Chimerism

Medical Definition and Introduction

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Chimerism designates a phenomenon in which one individual carries cells from another individual. A chimera is an individual carrying two genetically distinct types of cells. The term michrochimerism refers to low levels of chimerism. In microchimerism, the second genetically distinct cell population comprises less than or equal to 1–10% of all cells. Chimerism can be classified as congenital or acquired chimerism.

Congenital Chimerism

Congenital chimerism includes twin (placenta-associated), dispermic, and tetragametic chimerism.

1. Twin chimerism occurs due to transplacental exchange of hematopoietic stem cells between twins. Therefore, the dual population of cells in twin chimerism is restricted to hematopoietic cells.
2. Dispermic chimerism occurs due to dual fertilization of a parthenogenetic oocyte, fertilization of an oocyte and its polar body by two sperm, or postzygotc diploidization of triploids.
3. Tetragametic chimerism occurs due to postzygotic fusion of two embryos. Thus, in dispermic and tetragametric chimerism, a dual cell population is detected in both hematopoietic and non-hematopietic tissues.

Acquired Chimerism

Acquired chimerism includes feto-maternal and iatrogenic chimerism.

Feto-maternal chimerism occurs through the transplacental exchange of small amounts of hematopoietic stem cells between a mother and fetus in utero.

Maternal cells may persist in the children’s circulation long term and vice versa; in fact, fetal cells have been reported to persist in maternal blood for as long as 27 years postpartum.

Iatrogenic chimerism can be seen following blood transfusion, hematopoietic stem cell transplantation (HSCT), organ transplantation and in vitro fertilization (IVF).

Following blood transfusion, chimerism exists until the donor red blood cells (RBCs) are cleared.

In some cases of transfusion-associated chimerism in massively transfused trauma patients, the finding of donor white blood cells up to 1.5 years post transfusion has been attributed to engraftment of donor stem cells.

Similarly, in HSCT, chimerism occurs when donor stem cells engraft in the host bone marrow, causing a dual cell population in only hematopoietic cells.

In IVF, the likelihood of chimerism may be increased due to the common practice of transfer of more than one developing embryo into the uterus. Thus, chimerism due to transplacental exchange of hematopoietic stem cells between twins (or other number of offspring from multiple gestation) and amalgamations of two or more implanted embryos may occur more readily.

Chimeras resulting from IVF may therefore have a dual population of cells in either hematopoietic or non-hematopoietic cell populations, depending on how the dual cell population was deprived.

Blood typing discrepancies, which involve typing that is inconsistent with those expected for the majority of individuals, are frequently demonstrable in chimeric individuals.

In twin chimeras, the dual population is restricted to hematopoietic cells, making them phenotypically normal. It has been estimated that blood group chimerism occurs in 8% of twin individuals and 21% of triplet individuals.

Incidentally identified cases of twin chimerism, however, are uncommon and are typically discovered through a red blood cell (RBC) typing discrepancy (most common) or a HLA typing discrepancies.

In contrast to twin chimerism, dispermic and tetragametic chimerism affects both hematopoietic and non-hematopoietic cells and is therefore expected to accompany phenotypic anomalies.

Most dispermic and tetragametic chimeras are identified as a result of infertility, anomalies in sexual development such as ambiguous genitalia and hermaphrodism, or through identification of abnormal karyotypes. However, rare cases of phenotypically normal tetragametic and dispermic chimeras have been reported, the vast majority of which were brought to light by RBC or HLA typing discrepancies.