POTENTIATION OF MYELOID COLONY-FORMATION IN BONE-MARROW OF INTACT ANDNEONATALLY THYMECTOMIZED MICE BY THE THYMIC HORMONE THF-GAMMA-2

The effect of the thymic hormone THF-gamma 2 on committed stem cells o f bone marrow (BM) origin was determined using the myeloid progenitor cell clonal assay. Preincubation of normal BM cells with THF-gamma 2 f or 1 hour or 18 hours caused a 2- to 6-fold increase in the number of myeloid colonies in the presence of suboptimal concentrations of colon y-stimulating factor (CSF). The optimal dose of THF-gamma 2 causing th is enhancement was in the range of 25 to 100 ng/mL. THF-gamma 2 was no t able to replace CSF as an inducer in these experiments. THF-gamma 2 neither induced IL-6 activity upon 24-hour incubation with bone marrow cells nor enhanced LPS-induced IL-6 secretion by bone marrow cells in vitro. Neonatal thymectomy (NTx) of Balb/c mice caused a decrease in myeloid progenitors, which was repaired by serial injections of THF-ga mma 2. The repair of the stem cell compartment in the bone marrow corr elated with an increased percentage of Thy1(+) cells in the spleen of THF-gamma 2-treated NTx mice. These findings indicate that THF-gamma 2 is able to regulate committed stem cell functions in the bone marrow of immune-deprived NTx and of normal mice.