Mast cells

We begin, then, with mast cells. As I said at the beginning, the primary function of mast cells is the release of granules which contain chemicals such as histamine and heparin, which are a useful part of the immune response. Although histamine and heparin predominate somewhat, they are not the only substances released. Upon stimulation mast cells also release numerous other inflammatory mediators, as well as at least one type of cytokine. Together, these help to kick start the inflammatory response.

These substances, sadly, are not unconditionally useful - mast cells are somewhat infamous for their prominent role in allergy and anaphylaxis. Mast cells are a little trigger happy - this activity (degranulation, leading to the activation of the inflammatory response) is very important if there's a pathogen around, but mast cells can be stimulated to degranulate by harmless allergens. This will lead to the initiation of the inflammatory response in the absence of a pathogen and can be very harmful, as the substances I mentioned above can be dangerous to the body as well (see Inflammation).

Mast cells are produced in the bone marrow, from where they migrate in an immature form. Mast cells will either locate themselves in connective tissue, or in mucous membranes and there are slight differences which arise depending on where they are located. They develop in these locations, ready to be stimulated to degranulate as a result of an invading pathogen. These locations are obviously chosen as they are somewhat strategic.

There are three ways in which a mast cell can be stimulated to degranulate - the first is direct injury to the cell itself, the second is stimulation by complement proteins (one way in which the complement system complements other immune activities) and the third is by means of Immunoglobulin E (IgE). IgE is one of the five different isotypes of antibody. In addition, mast cells also express pattern recognition receptors and these may enable the mast cell to recognise pathogens and degranulate unassisted. We will concentrate on stimulation by means of IgE, as this is an informative example. As with all things we've been discussing, this is only one part of a larger picture and there are many other processes which combine to produce an effective response. Recall also that antibodies are part of the adaptive immune system and this is another instance of overlap between the two. Nevertheless, mast cells are part of the innate immune system as they respond generically, do not adapt and can be activated without the assistance of antibodies.

So, expressed on the surface of all mast cells is an important receptor called, helpfully, FcεRI. Now FcεRI has very high affinity for IgE and this tends to cause a build up of IgE on the surface of mast cells. An important subtlety to appreciate at this point is that antibodies will be produced in response to specific pathogens. We will discuss this in more detail later, but specific pathogens instigate the production of antibodies which bind well to an epitope on those particular pathogens. Therefore, when a pathogen - let's say a parasitic worm - instigates the production of IgE in this way, levels of IgE which bind well to that worm will build-up and will bind with the FcεRI receptors on the mast cells, which will then be able to recognise the worm. This can have a dark side if it should be a harmless allergen - like peanuts - which causes this, but it is plainly very useful in the case of harmful pathogens like the worm.

The FcεRI receptor is composed of four subunits - the name given to such a molecule is tetramer (remember that a dimer is a macromolecule formed of two smaller molecules; a tetramer is formed of four). It has one chain called the α chain, a β chain and two γ chains. The two γ chains are linked by a disulfide bridge, meaning that they are joined together by two sulfur atoms (there is one sulfur atom on each γ chain and the two sulfur atoms are bonded to each other, linking the γ chains):

Figure 1.42: A diagram showing FcεRI and illustrating the four chains which make it up

On the α chain there are two domains which form the binding site for IgE. This site binds with a part of the IgE called the Fc region, which we will discuss in more detail later. As I described above, IgE antibodies will bind to the FcεRI receptors and build up on the surface of the mast cells. These antibodies, remember, bind well to epitopes on particular pathogens. When pathogen antigens bind to two (or more) of these IgE antibodies, the mast cell is activated.

Figure 1.43: A diagram showing an antigen binding to the paratopes of two IgE antibodies, which are themselves bound to two FcεRI receptors of a mast cell. This will lead to the activation of the mast cell. This diagram is intended to show a very simplified and approximate view of how things might look at the instant of binding, before any subsequent changes have taken place

The mast cell activation occurs by means of a signalling pathway. The binding of the antigen to the two IgE antibodies (themselves bound to FcεRI receptors), will bring about changes to the receptors, initiating a signaling cascade, leading to the degranulation of the mast cell.

The β and γ chains have as part of the chain an activation motif called ITAM (immunoreceptor tyrosine-based activation motif). When the antigen epitopes bind with the IgE antibodies (which are themselves bound to the α chains of two FcεRI receptors), this causes the receptors to cross-link - that is the chains become linked. The ITAM motif contains a chemical called tyrosine and when the FcεRI receptors become cross-linked, this change allows the tyrosine to become phosphorylated by Lyn tyrosine kinase.

