Phagocytosis: Leukocyte Killing

00:00 So, how do leukocytes kill? Well they phagocytized first.

00:05 And they phagocytized because they have on their surface specific receptors.

00:10 So you see a mannose receptor there, you see a scavenger receptor there are a variety of receptors that are able to bind motifs on the surface of our pathogen.

00:19 And the pathogen here looks like Sputnik or Coronavirus.

00:23 It's not a virus, it's a bacteria or a fungus.

00:26 But there are specific motifs there that can bind two receptors.

00:29 Once the receptors bind, then the membrane wraps up around them, kind of zips up around them.

00:36 And we can internalize them in a phagosome, and then fuse lysosomes with them, and the lysosomes, and some of the things that are also recruited that time will break down the microbe.

00:49 So perfect, we not only ingested them, but we digested them.

00:53 Okay, there are specific receptors.

00:55 So these are pattern recognition receptors, we talked about mannose receptor and scavenger receptor, all those things.

01:01 Bound the antibody and complement will also work remember the obstinance, the things that make things tasty, and mannose-binding protein, for example.

01:10 So these are all ways that we can specifically eat just the bad guys.

01:15 So killing. It's not just a matter of dumping in lysosomal proteases, we actually have to do one extra step, to kill.

01:23 And it's kind of cool because bacteria figured out how to evade this.

01:26 I'll talk about that later.

01:29 But any then, here we are, we are looking at the lysosomal protein primary granule.

01:35 And there are a number of proteins that are present on the primary granule.

01:38 These are within neutrophils.

01:40 So we've ingested, but we haven't yet digested, killed, or digested the pathogen.

01:47 So what do we do? So, we've now delivered the pathogen to our primary granule.

01:52 It's basically a lysosome.

01:56 On the surface of the lysosomes, there is, as you can see in that diamond a membrane oxidase.

02:02 And once we have accumulated pathogens within the primary granule, we bring in from the cytoplasm, a cytoplasmic oxidase.

02:11 So now we have a complex that will pump protons into the lumen.

02:17 So this is an important step.

02:19 Now, next step will require the incorporation of oxygen plus the protons to generate oxygen free radicals and in particular hydrogen peroxide.

02:33 So this is going to be an important step on our way to killing.

02:38 Next that hydrogen peroxide interacts with myeloperoxidase, which is another cell with another component of the membrane in these primary granules.

02:50 And with chloride ions, we're going to generate from that hydrogen peroxide, the chloride and the myeloperoxidase hypochlorous acid, that's HOCl.

02:59 See that? HOCl. And we'll know what that is? That is Clorox, that is bleach.

03:06 So bleach works great on your toilet.

03:09 It also works great inside your primary granules to kill bacteria.

03:14 That's exactly we do the same thing.

03:16 When we clean out our toilet bowls, we make hypochlorous acid do the work to kill the cells and it's through free radical damage.

03:23 So we have free radicals from the superoxide from the hydrogen peroxide, from the hydroxide free radicals and importantly hypochlorous acid or bleach, and that kills bacteria.

03:35 And then now that we killed them proteases will be able to degrade them.

03:40 This uptake of oxygen to make the oxygen free radicals is called an oxidative burst.

03:47 So there will be a massive kind of uptake of oxygen utilization of oxygen, which we can measure oxidative burst.

03:54 The reactive oxygen species that get generated that are shown there on the box will then be responsible for killing the bacteria.

04:02 And finally lysosomal proteases will degrade them.

04:05 So we've ingested, killed, and digested.

04:10 A very cool process.

04:15 Another thing that we've just learned recently, about neutrophils is that they also extrude their intracellular contents into the extracellular space as a way to corral and kill microbes outside of them.

04:31 So they don't have just eat, but they can in fact kill by extruding their material.

04:36 So this is called a neutrophil cytoplasmic net.

04:41 So, or neutrophil extracytoplasmic net, or NET.

04:47 Contained within that is going to be DNA as shown here.

04:51 Then we're going to have histones as part of the DNA structure.

04:54 We're going to have myeloperoxidase.

04:56 Oh wow, we can even form potentially hypochlorous acid out there.

05:01 There are elastases.

05:02 There are cathepsin and there a whole variety of things that are part of the net that it's very sticky, and it goes out and sits out in the space and can capture up microbes.

05:13 And then we can kill them in that location.

05:16 So this is an X neutrophil extracellular trap or a NET.

05:23 It's a mechanism for neutrophils that have extracellular release and killing.

05:29 It includes histones and DNA.

05:33 There are a lot of attached granules that can also be potentially cytotoxic.

05:37 It's a questers and kills bacteria fungus and viruses.

05:41 And the cell dies when it does this.

05:44 So the neutrophil by extruding it's kind of guts into the space around it will die.

05:50 It's a distinct process from apoptosis which neutrophils will also undergo after a short period of time.