In a scientific review published in Molecular Biology Reports, a research team led by Yan Wang examined how progressive shifts in epigenetic regulation intersect with the innate immune system during aging. As human populations age, tissues undergo extensive molecular remodeling characterized by altered DNA methylation, shifting histone modifications, and non-coding RNA expression. These epigenetic disruptions frequently coincide with persistent low-grade inflammation, a systemic state termed inflammaging that underpins late-life morbidity. Central to this immunological architecture is the complement system, a cascade of innate immunity responsible for pathogen defense, continuous immune surveillance, debris clearance, and tissue homeostasis. While experimental literature demonstrates that epigenetic mechanisms can modify complement gene expression, the authors emphasize an essential physiological distinction: increased transcription or epigenomic marks do not inherently prove functional activation of the complement cascade, which fundamentally demands specific initiating signals, pathway-specific convertase assembly, and downstream proteolytic cleavage of core components.
To establish a clear conceptual foundation, the review distinguishes immunosenescence, cellular senescence, and systemic inflammaging, noting that while interconnected, they are not interchangeable terms. Immunosenescence defines the age-associated remodeling of the immune system across tissues, encompassing alterations in immune-cell composition, blunted responses to novel antigens, and shifts between pro-inflammatory and regulatory activities. Conversely, cellular senescence describes a stable state of proliferative cell-cycle arrest triggered by persistent stress and DNA damage. Senescent cells often develop a senescence-associated secretory phenotype, releasing cytokines, chemokines, growth factors, and proteases that alter neighboring microenvironments. Inflammaging represents the broader chronic, low-grade inflammatory state associated with aging, reflecting immune remodeling, senescent cell accumulation, and persistent inflammatory stimuli. Distinguishing these processes provides the biological context required to understand how age-associated chromatin alterations influence complement regulation and inflammatory homeostasis across diverse organ systems.
Complement activation initiates through the classical, lectin, or alternative pathways, converging on C3 and C5 activation steps. Pathway convertases cleave C3 into active fragments C3a and C3b. The C3b fragment mediates opsonization, amplification, and C5 convertase assembly, whereas C3a acts as an inflammatory anaphylatoxin. Subsequent C5 convertases cleave C5 into the potent anaphylatoxin C5a and C5b, which coordinates assembly of the terminal membrane attack complex C5b-9. In the central nervous system, complement proteins participate in vital non-inflammatory functions, including synaptic pruning during development and circuit plasticity, where C1q and C3 tag dispensable synapses for microglial clearance. However, during aging or neuropathology, complement activity can escape regulatory restraint, shifting homeostatic remodeling toward excessive, mislocalized synapse loss and neuroinflammation. The authors emphasize that complement activity depends strictly on local tissue context, cellular microenvironments, and endogenous regulatory capacity.
DNA methylation at cytosine residues, regulated by DNA methyltransferases, influences transcriptional accessibility depending on chromatin context and cell type. Promoter methylation typically enforces gene silencing, whereas hypomethylation permits active transcription. The review assesses studies linking DNA methylation to complement expression, noting that findings remain highly context-dependent. In the Norfolk Island cohort, altered methylation at cg13053396 in the C1S promoter associated with metabolic phenotypes, but this peripheral-blood finding does not establish tissue-specific regulation. In renal ischemia-reperfusion models, C5a stimulation induced genome-wide hypomethylation, yet this acute injury response cannot be generalized to physiological aging. Furthermore, in neural models where heterochromatin disruption provoked endogenous retroviral derepression, increased C3, C4b, and C1qa transcription accompanied glial activation, demonstrating indirect chromatin instability rather than direct promoter methylation control. In fatty liver disease, hypermethylation and hypomethylation coexist across distinct complement genes, underscoring tissue-specific heterogeneity.
Histone modifications represent another critical regulatory layer, where marks such as H3K4me3 associate with transcriptional activation, while H3K27me3 and H3K9me3 enforce repression. Mechanistic evidence from neurodegenerative models harboring the H3.3G34R mutation reveals that loss of H3K36me2 impairs DNMT3A recruitment, altering DNA methylation and sustaining brain C1q and C3 expression, demonstrating histone-DNA methylation crosstalk. Alongside chromatin marks, non-coding RNAs regulate complement networks post-transcriptionally. For instance, miR-146a, an NF-kappa-B-responsive microRNA altered in neurodegenerative conditions, has been reported to target or regulate complement factor H, an important regulator of the alternative pathway. Because complement factor H restrains the alternative pathway, altered expression may affect complement control and influence inflammatory responses, although the consequences depend on cellular context. Nevertheless, the authors caution that correlations between microRNA abundance and complement transcripts vary widely across experimental designs, necessitating direct functional validation before inferring causal relationships in aging tissues.
To synthesize these complex molecular findings, the authors propose a context-dependent framework comprising three distinct conceptual states rather than a uniform linear trajectory. The first state, physiological epigenetic remodeling, involves normal age-associated chromatin changes that remain compatible with baseline tissue homeostasis, wherein complement expression supports physiological clearance. The second state, reduced regulatory reserve, defines an intermediate condition where the capacity to maintain complement homeostasis against secondary stress diminishes, reflecting depleted regulatory proteins, blunted anti-inflammatory pathways, and impaired cellular repair. While baseline stability persists, tissues cannot properly constrain complement responses when challenged. The third state, disease-associated epigenetic-complement dysregulation, occurs when epigenetic alterations actively coexist with persistent complement activation, inappropriate convertase cleavage, and tissue injury. Transitions between these states depend on genetic background, somatic mutations, metabolic exposures, sex, cell identity, and microenvironmental factors rather than chronological age alone.
