A Plasma Proteomic Ageing Clock Reflects Advanced Ageing in People with Untreated HIV and its Reduction Under Antiretroviral Therapy

This longitudinal study demonstrates that untreated HIV infection drives accelerated proteomic ageing, which is significantly reversed and decelerated upon the initiation of suppressive antiretroviral therapy, highlighting the critical importance of minimizing the duration of untreated infection.

The Gist

The Big Picture: The Body's "Biological Clock" vs. The "Wall Clock"

Imagine you have two clocks.

1. The Wall Clock: This is your chronological age. It ticks forward at the same speed for everyone. If you were born in 1980, you are 44 years old, no matter what.
2. The Body Clock: This is your biological age. It measures how fast your cells are actually wearing out. Sometimes, due to stress, illness, or bad habits, this clock runs faster than the Wall Clock. This is called "advanced ageing."

This study is about people living with HIV. The researchers wanted to know: Does untreated HIV make the Body Clock run super fast? And does taking medication (ART) slow it back down?

The Experiment: A Long-Term Time Travel Study

The researchers used a unique group of people from the Swiss HIV Cohort Study. These were special participants who, for various reasons, waited a long time (up to 8 years) before starting HIV medication. This gave the scientists a rare "before and after" snapshot.

They used a high-tech tool called a Plasma Proteomic Ageing Clock (PAC).

• The Analogy: Think of your blood as a soup. Inside this soup are thousands of tiny ingredients called proteins. These proteins are like the "ingredients" of your body's health.
• The Tool: The researchers created a recipe (a computer model) that looks at the mix of these 416 specific protein ingredients in your blood soup. Based on the recipe, it predicts your "Body Clock" age.

What They Found

1. The "Untreated" Phase: The Body Clock Speeds Up

When these participants were living with HIV but not taking medication, their blood showed a chaotic mix of proteins.

• The Result: Their "Body Clock" was ticking much faster than their "Wall Clock." On average, their bodies looked 6 years older than they actually were.
• The Metaphor: Imagine a car engine running without oil. It's grinding, smoking, and wearing out parts way too fast. The HIV virus was causing inflammation (like the engine smoke), making the body age prematurely.

2. The "Treatment" Phase: The Clock Slows Down

When these same people started taking Antiretroviral Therapy (ART) and the virus was suppressed (stopped replicating), they gave blood samples again.

• The Result: The protein mix in their blood changed. The "Body Clock" immediately slowed down. The "extra" 6 years of ageing disappeared.
• The Metaphor: They put the oil back in the engine. The grinding stopped, the smoke cleared, and the engine started running smoothly again. The body began to heal and return to a normal pace of ageing.

The Surprise: It's Not Just About Immune Cells

For a long time, doctors thought that if HIV treatment fixed the immune system (specifically the CD4 and CD8 T-cells, which are the body's "soldiers"), then ageing would stop.

• The Finding: The researchers checked the "soldier" counts. They found that even though the soldiers were recovering, that wasn't the main reason the ageing clock slowed down.
• The Analogy: It's like fixing a house. You might fix the front door (the immune cells), but the real reason the house stopped falling apart was that you fixed the leaking roof and the broken pipes (the inflammation and protein signals).
• The Takeaway: The medication stops the virus, which stops the "engine smoke" (inflammation), which slows the ageing clock. This happens even if the immune cell counts haven't fully recovered yet.

Why This Matters

1. Start Early: The study shows that the damage of untreated HIV is real and speeds up ageing. Starting medication immediately is crucial to stop the "engine" from wearing out.
2. Reversibility: The good news is that this damage isn't permanent. Once treatment starts, the body can "rewind" the clock and start ageing normally again.
3. New Way to Measure Health: This study proves that looking at proteins in the blood is a very sensitive way to see how well a treatment is working, perhaps even better than just counting immune cells.

In a Nutshell

Think of HIV as a thief that steals time from your body, making you age faster than you should. This study proves that medication acts like a time machine. It stops the thief, repairs the damage, and helps your body get back to its natural, healthy pace of ageing. The best part? It works, and it works quickly.

Technical Summary

1. Problem Statement

People living with HIV (PWH) face an increased risk of age-related comorbidities (e.g., cardiovascular disease, osteoporosis) and serious disease endpoints, even when on effective antiretroviral therapy (ART). This phenomenon is linked to "advanced ageing," where biological age exceeds chronological age. While previous studies using telomere length and epigenetic clocks (DNA methylation) have shown that untreated HIV accelerates ageing and that ART can reverse this, the molecular mechanisms remain incompletely understood. Specifically, there is a lack of data on how the functional proteome responds to untreated HIV and ART initiation, and whether standard immunological markers (CD4+/CD8+ T-cell counts) fully explain the reversal of ageing. Current guidelines recommend immediate ART, making it difficult to study the natural history of untreated HIV in new cohorts; thus, researchers must rely on unique longitudinal cohorts with historical samples.

