welcome to the induced innate immune responses module and in this talk we're going to learn about the extravasation of immune cells we're going to first look at chemokines and different groups of chemokines and then learn about the different events that occur during the extravasation of monocytes and neutrophils inflammation normally occurs when the host is able to recognize pathogens that are present inside the role of inflammation is to direct molecules as well as cells to the site of infection and these molecules and cells will then play an important role in clearing the pathogen another role of inflammation is to activate the immune cells so that they do a better job at clearing the pathogen inflammation also induces localized blood clotting and this helps in curbing the spread of the pathogen to other parts of the body and finally inflammation also helps in repairing injured host tissues now inflammation is triggered by the production of different molecules like cytokines chemokines and other types of mediators like histamine now let us look a little more in depth into the role of chemokines and how they promote chemotaxis now chemokines are made in the earliest phase of infection and they induce directed chemotaxis and the result of this directed chemotaxis is the movement of cells towards the source of the chemokine now there are different groups of chemokines and we're going to look at four such groups the first one are called as the c chemokines because they have a cysteine two cysteines that form a disulfide bridge another group of chemokines are the cc chemokines because as you can see in the figure in the black box there are two cysteines that form two disulfide bridges and these two cysteines are right next to one another in the primary sequence of the chemokine example of another chemokine is the cxc chemokine family now in this case as we can see in the black box there are two cysteine amino acids but there is another amino acid that is between them hence the sequence is assisting another amino acid followed by another cysteine and the two cysteine once again form disulfide bridges the last one is going to be the cx3c in the case of the cx3c if you're following the pattern then you can already guess that there are two cysteines but between those two cysteines there are three amino acids and thus the sequence is a cysteine followed by one amino acid two amino acids three amino acids and then another cysteine chemokines are able to recruit immune cells to the site of infection and in the following slides we will look at the mechanism by which this occurs now immune cells need to leave the blood vessels so that they can come to the site of infection so the monocytes and neutrophils that are part of the innate immune system are usually circulating in the blood however when they have to go to a tissue where there is an infection they have to leave the blood and for that process is called as extravazation now in order for the cells to know where they're supposed to go to or where that site of infection is that is where chemokines play a role so chemokines help the immune cells to leave the blood vessels so that they can go to the site of infection so let us see how this process actually works for leukocytes to leave blood vessels they must be able to interact with the endothelial cells that are present in the blood vessels or that make up the blood vessels so this interaction requires protein protein interactions and the proteins that play a role in interactions between the endothelial cells and the immune cells are called adhesion molecules the different types of adhesion molecules that are involved in the extravasation of leukocytes are the first one is the selectance and the selectance that we're going to look at is the p selected and the e selected and they are expressed in endothelial cells another adhesion molecule that we will look at are the integrands the integrins in this situation are present in the innate immune cells another set of proteins that play a role in extravasation are the immunoglobulin superfamily for example the icam now icam can be present on both endothelial cells as well as activated immune cells let us look at the process of extravasation now now we all learned that when there is a pathogen present it can activate the complement pathway resulting in the production of anaphylatoxins like c5a we also learned that our own immune cells and epithelial cells can also produce pro-inflammatory cytokines like tnf-alpha in response to the presence of a pamp these two molecules can activate endothelial cells and let's look at an endothelial cell in the figure and if we look focus on the figure now in the endothelial cells there are granules present and these granules have the protein piece selected the presence of the molecules like c5a and tnf alpha induce the externalization of these granules and that results in p-selectin being present on the endothelial surface once piece selected is present on the endothelial surface it can interact with the leukocytes and we will see how that happens in the next few slides what should be noted is that p selected is the first selectin that is expressed on the endothelial cells during an infection once p-selectin is expressed on the cell surface there is a signal in the cell to induce the expression of another selected which is e-selectin and once e-selected is expressed it will also be transported to the cell surface in this