Selectively eliminated blood monocytes and splenic suppressor macrophages in mice depleted of bone marrow by strontium 89

The contribution of specific activity to the effects of the bone-seeking isotope, strontium 89 on radiosensitive components of mononuclear phagocyte populations was investigated in mice. CBA/J mice received a fixed dose of 2 microCi/g body weight of 89Sr with three different specific activities, 6 Ci, 100 microCi and 20 microCi per mg Sr. The estimated radioactivity located in the bone surface was 4,200, 3,000 and 2,400 cpm/mg bone when measured 2 days after the administration of 89Sr, and was lost with an estimated biological half-life of 27, 25, and 23 days, respectively. Bone marrow suppression was assessed by quantitation of the depletion of macrophage-colony forming cells (M-CFC) grown in vitro in the presence of macrophage growth factor. The decline in M-CFC closely paralleled the level of radioactivity in the bone. These effects were clearly reflected by the depletion of monocytes in the blood, which were reduced to 14%, 14%, and 21% of control levels corresponding to SA's of 6 Ci/mg, 100 microCi/mg and 20 microCi/mg when counted on day 10. By day 30 the respective monocyte levels were 15%, 31%, and 77%. Furthermore, the induction of prostaglandin E producing suppressor macrophages (M phi) by Corynebacterium parvum administration was found to vary inversely with the effects of radioactivity in the bone, with initial impairment followed by quantitative recovery. Resident-type M phi in peritoneal cavity, however, appear to be unaffected by 89Sr-treatment. These data suggest, as before, that the monocytes and suppressor M phi are dependent on radiosensitive marrow cells. The observations also lead to the conclusion that the specific activity of 89Sr preparations is an important determinant of the degree of suppression and of the rate of recovery of bone marrow from the effects of irradiation that follow the administration of this isotope.