IMMClock reveals immune aging and T cell function at single-cell resolution
Posted on: 19 January 2025 , updated on: 20 January 2025
Preprint posted on 15 November 2024
Categories: bioinformatics, genomics, immunology
Background
Age is not just a number anymore! Aging clocks, tools specifically designed to infer your biological age, how old your cells actually are, are gaining in popularity. So much so that the Biomarkers of Aging Consortium created a contest to benchmark and identify the top clock, or should I say clockmaker, in exchange for fame and a generous cash prize (1).
Methylation clocks, a popular type of aging clock, analyze methylation patterns at CpG sites in the DNA sequence. While accurate, obtaining whole-epigenome data at the single-cell level remains challenging. With this in mind, Schmidt and colleagues developed IMMClock (IMMune Cell clock), a machine learning model that infers the biological age of immune cells from single-cell (sc) and bulk transcriptomic data. The authors then used IMMClock to gain insight into the age-old question: “how does aging influence T cell functionality?”.
Key findings
Study design: embracing large and diverse cohorts
Schmidt and team developed individual IMMClocks for CD4+ T cells, CD8+ T cells, natural killer (NK) cells, B cells and monocytes. The development of each IMMClock can be split into a training, validation and testing phase. Unlike previous studies that trained aging clocks on small datasets, Schmidt et al. trained all IMMClocks on OneK1K- a massive single-cell dataset of ~1 million peripheral blood mononuclear cells (PBMCs) from a total of 982 healthy donors, ages 19-97 years-old. During the training phase, diversity is key. Training a model on representative unbiased data enables it to make accurate predictions later on.
Next came the validation phase, during which a different set of data was used to assess model accuracy and compare model performances during training. This step was used to finetune model parameters. The authors used scRNA-seq and bulk RNA-sequencing data from a total of seven large, diverse cohorts of healthy donors’ blood immune cells to validate the different IMMClocks. Finally, the testing phase approximated the model’s true performance on data it had not seen before.
The biological age of cells reflects the chronological age of individuals
Aging of the immune system can be attributed either to 1) differentiation, whereby a naïve cell differentiates into a specialized functional cell shifting the abundance of both cell states 2) intrinsic age-related changes that occur within the cell itself or 3) a combination of both. Consequently, Schmidt and colleagues generated these different models for each immune cell-type. The models were used to predict the biological age of cells and, subsequently, to assess whether these predictions correlated with chronological age. Predictions from the integrated IMMClock model, which considers both immune cell-type differentiation and intrinsic age-related changes, correlated highly with individual chronological age for CD4+ T cells, CD8+ T cells and NK cells. A lower correlation was observed for both B cells and monocytes, partially attributed to the diminished impact of age on these subsets.
The authors then focused their analysis on T cell aging. Four IMMClock models were trained to account for one of the following: (1) intrinsic age-related changes in naïve/central memory T cells, (2) intrinsic age-related changes in effector memory T cells, (3) shifts in cell-state abundance or (4) both intrinsic age-related changes and cell-state abundance, called the integrated model. All models were tested on six independent scRNA-seq datasets, a total of ~800 CD4+ and CD8+ T cell samples. The integrated IMMClock model was clearly the best predictor of biological age for both CD4+ and CD8+ T cells across cohorts, with the naïve/central memory T cell model not far behind. Models relying strictly on cell-state abundance generally performed poorly. Across cohorts, predictions from the integrated IMMClock model correlated highly with individual chronological age.
Aging hallmarks: CD8+ T cell vulnerability increases with age
Aged T cells exhibit gene signatures specific to telomere shortening, senescence and exhaustion. The authors wanted to assess the correlation between the predicted biological age of cells and age-related gene signatures specifically in CD8+ T cells. As expected, a higher biological age was associated with a reduced expression of telomere maintenance genes and with a higher expression of both senescence- and exhaustion-related gene signatures. Importantly, the predicted biological age of naïve CD8+ T cells was lower than that of memory CD8+ T cells owing to the fact that biological age increases as T cells differentiate.
