LIGAND GENE-EXPRESSION IN NORMAL AND SUBLETHALLY IRRADIATED MICE

The c-kit ligand (KL; Steel factor, mast cell growth factor, stem cell factor) is a hematopoietic factor that has been shown to act as a pot ent cofactor for hematopoietic growth and differentiation in vitro. Th e in vivo effects of KL, however, have been variable, To study the hem atopoietic role of KL in vivo, we evaluated KL gene expression in both normal mice and mice recovering from myelosuppressive radiation expos ure using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. In a single RNA sample, we found that the RT-PCR technique has high precision (coefficient of variation, 15.7%). Amplifications of serial 1:2 dilutions of template RNA precisely correlated with star ting RNA concentrations at 20 cycles or at 25 cycles, depending on the level of expression. Amplification of individual normal bone marrow a nd spleen cell RNA showed basal expression in all normal bone marrows but irregular expression in normal spleens. On day 2 after a sublethal 7.75-Gy (0.4 Gy/min) Co-60 irradiation, splenic KL gene expression in creased approximately 2.5-foId (P = .011), and bone marrow expression increased 15-fold (P = .004), During a 28-day postirradiation recovery period, KL expression increased in bone marrow on days 2 through 7. S plenic expression during the same period was more variable, In conclus ion, the KL gene is invariably expressed in all murine bone marrows an d variably expressed in normal murine spleens. Postirradiation, recove ring bone marrow and spleen both express increased levels of KL mRNA a t day 2 and continue to express increased levels for several days post exposure. These data support a role for KL in the endogenous recovery of hematopoiesis after hypoplastic injury. This is a US government wor k. There are no restrictions on its use.