Cytokine Measurements in Biological Fluids

IT IS now widely accepted that the investigation of cytokine expression during inflammation can provide information about the pathogenesis of disease and help to define key therapeutic targets [1]. These studies have been greatly facilitated by the availability of recombinant human cytokines, essential for the development of specific assays. As assays have become more widely available, rheumatologists have applied them to study arthritis, taking advantage of access to biological fluids such as synovial fluid at the site of inflammation, as well as those distant from the inflamed tissue. To date there is little doubt that in synovial fluid (SF), pro-inflammatory cytokines such as interleukin-1 (IL-1) [2], interleukin-6 (IL-6) [3], interleukin-8 (IL-8) [4], granulocyte macrophage colony stimulating factor (GM-CSF) [5] and tumour necrosis factor alpha (TNFa) [6] are detectable in abundance, whilst the T cell cytokines IL-2, IL-4, interferon-y and lymphotoxin are virtually undetectable [7]. However, an overview of the many studies cataloguing cytokine levels in biological fluids has revealed that there are differences regarding the relative levels of some of these cytokines. Furthermore, the presence of a cytokine at high levels, such as IL-6, does not necessarily indicate a pathogenic role. Over the past few years it has become clear that interpretation of these studies is dependant upon the assays used, and an awareness of the many factors which interfere within each specific system. Almost without exception, cytokines have been characterized on the basis of their biological actions. Thus, bioassays preceded the development of immunoassays, many studies focusing on detection of cytokines in tissue culture supernatants. However, the pleiotropic nature of these mediators is reflected in the observation that bioassays are not cytokine specific. For example, the mouse thymocyte proliferation assay used for measurement of human IL-1, also respond to human IL-2, IL-6, and IL-7 [8-10] and even more murine cytokines e.g. IL-4, TNF and LT. Furthermore when bioassays are used to measure cytokines in biological fluids, other problems are encountered. For example, the effects of body fluids on promoting (or inhibiting) cell growth has to be considered, and specificity has to be confirmed with neutralizing antibodies. As well as being labour intensive, bioassays lack reproducibility, an important criterion if assays are to be used for clinical evaluation. Furthermore, an awareness of the presence of specific inhibitors such as soluble cytokine inhibitors which interfere with cytokine bioactivity, is of importance (reviewed in [11]). The development of high affinity monoclonal antibodies (mAb) to cytokines for use in immunoassays meant that they could be performed rapidly, with a high degree of reproducibility and specificity, although