In this next section, we're going to look at the function of innate immune cells, primarily looking at phagocytosis. So, I had this really pretty artistic conceptual rendering of a phagocytic cell about to grab something, and I thought it was really beautiful. The next one, this one is a prize winning picture, one of Wikipedia's best featured pictures, and it shows a neutrophil in yellow phagocytizing anthrax bacteria in orange. Naturally, this is all false color to bring out the contrast, but you can see that this is really quite a wonderful picture and gives you an idea that the phagocytic cells are not little round lumps, but rather have very active extensions and are extremely active in the process of seeking out debris, and pathogens, and we'll see getting rid of them. Here, we have a drawing of the ongoing process of phagocytosis, and we have a macrophage that seems to have good cell. You can see up at the top that there are some receptors in the region where this cell is being grabbed, some of them are picking up the cell and some of them look as though they might be toll-like receptors which will be activated by the cell or viral surface components. So, here we put the phagocytized particle into a phagosome, and here, we're fusing it with a lysosome. So, that's going to put in a whole bunch of different compounds that will help to kill and to break up this particular pathogen, debris, iris, whatever. You can see after we've done that, we will again discharge some of the undigestible parts of this phagocytic vacuole. Now, what's going on in here is more than just capturing, and digesting. We're going to look at a lot of very toxic species just to make sure that whatever we've taken up especially some resistant bacteria are good, and dead. So, we're going to have an enzyme complex that will be activated when the phagosome fuses with the lysosome, and that activated complex is going to take this compound and use it to set off an oxidation process that will produce a bunch of toxic compounds to kill off the bacteria. This is NADP, and this is a representation of the compound that takes the NADPH, takes the hydrogen off of it, and uses it to generate, you'll see here, hydrogen peroxide. This process takes place by a membrane complex that is in the membrane of the phagolysosome. It's an oxidative process, do you see we're taking up oxygen? Actually, we're converting it into among other things, a highly reactive oxygen radical. So, because this involves the uptake of oxygen, it's considered a form of respiration. It is not however the respiration you have in the mitochondria, this is not something that generates ATP, this is respiration strictly in the sense of something that uses up oxygen, and uses that oxygen to produce toxic compounds. When your neutrophil or your macrophage phagocytizes a bacterium, there is something that's called a respiratory burst, and that is a rapid oxygen uptake because these enzymes have been activated. In the next one, we'll see some of the compounds, the toxic compounds that are produced here. Okay, so first of all, we've got the super oxide radical. Now, oxygen itself is fairly toxic, and the irony is these enzymes that work in this are actually used in non-phagocytic cells in order to detoxify oxygen and some of the compounds generated by it. But in this case, we're not just having oxygen, we're having oxygen with an extra electron on it that makes it highly reactive. This thing can then be used to add it to water and make hydrogen peroxide. Some of you have maybe used hydrogen peroxide as a disinfectant on an infected wound. Another thing that happens with this is this picks up chlorine and turns it into hypochlorous acid, that's another word for bleach. You actually generate bleach in your phagolysosomes, one of the most toxic compounds you can generate. In addition to that, as if that weren't enough, you will use these oxygens to generate an NO, nitric oxide which you see here has the extra electron on it. Reactive nitrogen species in general which include files, sulfur-containing compounds, sulfur's in a whole bunch of anti-bacterial compounds. Something called peroxynitrous oxide, this is an acid form of nitrogen. This thing can also kill off various bacteria. So, you have here basically enzymes that are activated by the fusion of these membranes, and you begin to produce these compounds, and of course you do that in the interior of your phagolysosome, because the last thing you want is to have these things running around in the cytoplasm. Now, in addition to that, when you oxidize this compound, you also kick off a pump that takes potassium from the outside, and rather from the cytoplasm, and puts it to the inside of the vacuole or vesicle. So, in addition to making lots of toxic stews and compounds, you have something that will generate really an increase in the ion concentration, and essentially this makes this hypertonic. Now, the tonicity in this case it comes first from the potassium, and but you might not understand that you use things like salt, and sugar to raise the tonicity of an environment if you want to keep bacteria from dividing, and kill them off, and so this is