By Hendra Lukman 
The intricate ecosystem within the bone marrow, a site traditionally recognized for its relentless production of millions of new blood and immune cells every moment, is now understood to play a far more complex and critical role in the genesis of age-related blood disorders. This continuous renewal is orchestrated by a delicate balance between hematopoietic stem cells (HSCs), the supportive stromal cells that create the niche, and a sophisticated network of immune signaling molecules. However, this vital equilibrium is not immutable. As individuals age, or in the presence of chronic inflammation or accumulated somatic mutations, the intricate communication channels within the bone marrow can become disrupted. This breakdown in cellular dialogue can impair normal stem-cell renewal and, crucially, allows for the expansion of mutated HSCs to go unchecked. This insidious process leads to a condition known as clonal hematopoiesis of indeterminate potential (CHIP). CHIP affects a significant portion of the aging population, appearing in approximately 10 to 20% of adults over the age of 60 and escalating to nearly 30% of those over 80.
While individuals diagnosed with CHIP often remain asymptomatic, the condition carries substantial, albeit silent, risks. It increases the likelihood of developing blood cancers by a tenfold margin and demonstrably doubles the risk of cardiovascular disease and premature mortality. A related, and more clinically advanced, disorder is myelodysplastic syndrome (MDS). MDS is characterized by the presence of clonal HSCs that result in inefficient blood-cell production and a progressive failure of the bone marrow’s regenerative capacity. This condition affects a notable segment of the elderly population, with an incidence rate of up to 20 in every 100,000 adults over 70. A concerning statistic associated with MDS is that approximately 30% of these cases will inevitably advance to acute myeloid leukemia (AML), an aggressive and often fatal form of blood cancer.
Despite the profound clinical implications of these disorders, the precise contribution of the bone marrow microenvironment – the complex web of cells, extracellular matrix, and signaling molecules that surrounds and supports HSCs – to their development has remained a significant enigma. Researchers have long suspected that the niche itself plays a role, but the specific mechanisms and the timing of these changes were not well understood.
Mapping Hidden Changes in the Bone Marrow Microenvironment: A Molecular and Spatial Deep Dive
To illuminate how mutated HSC clones gain a competitive advantage and ultimately dominate the bone marrow, an international consortium of researchers, co-led by Judith Zaugg from the European Molecular Biology Laboratory (EMBL) and the University of Basel, and Borhane Guezguez from the University Medical Center Mainz (UMC Mainz), embarked on an extensive molecular and spatial analysis of human bone marrow samples. This groundbreaking research drew upon data from the BoHemE cohort study, a valuable resource developed in collaboration with Uwe Platzbecker at the National Center for Tumor Diseases (NCT) Dresden.
The research team employed a sophisticated suite of cutting-edge technologies to achieve their objectives. Single-cell RNA sequencing allowed for the detailed analysis of gene expression profiles in individual cells, providing an unprecedented level of cellular resolution. Biopsy imaging techniques offered spatial context, revealing the physical arrangement of cells within the bone marrow. Proteomics provided insights into the protein landscape, while co-culture models allowed for the investigation of cellular interactions in a controlled laboratory setting. This multi-faceted approach enabled the researchers to construct a highly detailed map of the bone marrow microenvironment. The analysis encompassed samples from healthy donors, including those who had been identified as having CHIP, as well as patients diagnosed with MDS.
A particularly striking and unexpected revelation from this comprehensive analysis was the identification of a significant cellular shift that begins remarkably early, often long before any clinical signs or symptoms of disease become apparent. The research team observed that a distinct population of inflammatory stromal cells was gradually supplanting the normal mesenchymal stromal cells (MSCs). These typical MSCs are crucial for maintaining the health and function of HSCs, providing essential support for their self-renewal and differentiation.
"I was surprised to observe such pronounced remodeling of the bone marrow microenvironment already in individuals with CHIP, although the underlying cause-and-effect relationships remain unclear," stated Dr. Judith Zaugg, a co-senior author on the study, an EMBL Group Leader, and a Professor at Basel University. Her remark underscores the unexpected early onset of these microenvironmental alterations, even in the pre-symptomatic stages of CHIP.
The Rise of Inflammatory Stromal Cells and Their Cascade Effect
The newly identified population of inflammatory MSCs, or iMSCs, exhibited a fundamentally different functional profile compared to their healthy counterparts. Unlike healthy stromal cells, these iMSCs were found to produce substantial quantities of interferon-induced cytokines and chemokines. These potent signaling molecules have a well-documented role in attracting and activating immune cells, specifically T cells. In this context, the iMSCs essentially orchestrate an inflammatory response by recruiting and activating interferon-responsive T cells, which, in turn, further amplify the inflammatory activity within the bone marrow.
This creates a detrimental positive feedback loop, or a feed-forward loop, that perpetuates chronic inflammation within the bone marrow. This sustained inflammatory state is highly disruptive to the normal processes of blood formation. Moreover, it contributes to the development of vascular changes within the marrow, further compromising its regenerative capacity. This intricate interplay highlights how changes in the supportive niche can directly impact the function of the stem cells it harbors.
