Lab-grown human stem cells reveal a shared origin for the brain and body’s immune sentinels

Deep inside every human organ sits a population of long-lived immune cells that never appear on the front lines of infection the way roaming white blood cells do. These tissue-resident macrophages, from the microglia that nurse the brain’s neurons to the Kupffer cells that filter the liver, are established before birth and then maintain themselves locally for decades. Despite their importance to organ health and to diseases ranging from Alzheimer’s to fibrosis, scientists have never been able to watch these cells being born in a human, because the earliest stages of their development unfold in embryonic tissue that is almost impossible to access. A new study from researchers at the Flanders Institute for Biotechnology, or VIB, has now overcome that barrier with a stem-cell model that recreates a critical chapter of human immune development in a laboratory dish.

The research, led by Professor Renzo Mancuso of the VIB-UAntwerp Center for Molecular Neurology and Professor Bart De Strooper of the VIB-KU Leuven Center for Neuroscience and the UK Dementia Research Institute, was published in the journal Developmental Cell. The team’s strategy was to coax human pluripotent stem cells, which can in principle become any cell type in the body, into mimicking a structure most people never think about after birth: the yolk sac. This temporary embryonic organ supports the developing embryo before the placenta and permanent organs are fully formed, and it is where some of the very first immune precursors in human development are generated.

Using carefully timed developmental cues, the researchers generated a population they named inducible yolk sac myeloid progenitors, abbreviated iYSMPs. Molecular profiling showed that these laboratory-grown cells closely resemble the progenitors found during early human development, effectively giving scientists a reproducible snapshot of a developmental stage that has remained hidden from direct study. The technical achievement matters because tissue-resident macrophages are not simply macrophages that happen to settle in an organ; they follow a distinct developmental trajectory that begins in embryonic tissue, and many of their defining properties depend on that early origin.

The critical question was whether these lab-made progenitors would behave like their embryonic counterparts, not merely look like them under a molecular microscope. To find out, the team transplanted the human cells into mouse models. The results were striking: the transplanted progenitors colonized several organs and adapted to their new surroundings with remarkable fidelity. In the brain, they matured into microglia and border-associated macrophages, the two specialized populations that patrol the brain’s boundaries and parenchyma. In the liver, heart, and kidney, they acquired the identities of the macrophages normally found in those tissues, activating tissue-specific gene programs that closely matched those seen in genuine human macrophages from the corresponding organs.

Single-cell analysis, a technique that reads out the gene activity of individual cells rather than averaging across a whole population, confirmed that the transplanted cells were not just surviving but genuinely taking on the molecular character of their adopted tissues. This ability to respond to local environmental cues is what distinguishes a true developmental model from a simple cell replacement experiment. It suggests that the iYSMPs retain the intrinsic potential of their embryonic ancestors, and that the organ itself provides the instructions that shape the final identity of the cell.

One of the most consequential findings of the study concerns timing. The researchers observed that cultures containing a higher proportion of iYSMPs established themselves far more effectively after transplantation than cultures enriched for more mature macrophages. In other words, the younger, more developmentally plastic the cell, the better it could seed a tissue and complete its maturation in place. This principle, that progenitor state rather than terminal differentiation is the key to successful engraftment, could reshape how scientists approach cell-based therapies for the immune system.

Professor Mancuso emphasized the significance of this developmental window. What is exciting, he noted, is that the team can now generate a human progenitor population that not only resembles the cells seen during early development but also behaves like them, providing a way to study in much greater detail how these cells develop and how local tissue environments shape their identity. That combination of molecular fidelity and functional plasticity is what has been missing from previous attempts to model human macrophage biology in the laboratory.

The therapeutic implications extend well beyond basic developmental biology. Because many neurodegenerative and chronic diseases involve dysfunction of tissue-resident macrophages, the ability to generate progenitors that can home to the right tissue and differentiate into the right macrophage type opens a conceptual path toward cell replacement strategies. Mancuso suggested that being able to generate such progenitors could provide a starting point for exploring whether damaged or dysfunctional macrophage populations might one day be replaced, a possibility that until now has been largely theoretical for cells that cannot simply be injected as mature macrophages and expected to integrate properly.

De Strooper, whose research has long focused on the mechanisms of Alzheimer’s disease, pointed to the brain as an especially promising arena for this approach. Microglia and other macrophage populations are increasingly recognized as active contributors to the disease biology of Alzheimer’s, he explained, and having a system that allows researchers to study how these cells develop, adapt, and are potentially replaced offers a new way to explore their role in disease. The same logic applies to frontotemporal dementia, another devastating neurodegenerative condition in which microglial behavior is thought to influence disease progression.

Beyond neurology, the model could inform research into liver disease, cardiac inflammation, and kidney injury, all conditions in which resident macrophages participate in both tissue repair and pathological scarring. By providing a renewable, human-derived source of embryonic-like myeloid progenitors, the iYSMP platform gives researchers a controlled system for asking how genetic risk variants, environmental toxins, or therapeutic compounds alter macrophage development and function. It also offers a way to compare macrophage biology across organs within a single experimental framework, something that has been nearly impossible with primary human tissue. While clinical applications remain distant, the study establishes a crucial proof of principle: the earliest chapters of human immune development, long locked away in embryonic tissue, can now be replayed, observed, and potentially redirected in the laboratory.

Subject of Research: Human stem-cell-derived yolk sac myeloid progenitors and the developmental origins of tissue-resident macrophages

Article Title: Human stem-cell model reveals a shared origin for tissue-resident macrophages

Article References: Human stem-cell model reveals a shared origin for tissue-resident macrophages. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: tissue-resident macrophages, microglia, yolk sac, myeloid progenitors, pluripotent stem cells, Alzheimer's disease, neurodegeneration, Developmental Cell, VIB, cell therapy, single-cell analysis, immune development