The fetal immune system develops from stem cells in the liver, whereas the immune cells that protect adults form in the bone marrow. Moreover, early in life the immune system contains cells that quickly respond to only a limited number of foreign molecules; adult immune cells can recognize almost anything that might harm a host.

A 'master-switch' gene called Lin28b accounts for these differences, report Stefan Muljo and his team at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland. Lin28b — which blocks a class of gene-regulating RNA fragments called microRNAs — is active in the stem cells that form a mouse's immune system early in life, yet is absent from adult bone marrow. Marrow cells engineered to express a closely related gene, Lin28, and transplanted into adult mice form fetal-like immune cells.

Because the fetal-like immune cells are known to be effective against some pathogens, cancers and other diseases, coaxing transplanted bone marrow cells to take on fetal properties could be used to improve immune responses.

Science http://dx.doi.org/10.1126/science.1216557 (2012)