Within the IMMClock models, genes are categorized based on their weights. If a gene contributes to predicting the biological age of an immune cell it’s weight may increase. Conversely, if a gene does not contribute to predicting biological age it’s weight may decrease. Schmidt et al. selected genes considered predicters of biological age and performed a pathway enrichment analysis on these genes to identify pathways up and down regulated in different immune cell types. Focusing on CD8+ T cells, pathways associated to the positive regulation of cell processes, cell proliferation and translation were down regulated. Pathways related to the regulation of T cell cytotoxicity, differentiation and activation were also downregulated. Upregulated pathways included pathways related to adaptive immune responses and cytokine-mediated signaling, both contributors to “inflammaging”, a low-grade inflammatory state associated with age.
Biological age: an important determinant of health
Schmidt and colleagues investigated whether the biological age of cells can be linked to various clinical outcomes using data from the Framingham Heart Study. The IMMClock predicted an elevated biological age for CD4+ T cells, CD8+ T cells and NK cells in individuals with a history of cancer, heart disease and stroke. Next, the authors analyzed a scRNA-seq dataset of matched blood PBMCs and tumor-infiltrating lymphocytes to compare the biological age of circulating T cells to those of the tumor microenvironment in squamous cell carcinoma patients. Interestingly, the biological age of tumor microenvironment- associated CD8+ T cells was higher than that of circulating T cells.
The influence of T cell activation on biological age
The authors used a CRISPR activation (CRISPRa) perturb-seq dataset of a study by Schmidt, R. et al. to investigate the relationship between the biological age of T cells and their functionality. The naïve/central memory (CM) IMMClock was used to predict the biological age of T cells in this dataset. Plotting the biological age of unstimulated cells against re-stimulated cells revealed that most targets influence biological age in the same manner. However, targets associated with the negative regulation of cytokine production (e.g. MUC1, MAP4K1 and LAT2) behaved differently across conditions. These targets resulted in an elevated biological age in re-stimulated cells compared to unstimulated cells. The authors also demonstrated that biologically-young T cells generally have a higher activation score and vice versa. As suggested by the authors, it would be exciting to pinpoint which perturbations specifically generate young and highly activated T cells for therapeutic purposes.
Why is this work important
Unlike previous aging clocks, the IMMClock was trained on a large-scale scRNA-seq dataset contributing to its robustness. The authors manage to extensively demonstrate that their model accurately predicts the biological age of CD4+ T cells, CD8+ T cells and NK cells. The use of the IMMClock to analyze a previously published CRISPRa perturb-seq dataset to gain insight into T cell functionality with age, demonstrates the versatility of this tool in identifying “rejuvenated” T cells using solely their transcriptomic profile.
As we age, our innate and adaptative immune system fall short. This phenomenon, coined for the first time in 1969 by Dr. Roy Walford as “immunosenescence”, is characterized by a complete remodeling of the immune system resulting in an increased susceptibility to infections, cancers and autoimmune diseases in older individuals (2). Understanding aging at both the cellular and molecular level may pave the way for the development of novel therapeutic strategies that promote healthy aging- that is, growing older without disease onset.
Questions for the authors
1. It appears that most researchers develop aging clocks using only one data type, such as epigenomic or transcriptomic data. Do you envision developing a multi-omic aging clock that takes as input different “omics” – the epigenome, transcriptome and proteome? Would such a clock better predict the biological age of a cell or is a simpler model more appropriate?
2. Did the IMMClock discover any cell-type specific “aging signatures” that were largely conserved across individuals?
References
(1) “Biomarkers of Aging Challenge”. The Biomarkers of Aging Consortium, 2024, https://www.agingconsortium.org/challenge
(2) Liu, Zaoqu, et al. “Immunosenescence: molecular mechanisms and diseases.” Signal transduction and targeted therapy 8.1 (2023): 200.
doi: https://doi.org/10.1242/prelights.39515
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