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Sources for human (and other) cytokines are extremely poor and the expression of these factors may vary considerably, depending on the physiological state of the body. Generally, levels are too low to be considered good sources of factors for clinical use. As an example, 8.5 mg of M-CSF have been isolated from 10000 L of human urine.
Recombinant cytokines, i.e., cytokines produced by expression from suitable cloning vectors containing the desired cytokine gene, can be expressed in yeast (see: Saccharomyces cerevisiae expression system), bacteria (see: Escherichia coli expression system), mammalian cells (see: BHK, CHO, COS, Namalwa), or insect cell systems (see: Baculovirus expression system).
Expression in each system results in a protein that differs, to a varying extent, from native molecules. Alterations can include absence of glycosylation (E. coli), alterations in glycosylation pattern (yeast, mammalian and insect cells), slight alterations in amino acid sequence (all systems). Proteins expressed in mature form in different host cells can differ also in their specific activities for several reasons (see also: cytokine assays, bioassays).
A review of laboratory studies shows that differences in physiochemical properties can result in variations in the pharmacokinetics, biologic activity, and immunogenicity of cytokines expressed in different host cells. The expression system can influence the pharmacokinetic properties, biologic activity, and clinical toxicity of recombinant proteins. Protein variations may lead also to an increased clinical toxicity. On the other hand, expression vectors are useful for the construction of recombinant forms of cytokines to investigate structure/function relationships.
Heterologous expression systems have been employed also to express streamlined cytokines engineered for better clinical efficacy or to create novel specificities or combinations thereof (see, for example: Muteins, Antibody fusion proteins).
See REFERENCES for entry recombinant cytokines.
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