By Hendra Lukman 
Every moment, the bone marrow diligently generates millions of fresh blood and immune cells, a testament to its vital role in sustaining life. This ceaseless renewal hinges on a delicate equilibrium: hematopoietic stem cells (HSCs), the progenitors of all blood cells, must exist in harmony with supportive stromal cells and a complex network of immune signals. However, this intricate balance is increasingly recognized as vulnerable, particularly as individuals age or face chronic inflammatory conditions. Over time, disruptions in the communication pathways between these cellular components can compromise normal HSC renewal, allowing for the unchecked proliferation of mutated HSCs. This phenomenon, known as clonal hematopoiesis of indeterminate potential (CHIP), is not uncommon, affecting approximately 10 to 20% of adults over 60 and nearly 30% of those over 80. While CHIP itself is often asymptomatic, it significantly elevates the risk of developing blood cancers, increasing it tenfold, and doubles the likelihood of experiencing cardiovascular disease and premature death. A related disorder, myelodysplastic syndrome (MDS), characterized by clonal HSCs leading to inefficient blood cell production and progressive bone marrow failure, affects up to 20 in every 100,000 adults over 70. A concerning aspect of MDS is that around 30% of these cases can advance to acute myeloid leukemia (AML), an aggressive and frequently fatal malignancy. Despite the profound impact of these conditions, the precise contribution of the bone marrow microenvironment – the intricate ecosystem that nurtures HSCs – to their development has remained an area of intense scientific inquiry.
Mapping Hidden Cellular Shifts in the Bone Marrow
To shed light on how mutated HSC clones gain dominance within the bone marrow, an international research consortium, co-led by Dr. Judith Zaugg from the European Molecular Biology Laboratory (EMBL) and the University of Basel, and Dr. Borhane Guezguez from the University Medical Center Mainz, undertook an extensive molecular and spatial analysis of human bone marrow samples. This ambitious research was facilitated by the BoHemE cohort study, a collaborative effort with Professor Uwe Platzbecker at the National Center for Tumor Diseases (NCT) Dresden.
The researchers employed a multi-faceted approach, integrating single-cell RNA sequencing, advanced biopsy imaging techniques, proteomics, and sophisticated co-culture models. This comprehensive methodology allowed them to construct a highly detailed molecular and spatial map of the bone marrow microenvironment in both healthy donors, including those identified as having CHIP, and patients diagnosed with MDS. The analysis yielded a surprising revelation: a significant cellular shift occurs long before any overt clinical symptoms become apparent. Specifically, the team identified a gradual replacement of the normal mesenchymal stromal cells (MSCs), which are crucial for supporting HSC function, by a distinct population of inflammatory stromal cells.
"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. Zaugg, a co-senior author of the study, EMBL Group Leader, and Professor at Basel University. This observation underscores that the changes within the bone marrow’s supportive structure can precede the detection of any disease manifestation.
The Inflammatory Cascade: iMSCs and the Amplification Loop
The newly identified inflammatory MSCs (iMSCs) exhibit a distinct functional profile compared to their healthy counterparts. 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. Once activated, these T cells further intensify the inflammatory activity within the bone marrow. This creates a self-perpetuating, or feed-forward, loop. This chronic inflammation disrupts the normal processes of blood formation and contributes to adverse vascular changes within the marrow.
Deciphering the Drivers of Bone Marrow Inflammation
A critical question addressed by the research was whether mutated hematopoietic cells in MDS directly initiate this inflammatory cascade. Through the application of SpliceUp, a sophisticated computational method developed by co-lead author and EMBL alumnus Dr. Maksim Kholmatov in collaboration with Dr. Pedro Moura and Professor Eva Hellström-Lindberg from the Karolinska Institute, the researchers were able to meticulously separate mutated from non-mutated cells within single-cell datasets. SpliceUp achieves this by identifying abnormal RNA-splicing patterns, a hallmark of cellular mutations.
Intriguingly, the study found no direct evidence that mutated hematopoietic cells in MDS are the primary instigators of the observed inflammatory response. Instead, within the context of MDS, the inflammatory network operating within the microenvironment appears to become dominant, displacing a significant portion of the bone marrow’s normal regenerative architecture.
"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 Dr. Karin Prummel, a co-lead author and EMBL postdoctoral researcher. CXCL12 plays a crucial role in retaining blood stem cells within their protective niche in the bone marrow. Its absence or dysfunction could lead to stem cells migrating away from their optimal environment, hindering proper regeneration.
Dr. Maksim Kholmatov further elaborated on these findings: "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 suggests that the microenvironment’s inherent inflammatory state, rather than a direct attack by mutated cells, plays a more significant role in the early stages of disease development.
Inflammation as an Early Sentinel of Blood Disease
The collective findings of this research strongly indicate that inflammation acts as a central player in the earliest phases of bone marrow disorders, even preceding the manifestation of overt disease. This pivotal role positions the bone marrow microenvironment, often referred to as the bone marrow niche, as a critical target for future therapeutic interventions. By shifting the focus from solely targeting mutated cells to understanding and modulating the supporting ecosystem, the research opens new avenues for early intervention and prevention strategies.
The implications for treatment are significant. Anti-inflammatory drugs or therapies designed to modulate interferon signaling could potentially preserve bone marrow function in older adults who have CHIP. Furthermore, combining these microenvironment-targeted therapies with existing treatments could offer a powerful strategy to slow or even prevent the progression from CHIP to more aggressive conditions like MDS or AML. The unique molecular signatures of iMSCs and interferon-responsive T cells also hold promise as 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 Dr. Guezguez, a 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 could revolutionize the management of blood cancers.
‘Inflammaging’ and Broader Health Implications
Beyond its direct impact on blood disorders, this research contributes significantly to a broader understanding of ‘inflammaging.’ This term describes the low-level, chronic inflammation that is increasingly recognized as a contributing factor to a wide range of age-related conditions, including various cancers, cardiovascular diseases, and metabolic disorders. The bone marrow, once viewed primarily as a site for blood cell production, now appears to be both a recipient of and a contributor to systemic inflammatory aging processes.
By elucidating the intricate interactions between immune and stromal cells that drive these age-related inflammatory changes within the bone marrow, the study provides a compelling model for investigating similar inflammatory remodeling processes in other myeloid malignancies and advanced stages of leukemia.
"It will be crucial to study these processes over time; our current findings are based on cross-sectional data," emphasized Dr. Zaugg. "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." Understanding the persistence of these inflammatory signals, even after successful cell replacement therapies, is vital for optimizing long-term patient outcomes.
Adding further weight to these findings, the research is published alongside a complementary study that also delves into the MDS bone marrow microenvironment. This parallel investigation, published in the same issue of Nature Communications and led by Dr. Marc Raaijmakers from Erasmus MC Cancer Institute in Rotterdam, collectively provides a more comprehensive and nuanced view of inflammatory remodeling during the nascent stages of bone marrow disease. Together, these studies offer a powerful testament to the collaborative spirit of scientific inquiry and its potential to unlock new therapeutic strategies for devastating diseases.
The groundbreaking research involved a broad international collaboration, with contributions from 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. It also included participation from DKTK partner institutions, such as the German Cancer Research Center (DKFZ) and the NCT Dresden. The study received funding from the DKTK-CHOICE program, an ERC grant for EpiNicheAML awarded to Dr. Judith Zaugg, the MCSA-funded ITN ENHPATHY program, EMBO, the Swiss National Foundation, and the José Carreras Leukemia Foundation. This multidisciplinary and internationally supported effort underscores the global importance of addressing the complex mechanisms underlying blood disorders and age-related inflammation.
