Mechanisms of Age-Associated Variation in Immune Cell Regeneration Across Individuals

Geriatrics Medicine Immunology Life Sciences Cell Biology Hematology
aging hematopoietic stem cells clonal tracking lineage bias myelopoiesis

The functional decline of the immune system with aging is characterized by an increase in innate immune cells (e.g., granulocytes) and a decrease in adaptive immune cells (e.g., B cells). This imbalance in the immune system is closely related to the onset of various diseases, particularly myeloid leukemia and immune deficiencies common in the elderly. Although aging is a universal biological process, there are significant inter-individual differences in the pace of aging. The mechanisms underlying these differences, particularly the role of stem cells (e.g., hematopoietic stem cells, HSCs) in immune cell regeneration during aging, remain poorly understood. Understanding the factors driving these differences at the individual level can reveal triggers behind age-associated physiological decline and provide new therapeutic strategies for mitigating aging.

Hematopoietic stem cells (HSCs) play a key role in maintaining tissue homeostasis and regenerating damaged cells. As organisms age, HSCs undergo various functional changes, including increased self-renewal, enhanced myelopoiesis (myeloid differentiation), and reduced lymphopoiesis (lymphoid differentiation). These functional changes are closely linked to molecular alterations, such as DNA damage, epigenetic remodeling, translational defects, and changes in extracellular signaling. Despite the development of several models of HSC aging, the mechanisms underlying inter-individual differences in aging remain unclear. This study aims to use high-throughput single-cell techniques to compare mice of identical chronological ages but exhibiting early or delayed immune aging phenotypes. The results reveal heterogeneous changes in HSCs during aging and their contributions to immune aging phenotypes.

Source of the Paper

This paper was authored by Anna Nogalska, Jiya Eerdeng, Samir Akre, Mary Vergel-Rodriguez, Yeachan Lee, Charles Bramlett, Adnan Y. Chowdhury, Bowen Wang, Colin G. Cess, Stacey D. Finley, and Rong Lu. The authors are affiliated with the Department of Stem Cell Biology and Regenerative Medicine, the Biomedical Engineering Department, and the Quantitative and Computational Biology Department at the University of Southern California (USC). The paper was published online on October 24, 2024, in the journal Cellular & Molecular Immunology, DOI: 10.1038/s41423-024-01225-y.

Research Process and Results

Research Process

  1. Mouse Models and Classification of Immune Aging Phenotypes
    The study utilized C57BL/6J mice and assessed the aging phenotype by calculating the B-cell-to-granulocyte (BG) ratio in peripheral blood. It was found that at 30 months of age, some mice exhibited an early aging phenotype (reduced BG ratio), whereas others showed a delayed aging phenotype (BG ratios similar to younger mice). Based on this, mice were categorized into early aging and delayed aging groups.

  2. Single-Cell Transcriptome Analysis
    Through single-cell RNA sequencing (scRNA-seq) of HSCs from 30-month-old mice, the study compared gene expression profiles in the early and delayed aging groups. It was observed that HSCs from the early aging group significantly upregulated genes associated with aging, myelopoiesis, and stem cell proliferation. In contrast, HSCs from the delayed aging group upregulated genes linked to stem cell regulation and responses to external signals.

  3. Clonal Tracking Experiments
    By employing genetic barcode technology to track HSC clones in young mice transplanted into older mice, the study observed that 30–40% of HSC clones experienced lineage bias changes during aging. HSCs in early aging mice shifted their lineage bias toward the myeloid lineage, while in delayed aging mice, they shifted toward the lymphoid lineage.

  4. CRISPR Gene Knockout Experiments
    The study validated the functional significance of differentially expressed genes using CRISPR knockouts. Deleting genes upregulated in the early aging group (e.g., LGALS9, NME1, SLC25A5) increased the BG ratio, whereas deleting genes upregulated in the delayed aging group (e.g., NEDD4, PREX2) reduced the BG ratio.

