Natural Resistance Systems

It has become clear during the last decade that in mice and other mammalian species commonly used for experimental transplantation there is a natural (i.e., nonintentionally induced) resistance to the engraftment of foreign hemopoietic cells, even after heavy total-body irradiation of prospective recipients. This natural resistance manifests itself as a cytostatic effect exerted by the host environment on transplanted cells after the latter have settled into sites of hemopoiesis. In strongly resistant hosts such cells fail to proliferate and differentiate; in weakly resistant hosts the proliferation is deficient; and in susceptible hosts exponential proliferation begins about 48 hr after transplantation, with early differentiation occurring primarily in the direction of erythropoiesis, granulocytopoiesis, and megakariocytopoiesis. For a short period after transplantation, two to three days, viable transplanted stem cells can be recovered from spleens of susceptible and strongly resistant recipients. Later on, the transplanted cells disappear from the hemopoietic sites of the resistant mice.