Chimaerism in the Immune System of Tetraparental Mice

Abstract

TETRAPARENTAL mice, formed by combining two eight-cell embryos derived from different inbred strains^1,2, have been used for a variety of immunological studies^3,6. These animals have immune cells capable of destroying both parental cell lines and specific blocking factors in the serum capable of preventing such destruction^3,4. In view of this surprising result, it is desirable to clarify the nature of the chimaerism in the immune organs of these mice. One important point is whether a tetraparental mouse could have a mixed spleen, but a thymus derived from only one parent. This would also have implications for the development of these organs. As a first step in this direction we have fused pairs of embryos differing by a cyto-logically distinct chromosome marker, the T6 chromosome⁷. By examining cells in mitosis in the resultant adult thymus, spleen and bone marrow, we have found that within each animal the ratio of the two cell types is remarkably constant from organ to organ although it varies extensively from one animal to another.