Introduction of Mature Mast Cells into Bone Marrow Alters Bone Metabolism in Growing Mice
Do mast cells (immune cells) that live in bone marrow actually affect bone health, and can we create a mouse model to study this?
College of Health researcher(s)
Abstract
There is evidence that mast cells contribute to skeletal response to injury, but it is less clear whether these immune cells directly influence normal bone growth and turnover. Mature mast cells are common in the bone marrow of humans and rats, but have not been convincingly demonstrated to be present in the bone marrow of healthy mice, potentially limiting the mouse as a model for characterizing the full range of mast cell/bone cell interactions. An initial goal of this investigation was to comprehensively screen seven strains of mice for mature mast cells in bone marrow. Finding none, we then investigated three approaches to home these cells to the marrow of mice unable to generate mast cells: (1) administration of soluble kit ligand to membrane kit ligand-deficient KitSl/Sld mice, (2) adoptive transfer of wild-type hematopoietic stem cells to kit receptor-deficient KitW/W−v mice, and (3) adoptive transfer of wild-type mouse bone marrow-derived mast cells generated in vitro and delivered intravenously to KitW/W-v mice. Only the third approach was successful. Using this method, we then evaluated the impact of bone marrow-derived mast cells on bone mass, architecture, turnover, and gene expression. The adoptive transfer of mast cells resulted in alterations in cancellous bone microarchitecture and cell populations in the vertebra, and in differential expression of genes associated with bone metabolism in the tibia. Taken together, our results support the concept that bone marrow mast cells influence bone metabolism and suggest that homing mast cells to the bone marrow of mice is a useful model to understand the role of these cells in skeletal health and disease.
Mast Cells and Bone Health: A 5-Question Guide to New Research
The immune system and the skeletal system, though seemingly distinct, are surprisingly interconnected. Our bones are not just a static framework but a dynamic organ in constant communication with other parts of the body, including the cells that defend us from disease. Recent research has uncovered new details about this relationship by focusing on a specific immune cell: the mast cell. This guide breaks down the key findings from a groundbreaking study that developed a new way to explore how these enigmatic cells influence bone health.
What Are Mast Cells and How Do They Affect Bones?
Mast cells are specialized immune cells that play a key role in the body's defense systems. They originate from precursor cells in the bone marrow, but then travel through the bloodstream to mature and take up long-term residence in various body tissues, such as the skin and gut. Once mature, these cells are not found in circulation, suggesting they remain embedded within their chosen tissue for the long haul.
They are best known as initiators of the inflammatory cascade, crucial players in both innate and adaptive immune responses. This function extends to the skeleton, where they contribute to the body's response to injury. In humans and rats, mature mast cells are commonly found within the bone marrow itself, where they can directly interact with bone-forming cells (preosteoblasts) and bone-resorbing cells (preosteoclasts), influencing their function and activity.
Despite this, the exact role of mast cells in normal, day-to-day bone growth and maintenance has been a subject of debate. This uncertainty has been partly due to challenges in studying them in one of the most common animal models used in biomedical research: the mouse.
Why Is It Difficult to Study Mast Cells in Mouse Bones?
The central difficulty, as confirmed by this study, is that mature mast cells are not normally present in the bone marrow of healthy mice. This is a fundamental biological difference compared to humans and rats, where these cells are a common feature of the bone marrow environment.
The researchers came to this conclusion after an exhaustive review of over 1000 histological bone specimens from seven different mouse strains (B6, BALB/cJ, C3H/HeJ, DBA, ICR, Swiss Webster, and WBB6F1/J). In every case, they found no evidence of mature mast cells.
Furthermore, this wasn't just true for healthy mice. The researchers confirmed this absence even after subjecting the mice to a battery of interventions known to dramatically alter bone, including hormonal changes from ovary removal, high-fat diets, simulated microgravity, high-dose radiation, and even spinal cord injury. This stark difference makes the standard mouse a problematic model for investigating the direct influence of bone marrow mast cells on skeletal health.
How Did Researchers Successfully Introduce Mast Cells into Mouse Bone Marrow?
To solve this long-standing problem, the scientists embarked on a classic scientific detective story, testing three different methods to populate the bone marrow of mast-cell-deficient mice (KitW/W-v mice).
