Biological standardization of cytokines

Cytokines are a heterogenous group of biologically active proteinaceous molecules which regulate cell growth, differentiation and function. They are secreted, soluble (non-antibody) mediators which are active in very small quantities and thus may be considered to be a subset of hormones. Many cytokines, especially those affecting the immune system, are being evaluated in the clinic, mainly in the area of cancer therapy. Cytokines constitute a group of proteins which cannot be completely characterized by chemical and physical features alone. Since they are highly biologically active and generally have wide-ranging pharmacological effects, they must necessarily be characterized from a biological standpoint. For convenience, the biological properties of cytokines are assessed in responsive in vitro cell systems. The development of specific biological assays to measure cytokine activities has rapidly progressed in the last decade and has led to attempts to standardize these activities. Standardization of interferons in terms of antiviral activity, for example, has been largely successful in establishing a common international unitage for interferon activity. The latter was dependent on the development and establishment of WHO international interferon reference preparations or standards. Standardization of other cytokines including interleukin-1, interleukin-2 and tumour necrosis factor is currently underway and interim reference preparations have been made available. It is likely that there will be a growing requirement for biological standardization of colony stimulating factors and other growth and differentiation factors.