2. Methodology

The study utilized a longitudinal design leveraging the Swiss HIV Cohort Study (SHCS), which contains banked plasma samples spanning over 17 years.

• Cohorts:
  • Training Cohort: 727 PWH on long-term suppressive ART (undetectable HIV viraemia). Plasma samples were used to train the ageing clock.
  • Test Cohort: 80 PWH with 4 longitudinal samples each:
    • T1 & T2: During untreated HIV infection (>3 years apart).
    • T3 & T4: During suppressive ART (>3 years apart, with HIV RNA <100 copies/mL).
• Proteomic Profiling:
  • Automated sample preparation and high-throughput mass spectrometry (Data Independent Acquisition - DIA).
  • 1,456 proteins detected; 626 common across cohorts; 416 proteins selected for the final model.
  • Batch correction performed using ComBat.
• Modeling (Proteomic Ageing Clock - PAC):
  • Feature Selection: ElasticNet regression to select predictive proteins.
  • Prediction Model: Ridge regression trained to predict chronological age based on protein abundance.
  • Definition of Advanced Ageing: Calculated as PAC-predicted age minus chronological age.
  • Adjustments: Models were adjusted for sex, study site, and genetic ancestry (principal components).
• Statistical Analysis:
  • Linear regression to compare advanced ageing between untreated and treated states.
  • Mediation Analysis: Used to determine if the reduction in ageing was driven by HIV viraemia reduction or immune reconstitution (CD4+/CD8+ counts and ratios).
  • Comparison: Results were cross-validated against a previously published epigenetic ageing clock (EAC) study on the same cohort.

3. Key Contributions

• First Proteomic Clock for HIV: This is the first study to apply a proteomic ageing clock specifically trained on PWH to quantify the impact of untreated HIV and ART on the functional proteome.
• Temporal Resolution: Demonstrated that the proteome captures acute biological changes (immediate response to infection and treatment) faster than epigenetic clocks, which integrate signals over longer time scales.
• Mechanistic Insight: Provided evidence that the reversal of advanced ageing under ART is not mediated by standard immune reconstitution markers (CD4+/CD8+ counts), suggesting the proteome captures ageing signals beyond T-cell recovery.
• Pathway Identification: Identified coagulation, complement, and xenobiotic metabolism pathways as key drivers of the ageing signal in the PAC.

4. Key Results

• Accelerated Ageing in Untreated HIV: Participants showed significantly higher proteomic age during untreated infection compared to when on suppressive ART.
  • Mean Difference: 5.99 years (95% CI 4.25, 7.72; p = 0.0001).
• Rapid Reversal with ART:
  • A statistically significant drop in advanced ageing was observed immediately at T3 (first sample after viral suppression, median 1.55 years post-ART start).
  • Linear interpolation suggests that with long-term suppressive ART, advanced ageing progressively normalizes, converging toward chronological age approximately 16.4 years after ART initiation.
• Mediation Analysis Findings:
  • Viraemia: Reduction in HIV viral load significantly mediated the decrease in advanced ageing (ACME $\beta$ = -0.079, p < 0.001).
  • Immune Reconstitution: CD4+ count, CD8+ count, and CD4:CD8 ratio did not significantly mediate the reversal of ageing (p > 0.05 for all). This indicates that the proteomic improvement is driven by factors other than just T-cell count recovery.
• PAC vs. EAC Comparison:
  • Moderate correlation between PAC and EAC (r = 0.44).
  • T1 (Untreated): PAC showed significantly higher advanced ageing than EAC, suggesting the proteome reacts more rapidly to acute infection.
  • T3 (Post-ART): PAC showed slightly lower (though not statistically significant) advanced ageing than EAC, indicating faster normalization.
  • Variability: PAC had higher standard deviation, likely due to a smaller training set (727 vs. 8000 for Horvath EAC).

5. Significance and Implications

• Clinical Urgency: The findings reinforce the critical importance of prompt ART initiation. The ~6-year reduction in proteomic age upon viral suppression highlights the reversibility of HIV-induced biological ageing.
• Beyond Immune Counts: The study challenges the assumption that immune reconstitution (CD4/CD8 recovery) is the sole driver of health improvement in PWH. It suggests that persistent inflammation or other proteomic pathways (coagulation/complement) remain active drivers of ageing even when T-cell counts recover.
• Biomarker Potential: The plasma proteomic clock serves as a sensitive, clinically interpretable endpoint for monitoring recovery and evaluating adjunctive interventions aimed at reducing residual inflammation or comorbidities in PWH.
• Future Directions: The authors call for future studies to link these proteomic changes to hard clinical outcomes (e.g., cardiovascular events) and to train PACs on HIV-negative cohorts to better isolate HIV-specific ageing signals.

Conclusion: The study successfully demonstrates that untreated HIV infection causes a significant, measurable acceleration of biological ageing in the plasma proteome, which is substantially reversed upon successful viral suppression. This reversal occurs independently of standard T-cell count recovery, pointing to the need for therapeutic strategies that target broader inflammatory and metabolic pathways in addition to viral suppression.