By Hendra Lukman 
Every moment, the bone marrow hums with activity, a tireless engine generating millions of fresh blood and immune cells. This constant renewal, crucial for maintaining health, relies on a delicate and intricate balance. Hematopoietic stem cells (HSCs), the originators of all blood cells, are supported by a specialized network of stromal cells and a complex symphony of immune signals. This tightly regulated ecosystem ensures that the body has a continuous supply of oxygen-carrying red blood cells, infection-fighting white blood cells, and clot-forming platelets. However, as individuals age or face chronic inflammatory conditions, this finely tuned equilibrium can falter. Disruptions in the communication pathways between these cellular components can lead to reduced normal stem cell renewal and, more ominously, allow for the expansion of mutated HSCs that go unnoticed by the body’s surveillance systems. This silent proliferation of genetically altered stem cells gives rise to a condition known as clonal hematopoiesis of indeterminate potential, or CHIP.
CHIP, once a poorly understood phenomenon, is now recognized as a significant risk factor for serious health issues. Studies have shown that it appears in approximately 10% to 20% of adults over the age of 60 and its prevalence nearly doubles to close to 30% in those over 80. While individuals with CHIP often remain asymptomatic, their underlying risk profile is substantially elevated. They face a tenfold increased risk of developing blood cancers, such as leukemia and lymphoma, and a doubled likelihood of experiencing cardiovascular disease, a leading cause of mortality worldwide. Furthermore, CHIP has been linked to an increased risk of early death.
A related, yet more advanced, disorder is myelodysplastic syndrome (MDS). MDS also involves the proliferation of clonal HSCs but is characterized by inefficient blood cell production and a gradual failure of the bone marrow’s ability to generate healthy blood components. This can lead to severe anemia, increased susceptibility to infections, and bleeding problems. MDS affects a notable portion of the elderly population, with estimates suggesting it occurs in up to 20 out of every 100,000 adults over 70. A particularly concerning aspect of MDS is its potential to transform into acute myeloid leukemia (AML), an aggressive and often fatal form of blood cancer. Approximately 30% of MDS cases will progress to AML, underscoring the critical need for early detection and intervention.
Despite the profound impact of CHIP and MDS on patient health and the clear connection to stem cell dysfunction, the precise contribution of the bone marrow microenvironment – the complex milieu of cells, molecules, and physical structures that surrounds and supports HSCs – to the development and progression of these disorders has remained largely elusive. Understanding this intricate interplay is paramount to unlocking new therapeutic strategies.
Mapping Hidden Changes in the Bone Marrow Microenvironment
To shed light on how mutated HSC clones gain dominance within the bone marrow and to unravel the role of the microenvironment, an international research consortium, 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 (UMC) Mainz, embarked on an extensive molecular and spatial analysis of human bone marrow. This groundbreaking research utilized samples from the BoHemE cohort study, a significant endeavor conducted in collaboration with Uwe Platzbecker at the National Center for Tumor Diseases (NCT) Dresden.
The researchers employed a multi-pronged approach, integrating cutting-edge technologies. Single-cell RNA sequencing allowed for the detailed analysis of gene expression within individual cells, providing unprecedented resolution. Biopsy imaging offered spatial context, revealing the physical arrangement of cells within the bone marrow. Proteomics provided insights into the protein landscape, indicating cellular function and communication. Finally, co-culture models enabled the investigation of cellular interactions in controlled laboratory settings. This comprehensive suite of techniques allowed the team to construct a remarkably detailed map of the bone marrow microenvironment. The analysis included samples from healthy donors, crucially including individuals diagnosed with CHIP, as well as patients suffering from MDS.
Their meticulous investigation revealed a startling and unexpected cellular shift that commences long before any overt clinical signs of disease manifest. The study identified a gradual replacement of the normal mesenchymal stromal cells (MSCs), which are vital for supporting healthy stem cell function, by a distinct population of inflammatory stromal cells. This cellular remodeling, occurring at the fundamental level of the bone marrow’s supportive infrastructure, appears to be a critical early event.
"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 Zaugg, co-senior author of the study, an EMBL Group Leader, and Professor at Basel University. This observation underscores the notion that significant biological changes are underway even when patients feel perfectly healthy.
Identifying the Drivers of Bone Marrow Inflammation
The newly identified inflammatory MSCs (iMSCs) exhibit a distinct functional profile compared to their healthy counterparts. Unlike normal MSCs, these iMSCs secrete substantial quantities of interferon-induced cytokines and chemokines. These signaling molecules act as potent attractants and activators for interferon-responsive T cells, a subset of immune cells. The recruitment and activation of these T cells, in turn, amplify the inflammatory activity within the bone marrow. This creates a self-perpetuating, or feed-forward, loop that sustains chronic inflammation. Such persistent inflammation is detrimental, as it disrupts the normal processes of blood formation and can contribute to pathological vascular changes within the bone marrow, further compromising its function.
