Amplification and High-Level Expression of a cDNA for Human Granulocyte-Macrophage Colony-Stimulating Factor in Human Lymphoblastoid Namalwa Cells

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Abstract
We have achieved high-level expression of human granulocyte-macrophage colony-stimulating factor (GM-CSF) in human lymphoblastoid Namalwa cells by introducing and subsequently amplifying an expression vector encoding human GM-CSF and mouse dihydrofolate reductase (DHFR), Transformants expressing elevated levels of both GM-CSF and DHFR were selected by a step-wise increase in the concentration of methotrexate (MTX). Several cell lines resistant in 800nM MTX have been established that express GM-CSF at a rate of 10–20 μg/106 cells/day. The amplified genes are integrated and stably maintained in the chromosomes of these cell lines. Analysis of the GM-CSF produced in the amplified Namalwa cells revealed that the molecular-size distribution due to varied degrees of glycosylation was similar to that observed in the naturally-occurring molecules.