In Alzheimer’s disease and the aging brain, epigenetic remodeling and complement activity exhibit marked cell-type specificity. Epigenomic analyses in CK-p25 neurodegeneration models identified chromatin alterations at immune enhancers enriched for PU.1 transcription factor motifs. In human Alzheimer’s brain tissue, differential DNA methylation occurs at ANK1, BIN1, and ABCA7, with ANK1 hypermethylation correlating with Braak stages across cortical regions while sparing the cerebellum and blood. Single-nucleus transcriptomic profiling of prefrontal cortex reveals divergent, cell-type-specific transcriptional responses across neurons, astrocytes, and microglia during early disease stages. Furthermore, membrane microdomain integrity regulates complement thresholds: disruption of lipid rafts in ganglioside-deficient models impairs GPI-anchored complement inhibitors, including DAF and CD59, promoting complement-dependent neuroinflammation and neurodegeneration. In cerebral microvessels, endothelial C3a receptor signaling mediates blood-brain barrier permeability during aging, demonstrating that neurovascular complement interactions contribute to inflammatory vulnerability across distinct cellular compartments.
Age-related macular degeneration provides a clear example of complement involvement in chronic disease, illustrating both genetic foundations and epigenetic uncertainties. As an immune-privileged tissue, the retina depends on strict alternative pathway regulation to maintain homeostasis. Genetic polymorphisms in complement genes, including CFH, CFB, C2, and C3, represent major risk factors for macular degeneration. Affected ocular tissues frequently show elevated anaphylatoxins C3a and C5a and deposition of C3 fragments and terminal C5b-9 complexes. Although epigenetic alterations, such as IL17RC promoter hypomethylation and methylation changes at SOCS3 and AQP3, have been documented, these findings reflect cytokine signaling rather than direct complement regulation. The authors emphasize that genetic risk should not be conflated with epigenetic causality. Current evidence does not demonstrate that histone acetylation or DNA methylation determines complement activation thresholds or anti-VEGF treatment responses in macular degeneration, highlighting the need for cell-resolved mechanistic studies.
In vascular biology, atherosclerosis demonstrates marked spatiotemporal specificity in complement function throughout plaque progression. During advanced aging, systemic genomic instability introduces clonal hematopoiesis of indeterminate potential, wherein somatic mutations in epigenetic regulators like TET2 and DNMT3A drive clonal expansion of inflammatory myeloid cells. These altered leukocytes activate NLRP3 and AIM2 inflammasomes, elevating interleukin-1-beta and interleukin-18 secretion and accelerating plaque instability. Advanced single-cell multi-omics technologies, including single-cell RNA sequencing and ATAC sequencing, are uncovering these heterogeneous immune states across aging tissues. Notably, human microglial enhancer landscapes rapidly disintegrate in culture, accompanied by marked downregulation of complement genes C1QA, C1QB, C1QC, and C3, demonstrating the critical role of environmental cues in preserving immune cell identity.
Therapeutic modulation of the epigenetic-complement axis requires shifting from broad systemic inhibition toward precision intervention strategies. Downstream complement-directed therapies, including C3 and C5 inhibitors evaluated in geographic atrophy clinical trials, can modify disease progression but carry risks of disrupting homeostatic immune surveillance and clearance. Conversely, upstream broad-spectrum epigenetic drugs, such as generic DNA methyltransferase or histone deacetylase inhibitors, produce non-specific effects across pro- and anti-inflammatory loci, precluding selective complement normalization. To resolve these challenges, emerging preclinical approaches utilize catalytically dead Cas9 platforms fused with epigenetic modifiers, enabling locus-specific epigenome editing to alter targeted complement genes without disturbing global chromatin programs. Future clinical translation requires developing tissue-specific delivery vectors, establishing functional biomarkers to detect reduced regulatory reserve before irreversible damage occurs, and verifying that targeted chromatin modifications functionally normalize complement proteolytic activity within defined disease stages.
Subject of Research: Context-dependent epigenetic regulation of the complement system in aging and chronic inflammation
Article Title: Epigenetic regulation of the complement system in age-related chronic inflammation: mechanisms and therapeutic prospects
Article References: Wang, Y., Zhao, H., Kong, M., Qin, K., Teng, Y., Zhu, Z., Li, Y., Liu, D., Cui, L., & Sun, Y. (2026). Epigenetic regulation of the complement system in age-related chronic inflammation: mechanisms and therapeutic prospects. Molecular Biology Reports, 53(1), Article 1949. https://doi.org/10.1007/s11033-026-13148-0
Image Credits: AI Generated
DOI: 10.1007/s11033-026-13148-0
Keywords: Epigenetic regulation, Complement system, Inflammaging, DNA methylation, Histone modifications, Non-coding RNA, Synaptic pruning, Microglial activation, Alzheimer's disease, Age-related macular degeneration, Atherosclerosis, Clonal hematopoiesis, Regulatory reserve, C3 convertase, C5b-9 complex, Epigenome editing, Single-cell multi-omics, Central nervous system
News Source: Beatrice Stafford. (November 13, 2026). Epigenetic Alterations Influence Complement Activation Across Context-Dependent Stages of Aging. Scienmag.
