Joint influence of natural killers (NK) and hemopoietic stroma on rejection of parent stem cells.

The growth of hemopoietic stem cells of C57BL/6 mice was explored in syngeneic and semi-syngeneic foci of heterotopic hemopoiesis, formed under the kidney capsule of (CBA x C57BL/6)F1 recipients with and without prior treatment with cyclophosphamide or fractionated irradiation, which is known to abrogate hybrid resistance and NK cell activity. Our results show that the suppression of the growth of parent stem cells in semi-syngeneic F1-recipients is due to NK cell participation. However, no inhibition was found in the growth of identical cells in the foci of syngeneic hemopoietic stroma in the same recipients, despite the presence of NK cell activity. The experimental model used shows clearly that hybrid resistance is the result of the joint involvement of NK cells and the semi-syngeneic stroma.