Microchimerism describes a biological phenomenon where a small number of cells from one individual persist and function within another genetically distinct individual. This presence of foreign cells highlights a biological connection that can endure for decades, influencing various physiological processes.
Understanding Microchimerism
Microchimerism involves the presence of a small population of cells within an individual that originated from a different person and are therefore genetically distinct. The term is derived from the Greek mythological creature, the Chimera, a hybrid beast. These foreign cells integrate into the host’s body, often persisting alongside the host’s own cells. The phenomenon can be categorized into naturally acquired forms, such as during pregnancy, and artificially acquired forms, like those resulting from organ transplantation.
How Chimeric Cells Arise
The most common natural occurrence of microchimerism is maternal-fetal cell exchange during pregnancy. This bidirectional transfer of cells between mother and fetus across the placenta begins early in gestation and continues throughout pregnancy. Fetal cells can transfer to the mother, and maternal cells can transfer to the fetus.
Other sources include twin-to-twin transfusion syndrome, where dizygotic twins exchange cells through placental connections. Organ transplantation also leads to microchimerism, as the recipient’s body incorporates donor cells. Blood transfusions can introduce foreign cells, contributing to artificially acquired microchimerism. The “vanishing twin” phenomenon, where one embryo does not survive and its cells are absorbed by the surviving twin, can also result in chimerism.
Where Chimeric Cells Reside
Once introduced, these foreign cells can reside in various tissues and organs throughout the host’s body. Studies have detected chimeric cells in locations such as the bone marrow, skin, brain, heart, liver, and lungs. These cells can integrate into the host’s existing tissues and, in some cases, differentiate into different cell types. For example, fetal cells have been observed to cross the blood-brain barrier in mice and differentiate into neural cells.
In mothers, fetal microchimeric cells can persist in the bone marrow for many years after birth, from where they can then migrate to the blood and other tissues. This persistence suggests these cells find a hospitable microenvironment that supports their long-term survival. Their ability to integrate and differentiate into various cell lineages points to their adaptability within the host.
The Impact of Microchimerism on Health
Microchimerism has an influence on human health, with research suggesting both beneficial and complex associations. These foreign cells may contribute to tissue repair. Observations include microchimeric cells migrating to damaged areas like skin wounds and participating in healing, either by secreting molecules that promote healing or by adopting the characteristics of the damaged tissue’s cells. Fetal microchimerism has also been implicated in the repair of brain lesions and improved post-stroke recovery in animal models.
Conversely, microchimerism has been linked to certain autoimmune diseases, such as systemic sclerosis and lupus. Some studies indicate that fetal microchimeric cells might contribute to these conditions, while other research suggests they may simply be present in inflamed tissues without causing the disease. The presence of these cells is also being explored for their role in immune tolerance, protection against some cancers, and cardiovascular disease. These remain areas of active investigation with often contradictory findings.