The first two attempts were based on the plausible idea of encouraging the mouse's own cells to mature and settle in the bone marrow. These approaches, which included giving mice a special protein to induce mast cell differentiation and transferring healthy stem cells to restore normal signaling, were ultimately unsuccessful.
Where these attempts to coax the mouse's own cells into maturing in the right place failed, the researchers decided to bypass that step entirely by growing the mast cells outside the body and introducing them fully formed. This clever pivot, the third method, proved successful. It involved a multi-step process:
- Culturing Mast Cells: Researchers first harvested bone marrow cells from healthy, wild-type mice. They grew these cells in a laboratory dish for four weeks with specific growth factors (IL-3 and kit ligand) that guided them to differentiate into mature mast cells.
- Verification: They ran two crucial checks. First, they confirmed the purity of their culture, finding that 96% of the cells were indeed mast cells identified by specific surface markers. Second, and just as importantly, they confirmed the cells were functional. By stimulating the cells with inflammatory triggers, they showed the cells produced TNFα, a key inflammatory molecule, proving they could perform their expected duties.
- Adoptive Transfer: Finally, they injected these lab-grown, fully functional mast cells (a total of 6 million cells) intravenously into the mast-cell-deficient KitW/W-v mice.
This third approach worked. The mature mast cells successfully "homed" to and populated not only the bone marrow but also the skin, spleen, and adipose tissue of the recipient mice, creating what the researchers believe to be the first mouse model with a significant and stable mast cell presence in the bone.
What Were the Effects of Adding Mast Cells to Mouse Bone Marrow?
The introduction of mast cells into the bone marrow had specific, localized effects on the bones. Importantly, these changes occurred without altering the mice's overall body composition, including their body mass, fat mass, or lean mass.
The key findings on bone structure and cell populations were:
- Improved Bone Structure: The cancellous (spongy) bone in the vertebrae became more robust and interconnected. This was measured as a significant increase in connectivity density (a measure of how well-connected and web-like the inner spongy bone is).
- Increased Bone Area: The total area of bone tissue within the marrow space grew. This was confirmed by a higher bone area fraction (the percentage of the marrow space that is occupied by bone tissue).
- More Bone-Resorbing Cells: The perimeter of the bone surface covered by osteoclasts—the cells responsible for breaking down old bone—increased.
At first glance, an increase in bone-resorbing cells might seem to contradict the finding of improved bone structure. However, the genetic data told a more nuanced story. The presence of mast cells triggered the differential expression of 40 genes related to bone metabolism in the tibia. Deeper analysis of these genes suggested an overall reduction in bone turnover and decreased osteoclast function. This indicates that while the number of osteoclasts on the bone surface increased, their actual bone-resorbing activity may have been modulated or even reduced, leading to a net positive effect on bone architecture.
The results paint a complex picture of mast cells as active remodelers. Rather than simply promoting bone loss or formation, they appear to accelerate bone turnover, simultaneously increasing the presence of bone-resorbing cells while spurring structural improvements—a dynamic process that reshapes the entire bone environment.
What Is the Significance of This Study for Human Bone Health?
This research carries several important implications for our understanding of bone biology and disease.
First, it provides strong, direct evidence that mast cells residing within the bone marrow influence bone metabolism. This reinforces the broader concept that immune cells play a critical and active role in maintaining skeletal health.
Second, by successfully introducing mast cells into mouse bone marrow, scientists have created a valuable new tool. This novel mouse model will allow researchers to precisely investigate the specific roles these cells play in both healthy bone maintenance and in the progression of various diseases.
Finally, the findings have a direct connection to human health. In humans, the number of mast cells in the bone marrow increases in conditions like osteoporosis. A rare disease called systemic mastocytosis, where mast cells form compact, multi-focal aggregates in bone marrow, is also often associated with osteoporosis. Crucially, the new mouse model shows a diffuse, spread-out distribution of mast cells, which is more similar to what is seen in healthy humans and rats. This distinction means the model is not just a tool for studying a rare disease, but a more physiologically relevant way to understand the normal role these cells play. This research could be instrumental in helping scientists uncover the mechanisms behind these conditions and explore how targeting mast cells could potentially lead to new treatments for a range of bone disorders.