chatGPT:
Paper
Triggianese & Della-Morte, 2026 — “The inner fire: the shifting paradigm of complement system in aging”, Frontiers in Immunology. This is a mini-review on how the complement system changes with aging, with particular focus on inflammaging, neuroinflammation, autoimmunity, and the intracellular “complosome.”
Summary
The paper argues that the complement system is not simply a pro-inflammatory ageing mechanism, but a dual-function immune network whose effects depend on context, tissue, activation level, timing, and regulation.
The authors begin by reviewing the classical role of complement: activation through the classical, lectin, and alternative pathways, convergence on C3 convertase, production of C3a/C3b, and downstream formation of the membrane attack complex. They also emphasize that complement is not only plasma-based: many complement components act locally in tissues and even inside cells as the complosome, influencing metabolism, oxidative stress, cell survival, sterile inflammation, and NLRP3 inflammasome activity.
In aging, the paper presents complement as part of the broader inflammatory phenotype of ageing, or inflammaging. Persistent low-grade inflammation can continually activate complement, while oxidative stress and mitochondrial dysfunction may reinforce complement activation. The authors suggest a vicious cycle: aging increases ROS and tissue damage; damaged tissues activate complement; complement fragments recruit inflammatory cells and amplify ROS production; this worsens chronic tissue injury.
However, the central argument is that complement upregulation in older people may not be purely harmful. Higher complement activity may sometimes reflect immune adaptation, helping maintain pathogen defence, apoptotic-cell clearance, tissue repair, and immune surveillance despite immunosenescence. The paper highlights evidence that C3 and C4 levels rise with age and are linked with metabolic syndrome, obesity, lipid metabolism, and liver disease, but also that lower C3 has been associated with sarcopenia risk in older adults.
The neuroinflammation section develops the “double-edged sword” argument. Complement activity in the CNS can contribute to synaptic pruning, clearance of debris and amyloid, axon regeneration, neuroplasticity, and tissue repair. But chronic C3/C3aR and C5a/C5aR activation can also damage the blood-brain barrier, recruit inflammatory cells, promote neurodegeneration, and worsen diseases such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, ALS, and peripheral neuropathies. The paper stresses that complement inhibition could be beneficial in some neuroinflammatory contexts but harmful if it blocks protective clearance or repair functions.
The autoimmunity section argues that aging creates a paradox: older people often accumulate autoantibodies and immunosenescent T-cell phenotypes, yet centenarians can show immune profiles that appear relatively well-regulated and may resist overt autoimmune disease. The authors suggest that complement may contribute to this “immune fitness” by helping clear apoptotic material and immune complexes, thereby reducing autoimmune triggering, while also sustaining defence against infection.
The conclusion is that future complement-targeted geroscience should not simply suppress complement. Instead, it should aim to selectively inhibit destructive inflammatory pathways while preserving protective complement functions involved in host defence, tissue repair, debris clearance, and immune equilibrium.
Novelty
The paper’s main novelty is conceptual rather than experimental. It reframes complement in aging from a mostly harmful inflammatory cascade into a regulated, context-dependent ageing-adaptation system.
The most distinctive elements are:
| Novel point | Why it matters |
|---|---|
| Complement as “two-faced Janus” in aging | The paper argues that complement both drives inflammaging and may protect against infection, autoimmunity, neurodegeneration, and tissue damage. |
| Integration of extracellular complement and intracellular complosome | It links systemic complement activation with intracellular metabolism, oxidative stress, NLRP3 inflammasome activation, and cellular survival pathways. |
| Complement as part of immune fitness in the elderly | Rather than treating increased complement proteins as simply pathological, the authors suggest they may sometimes represent adaptive immune recalibration. |
| Neuroinflammation as both destructive and reparative | The review emphasizes that C3/C5 pathways can contribute to BBB dysfunction and neurodegeneration, but also to debris clearance, synaptic remodeling, and axon repair. |
| Centenarian/autoimmunity paradox | The authors connect complement biology to the observation that centenarians may have autoantibodies without overt autoimmune disease, possibly because regulatory immune and complement networks remain balanced. |
| Therapeutic caution | The paper argues against blanket complement inhibition in older adults and favours phenotype-specific modulation. |
The paper also includes a useful Table 1 summarizing ongoing or recent clinical studies in older adults involving immune age, immune fitness, inflammatory markers, innate immunity, complement activity, HLA-DR expression, cytokines, and immune-cell phenotyping. The figure on page 5 visually presents complement activation, effector generation, and biological effects in neuroprotection, host defence, and autoimmunity.
Critique
The paper is useful as a hypothesis-generating review, but its evidential strength is uneven.
The strongest part is the synthesis. The authors successfully bring together complement biology, inflammaging, neuroinflammation, autoimmunity, metabolism, and centenarian immune profiles into one framework. The “dual role” framing is biologically plausible and clinically important, because indiscriminate complement blockade in older adults could plausibly impair infection defence, clearance of apoptotic cells, or neural repair.
The main weakness is that the paper often moves from general complement biology to aging-specific claims without enough direct aging data. For example, oxidative stress can activate complement, complement can amplify ROS, and mitochondria decline with age; but the paper frequently treats these links as a likely aging mechanism rather than as a fully demonstrated causal pathway in old tissues. The authors sometimes acknowledge this, but the review would be stronger if it separated established human aging evidence from mechanistic extrapolation more explicitly.
A second limitation is that “protective complement upregulation” remains underdefined. Higher C3/C4 levels in older adults can correlate with metabolic syndrome and abdominal obesity, which are not obviously beneficial. The paper also notes that healthy centenarians may have more favourable immune regulation, but it is difficult to tell whether complement changes are causal, compensatory, or simply biomarkers of broader immune status.
Third, the review lacks a clear intervention framework. It says complement therapies should preserve protective functions while blocking destructive inflammation, but does not provide much practical detail about which components should be targeted in which phenotypes. For instance, C3 inhibition, C5 inhibition, C5aR1 antagonism, C3aR modulation, CR1 restoration, and local tissue-specific approaches could have very different consequences in brain, muscle, kidney, vasculature, and autoimmune disease.
Fourth, the clinical-trials table is useful but somewhat broad. Many listed studies are not primarily complement-focused; they measure general immune or inflammatory markers. This supports the broader immune-aging context, but less directly supports the paper’s specific complement thesis.
Finally, the paper would benefit from more discussion of measurement problems. Serum C3/C4 levels are crude markers. Complement biology depends on activation fragments, convertase activity, tissue localization, receptor expression, regulatory proteins, intracellular complosome activity, and local inflammatory context. A person could have normal C3/C4 but abnormal complement activation, or high complement proteins without harmful activation. This distinction is central for translating the argument into geroscience or clinical practice.
Bottom line
This is a thoughtful mini-review proposing that complement in aging should be viewed not simply as an inflammatory villain, but as a context-dependent immune-regulatory system. Its key contribution is the “dual role” model: complement can worsen inflammaging and neuroinflammation, but may also support pathogen defence, debris clearance, tissue repair, and resistance to overt autoimmunity. The critique is that much of the argument remains inferential, with limited direct causal evidence in aged humans, and future work needs better biomarkers, tissue-specific analysis, and clearer rules for when complement should be inhibited versus preserved.