THE GROWTH OF HEMATOPOIETIC STEM-CELLS IS INHIBITED BY NATURAL KILLERS ONLY IN THE NONSYNGENEIC MICROENVIRONMENT

The growth of hemopoietic cells of some inbred animal lines is repress ed in hybrids of the first generation (F-1)-the phenomenon of hybrid r esistance. We have investigated the mechanism of hybrid resistance by studying the growth of hemopoietic cells of C57BL/6 mice in syngeneic and semisyngeneic heterotopic hemopoietic foci formed under the kidney capsule in (CBAC57BL/6)F-1 recipient mice. We found that hybrid resi stance depended upon the joint involvement of NK cells and semisyngene ic he mopoietic stroma. It is concluded that NK cells recognize antige nic markers appearing on target cells located in a nonsyngeneic microe nvironment.