Humoral factor in irradiation protection: modification of lethal irradiation injury in mice by injection of rat bone marrow.

I N EARLIER REPORTS it was demonstrated that the injection of suspensions of normal bone marrow from the same species would prevent the death of animals exposed to total body irradiation. Data indicating protection are available on mice (I), rats (2), hamsters (3), and guinea pigs (I). The results obtained with bone marrow suspensions in irradiated rabbits and dogs were equivocal (49. 5>* Although the hypothesis that seeding of the damaged hematopoietic tissue with nonirradiated bone marrow is responsible for the recovery of the irradiated host is an attractive one, there is mounting evidence that other noncellular factors may be equally, if not more, important. Jacobson (6) postulated such factors following his observations that splenectomy of the shielded spleen I hour after irradiation still gave survival of his irradiated mice. Furthermore, he found evidence indicating that hematopoiesis in irradiated rabbits could be stimulated by introducing the spleens of immature mice intraperitoneally. Lorenz et al. (I, 7) also suspected that noncellular factors might be operating following their observation that intravenous injections of guinea pig bone marrow into irradiated mice gave protection, although sporadic. No effect, however, was observed from using dog and rabbit bone marrow in irradiated mice or from using rabbit bone marrow in irradiated guinea pigs. Nevertheless, since guinea pig bone marrow could not repopulate the damaged bone marrow of the mouse (7), a humoral factor, stimulating the damaged mouse hematopoietic tissue to regrowth was postulated. Experiments in which the guinea pig bone marrow failed were explained by the observa-