Interestingly, the research team did not find evidence suggesting that the mutated hematopoietic cells themselves directly trigger this widespread inflammatory response in MDS. To dissect this complex interaction, they developed and employed a sophisticated computational method called SpliceUp. This innovative tool, developed by co-lead author and EMBL alumnus Maksim Kholmatov in collaboration with Pedro Moura and Eva Hellström-Lindberg from the Karolinska Institute, is capable of identifying mutated cells within single-cell datasets by detecting abnormal RNA-splicing patterns. By separating mutated from non-mutated cells, the researchers could isolate the contributions of each. Their analysis revealed that in MDS, the inflammatory network within the microenvironment becomes the dominant force, effectively supplanting much of the marrow’s normal regenerative structure.
"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," remarked Karin Prummel, co-lead author and an EMBL postdoc. CXCL12 plays a crucial role in anchoring hematopoietic stem cells to their niche, ensuring their retention and proper functioning. Its absence or significant reduction would likely lead to stem cell loss and impaired blood production.
"It was quite surprising to see the lack of a direct inflammatory effect that we could attribute to the mutant cells," added Maksim Kholmatov, co-lead author and EMBL alumnus. "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 sentiment highlights a paradigm shift in understanding these diseases, moving beyond a sole focus on the mutated cells to acknowledging the critical influence of their surrounding environment.
Inflammation as an Early Driver of Blood Disease
These pivotal findings strongly indicate that inflammation plays a central and perhaps initiating role in the earliest phases of blood disorders like CHIP and MDS. The research thus shines a spotlight on the bone marrow microenvironment, often referred to as the bone marrow niche, as a prime therapeutic target. By shifting the focus from solely targeting the mutated stem cells to addressing the entire ecosystem that supports them, the study opens up novel avenues for early intervention and prevention strategies.
The implications for future treatments are significant. Anti-inflammatory drugs, or therapies designed to modulate interferon signaling pathways, could potentially preserve bone marrow function in older adults who are identified as having CHIP. Furthermore, combining such targeted anti-inflammatory approaches with existing therapies that act on the microenvironment could prove to be a powerful strategy to slow down or even prevent the transition from CHIP to more aggressive conditions like MDS and AML. The specific 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 years before clinical symptoms emerge.
"Our findings reveal that the bone marrow microenvironment actively shapes the earliest stages of malignant evolution," stated Guezguez, Principal Investigator in the Department of Hematology at UMC Mainz and co-senior author. "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 proactive approach to disease management represents a significant leap forward in hematological oncology.
Inflammaging and the Wider Impact on Age-Related Disease
The significance of these findings extends beyond the realm of blood disorders. The study contributes to a broader understanding of ‘inflammaging,’ a term used to describe the low-level, chronic inflammation that is increasingly recognized as a fundamental contributor to a wide array of age-related conditions. These include not only cancers but also cardiovascular disease, metabolic disorders, and neurodegenerative diseases. The bone marrow, once viewed primarily as a site of blood cell production, now emerges as a crucial player in both being affected by and contributing to systemic inflammatory aging processes. By elucidating how the intricate interactions between immune cells and stromal cells drive these inflammatory changes within the bone marrow, the study provides a compelling model for investigating similar inflammatory remodeling processes in other myeloid malignancies and in advanced stages of leukemia.
"It will be crucial to study these processes over time; our current findings are based on cross-sectional data," cautioned Zaugg. This highlights the need for longitudinal studies to fully grasp the dynamic nature of these changes. "This has important implications for therapies that replace malignant cells but leave the bone marrow niche intact, such as blood stem cell transplantation. 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 a "niche memory" is particularly intriguing, suggesting that the bone marrow microenvironment might retain a predisposition to disease even after the introduction of healthy stem cells, posing a challenge for successful engraftment and long-term remission.
The research appears in tandem with a complementary study, also published in Nature Communications, which further examines the MDS bone marrow microenvironment. This related work, led by Marc Raaijmakers from Erasmus MC Cancer Institute in Rotterdam, complements the findings of the EMBL-led consortium by providing an even more comprehensive view of inflammatory remodeling during the early phases of bone marrow disease. Together, these two studies offer a robust and multifaceted understanding of a critical aspect of aging and disease pathogenesis.
This extensive research initiative involved a wide array of international collaborators from institutions including UMC Mainz, the University of Basel, University Hospital Dresden, the Karolinska Institute in Sweden, The Jackson Laboratory in the USA, and Sorbonne University in France, alongside partner institutions of the German Cancer Consortium (DKTK), including the German Cancer Research Center (DKFZ) and NCT Dresden. The project was made possible through substantial funding from the DKTK-CHOICE programme, an ERC grant (EpiNicheAML) awarded to Judith Zaugg, the Marie Skłodowska-Curie Actions (MSCA)-funded ITN ENHPATHY program, EMBO, the Swiss National Science Foundation, and the José Carreras Leukämie-Stiftung. This collaborative effort underscores the global commitment to unraveling complex biological mechanisms and developing innovative treatments for devastating diseases.
