Growth-factor mRNA expression in AML blasts. RT-PCR was used to generate total cDNA from AML samples. (A) Southern blots of total cDNA from the CD34⁺CD38⁻, CD34⁺CD38⁺, and CD34⁻fractions of cells from pt 8 in Table 1 hybridized with probes specific for IL-3, SF, G-CSF, GM-CSF and, as a positive control, β-actin. IL-3 expression was detected in all 3 subpopulations. SF, G-CSF, and GM-CSF expression was undetectable. RT, reverse transcriptase. (B) Total cDNA from 15 AML samples was subjected to a second round of PCR using cytokine specific primers. Patients A through H showed less than 10% human AML cells in mouse marrow 8 weeks after injection of 10⁷ cells; patients I through O showed greater than 10% human AML cells in mouse marrow under the same conditions. For each cytokine, the positive control was a murine cell line transfected with the corresponding human cytokine cDNA. Southern blots of PCR products were hybridized with the corresponding cytokine-specific probes. Expression of 1 or more cytokines was detected in 2 of the 8 samples shown that achieved less than 10% engraftment, and in 5 of the 7 samples shown that achieved greater than 10% engraftment. In a separate experiment, 2 additional samples with less than 10% engraftment showed no detectable growth-factor expression while 1 sample with greater than 10% engraftment showed expression of G-CSF message.