SECTION OF BIOLOGICAL AND CHEMICAL SCIENCES: THE ACTION OF THYMUS DNA ON L1210 LEUKEMIA CELLS*,†

Abstract

We have recently reported the inhibition of L1210 tumor growth in mice by preincubating the L1210 cells with mouse thymus DNA.l Treating the DNA with DNase abolished its growth inhibitory effect. Intact L1210 DNA was also ineffective. This work and additional ~t u d i e s~'~ have indicated that the base sequence, helical structure, and size of the thymus DNA a r e all important factors in causing inhibitionof tumor growth. Our current experiments deal with the action of DNA on L1210 cells in vitro. MATERIALS AND METHODSMouse thymus DNA was isolated and treated with RNase by the method of Szybalska and S~y b a l s k i .~ DNA was similarly prepared from L1210 leukemia cells. Except where indicated, the L1210 cells were grown in spinner cultures containing medium RPMI 1630 + 5 per cent fetal calf serum.6 (This medium includes 20 amino acids, a number of vitamins, but no purine or pyrimidine derivatives.) In most cases the cells were washed and incubated at 37°C in 4 ml. of a phosphate-buffered saline (pH 7.5)5 containing 5.5 mM glucose and 0.2 mM spermine tetrahydrochloride. The cells were usually treated for 30 minutes with DNA (30 to60 pgper lo6 cells) and then washed and resuspended in DNA-free PBSG (phosphate-buffered saline -t glucose). Control cells were incubated for 30 minutes without DNA and then washed and resuspended in fresh PBSG. Between two and 20 x lo6 cells per ml. were used for experiments in vivo. About 106 cells per ml. were used for the experiments in vitro, with the exception of the isotope-incorporation experiments in which 2-4 x lO'.cells per ml. were utilized.Cell viability was determined by adding 0.2 ml. of 0.1 per cent trypan blue (dissolved in Hanks Balanced Salt Solution) to 0.2 ml. of cell suspension. Only unstained cells were considered ~i a b l e .~,~ Cells were counted microscopically with a Speirs-Levy Chamber and electronically by means of a Coulter Counter.