"Xenogeneic resistance" to rat bone marrow transplantation. III. Maturation age, and abrogation with cyclophosphamide, Corynebacterium parvum and fractionated irradiation.

Lethally irradiated C57 Bl/6 mice and (C57 X A) F1 hybrids fail to accept doses of rat bone marrow cells (5 X 10(6)) which give confluent splenic repopulation in "non-resistant" strains of mice. This phenomenon has been termed "xenogeneic resistance" (XR). XR in (C57 X A) F1 mice can be overridden by a very large inoculum of rat bone marrow (26 X 10(6) cells). XR is not manifest in mice of a resistant strain at ages of 18 days or younger, but is manifest at ages of 22 days and older. XR can be abrogated by agents as varied as: 1) cyclophosphamide, which abrogates XR in a dose dependent manner when given 1 hr prior to lethal irradiation and bone marrow transplantation 2) C. parvum, which abrogates resistance when given 7 days prior to lethal irradiation and bone marrow transplantation, and 3) Fractionated irradiation, which, while capable of abrogating XR, is much less potent than either cyclophosphamide or C. parvum.