Researchers discover how stress signals weaken the aging immune system

As we age, our ability to maintain healthy blood and a strong immune system gradually declines, largely because hematopoietic stem cells (HSCs), the cells responsible for producing all blood cell types, begin to lose their effectiveness. Normally, HSCs can both self-renew and generate a balanced mix of blood cells, but over time they produce fewer new cells, favor certain cells such as myeloid cells over lymphoid cells, and struggle to support a robust immune response.

Accumulated cellular damage, shifts in gene activity, ongoing low-level inflammation, and changes in the bone marrow environment, all appear to contribute to this decline. However, the precise mechanisms by which these diverse stresses converge to weaken HSCs have remained unclear.

Researchers from The University of Tokyo, Japan, and St. Jude Children's Research Hospital, USA, sought to uncover a mechanism explaining how age-related stresses drive HSC functional deterioration, focusing on the receptor-interacting protein kinase 3 (RIPK3)-mixed lineage kinase like (MLKL) signaling axis-a pathway traditionally associated with necroptosis, or programmed cell death.

The study was led by Dr. Masayuki Yamashita, an Assistant Member at St. Jude Children's Research Hospital, who, at the time of the investigation, was an Assistant Professor at The Institute of Medical Science, The University of Tokyo. The other co-authors include Dr. Atsushi Iwama from The Institute of Medical Science, The University of Tokyo, and Dr. Yuta Yamada from St. Jude Children's Research Hospital, who was a graduate student at The Institute of Medical Science, The University of Tokyo.

Explaining the motivation behind the study, Dr. Yamashita says, "We discovered an unexpected phenotype in HSCs of MLKL-knockout mice repeatedly treated with 5-fluorouracil, where aging-associated functional changes were markedly attenuated despite no detectable difference in HSC death, prompting us to investigate whether this pathway might induce functional changes beyond cell death." This observation shifted the research focus toward a non-lethal role of MLKL-a concept later highlighted in their study, published in Volume 17 of the journal Nature Communications on April 6, 2026.

To investigate this, the team employed a combination of genetic mouse models, stress treatments, and functional assays. They used wild-type, MLKL-deficient, and RIPK3-deficient mice, along with specialized reporter mice capable of detecting MLKL activation through a Förster resonance energy transfer-based biosensor. Mice were exposed to stressors mimicking aging, including inflammation, replication stress, and oncogenic stress. HSC function was then assessed primarily through bone marrow transplantation, which measures the ability of stem cells to regenerate the blood system.

Complementary analyses included flow cytometry, ex vivo expansion, RNA-seq, assay for transposase-accessible chromatin-seq, high-resolution microscopy, metabolic assays, and mitochondrial analyses, enabling a detailed understanding of how non-lethal MLKL activation impairs HSC function at molecular, cellular, and organelle levels.

The results revealed a novel, non-necroptotic role for MLKL in HSC aging. While MLKL is typically linked to cell death, its activation in HSCs did not increase cell death or reduce cell numbers. Instead, stress-induced MLKL activation was transient and localized to mitochondria, where it caused direct damage, reducing membrane potential, altering mitochondrial structure, and impairing energy production. These changes led HSCs to exhibit hallmark features of aging, such as diminished self-renewal, reduced lymphoid differentiation, and a shift toward myeloid-biased output.

Crucially, deletion or inactivation of MLKL significantly alleviated these defects. MLKL-deficient HSCs maintained regenerative capacity, produced healthier immune cells, displayed lower DNA damage, and preserved mitochondrial function, even under stress or in aged animals. Interestingly, these improvements occurred without substantial changes in gene expression or chromatin accessibility, suggesting that MLKL drives HSC aging primarily through post-transcriptional and organelle-level mechanisms, rather than through transcriptional regulation or inflammation.

These findings have broad implications for understanding aging and potential therapies. By linking diverse stress signals to mitochondrial dysfunction via MLKL, the study identifies a common pathway underlying HSC aging.

In the longer term, this research could lead to therapies that preserve the function of hematopoietic stem cells, ultimately improving recovery and long-term health for patients undergoing chemotherapy, radiation, or transplantation. By revealing how non-lethal activation of cell-death pathways drives stem cell aging, these findings may inspire new classes of mitochondrial-protective or necroptosis-modulating drugs."

Dr. Masayuki Yamashita, Assistant Member, St. Jude Children's Research Hospital

In conclusion, this study uncovers a previously unrecognized role of MLKL as a non-lethal regulator of stem cell aging. Rather than inducing cell death, MLKL acts as a stress-responsive factor that damages mitochondria and drives functional decline in HSCs. These insights not only redefine the role of necroptosis-related proteins but also open new avenues for understanding and potentially intervening in the aging of the hematopoietic system.