Background

Aging profoundly remodels the immune system. Adaptive immunity becomes less efficient, chronic low-grade inflammation accumulates, and the homeostasis of tissue-resident immune cells is disrupted. This immune aging is directly linked to increased susceptibility to infection, impaired vaccine responses, multimorbidity, and frailty, making it a key target for geroscience intervention clinical trials.

The question was, 'What should be measured?' Dozens of candidate biomarkers exist, including T cell receptor diversity, CD4+/CD8+ ratio, inflammatory cytokine panels, and epigenetic clocks. However, there was no systematic way to compare and select these biomarkers in the context of clinical trials. Given that each biomarker has different measurement platforms, levels of analytical standardization, and predictive power, comparing results across different trials was virtually impossible.

Key Findings

A Framework of Five Selection Criteria

This Perspective article, co-authored by researchers from more than 26 international institutions, including Stanford, Duke, and Mount Sinai, proposes five evaluation axes for applying immune aging biomarkers to clinical trials: mechanistic validity, analytical rigor, predictive power, practical applicability, and translational potential. Each axis is designed to be independent yet complementary, revealing the strengths and weaknesses of a single marker in a multidimensional way.

Mapping the Five Dimensions of Immune Aging and Biomarkers

The authors divide immune aging into five dimensions: immunosenescence, inflammaging, hematopoietic dysfunction, stromal/niche changes, and changes in tissue-resident immune cells. They systematically arranged and evaluated key candidate biomarkers, such as T cell receptor repertoire diversity, CD28⁻ T cell frequency, thymic output (TREC-based), neutrophil-to-lymphocyte ratio (NLR), IgG N-glycosylation patterns, senescence-associated secretory phenotype (SASP) markers, and DNA methylation inflammatory clocks (iAge), within this framework.

The evaluation revealed that composite markers, such as multidimensional immune profiles and integrated inflammaging scores, which encompass multiple axes simultaneously, performed better than single markers. It also clarified that longitudinal assessment is more suitable than a single snapshot, and that functional readouts that reflect actual immune function are more appropriate as endpoints for clinical trials than cell counts.

Significance and Prospects

This framework can be used as a standard tool for quantifying immune fitness and resilience in large-scale aging intervention competitions, such as XPRIZE Healthspan. It opens the door to comparing the effects of different anti-aging interventions—such as metformin, rapamycin analogs, and senolytics—using the same criteria.

However, the authors also clearly identify the challenges that need to be addressed. The existing biomarker validation cohorts lack population diversity, the standardization of measurement protocols is insufficient, and the understanding of the mechanistic interrelationships between biomarkers is still limited. Emerging approaches, such as single-cell multiomics, systems-level immune network analysis, and sex-specific immune aging patterns, have great potential, but there are still technical and economic barriers to scaling them up to clinical trial size. Long-term predictive validation also requires the accumulation of longitudinal data.

Ultimately, this paper presents not a definitive answer, but a coordinate system that poses the right questions. Its significance lies in the fact that it is the first attempt to reach a consensus on the criteria that should be met in order to implement immune aging biomarkers in clinical trials.