Cytokine bioassays

Meager, A. 2006. Measurement of cytokines by bioassays theory and applications. Methods 38, 237-252. 

Incubation periods in excess of 2 h were required before this activity was detected in cell-free supernatants. More recently, the use of cDNA probing of Northern transfers (to detect specific mRNA levels), the use of ELISA techniques (to detect protein levels immunologically) and the development of more specific bioassays (culture techniques in which a biomolecule stimulates proliferation in a particular cell line) have resulted in a more thorough analysis of IL-1 production by neutrophils. IL-1 is only poorly expressed in blood neutrophils because mRNA for this cytokine is detectable only at very low levels (if at all), and protein production is usually below the level of detection of most assays. However, exposure of neutrophils to lipopolysaccharide (LPS), or to cytokines such as GM-CSF, TNF or IL-1 itself, results in a rapid but transient increase in IL-1 expression. 

Endotoxicity results from the interaction of a bacterial cell envelope component (e.g., LPS or PG with a cell surface receptor constituting part of the nonspecific immune system, (i.e., a toll-like receptor on white blood cells). This results in the production of cytokines [e.g., interleukin 1 (IL-1) or tumor necrosis factor (TNF)] as part of an intracellular enzyme cascade which can cause severe tissue injury. Bioassays or immunoassays can be used to detect such reactions respectively. As noted above the most widely used bioassay is the LAL assay. A lysate of amoebo-cytes of the horseshoe crab (Limulus) contains an enzymatic clotting cascade which is activated by extremely low levels of LPS (nanogram levels or lower). There are variants of this assay that can detect PG, but they are not as widely used. As noted above, other bioassays employ cultured cell lines that respond to LPS or PG, respectively. Unfortunately bioassays are highly amenable to false positives (from the presence of cross-reactive substances) or false negatives from inhibition (by contaminants present in the sample) [10]. A detailed discussion of these assays is beyond the scope of this chapter and has been reviewed elsewhere [1]. 

Cytokines, which are protein mediators produced by immune cells, are involved in the regulation of cell activation, growth and differentiation, inflammation, and immunity. Measurement of cytokine production, as determined by techniques such as bioassay, radioimmunoassay, and enzyme-linked immunosorbent assay, has been used to examine various immune functions. 

As with the measurement of other analytes, cytokine assay methods changed from the original bioassays to immunoassays, flow cytometric analysis, and mi-croarray technology. 

There are four major types of in vitro cytokine bioassay, according to the reaction elicited by the cytokine on the reporter cell. 

The most relevant bioassays measure specific activities on immune function, either as effector or regulatory, for example, assay of cytokine IL-12 by the augmentation of the cytolytic function of natural killer cells (B9), IL-10 by the inhibition of the production of TNF-o from stimulated monocytes (B13), and IL-8 and eotaxin by the chemotaxis of neutrophils and eosinophils, respectively (C2). 

Ml 8. Mire-Sluis, A. R., Page, L., and Thorpe, R., Quantitative cell line based bioassays for human cytokines. J. Immunol. Meth. 187, 191-199 (1995). 

Methods to determine the potential biological activity of products obtained through recombinant DNA techniques are of fundamental importance. Despite the existence of numerous physicochemical techniques to characterize the protein product structure and the presence of contaminants, they provide little, if any, information about its biological potency. A bioassay is defined as a functional test, and no physicochemical test can measure the function. However, for some peptide hormones, which are less complex in structure than most cytokines, well defined physicochemical tests may be used to estimate biological activity for instance, the capillary electrophoresis analysis of a protein s isoform content if the specific activity of each one is known. 

The first bioassay type evaluated the response of cytokines directly in animals. However there are many disadvantages to animal tests including inter-animal variability, their expense, demand for intensive work, and ethical concerns when many animals may have to be sacrificed to provide statistically valid data. Significant variability can result from the sex, age, species, and health of the animals used. 

Despite the diverse existing varieties of bioassays for cytokines, all are based on the protein s capacity to induce a measurable activity in cells and tissues. The cell responds in various ways including enhanced growth, growth inhibition, expression of cellular markers, cytotoxicity, or antiviral activity (Wadhwa et al., 1995). 

The ability to obtain cell clones that respond to cytokines and serve as source material for bioassays has aided the development of bioassays. The cell lines, obtained mainly from human or murine cancers, often depend on a cytokine for their growth and tend to be immortal, thus providing a unique homogeneous cell source, which may be distributed among laboratories (Mire-Sluis and Thorpe, 1998). The advent of recombinant DNA technology has enabled the cloning of specific cytokine receptors and their expression in any cell line (Canosi et al., 1996). In this way, a cell line can be created that responds specifically to almost any cytokine, and avoids the necessity to isolate a cell line with an endogenous receptor. 

Mire-Sluis AR, Thorpe R (1998), Laboratory protocols for the quantization of cytokines by bioassay using cytokine responsive cell lines, J. Immunol. Methods 211 199-210. 

Cytokines are peptides used by cells for interceUular communication and for controlling the inner environment of the cells in which they operate2 They are produced by cell types that have important roles in the immune response, inflammation, hemopoiesis, healing, and systemic response to injury. Many cytokines are measured by bioassay and immunoassay,. ... 

Principles of Cytokine Assays Bioassay and immunoassay are the analytical techniques of choice to measure cytokines. However, newer instrumental techniques are also beginning to be used to measure cytokines and CKs. These techniques are used to quantify their (1) concentration and activity in biological fluids, (2) production by whole blood cells,(3) concentration of receptors, and (4) intracellular levels. 

Historically the functions of cytokines have been elucidated first with bioassays preceding immunoassays for cytokine quantification. The bioassay of a given cytokine is based on its bioactivity in a defined biological model, normally based on a certain cell line. Various approaches have been reported (1) proliferation tests— induction of cell growth (e.g., B9 cell line for IL-6) (2) tests for cytotoxicity— TNFa on WEHI164... 

A standard method for the quantitation of cytokines is to perform a bioassay in which aliquots of samples are com-pared with known concentrations of a cytokine in supporting the proliferation of a cytoldne-dependent cell line. In most instances, however, these cell lines are dependent on the cytokine not only for proliferation, but also for survival. ... 

The main disadvantage of immunoassays is that they measure total cytokine, which contains both functional and nonfunctional levels. Cross-reactivity with the precursors or degradation products of the cytokine is frequently observed. The detection limit (about 1 to lOpg/mL) of immunoassays is higher than bioassays. A 1995 study reported a lack of comparability between the different kits for IL-2, IL-6, IL-8, and TNFa. The major advantages of immunoassays are their excellent analytical performance and their ability to be automated. 

The comparative characteristics of bioassays and immunoassays when applied to cytokine measurements in biological fluids are listed in Table 22-9. ... 

Lansky D. Validation of bioassays for quality control. In Brown F, Mire-Sluis AR, eds. Biological Characterization and Assay of Cytokines and Growth Factors. Dev Biol Stand. Basel Karger, 1999 97 157-168. 

The multiple actions of individual cytokines mean that cell lines rarely respond only to one cytokine. This is well illustrated by the efforts to develop specific bioassays for lL-1. Although the original mouse thymocyte proliferation assay was thought to be IL-1 specific, it is now clear that many cytokines influence the assay. IL-6 and TNF can replace IL-1 (G7, U4), and IL-2 and IL-4 can sygerize with... 

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