Mapping the Human Immunobiography: A Systematic Review of the Pathophysiology, Sex-Specific Shifts, and Metabolic Anchors of Inflammaging

Summary
Human immunobiography represents the cumulative history of an individual’s immune experiences—from prenatal stimuli to late-life stressors—that shapes the development of inflammaging, a state of chronic low-grade inflammation driven by cellular senescence, metabolic reprogramming, and sex-specific biological shifts. Evidence indicates that while older women exhibit higher vulnerability to acute inflammatory stressors as measured by neutrophil-to-lymphocyte ratios, young men are more susceptible to sedentary-induced metabolic multimorbidity, highlighting a distinct sexual dimorphism in immune and metabolic aging.

Pathophysiology of Inflammaging and Immunosenescence

• Mechanistic Drivers: Inflammaging is characterized by a chronic, sterile, low-grade inflammatory state resulting from "garb-aging" (accumulation of cellular debris), mitochondrial dysfunction, defective autophagy/mitophagy, and the senescence-associated secretory phenotype (SASP) of senescent cells (Direct, High; PMID: 29375577, PMID: 35378106) «✓ PMID:29375577» «✓ PMID:35378106».
• Innate Immune Memory: The innate system maintains a "trained memory" through epigenetic reprogramming, allowing for sustained activation (Direct, High; PMID: 29375577) «✓ PMID:29375577». This results in elevated basal cytokine production in older adults but a reduced dynamic range when responding to new stressors, such as TLR ligands (Direct, High; PMID: 20100933) «✓ PMID:20100933».
• Cellular Shifts: Immunosenescence involves thymic involution leading to a shrinking T-cell receptor (TCR) repertoire and a shift from naive to memory T-cell populations (Direct, High; PMID: 29375577, PMID: 35378106) «✓ PMID:29375577» «✓ PMID:35378106».
• The "Bow Tie" Architecture: The immune system functions as a complex network where many input signals (pathogens, DAMPs) converge on a few sensors to produce diverse effector responses (Direct, Medium; PMID: 29375577, PMID: 37556623) «✓ PMID:29375577» «✓ PMID:37556623».

Metabolic Anchors and Reprogramming

• Sedentary Behavior and Multimorbidity: In young adults ($\le$45 years), prolonged sedentary time ($\ge$8 hours/day) is a significant risk factor for early-onset metabolic multimorbidity, which includes the coexistence of conditions such as hypertension, T2D, dyslipidemia, and NAFLD (Direct, High; PMID: 30046148) «✓ PMID:30046148».
• Energetic Trade-offs: Activated immune cells utilize aerobic glycolysis (Warburg effect) for rapid energy, whereas quiescent cells rely on oxidative phosphorylation (OX-PHOS) (Direct, High; PMID: 29375577) «✓ PMID:29375577». Aging T cells often suffer from energy deprivation, shunting glucose into the pentose phosphate pathway rather than breaking it down for ATP (Direct, Medium; PMID: 29375577) «✓ PMID:29375577».
• Sarcopenia and Malnutrition: In hospitalized older adults, malnutrition and sarcopenia interact with inflammaging to create a background pro-inflammatory state that increases acute vulnerability to infections like COVID-19 (Direct, Medium; PMID: 41462275) «✓ PMID:41462275».

Sex-Specific Shifts in Immunobiography

• Predictors of Mortality: The neutrophil-to-lymphocyte ratio (NLR) acts as a sex-specific predictor of short-term mortality ($\le$90 days) in older adults. This association is significantly stronger in females (HR 2.50) compared to males (HR 1.34) (Direct, High; PMID: 41462275).
• Metabolic Susceptibility in Young Adults: Young men exhibit a clear dose-response relationship between sedentary time and metabolic multimorbidity (OR 1.26 for $\ge$8 h/day), whereas young women do not show a significant association (Direct, High; PMID: 30046148) «✓ PMID:30046148».
• Hormonal Influence: Estrogen ($17\beta$-estradiol) in premenopausal women promotes energy homeostasis, improves body fat distribution, and ameliorates insulin resistance, potentially counteracting the metabolic risks of sedentary behavior (Direct, Medium; PMID: 30046148) «✓ PMID:30046148».
• Life-Course Dimorphism: Immune profiles shift with age; while men generally have higher NLR values, women experience distinct shifts in leukocyte composition post-menopause that may amplify their inflammatory vulnerability (Direct, Medium; PMID: 41462275) «✓ PMID:41462275».

Developmental Origins of Immunobiography

• Prenatal Influences: The "Biodiversity Hypothesis" suggests that interaction between external and internal microbiotas during pregnancy promotes immune tolerance. Maternal diet (e.g., fish oil, vitamins A, C, D, and E) and avoidance of excess folate are linked to reduced atopy in offspring (Direct, High; PMID: 37556623) «✓ PMID:37556623».
• Birth and Early Life: Vaginal delivery and exclusive breastfeeding are critical for vertical microbiome transfer, enriching the infant's "second genome" and protecting against inflammatory and allergic disorders (Direct, High; PMID: 37556623) «✓ PMID:37556623».
• COVID-19 and Immunobiography: SARS-CoV-2 infection may accelerate immunosenescence and inflammaging, potentially leading to earlier onset of age-related diseases (ARDs) through "long COVID" and chronic immune dysregulation (Derived, Medium; PMID: 35378106) «✓ PMID:35378106».

