Phytohemagglutinin-enhanced hybrid colony formation by cells from aged mice: expression of TH 4Mod-1 mouse locus in human-mouse hybrids.

Cells from a continuous human line and freshly isolated cells from old adult mice heterozygous at the Mod-1 locus were fused in the presence of polyethylene glycol (PEG). The production of hybrid cells, as a function of PEG concentration in the presence and absence of phytohemagglutinin (PHA), was measured by cell survival and proliferation on selective medium. The incorporation of PHA into the fusion mixture allowed cell fusion to take place at nontoxic concentrations of PEG. PHA increased the frequency of cell fusion and increased the production of viable hybrid cells from 138- to over 2800-fold depending on cell type. The results suggest that the procedure may have broad application in promoting the fusion of cells sensitive to PEG. Clones were analyzed for isozymes of malic enzyme and glucose-6-phosphate dehydrogenase. The expression of the gene encoding X-linked mouse glucose-6-phosphate dehydrogenase confirmed that the cells were hybrids. These cells lost other mouse isozymes rapidly. In those clones in which the mouse malic enzyme gene was expressed, the product of Mod-1a was detected significantly more frequently than that of Mod-1b.