blockers to B-cell-depleting agents and, increasingly, CAR T-cell therapies, crowd the neurological treatment landscape, complement inhibitors will need to demonstrate not just efficacy but a rational, mechanistically defined place in therapeutic algorithms. What is no longer in doubt is the direction of travel: from a system once studied almost exclusively by immunologists, complement has become a frontier of clinical neurology, and the coming decade will determine how precisely medicine can wield its power without extinguishing its gifts.

Subject of Research: The role of the complement system and complement-targeted therapeutics in autoimmune and neurodegenerative neurological diseases

Article Title: Role of complement and complement-targeted therapeutics in neurological diseases

Article References: Dalakas, M. C., & Lünemann, J. D. (2026). Role of complement and complement-targeted therapeutics in neurological diseases. Nature Reviews Neurology. https://doi.org/10.1038/s41582-026-01261-4

Image Credits: AI Generated

DOI: 10.1038/s41582-026-01261-4

Keywords: complement system, neuroimmunology, myasthenia gravis, neuromyelitis optica, multiple sclerosis, Guillain-Barré syndrome, Alzheimer disease, amyotrophic lateral sclerosis, eculizumab, ravulizumab, membrane attack complex, complement inhibitors