Inflammation (part 3)

This, then, is the "nuts and bolts" behind the initiation of the inflammatory response. The PAMPs of pathogens bind with the PRRs of cells in the immune system, which will either lead to the release of molecules which initiate or mediate an inflammatory response; or it will simply result in the destruction on the pathogen in and of itself. (Alternatively, DAMPs released by damaged cells can initiate the response.) But what happens when pathogens are recognised? We know broadly what happens when a pathogen is recognised (vascular changes (such as vasodilation) and cellular changes (such as recruitment of immunocompetent cells) take place). It is time to consider how those changes are brought about. These changes, as we have seen, are brought about by specific chemicals released by cells of the immune system when they are activated. We now turn to these mediators and what they do.

One of the most crucial inflammatory modulators is histamine, which looks like this:

Figure 1.18: The structural formula of 2-(1H-imidazol-4-yl)ethanamine (histamine). (Image courtesy of "NEUROtiker," via Wikipedia)

Histamine is released by mast cells, basophils and platelets, where it is stored in granules prior to release. The role of histamine in inflammation is vasodilation and it is also responsible for the increase in permeability of the endothelium. This, you will recall, is important for allowing antibodies - which can clear the infection - and clotting factors - which can contain it - to reach the required location. It also has a role to play in recruiting leukocytes.

Histamine works by binding with certain G protein-coupled receptors called histamine receptors. There are four such receptors, imaginatively named H₁ histamine receptor, H₂ histamine receptor, etc. This induces a signal transduction cascade which brings about the required response.

Naturally, histamine is not the only compound involved in this process. Indeed, numerous cytokines involved in various elements of the inflammatory response (some of which we will soon see) have vasoactive properties. Lysosome granules, which are enzymes used to break down molecules as part of the response (either as a direct defence mechanism, or to break down molecules involved in initiating the response, in order to regulate it, or to instigate another aspect of the response), are a good example.

Another important group of molecules (specifically, they are fatty acids) are the prostaglandins, which - as we have already seen - are partly responsible for the experience of pain. There are numerous different prostaglandins and they perform many roles, not all of which are related to the immune response. Some of them are also involved in increasing vascular permeability. Others of them cause the aggregation (and others, still, cause the disaggregation) of platelets. This is important in the process of clotting, which helps to contain the infectious agent and keep it from spreading.

All of this is achieved by the binding of specific prostaglandins to specific receptors. We know of ten prostaglandin receptors, all of which are G-protein coupled receptors. Naturally, the receptors are selective with regards to the prostaglandins they bind with. This accounts for the range of effects of the prostaglandins. Different cell types will exhibit different (combinations of) prostaglandin receptors, which - of course - bind with particular prostaglandins, stimulating particular cells to produce particular responses.

A class of mediators which we have mentioned quite frequently, but only briefly touched on, are cytokines, which are vital in cell signalling processes. Each cytokine has a specific role to play, but they may serve to promote inflammation, or as anti-inflammatories. Due to the dangers of inflammation mentioned above, it is important that it be somewhat limited. Indeed, most of the body's immune responses involve impressive feedback systems and processes which are self-limiting. Also, the mediators we are looking at are quickly degraded, hence inflammation is usually self-regulating and clears up when the stimulus is removed. It is the failure of this which precipitates chronic inflammation. Now, cytokines are involved in upregulation and/or downregulation of certain genes. In a fascinating process, which is another example of where complex feedback mechanisms are employed, they serve to increase production of other cytokines, inhibit their own effects, change cell behaviour to more effectively combat disease and attract cells which can help fight the infection.

Two of the most notable cytokines released (primarily by macrophages) are interleukin-1 (IL-1) and tumour necrosis factor alpha (TNF-α). TNF-α causes fever, cell death and inflammation. It also inhibits the growth of malignant tumours and viral replication. An important cytokine,¹ TNF-α looks like this:

Figure 1.19: Crystal structure of tumour necrosis factor alpha. (Image courtesy of Ramin Herati, via Wikipedia)

The receptors which TNF can bind to are TNFR1 (TNF receptor type 1) and TNFR2. These receptors can actually initiate three signalling pathways, one of which is the NF-κB pathway.

IL-1 cytokines are deeply involved in producing fever. IL-1 also leads to vasodilation and enables immunocompetent cells, which can help the immune response, to reach the site of infection.

IL-1 looks like this:

Figure 1.20: Crystal structure of interleukin-1 alpha (IL-1α) (the interleukin-1 family actually consists of 11 cytokines, but these are beyond the scope of this text). (Image courtesy of "Boghog2," via Wikipedia)

Another important cytokine is IFN-γ (interferon gamma), which looks like this:

Figure 1.21: Crystallographic structure of interferon gamma. (Image courtesy of "K.murphy," via Wikipedia)

Released by natural killer cells (and T-cells, to which we will come), IFN-γ activates macrophages, can inhibit viral replication and combat tumours. IFN-γ binds to a heterodimeric receptor - interferon-gamma receptor (IFNGR) - composed of two macromolecules - interferon gamma receptor 1 (IFNGR1) and interferon gamma receptor 2 (IFNGR2).