What specific epigenetic markers, such as ELOVL2 methylation, best distinguish successful from pathological aging in the provided studies?

How does the interplay between sedentary behavior and sex hormones influence the development of metabolic multimorbidity in young populations?

What role do specific TLR signaling deficiencies play in predicting vaccine non-responsiveness among older adults with high basal inflammaging?

Hypothesis 1

Vaccine non-responsiveness in older adults with high basal inflammaging is mechanistically driven by a TLR-signaling 'ceiling effect' in dendritic cells, where chronic baseline elevation of pro-inflammatory cytokines limits the dynamic induction of IL-12p40 and IFN-α required to link innate activation to adaptive B-cell antibody production.

Mechanistic rationale

• Dendritic cells from older individuals exhibit markedly elevated basal levels of TNF-α, IL-6, and IL-12p40 in mDCs, as well as TNF-α and IFN-α in pDCs, compared to younger controls. (Derived, Low; PMID: 20100933)
• High basal inflammaging is associated with a 'reduced dynamic range' of the innate immune system, characterized by significantly smaller percentage increases in cytokine production following TLR ligand stimulation. (Derived, Medium; PMID: 20100933, PMID: 29375577)
• Specific transcriptional and post-transcriptional deficiencies lead to decreased protein and mRNA expression of TLR3 and TLR8 in mDCs and TLR7 mRNA in pDCs with advancing age. (Derived, Low; PMID: 20100933)
• The inability of mDCs to produce adequate IL-12p40 and pDCs to produce IFN-α upon TLR stimulation is a significant predictor of the failure to achieve seroprotection titers (HAI ≥ 1:64) in older adults. (Derived, Low; PMID: 20100933)

Predictions

• In a cohort of older adults with high inflammaging, individuals who fail to seroconvert will exhibit higher basal levels of intracellular IL-6 in mDCs and lower fold-change induction of IL-12p40 following TLR8 stimulation compared to responders. (Derived, Low; PMID: 20100933)
• Metabolic reprogramming toward aerobic glycolysis in mDCs will be impaired in vaccine non-responders, correlating with reduced mitochondrial respiration substrate availability. (Indirect, Low; PMID: 29375577)

Study design

A longitudinal study of 100 community-dwelling adults (age ≥65) undergoing seasonal influenza vaccination. Primary mDCs and pDCs will be isolated from blood collected pre-vaccination and 48 hours post-vaccination. Cells will be stimulated ex vivo with TLR7/8 (R848) and TLR3 (Poly I:C) ligands. Readouts include flow cytometry for intracellular cytokine production (TNF-α, IL-6, IL-12p40, IFN-α), basal cytokine concentrations via high-sensitivity ELISA, and post-vaccination HAI titers at 28 days to determine seroprotection. (Derived, Low; PMID: 20100933)

Confounders & controls

• Biological sex and reproductive history (menopause status) must be controlled, as women exhibit different pro-inflammatory profiles and mortality predictors in older age. (Derived, Medium; PMID: 41462275, PMID: 37556623)
• CMV serostatus and latent viral load are critical confounders that can alter the T-cell memory space and chronic innate activation. (Direct, High; PMID: 29375577)

Risks/limitations

• Primary blood dendritic cells are present in low frequencies, potentially limiting the number of TLR ligands that can be tested per subject. (Direct, High; PMID: 20100933)
• The use of ex vivo stimulation may not fully replicate the complex systemic 'bow tie' architecture where metabolic and endocrine systems interact during an in vivo vaccine response. (Derived, Medium; PMID: 29375577, PMID: 37556623)

Falsification criteria

• The hypothesis is falsified if vaccine responders and non-responders with similar basal inflammaging levels show no significant difference in the fold-change of TLR-induced IL-12p40 or IFN-α production. (Derived, Low; PMID: 20100933)

Methodology

Design

This is a longitudinal cohort study of community-dwelling older adults (aged ≥65) receiving the seasonal trivalent inactivated influenza vaccine. Participants will provide heparinized blood samples at baseline (Day 0, pre-vaccination) and 48 hours post-vaccination to assess acute innate activation. A final serum sample will be collected 28–42 days post-vaccination for antibody titer assessment. The study will evaluate the correlation between pre-existing basal inflammaging (cytokine profiles) and the subsequent dynamic range of the TLR-mediated innate response.