Identifying What Drives Bone Marrow Inflammation: A Shift in Focus
A critical question for the researchers was whether the mutated hematopoietic cells themselves were the primary instigators of this escalating inflammatory response. To address this, the team employed an innovative computational method called SpliceUp, developed by co-lead author and EMBL alumnus Maksim Kholmatov in collaboration with Pedro Moura and Eva Hellström-Lindberg from the Karolinska Institute. SpliceUp is designed to identify mutated cells within single-cell datasets by detecting subtle but distinct abnormal RNA-splicing patterns. This powerful tool allowed the researchers to effectively separate mutated cells from their non-mutated counterparts.
Using this approach, the study revealed that in MDS, the inflammatory network within the microenvironment emerges as the dominant force, effectively replacing much of the bone marrow’s normal regenerative architecture. Crucially, the researchers did not find direct evidence that the mutated hematopoietic cells in MDS were the initial trigger for this widespread inflammatory cascade. This finding suggests a paradigm shift in understanding the disease’s origins.
"Another striking observation was that MDS stem cells couldn’t trigger stromal cells to produce CXCL12, an important signal that triggers blood cells to settle in the bone marrow. This failure may help explain why the bone marrow stops working properly," explained Karin Prummel, a co-lead author and an EMBL postdoc. CXCL12, also known as SDF-1, is a key chemokine that guides HSCs to their niche and promotes their retention. Its absence would severely impair the homing and maintenance of functional HSCs.
Dr. Maksim Kholmatov further elaborated on this pivotal finding: "It was quite surprising to see the lack of a direct inflammatory effect that we could attribute to the mutant cells. However, when viewed in the context of changes in the T cell and stromal compartments, it underlines the importance of the bone marrow microenvironment in shaping disease progression." This perspective emphasizes that while the mutated cells are the ultimate culprits in the progression to leukemia, the environment they inhabit plays a crucial role in facilitating their ascent.
Inflammation as an Early Driver of Blood Disease: Therapeutic Implications
The collective findings of this research strongly indicate that chronic inflammation plays a central and causative role in the earliest phases of these blood disorders. This underscores the bone marrow microenvironment, often referred to as the bone marrow niche, as a critical target for therapeutic intervention. By shifting the focus from solely targeting the mutated stem cells to understanding and modulating the entire ecosystem that supports their proliferation, this research opens new avenues for early diagnosis, treatment, and even prevention strategies.
The implications for clinical practice are significant. The study suggests that anti-inflammatory drugs or therapies specifically designed to modulate interferon signaling pathways could be beneficial in preserving bone marrow function in older adults diagnosed with CHIP. Furthermore, combining such targeted anti-inflammatory treatments with therapies that directly address the microenvironmental changes could potentially slow down or even prevent the transition from CHIP to more aggressive conditions like MDS or AML. The unique molecular signatures of iMSCs and interferon-responsive T cells identified in this study may also serve as valuable early biomarkers, enabling the identification of individuals at elevated risk for developing these hematological malignancies.
"Our findings reveal that the bone marrow microenvironment actively shapes the earliest stages of malignant evolution," stated Dr. Borhane Guezguez, Principal Investigator in the Department of Hematology at UMC Mainz and a co-senior author. He added, "As advances in molecular profiling allow us to detect pre-leukemic states years before clinical onset, understanding how stromal and immune cells interact provides a foundation for preventive therapies that intercept disease progression before leukemia develops." This forward-looking perspective highlights the potential of harnessing this new knowledge to intercept disease at its nascent stages.
Inflammaging and the Wider Impact on Age-Related Disease
Beyond the specific context of blood disorders, the findings of this study contribute to a broader and evolving understanding of ‘inflammaging’. Inflammaging refers to the low-level, chronic, and systemic inflammation that is a hallmark of aging and is implicated in the pathogenesis of numerous age-related conditions, including cancer, cardiovascular disease, and metabolic disorders.
The bone marrow, once viewed primarily as a passive site of blood cell production, now appears to be both a victim of and a contributor to systemic inflammatory aging. By elucidating the intricate interactions between immune cells and stromal cells that drive these age-related inflammatory changes within the marrow, this study provides a powerful model for investigating similar inflammatory remodeling processes in other myeloid malignancies and even in advanced stages of leukemia.
Dr. Zaugg acknowledged the need for longitudinal studies to fully grasp the dynamic nature of these processes. "It will be crucial to study these processes over time; our current findings are based on cross-sectional data," she remarked. This observation has profound implications for therapies that aim to replace diseased cells, such as blood stem cell transplantation. A critical question remains: "We are now investigating to what extent the niche retains a ‘memory’ of disease, which could shape how it responds to new, healthy stem cells." This concept of niche memory could influence the long-term success of transplantation and the potential for relapse.
This significant research appears alongside a complementary study that also examines the MDS bone marrow microenvironment, published concurrently in Nature Communications. This parallel study, led by Marc Raaijmakers from the Erasmus MC Cancer Institute in Rotterdam, offers further insights into the inflammatory remodeling occurring in the early phases of bone marrow disease, and together, these two studies provide a more comprehensive and holistic view of these complex pathological processes.
The collaborative nature of this research is evident in the extensive list of participating institutions, including UMC Mainz, University of Basel, University Hospital Dresden, Karolinska Institute Sweden, The Jackson Laboratory USA, Sorbonne University, France, and DKTK partner institutions such as DKFZ and NCT Dresden. Funding for this pivotal work was generously provided by the DKTK-CHOICE programme, an ERC grant (EpiNicheAML) awarded to Judith Zaugg, the MCSA-funded ITN ENHPATHY program, EMBO, the Swiss National Foundation, and the José Carreras Leukämie-Stiftung.