another way of making life hard for your bacteria. In addition to that, a very interesting phenomenon happens. When I put in that extra potassium, I'm going to disperse some granules in side the phagolysosome. Now, interestingly enough, you can see that's happening here, we put in some granules and we're going to, and of course you really can't see what's going on, but essentially we're going to turn these into soluble enzymes, and we'll see, other molecules as well. So, we're making toxic reactive oxygen species, toxic reactive nitrogen species, we're pumping in a hypertonic solution. In addition to that, ordinarily water would just follow with it, but what happens is around the outside, we will also polymerize a bunch of actin, and that will act like a mini-cell wall to keep this thing from swelling. I always think of the scene in Star Wars where our heroes wind up in the trash compactor, and they're in there with this horrible toxic stew, in that case the walls are closing in. So, we're hoping that our little pathogen bacterium virus whatever is experiencing the same stress, it's being enclosed in a nice tight little compartment with a high concentration of very unpleasant molecules. Now, the other thing that the potassium does is it begins to disperse these granules, and that will release hydrolytic enzymes. Usually more than one kind because what happens is some bacteria binary way to resist one of them, so then you make another different kind. In addition to that, we also will solubilize defensins. Here at the top is a monomer, here it is as a dimer, it's really a cylindrical molecule now that is going to poke holes in membranes. It may poke holes in the capsids of viruses, it may poke holes in bacterial membranes. But in addition, you're poisoning them, you're digesting them, and in this one, you're stabbing them. It's you're doing every single thing you can in a tightly enclosed space. Now, here's the irony. Okay, the irony is some of them can still survive this, and so there are some bacteria that will not only survive in macrophages, but use the macrophages to transport themselves about the body. The macrophages can be used by both tuberculosis, and by leprosy. Again, it often depends on the strength of the immune response mounted. If you mount a strong response, the macrophages will respond by making even nastier versions of their phagolysosomes, whereas if you give a less strong response, for example a Th2 response, then the macrophages may not be sufficiently vicious to their phagocytized pathogens, and these pathogens will actually survive and multiply, and be disseminated throughout the body. So, but on the whole, this is pretty effective, you can see that your phagolysosomes don't miss a trick if they could think of something nasty, they will. I probably haven't even put in all the hydrolytic enzymes, they're going to be nucleases. Again, when I chew up the nucleic acids, they may set off the Toll-like receptors that are also poking in here. That will give your macrophages clue that it needs to secrete a bunch of chemical signals, cytokines which we'll look at later to upregulate the inflammatory response. So, a reminder of the cells involved. Before we mentioned the neutrophils which were the first to arrive on the scene, you can see as here's a reminder this neutrophil will begin to stick to the inflamed epithelium by its carbohydrates. It also has an Fc receptor at the top, so if there are antibodies stuck to things, it will be attracted to them and have an easier time phagocytizing them. The macrophages however are going to not only phagocytize things, but they're going to truck off, and present pieces of their hydrolyzed proteins to the Th cells, so that these things are not only phagocytic, they are also professional antigen presenters. When they do that, especially if they're toll-like receptors, allow them to sense a serious threat, they're going to secrete cytokines, highly inflammatory cytokines and set off an inflammatory response, both generally and in particular to alert the Th cells. Now, another thing we have is the dendritic cells. The dendritic cells, I have a fuzzy picture of it here, these are the ones that we said serve as sentinels in various parts of the body. If they do sense a pathogen, they can then go also looking for Th cells. So, you can see that these things have lots of extensions that allow them to tap into their environment. All of these cells can phagocytize, only the macrophages and dendritic cells can present antigen. The dendritic cells are particularly effective at alerting naive T cells, whereas macrophages do much better once the T-cells are already up and running. The final one that we've looked at before that's an innate defense actually doesn't do phagocytosis. So, we're looking at the function of innate cells, and so this is a reminder you also have a cell that uses innate recognition to induce apoptosis in cells that might be infected by virus or might have turn cancerous. So, they're here is a selection of cells in the process, most of the ones we looked at do phagocytosis which is a very interesting complex and highly effective way of getting rid of bad things.
