Cytokines and Chemokines
Cytokines
In order to mount and coordinate an effective immune response, a mechanism by which lymphocytes, inflammatory cells and haematopoietic cells can communicate with each other is required. Cytokines perform this function. Cytokines are a large, diverse family of small proteins or glycoproteins (usually smaller than 30 kDa). Although initially described for their immunomodulatory capabilities, additional roles separate from the immune system in developmental processes are also documented, such as cell differentiation and directed migration. Influencing both innate and adaptive immune responses, the two principal producers of cytokines are helper T cells (Th cells) and macrophages, although they can be transiently induced and secreted by virtually all nucleated cells.
The downstream effects of a particular cytokine occurs through its high-affinity binding of its receptor expressed on the surface of a target cell. This action may occur in an autocrine (acts on same cell), paracrine (acts on nearby cell) or endocrine (acts on distant cell; not the normal manner for cytokine responses) manner. Receptor engagement triggers intracellular signalling cascades leading to altered gene expression in the target cell, which lead to a biological effect. Differentiation, proliferation and activation of the target cell are all effects which can be detected after cytokine stimulation.
The multiple cytokines detected in the extracellular milieu at any given time during an immunological response can interact in pleiotropic (different effects on different types of target cells), redundant (multiple cytokines have same effect), synergic (cooperative effect of multiple cytokines), antagonistic (inhibition of one cytokines effects by another) and cascade induction (multiple-step feed-forward mechanism for the amplified production of a particular cytokine) manners. These interactions make it possible for the coordinated recruitment and activation of an entire network of immune cells by a relatively small number of cytokines produced by a single cell type (e.g. macrophages or Th cells).
Activation by cytokines occurs in an antigen-non-specific manner and must, therefore, be regulated to avoid inappropriate responses in a host’s system which would be detrimental to health. In healthy individuals, cytokine action is regulated by their transient production only in response to either antigen or potent inflammatory stimuli, the short half-life of cytokines in extracellular fluids and compartments, and the restricted receptor expression profiles on the surface of both activated and unactivated target cells, as well as other mechanisms. Of course, there are examples of cytokine dysregulation which result in pathological disease. Such an example is the role of tumour necrosis factor alpha (TNFα) in the development of rheumatoid arthritis; blockade of this cytokine’s effect through the administration of a recombinant soluble TNF-receptor also exemplifies how understanding these molecules can be exploited with medical benefits.
Examples of three cytokines and their actions
Colony stimulating factors (CSF) are part of the haematopoietin family of cytokines. Macrophage-CSF (M-CSF) and granulocyte/macrophage CSF (GM-CSF) induce the proliferation of naïve bone marrow precursors and their differentiation into macrophage colonies and granulocyte and macrophage colonies, respectively.
CXCL10 (also known as IP-10) is a chemokine and is secreted by IFNγ-stimulated cells. Only T helper-1 (Th1) cells expressing CXCR3, the receptor to which CXCL10 binds, are able to detect this chemokine and migrate towards it. CXCL10 induces the migration of Th1 cells from areas of low CXCL10 concentrations towards areas with high concentrations of the molecule, such as the site of an infection and inflammation
Chemokines
Chemokines are a family of chemoattractant cytokines (small proteins secreted by cells that influence the immune system) which play a vital role in cell migration through venules from blood into tissue and vice versa, and in the induction of cell movement in response to a chemical (chemokine) gradient by a process known as chemotaxis. In addition, chemokines also regulate lymphoid organ development and T-cell differentiation, mediate tumour cell metastasis, and have recently been shown to have a function in the nervous system as neuromodulators.
In order for a cell to respond to a chemokine it must express a complementary chemokine receptor. Chemokine receptors belong to the vast family of G-protein coupled receptors (GPCRs): seven transmembrane receptors which bind extracellular ligands and consequently initiate intracellular signalling. When a chemokine binds its receptor a calcium signalling cascade is created, resulting in the activation of small GTPases. This then has downstream effects such as activation of integrins (molecules involved in cell adhesion) and actin polymerisation, resulting in the development of a pseudopod (cellular projection), polarised cell morphology and ultimately cell movement.
