Genetically distinct cell populations in naturally occurring bone marrow-chimeric primates express similar MHC class I gene products.

The cotton-top tamarin (Saguinus oedipus) is a naturally occurring "A" + "B"----"A" bone marrow-chimeric species. These primates usually are born as dizygotic twins and, due to placental vascular anastomoses, develop sharing each others' bone marrow elements. Strikingly, almost 50% of the PBL of a member of a twin pair are derived from the hematopoietic stem cells of its cotwin. To clarify the mechanisms underlying the maintenance of tolerance in these stable chimeras, MHC gene products have been biochemically characterized in cloned, genetically distinct, male and female lymphocytes from two male/female cotton-top tamarin twin pairs. Extensive MHC class II sharing between the genetically distinct cell populations was not seen in the two twin pairs. This was consistent with the MHC class II polymorphism seen in the species. However, the MHC class I gene products expressed by one member of a twin pair were almost identical to those expressed by its cotwin. A human minisatellite probe demonstrated restriction fragment length polymorphism in DNA from these animals, indicating extensive polymorphism. Thus, MHC class I sharing did not occur due to inbreeding in these animals. Additionally, another bone marrow-chimeric primate species, the common marmoset (Callithrix jacchus), expresses MHC class I molecules with low levels of variation. These studies suggest that the stable chimerism of bone marrow-chimeric primates may be facilitated by MHC class I similarity between the genetically distinct bone marrow derived-cell populations in their circulation.