Cytokines are synthesized in the Golgi and may traffic through the endoplasmic reticulum to be released as soluble mediators, or they may remain membrane bound, or they may be processed into cytosolic forms that can traffic intracellularly, even returning to the nucleus where they can act as transcriptional regulators. Numerous factors promote cytokine expression in vivo, including cell-cell contact, immune complexes / autoantibodies, local complement activation, microbial sp ecies and their soluble products, reactive oxygen and nitrogen intermediates, trauma, sheer stress, ischemia, ra diation, ultraviolet light, extracellular matrix components, DNA (mammalian or microbial), heat shock proteins, and cytokines themselves in autocrine loops.
Figure 1. Overview of cytokine regulatory function. Numerous and diverse stimuli (1) promote cytokine expression arising either from novel gene expression (2) or from activation of preformed cytokine (3) Cytokine proteins are thereafter expressed in the cytosol, on the cell membrane, or in soluble form in the extracellular environment (4) Cytokines bind to reciprocal receptors that reside either on the membrane of a target cell or in the soluble phase (5) Membrane receptors, on cytokine ligation, signal to the recipient cell nucleus (6) and drive novel gene expression to promote effector function. Each phase of cytokine function offers rich therapeutic potential. IC, immune complexes.
Transcription factor binding allows for numerous signal pathways to regulate cytokine expression across a range of stimuli. Several transcription factors (e.g., nuclear factor κ B [NF κ B], activator protein-1 [AP-1], nuclear factor of activated T cell) are crucial in cytokine production. Sequence polymorphism within cytokine promoters offers potential for differential cytokine expression between individuals that could confer selective advantage against infection, but also could increase susceptibility to, or progression of, autoimmunity. Post-transcriptional regulation is important in determining longevity of cytokine expression. This regulation may operate by promoting translational initiation, mRNA stability, and polyadenylation.
Cytokine secretion is a widely studied process, although little is known regarding the specific mechanisms that regulate cytokine release. Recent findings have shed light on some of the precise molecular pathways that regulate the packaging of newly synthesized cytokines from immune cells. These findings begin to elucidate pathways and mechanisms that underpin cytokine release in all cells. The secretion of cytokines and chemokines from cells is a fundamental response to injury and infection in the body. Cytokines profoundly alter the body's response to cellular damage or invasive pathogens and are secreted by a wide range of cell types.
Recently, more information has come to light from innate immune cells regarding the distal steps of cytokine secretion from the Golgi complex through membrane-bound organelles for classical secretion involving membrane fusion and exocytosis. Cytokines may also be released through alternative pathways, such as molecular transporters, in nonclassical secretion.
Until recently, very little was known about how cytokines are secreted. New findings have shown that most cytokines are released through classical secretion. In this form of secretion, cytokines may be packaged in the Golgi for storage in secretory vesicles or granules and then secreted only during receptor-mediated release in a form of “regulated exocytosis” , or they may be released rapidly upon their synthesis through recycling endosomes (REs) and small secretory vesicles through “constitutive exocytosis”. Regulated exocytosis involves ligand-receptor signaling to secretory granules or vesicles, which contain preformed cytokines, that are mobilized to the cell surface for release. Constitutive exocytosis is instead dependent on receptor-mediated transcription events in the nucleus, leading to trafficking via Golgi through recycling endosomes to the cell surface. The pathway for regulated exocytosis is general for granulocytes, whereas the constitutive pathway shown here is an example of this type of trafficking for TNF release from macrophages. The R-SNAREs involved in regulated and constitutive exocytosis are distinct, with VAMP-2, -7, and -8 implicated in regulated secretion, whereas Vti1b and VAMP-3 are essential for constitutive release of recycling endosomes. A number of GTPases are also associated with cytokine release.
Cytokine release syndrome is a symptom complex associated with the use of many monoclonal antibodies. Commonly referred to as an infusion reaction, it results from the release of cytokines from cells targeted by the antibody as well as immune effector cells recruited to the area. When cytokines are released into the circulation, systemic symptoms such as fever, nausea, chills, hypotension, tachycardia, asthenia, headache, rash, scratchy throat, and dyspnea can result. In most patients, the symptoms are mild to moderate in severity and are managed easily. However, some patients may experience severe, life-threatening reactions that result from massive release of cytokines. Severe reactions occur more commonly during the first infusion in patients with hematologic malignancies who have not received prior chemotherapy; severe reactions are marked by their rapid onset and the acuity of associated symptoms. Massive cytokine release is an oncologic emergency, and special precautions must be taken to prevent life-threatening complications.
Macrophages secrete the following cytokines under different conditions:
|Cytokines||Macrophage cytokine release under different conditions|
Two major pathways for TH1 cytokine production have been identified. IL-12 signaling via its receptor activates Stat4, which upregulates IFN-gamma transcription. IFN-gamma, on the other hand, activates Stat1, which upregulates the leading TH1 transcription factor, T-bet, further enhancing IFN-gamma production. Both pathways upregulate each other via positive feedback mechanisms. For TH2 cytokine production, two major pathways are identified: IL-4–mediated signaling through the IL-4 receptor activating Stat6 and GATA-3, leading to IL-5 and IL-13 production; and signaling through the TCR-CD4 complex upregulating c-Maf, which in turn initiates and enhances IL-4 transcription. The response controlled by GATA-3 is further enhanced by an autoactivation process and a positive feedback on c-Maf expression. All three factors for TH1 cytokine production (Stat4, Stat1, and T-bet) inhibit GATA-3, which in turn downmodulates T-bet. (Fig. 3)
Figure 3 丨 Cytokine signaling in T lymphocytes via IFN-γ, IL-12 and IL-4. Upon binding to its receptor on the T-cell surface, IFN-γ induces activation of STAT1 and consecutively of T-bet. T-bet is a master transcription factor for Th1 T cells that induces Th1 cytokine production as well as IL-12 receptor β2 chain expression while it simultaneously suppresses Th2 cytokine production. IL-12 induces Th1 T-cell differentiation via activation of STAT4 and consecutive induction of IFN-γ production, but it does not induce T-bet activation directly. In contrast, IL-4 induces Th2 cytokine production in mucosal T cells by activation of STAT6 followed by activation of the master transcription factor GATA-3. GATA-3 has been shown to exert STAT6-independent autoactivation, creating a feedback pathway stabilizing Th2 commitment (blue arrows). In addition to GATA-3, c-maf and NFATc1 have been shown to regulate IL-4 production in T cells. In recent years, there is a growing interest in cytokine- or cytokine signaling-directed therapies for T cell-mediated mucosal diseases such as Crohn disease and allergic asthma using either recombinant cytokines or anti-cytokine strategies. The latter strategies have proven more beneficial in clinical trials so far and include, for example, neutralizing antibodies (such as against IL-4 in asthma and against TNF in Crohn disease) and soluble receptor antagonists (for example, IL-4 receptor antagonists in asthma).