Hemopoietic activity in bone marrow chimeras prepared with total lymphoid irradiation (TLI)

The hemopoietic status was examined in the blood, bone marrow and spleen of (BALB/c X C57BL/6)F1 hybrid mice treated with fractionated total lymphoid irradiation (TLI), with or without reconstitution with 30 X 10(6) syngeneic or parental C57BL/6 marrow cells. Colony forming units of pluripotent (CFUS) and granuloid committed stem cells (CFUC) were assayed in non-irradiated (femur) and irradiated bone marrow (humerus), and in spleen 9, 18, 73 and 109 days following TLI. Recovery of CFUS and CFUC was nearly complete as early as day 18, and fully recovered on day 73 in endogenously as well as exogenously (syngeneic and semiallogeneic) reconstituted mice. Mice transplanted with C57BL/6 bone marrow exhibited a 10-fold increase of CFUC in the spleen on day 9 and maintained a 4-fold increase of CFUC on day 109, with normal CFUC in shielded, and near normal CFUC in irradiated bones. In contrast, the number of CFUS in the irradiated humerus was below 10% of all controls on days 18-73 and below 30% on day 109. Also, the number of CFUS in the spleen was very much reduced at days 73-109. Unirradiated femora contained normal CFUC and only mildly depressed CFUS. The data suggest that repopulation of bone marrow is adequate although somewhat less efficient in irradiated as compared to shielded bones, whether or not exogenous marrow is transplanted. Semiallogeneic chimeras (C57BL/6 leads to F1) show low CFUS relative to high CFUC levels, particularly in irradiated organs (spleen and humerus). This may suggest an advantage, from a hemapoietic standpoint, to selective irradiation of lymphoid tissues, rather than the less selective wide fields that overlap marrow containing bones.