Exactly how this happens is not entirely clear to me, but I believe that Lyn is found in lipid rafts, which are essentially small domains, or regions, of cell membranes. These regions are useful for cell signalling, as they serve to collect the necessary signaling molecules. So, when the receptors cross-link, Lyn is recruited and it associates with the FcεRIβ chain. Lyn becomes activated and phosphorylates the ITAM motifs of FcεRIβ and FcεRIγ. I will confess to not being entirely confident about the finer points of this process, but it seems that the cross-linking causes Lyn to associate with the tail of the FcεRIβ chain and I am given to understand that, since the cross-linking links the chains and brings them close together, this allows Lyn to phosphorylate the ITAM motifs of the FcεRIβ and FcεRIγ chains of all the receptors involved (in our case two, but more than two receptors may be cross-linked).

Figure 1.44: At the risk of sounding like a broken record, I would just like to point out once more that the main purpose of my diagrams is to help keep track of the chemicals we're dealing with, since they very often have unhelpful names. I have chosen not to attempt to diagram the cross-linking, instead I have kept everything separate for clarity. The important thing to appreciate, however, is the cross-linking resulting from the antigen binding to the IgE causes a change in the two FcεRI receptors. This change has caused Lyn to phosphorylate the ITAM motifs on the FcεRIβ and FcεRIγ chains.

I mentioned above that my diagram is highly simplified and does not show the cross-linking. Some of you may prefer this more realistic alternative from Sari Sabban:

Figure 1.45: A much more realistic, highly detailed diagram showing the cross-linking of two FcεRI receptors and the resulting phosphorylation of the ITAM motifs by Lyn. The original diagram was included by Sari Sabban in their PhD thesis "Development of an in vitro model system for studying the interaction of Equus caballus IgE with its high-affinity FcεRI receptor." It was then uploaded to Wikipedia. I have cropped that image so that it shows only what is relevant at this stage and added a key and a few labels. The original image, and therefore this one, is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license

Now, once this has happened - once Lyn has phosphorylated the ITAM motifs - a tyrosine kinase called Syk is recruited. Syk binds to the phosphorylated ITAM motifs of the γ chains and this causes Syk to become activated and subsequently phosphorylated:

Figure 1.46

This is a very important stage as activated Syk can go on to activate many other proteins downstream. This, then, is the initiation of the cascade. One of the proteins so activated is LAT (linker for activation of T cells). When LAT is activated, phospholipase C (PLCγ) can bind to it and becomes phosphorylated too. This catalyses the breakdown of another protein called phosphatidylinositol bisphosphate (PIP₂), which produces inositol trisphosphate (IP₃) and diacyglycerol (DAG):

Figure 1.47: Phosphorylated PLCγ catalyses the breakdown of PIP₂ (left) into DAG and IP₃ (right)

IP₃ serves to increase the level of Ca²⁺ ions in the cell. Increased levels of both Ca²⁺ and DAG activates a kinase called protein kinase C (PKC). PKC and the increased levels of Ca²⁺ combine to direct the reorganising of the cytoskeleton through various intermediaries. The cytoskeleton is like the skeleton of the cell and it also provides the structure along which the secretory granules move to the cell membrane. The secretory granules consist of the chemicals - such as histamine - we mentioned above contained within a vesicle. The vesicle is a bit like a bag containing these proteins and facilitating their movement through the cell and into the outside environment. The vesicle binds with the cell wall and disperses its contents like so:

Figure 1.48: On the left we have a vesicle. On the right we see it has moved (this will have been done by moving along the cytoskeleton) to the cell membrane, where is has merged with the membrane itself. You can see that, since the outer surface of the vesicle has merged to become a part of the cell membrane, its contents have been released into the external environment

I have read suggestions¹ that PKC may be involved in the process of the secretory granules fusing with the cell membrane. Wikipedia suggests that PKC is involved in severing the links between the surface of the secretory granules and the exoskeleton, allowing it to merge with the cell membrane.

I should also stress that it is difficult to understate how many other processes and sub-processes are involved. It is far beyond me - and this text - to go into all of them, I merely seek to give an overview. However, as I've mentioned before, a number of the the components I've already mentioned go on to do other things and take part in other downstream signaling events and other cascades which help to produce the effect, or produce other effects.

Before I wrap up the discussion of mast cells, I wanted to add a picture:

(a) A photograph showing cultured mast cells (100X magnification). The cells have been stained (using Tol Blue) and were activated in the course of an experiment. (Image courtesy "Kauczuk" (via Wikipedia))

(b) A schematic representation of a mast cell. (Image courtesy "A. Rad" (via Wikipedia))

Figure 1.49: The mast cell

¹http://en.wikipedia.org/wiki/Mast_cell#Degranulation_and_fusion