We also have to consider another member of the interleukin group - interleukin 8 (IL-8), which is a chemokine (a chemotactic cytokine, or a cytokine which induces chemotaxis). IL-8 is also predominantly released by macrophages. IL-8 induces chemotaxis in a number of cells, but primarily in neutrophils (as we have, in fact, mentioned already). As well as this, it is important in phagocytosis, inducing changes in the cells which are conducive to phagocytosis, such as increasing the concentration of Ca²⁺ within the cell. It also promotes angiogenesis - the creation of new blood vessels from old ones.

Another mediator which we have mentioned before is leukotriene B4 - an eicosanoid (signalling molecules derived from fatty acids (by oxidation)) like the prostaglandins. Leukotriene B4 is released by leukocytes and actually affects leukocytes themselves. It is important in their activation and in mediating their adhesion to the endothelium and their migration across it. It is, too, an important chemotactic factor of neutrophils. The structural formula of leukotriene B4 is given below:

Figure 1.22: 5,12-dihydroxyicosa-6,8,10,14-tetraenoic acid (leukotriene B4). (Image courtesy of "Calvero," via Wikipedia)

The final cell derived mediator - that is, the final mediator released by cells - which we shall consider is simple nitric oxide. Nitric oxide is released by macrophages, as well as endothelial cells and some neurons. Its role is vasodilation and it also reduces platelet aggregation, in order to regulate the response. As well as this it aids leukocyte recruitment and can destroy microbes when it is sufficiently concentrated.

This, then, is the cellular side of things; but, when discussing mediation, we again need to consider the exudate - the fluid containing substances such as clotting factors and antibodies which carries them into the interstitial fluid - as well. Contained within the exudate are numerous plasma derived mediators of inflammation, which we consider here. These mediators can be categorised as belonging to four systems:

•  The complement system.


•  The kinin system.


•  The coagulation system.


•  The fibrinolysis system.

The complement system is actually the subject of our next "section," for want of a better word, coming up in the post after next. For now, we will consider three elements of it - the first of which is C3 (complement component 3). In order to function as part of the complement system, C3 actually needs to be cleaved into two smaller proteins - C3a and C3b. This is done by a protease called C4b2a which is a type of C3-convertase. C3a induces mast cells to release histamine, while C3b is an opsonin - these bind to pathogens and "mark" them for phagocytosis.

The second element we shall consider is C5a. We have already seen that this is a chemotactic factor and it also induces mast cells to release histamine.

The final element that we will consider here, and perhaps my favourite, is the membrane attack complex, which really does seem like something out of a military. The membrane attack complex is a combination of C5b (obviously this is short for complement component 5b), C6, C7, C8, and multiple units of C9. Pleasingly, "unit" really is the word used in the associated Wikipedia article. The complex formed of these proteins creates pores in the cell membrane of invading pathogens. These holes allow molecules to move in and out of the cell - they are very much physical wounds of the cell. Enough of them will utterly destroy the cell in a process called lysis.

Next we turn to the kinin system and bradykinin. Bradykinin is very important in vasodilation and increasing vascular permeability. Not a whole lot is known about it, let alone the "kinin system" to which it belongs. However, we do know that this is the bugger that's responsible for a large amount of the pain we feel from inflammation. Just look at it:

Figure 1.23: The structural formula of bradykinin. (Image courtesy of "Yikrazuul," via Wikipedia)

Turning, now, to the coagulation system, we look at thrombin. Thrombin cleaves the protein fibrinogen - a soluble protein which is part of the blood plasma - into fibrin. Fibrin is insoluble and aggregates, forming a clot and preventing the spread of the infection. Thrombin also works to produce chemokines and nitric oxide.

Alongside this, we have the fibrinolysis system. The primary component of this is an enzyme called plasmin and plasmin breaks down the fibrin clots, regulating the immune response. One does not want to overdo it with clots, they can be very damaging and, in extreme cases, can be deadly. Plasmin is, then, very important. Additionally, the products of breaking down fibrin can have some effects on vascular permeability. Plasmin is also involved in the cleaving of C3 (see above) and it activates Factor XII.

Factor XII is the final component and it is produced by the liver. Factor XII is a protein which usually circulates in the blood flow in its inactive state. When it is activated, it, in turn, participates in the activation of the kinin, fibrinolysis and coagulation systems.

¹Technically, it is an adipokine, a specific type of cytokine distinguished by being secreted by adipose tissue.