Sanne Hoeks

Cytokine assays 81 6 INTRODUCTION Cytokines are small protein, glycoprotein or peptide molecules that, through cell signaling, allow intricate cellular communication. These molecules are produced by cells of various embryological origin and are classified according to structure or function. 1 Structural classification is used mainly for cytokines that do not display a high level of redundancy, allowing these members to be separated into distinct groups. In general, however, cytokines are divided into functional families based on the enhancement of immunological responses, these include interleukin-1 (IL-1), IL-6, IL-10, IL-12, IL-17, TNF, IFN, TGF, PDGF, gamma chain (IL-2, IL-4, IL-7, IL- 9, IL-15, TLSP), and beta chain (IL-3, IL-5) families and chemokines (C, CC, CX and CXC3). They exhibit a wide variety of activities but are most often seen as effector molecules that can alter the behavior of the immune system instantly during an immune response. Cytokines can act in an autocrine, paracrine, endocrine or juxtacrine manner (Fig. 1). Upon binding with their cell surface receptor, these molecules initiate an intracellular signaling cascade that may result in the up regulation and/or down regulation of gene expression or may take on transcription factor activity all subsequently inducing further cytokine production or curbing their own activity via feedback inhibition. The effect exerted by a cytokine depends on its extracellular levels, the expression of its complementary receptor and the type of signaling cascade initiated by this receptor binding, with cell type particularly influencing receptor expression and downstream signaling. These molecules exhibit redundancy in their activity as well as dual functionality. This is exemplified by their ability to act in one case as a receptor ligand and in another as a transcription factor. 2 Cytokines produced by cells of the innate immune system play an essential role in influencing the immune response towards protective immunity and act as a key link between the innate and adaptive immune system. In response towards pathogens these molecules are secreted by immune cells and act as first danger signals, alerting, and initiating immunological pathways. Disturbance in this balance, however, plays a role in chronic disease progression by influencing auto-inflammatory or auto-immune pathways. 3, 4 The specific role of cytokines in these pathological disorders has been the subject of extensive research over the past few decades with molecular cloning, specific blockage of activity, gene deletion and receptor identification as well as the testing of recombinant cytokines illustrating how essential cytokines are to disease pathogenesis. 5-7 As a result of this work, the use of new drugs which modulate the inflammatory processes of the immune system for many disease types in the field of oncology and immunology have been shown to result in major changes in the downstream cytokine milieu. The most elegant illustration of the functionality of cytokines comes from the blockage of TNFα in rheumatoid arthritis, this specific blockage results in the effective reduction of disease activity and joint destruction. 8, 9 Contradictory to these positive intervention strategies, in 2006, a phase 1 clinical trial involving a CD28 receptor monoclonal