slide let's focus on the figure so in the figure in the black box we can see the endothelial cells that are expressing e-selecting on the side that is facing the blood so the part on the black box right now is where the blood is flowing and in the blood we will have our blood cells including monocytes and neutrophils the presence of e-selecting on the endothelial cell is going to trigger a process called as rolling and let us see how that happens so let us focus on this neutrophil and look at its cell surface and at the cell surface we see a carbohydrate moiety that's called as the sulfated cylinder lewis x the presence of the sulfated silent lewis x on the surface of the leukocytes like neutrophils and monocytes enables the cell to interact very weakly with p-selectins and e-selectants now ideally we would have the blood cells simply whizzing through the blood vessel so they travel very fast however when e-selectings are expressed on the endothelial cells the presence of these e-selectins or piece selectance for that matter will facilitate an interaction with the sulfated salad lewis x present on the neutrophils and hence the neutrophils and monocytes will start associating with the endothelium in a very weak fashion this slows them down and what we see is a process called as rolling so instead of passing through the blood vessel very quickly the leukocytes like the neutrophils and the monocytes are going to show rolling after the step of rolling comes the step of tight binding or margination before we look into the step of tight binding let us first focus on the chemokines that are secreted now chemokines are secreted at the site of infection and because they are soluble proteins they can diffuse in the surrounding area as well as into the blood vessels once they cross the endothelial barrier and are able to reach the blood vessels on the side where the blood is the chemokines can bind to proteoglycans that are associated with the endothelial cells if you remember from cell biology proteoglycans are proteins that form the extracellular matrix now from our previous slide we saw that the neutrophils have slowed down in their speed and they're rolling across the blood vessel because of the association of the selecting that is present on the blood vessel with the silent lewis x carbohydrate moiety that is present on the surface of the neutrophil now binding of the chemokine to the chemokine receptor can happen because of the slow movement of the neutrophil in this example now even though i see neutrophil all the time the same thing happens with monocytes as well so going back to the figure we can now see that that slow moving neutrophil is now able to associate with the cytokine or the in this aspect the chemokine that is present on the proteoglycans because it expresses the chemokine receptor this association triggers a change in the confirmation of the integrins that are present on the neutrophil the integrins can now interact with an icam adhesion molecule that is present on the endothelial cells and that is what is shown in the black box the interaction between the integrin of the neutrophil and the eye cam of the endothelial cell is a very strong interaction and hence we see tight binding between the neutrophil or a tight association between the neutrophil and the endothelial cell this allows the monocytes and neutrophils to attach firmly to the endothelial cells the next step of extravasation is called diapedesis in this step the monocytes and neutrophils are able to cross the endothelial barrier through the endothelial junctions and this is what is shown in the figure in the black box where we can see an immune cell squeezing through the junctions between two endothelial cells this is facilitated with the help of integrins as well as other molecules called pcamps now the pcam is present in immune cells as well as the intercellular junction of the endothelial cells and these interactions of integrins and picams allow the monocytes and neutrophils to squeeze between the endothelial cells once the immune cells are able to get access to the other side of the blood vessel which is where the tissues are and most likely the site of infection is the immune cells have to break through the basement membrane and the way they do that is they secrete or they make extracellular matrix proteases that are able to break down that extracellular matrix and can enter into the tissues the final step is migration where the chemokines produced by the cells they form a concentration gradient and so the monocytes and neutrophils follow that concentration gradient because they express receptors for the chemokines and now are able to go to the site of infection during an infection process neutrophils are the first cells that will go to the site of infection and they rapidly migrate to the site of infection in very large numbers monocytes also migrate but however they migrate a little more slowly and you don't see them as rapidly as you see neutrophils present simply because neutrophils are a lot more in number when it comes to how many cells are there in blood with this we come to the end of our talk where we learned about different groups of chemokines and functions of chemokines we also learned about the different steps that are involved during the extravasation of monocytes and neutrophils which include the processes of rolling tight binding which is also called as margination sometimes diapedesis and migration