Advanced Topics in Genetics Microchimerism Major reference: J.L. Nelson, “Your Cells Are My Cells,” Scientific American, Feb. 2008

Chimera In Greek mythology, a creature composed of parts of a lion, goat, and snake.

Advanced Topics in Genetics: Micochimerism Definition: a situation in which an individual has a small number of cells that arose from a different individual These cells are genetically distinct from the host.

Micochimerism The most common type arises during pregnancy. A few cells can cross from the mother to the fetus, and from the fetus to the mother. (When did we hear about this before?) If these are stem cells, they may be able to grow and persist in the other individual.

Detection of Micochimerism Microchimerism is most obvious when the sexes of the individuals differ What would be an obvious microscopic difference for female cells in a male? For male cells in a female?

Micochimerism Another type arises from organ transplant or blood donation. The donated tissue may contain stem cells that migrate to another part of the recipient body and reproduce.

Micochimerism A third type arises from exchange of cells between twins. There may also be cells obtained from a “vanished twin” that was lost early in pregnancy. Some evidence for transfer from mother to child during breast feeding.

Micochimerism: How Much? Some studies suggest we all contain some microchimeric cells from our mothers. All mothers may have some microchimeric cells from their children. May amount to about 1 in 1000 cells.

Micochimerism: Role in Disease Because they may have some distinct antigens, microchimeric cells might be attacked by the immune system. This could lead to damage of the person’s tissues. Autoimmune disease – a disease in which the person’s immune system attacks his or her own tissues.

Micochimerism: Role in Disease There is evidence that microchimerism is involved in at least some cases of these autoimmune diseases: – Scleroderma (results in thickening of skin and other damage) – Systemic lupus erythematosus (many organs are attacked)