Model/system (justification)

The study uses primary human peripheral blood mononuclear cells (PBMCs) and enriched dendritic cell (DC) populations. This system is chosen because primary myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) are the critical antigen-presenting cells linking innate and adaptive immunity, and their function in aging cannot be fully replicated in immortalized lines or monocyte-derived models which may mask age-specific signaling defects. (Direct; PMID: 20100933, PMID: 29375577)

Sample size & power

Based on previous research showing that approximately 55% of older adults fail to seroconvert to at least one vaccine strain, a sample size of n=100 older adults is estimated to provide >80% power at alpha=0.05 to detect a 20% difference in the dynamic range of TLR-induced IL-12p40 production between vaccine responders and non-responders. Participant recruitment will target a balanced sex distribution to account for known sexual dimorphism in inflammaging biomarkers. (Derived; PMID: 20100933)

Interventions & assays

PBMCs isolated via Ficoll-Hypaque gradient will be stimulated for 6 hours with selective TLR ligands: Poly I:C (TLR3, 50 µg/ml) and R848 (TLR7/8, 3 µg/ml). Intracellular cytokine staining (ICS) will quantify TNF-α, IL-6, IL-12p40, and IFN-α in mDCs (Lin- HLA-DR+ CD11c+) and pDCs (Lin- HLA-DR+ CD123+) using multicolor flow cytometry. Hemagglutination inhibition (HAI) assays will determine strain-specific antibody titers against the vaccine components to classify subjects as seroprotected (post-vaccine titer ≥1:64) or seroconverted (≥4-fold increase). (Direct; PMID: 20100933)

Controls & replicates

Experimental controls include unstimulated (vehicle-only) cultures for each subject to determine basal cytokine production and isotype-matched controls for all flow cytometry antibodies. Technical replicates will be performed for HAI assays and qPCR. Biological replicates are inherently addressed by the 100-subject cohort. Boolean gating strategies will be employed to identify multifunctional (triple-positive) cytokine-producing cells. (Direct; PMID: 20100933)

Endpoints & Go/No-Go

The primary decisive metric is the fold-change of TLR-induced IL-12p40 production in mDCs and IFN-α in pDCs relative to baseline. Success is defined as a statistically significant inverse correlation between basal (unstimulated) cytokine levels and the stimulated dynamic range (p<0.05). A Go/No-Go threshold for the mechanistic link is established if the 'ceiling effect' (low induction in high-basal individuals) significantly predicts HAI seroprotection status. (Derived; PMID: 20100933)

Statistical analysis

Data will be analyzed using mixed-effects multivariable models to estimate the effect of basal inflammaging on TLR-induced cytokine percentage changes, adjusting for age, sex, BMI, and comorbid conditions. Generalized linear models and Poisson regression will estimate the effect of cytokine dynamic range on the number of strains achieving seroprotection. All-cause mortality risk signals associated with high baseline NLR will be analyzed using Cox proportional hazards regression in secondary analyses.

Confounders & handling

Potential confounders including biological sex, BMI, and pre-vaccination antibody titers will be handled via multivariable adjustment. Chronic CMV infection status, which significantly influences the T-cell memory space and innate activation levels, will be assessed via IgG serology and included as a covariate. The analysis will specifically test for sex-by-NLR interaction terms to determine if the inflammatory 'ceiling' differs significantly between men and women.

Risks/limitations

The primary risk is the low frequency of circulating pDCs in older adults, which may lead to insufficient cell counts for complex multi-ligand stimulation panels. This will be mitigated by high-volume blood collection (50-100ml) and the use of an LSR II instrument to collect 0.5–1 million events per sample. Another limitation is that ex vivo stimulation might not capture the full systemic 'bow tie' metabolic environment present in vivo.

Bioethics & QC

The study will be conducted in accordance with the Declaration of Helsinki and Good Clinical Practice. All participants must provide written informed consent under a protocol approved by the local Ethics Committee. Quality control includes routine calibration of automated hematology analyzers and flow cytometers, and negative mycoplasma testing for all cell processing environments. Reagent lot traceability will be maintained via an electronic lab notebook. (Direct; PMID: 20100933, PMID: 41462275)

Unverified Citations

The following sources failed to support their assigned claims after 3 verification rounds designed to ensure only high-confidence, relevant references are retained:

• PMID: 20100933 — This is a longitudinal cohort study of community-dwelling older adults (aged ≥65) receiving the seasonal trivalent inact...
  Failed: mechanism — The paper does not collect samples at 48 hours post-vaccination; it repeat tests TLR responses 4-6 weeks post-vaccination.
• PMID: 41462275 — This is a longitudinal cohort study of community-dwelling older adults (aged ≥65) receiving the seasonal trivalent inact...
  Failed: mechanism,disease — The paper studies COVID-19 patients, not healthy community-dwelling adults receiving influenza vaccines, and does not involve vaccine-response dynamics.
• PMID: 41462275 — Based on previous research showing that approximately 55% of older adults fail to seroconvert to at least one vaccine st...
  Failed: conclusion,entities — The paper supports sexual dimorphism in NLR-associated mortality but contains no data on vaccine seroconversion or IL-12p40.
• PMID: 20100933 — Data will be analyzed using mixed-effects multivariable models to estimate the effect of basal inflammaging on TLR-induc...
  Failed: entities — The paper does not measure or analyze Neutrophil-to-Lymphocyte Ratio (NLR).
• PMID: 41462275 — Data will be analyzed using mixed-effects multivariable models to estimate the effect of basal inflammaging on TLR-induc...
  Failed: conclusion — The paper supports the Cox regression analysis of NLR but does not contain data or models regarding TLR-induced cytokine percentage changes or vaccine seroprotection.