  5. Analysis of Clonal Expansion and Exhaustion
    Further analysis revealed pronounced expansion of myeloid clones in early aging mice and lymphoid clones in delayed aging mice. Additionally, some HSC clones in early aging mice exhibited signs of exhaustion, which were absent in delayed aging mice.

Key Results

  1. Inter-Individual Differences in Aging Phenotypes
    The study identified significant inter-individual differences in immune aging phenotypes. Early aging mice showed dramatically reduced BG ratios, while delayed aging mice maintained ratios similar to young mice.

  2. Heterogeneity in HSC Gene Expression
    Transcriptome analysis revealed significant differences between HSCs from early and delayed aging groups. Early aging HSCs were enriched in genes related to myeloid differentiation and stem cell proliferation, whereas delayed aging HSCs exhibited increased expression of genes tied to external signal regulation and stem cell homeostasis.

  3. Lineage Bias Shifts in HSC Clones
    Clonal tracking experiments highlighted that during aging, 30–40% of HSC clones experienced a lineage bias shift. Early aging mice demonstrated myeloid-biased HSC shifts, whereas delayed aging mice showed lymphoid-biased shifts.

  4. Gene Knockout Impact on BG Ratio
    CRISPR knockouts revealed that knocking out genes upregulated in early aging HSCs increased the BG ratio, while knocking out genes upregulated in delayed aging HSCs decreased the BG ratio. These genes play critical roles in regulating immune aging.

  5. Differences in Clonal Expansion and Exhaustion
    Myeloid clonal expansion was more pronounced in early aging mice, whereas lymphoid clonal expansion dominated in delayed aging mice. Additionally, some HSC clones in early aging mice experienced exhaustion, which was not evident in delayed aging mice.

Conclusions and Implications

This study reveals that inter-individual differences in immune aging stem primarily from changes in myelopoiesis within HSCs. While early aging HSCs show a propensity toward myeloid differentiation, delayed aging HSCs lean toward lymphoid differentiation. These changes are closely tied to alterations in HSC clone gene expression and function. Additionally, the study uncovers several key genes linked to immune aging, suggesting potential therapeutic strategies for delaying aging by modulating HSC differentiation.

Scientific and Practical Significance

  1. Scientific Significance
    Using advanced single-cell and clonal tracking technologies, this study provides new insights into the heterogeneous changes in HSCs during aging and their impacts on the immune aging phenotype. It offers a novel perspective on the mechanisms behind inter-individual differences in aging and identifies molecular markers for age-associated physiological decline.

  2. Practical Significance
    By highlighting myelopoiesis as a critical driver of immune aging, the study provides a foundation for the development of anti-aging interventions. Targeted therapies that modulate HSC differentiation could represent a promising avenue for delaying aging-associated immune decline.

Research Highlights

  1. Mechanisms of Inter-Individual Aging Differences
    This study is among the first to uncover how HSC heterogeneity drives significant inter-individual differences in immune aging phenotypes.

  2. Lineage Bias Shifts in HSC Clones
    The discovery that HSC clones undergo lineage bias shifts during aging, shaping immune phenotypes, provides new avenues for aging research at the cellular level.

  3. Key Gene Identification
    Key genes implicated in early and delayed aging were identified and verified through CRISPR knockouts, offering molecular targets for future aging interventions.

  4. Clonal Expansion and Exhaustion Analysis
    A systematic analysis of HSC clone expansion and exhaustion illuminates their differential roles in immune aging pathways.

Summary

Using cutting-edge single-cell and clonal tracking approaches, this study reveals that inter-individual differences in immune aging are largely driven by HSCs and their myeloid differentiation patterns. It highlights the critical impact of a small subset of HSC clones on aging phenotypes and identifies potential therapeutic strategies for delaying immune aging. These findings mark significant progress in understanding the biology of aging and advancing targeted anti-aging treatments.