Chemokines are grouped and named according to their amino acid composition, particularly on the first two cysteine residues of a conserved tetra-cysteine motif. The CC and CXC chemokines form the two largest groups. The molecules CX3CL1, XCL1 and XCL2 are also regarded as chemokines. There are forty-seven known chemokines and nineteen chemokine receptors , and this numerosity results in a high degree of specificity. In fact, the particular molecules expressed on a cell determine which tissue a cell will migrate into. For example, cells expressing the chemokine receptor CCR7 migrate to lymph nodes, where their ligands, CCL19 and CCL21, are expressed.
Chemokines may also be grouped according to their function, such as whether they are inflammatory or homeostatic. Inflammatory chemokines are produced when inflamed tissue releases cytokines such as tumour necrosis factor (TNF), and they function to recruit leukocytes. Homeostatic chemokines are expressed constitutively and play a key role in lymphocyte migration to, and the development of, lymphoid organs. Furthermore the CXC chemokines can be grouped as to whether they are angiogenic or angiostatic. The CXC chemokines containing the ELR amino acid motif (CXCL1-3, 5-8, 14 and 15) tend to be angiogenic, whereas ELR negative CXC chemokines are mainly angiostatic, with CXCL12 being a possible exception.
Chemokine receptors are G-protein coupled receptors (GPCRs) which enable a cell to respond to its chemokine ligand(s). Binding to a chemokine induces a conformational change in the receptor which leads to intracellular signalling, resulting in chemotactic movement, increased avidity and affinity of integrins, and, in some cases, cell activation. The interaction between the chemokine receptors on a cell and chemokine is fundamental to the recruitment of the cell to the appropriate site for action.
Monocytes, derived from myeloid progenitors in the bone marrow, circulate in the blood. They are important for an inflammatory response, differentiating into macrophages or dendritic cells (DC) under certain circumstances. Two major subpopulations of monocytes have been identified in mice on the basis of their expression of Ly6C and the chemokine receptors, CCR2 and CX3CR1. The largest population under steady state conditions is Ly6C+, CCR2high, CX3CR1low, and the second Ly6C-, CCR2low, CX3CR1high. These correspond phenotypically to populations found in human blood, CD14++, CD16-, CCR2high, CX3CR1low (classical), and CD14+, CD16++, CX3CR1high, CCR2low (non-classical) respectively. However, in humans a third subset can be identified, CD14++, CD16+, CX3CR1high, CCR2low (intermediate).
In mice, the CCR2high subset is often referred to as inflammatory and is usually the predominant subset recruited at an inflammatory site. CCR2 is important for this recruitment. However, CCR2high monocytes, depending on environmental signals, may differentiate in tissue into a variety of subtypes of macrophages or DC which may be pro- or anti-inflammatory. CCR2 is also important for the release of CCR2+ monocytes from the bone marrow in response to CCL2 produced by bone marrow stromal cells.
CCR2 is important for monocyte release from the bone marrow. It is suggested that this may be via desensitization of another chemokine receptor, CXCR4, which normally anchors monocytes in the bone marrow via CXCL12.
Patrolling CX3CR1high monocytes and circulating CCR2high monocytes can both be recruited to an inflammatory site.
CX3CR1high monocytes in both mouse and human have been shown to adhere and crawl along the luminal side of the vascular endothelium in a CX3CR1 and LFA-1 dependent manner, independent of blood flow direction. CX3CR1 also provides a survival signal for these monocytes. They are known as patrolling monocytes and long-range crawling on the vasculature allows them to act as ‘housekeepers’ of the vasculature, clearing debris, scavenging microparticles and scanning for any inflammatory stimulus. They can recruit neutrophils and may interact with other cells of the immune system. With certain stimuli they are able to extravasate into tissue rapidly and can be inflammatory.
Monocyte recruitment may also be influenced by their expression of other chemokine receptors such as CCR1 and CCR5 and the relative importance of the chemokine receptors is likely to depend upon the particular microenvironment prevailing at the inflammatory site.