Be able to describe antigen-presenting cells
As I've probably mentioned several times before, antigen-presenting cells are cells that present antigens to T-cells. I guess you could say that a lot of cells of the body are antigen-presenting cells in a way, since in my last post I mentioned that all nucleated cells display MHC-I, which present antigens to cytotoxic T-cells, ultimately resulting in destruction of infected cells. However, there's a special subset of cells known as professional antigen-presenting cells (pAPCs), which display both MHC-I and MHC-II and are able to activate naïve T-cells.
Some of the most common types of pAPCs are dendritic cells, macrophages and B-cells. Dendritic cells are the most effective in priming naïve T-cells, followed by macrophages and B-cells. Aside from MHC-II, these cells can also express a co-stimulatory molecule known as B7. Dendritic cells express both MHC-II and B7 constitutively (i.e. all the time), macrophages must be activated to express these, and B-cells constitutively express MHC-II but must be activated to express B7.
Some of the most common types of pAPCs are dendritic cells, macrophages and B-cells. Dendritic cells are the most effective in priming naïve T-cells, followed by macrophages and B-cells. Aside from MHC-II, these cells can also express a co-stimulatory molecule known as B7. Dendritic cells express both MHC-II and B7 constitutively (i.e. all the time), macrophages must be activated to express these, and B-cells constitutively express MHC-II but must be activated to express B7.
Be able to describe the cytosolic pathway for endogenous antigens
As mentioned in my previous post, MHC-I molecules bind peptides derived endogenously (i.e. from within the cell). These peptides are mainly derived from old cytosolic proteins, or from DRiPs (Defective Ribosomal Products). DRiPs are basically poorly translated proteins that have errors in them. The rate of DRiP production increases in cells that have been infected with viruses.
So first let's look at how proteins are broken down into peptides! Firstly, proteins due to be degraded are tagged with ubiquitin, as described here. They are then degraded by a proteosome called Large Molecular Proteosome (LMP), which is encoded in the same region as class II MHC molecules, as mentioned here. Large Molecular Proteosome, true to its name, is pretty large: it's comprised of 28 subunits.
After a protein is broken down into peptides, the peptides need to be transported into the endoplasmic reticulum, where they will meet and bind to newly-synthesised MHC-I molecules. The transporter proteins that allow this to happen are called TAP-1 and TAP-2 (Transporters associated with Antigen Processing-1 and -2). As also mentioned in my previous post, TAP, like LMP, is also encoded near the MHC-II genes. TAP forms heterodimers of TAP-1 and TAP-2, and mutations in either can adversely affect antigen presentation, leading to symptoms such as skin lesions, chronic sinusitis and chronic bacterial infections of the lungs.
Now let's switch focus for a bit and look at the newly-synthesised MHC-I molecules. Firstly, the α-chain is synthesised. This is stabilised by a chaperone protein called calnexin until β2-microglobulin is able to bind. After this binding occurs, the complete MHC-I molecule is released from calnexin, and binds instead to some other chaperones. Calreticulin stabilises the MHC-I molecule, tapasin moves it close to the TAP transporters and Erp57 helps to load the peptide onto MHC-I. Erp57 achieves its goal by breaking and reforming a disulfide bond in the MHC-I α2 domain.
Another important enzyme in the ER is ERAAP, short for Endoplasmic Reticulum Aminopeptidase associated with Antigen Processing. (So glad that there's an acronym for that!) ERAAP trims off the amino terminus of the peptide, which enhances binding.
Once the peptide is bound, MHC-I folding is completed and calreticulin is released. MHC-I and its bound peptide are then exported to the Golgi apparatus, and from there they travel to the cell membrane.
As mentioned in my previous post, MHC-I molecules bind peptides derived endogenously (i.e. from within the cell). These peptides are mainly derived from old cytosolic proteins, or from DRiPs (Defective Ribosomal Products). DRiPs are basically poorly translated proteins that have errors in them. The rate of DRiP production increases in cells that have been infected with viruses.
So first let's look at how proteins are broken down into peptides! Firstly, proteins due to be degraded are tagged with ubiquitin, as described here. They are then degraded by a proteosome called Large Molecular Proteosome (LMP), which is encoded in the same region as class II MHC molecules, as mentioned here. Large Molecular Proteosome, true to its name, is pretty large: it's comprised of 28 subunits.
After a protein is broken down into peptides, the peptides need to be transported into the endoplasmic reticulum, where they will meet and bind to newly-synthesised MHC-I molecules. The transporter proteins that allow this to happen are called TAP-1 and TAP-2 (Transporters associated with Antigen Processing-1 and -2). As also mentioned in my previous post, TAP, like LMP, is also encoded near the MHC-II genes. TAP forms heterodimers of TAP-1 and TAP-2, and mutations in either can adversely affect antigen presentation, leading to symptoms such as skin lesions, chronic sinusitis and chronic bacterial infections of the lungs.
Now let's switch focus for a bit and look at the newly-synthesised MHC-I molecules. Firstly, the α-chain is synthesised. This is stabilised by a chaperone protein called calnexin until β2-microglobulin is able to bind. After this binding occurs, the complete MHC-I molecule is released from calnexin, and binds instead to some other chaperones. Calreticulin stabilises the MHC-I molecule, tapasin moves it close to the TAP transporters and Erp57 helps to load the peptide onto MHC-I. Erp57 achieves its goal by breaking and reforming a disulfide bond in the MHC-I α2 domain.
Another important enzyme in the ER is ERAAP, short for Endoplasmic Reticulum Aminopeptidase associated with Antigen Processing. (So glad that there's an acronym for that!) ERAAP trims off the amino terminus of the peptide, which enhances binding.
Once the peptide is bound, MHC-I folding is completed and calreticulin is released. MHC-I and its bound peptide are then exported to the Golgi apparatus, and from there they travel to the cell membrane.
Be able to describe the endocytic pathway for exogenous antigens
MHC-II, in contrast to MHC-I, binds peptides from antigens derived exogenously, or from outside the cell. These antigens are initially taken into the cell by phagocytosis or by endocytosis, which are pretty similar processes aside from the size of the particles which they take in (phagocytosis is the taking in of larger particles, sometimes even whole cells, whereas endocytosis involves smaller particles). Phagosomes and endosomes can fuse with lysosomes, forming phagolysosomes or endolysosomes, respectively. Antigens can be degraded into peptides in these vesicles.
Firstly, let's have a slightly closer look at uptake of antigens. Uptake of antigens is often mediated by receptors on the cell membrane, such as scavenger receptors like SR-A1. Antigens remain intact in early endosomes, as the pH of early endosomes is not low enough to activate proteases. As the endosomes mature, however, the pH decreases from 7 to around 3, allowing proteases such as cathepsin-S to become active and chew up the antigen.
Now let's backtrack a bit and look at MHC-II. The two chains of MHC-II are synthesised in the endoplasmic reticulum, where they become bound to the invariant chain (Ii). This chain prevents endogenous peptides from binding to MHC-II, forcing them to bind to MHC-I instead. MHC-II bound to Ii can then bud off into an endosome, which becomes progressively more acidified, just like the endosomes taking in antigens. In fact, the endosomes taking in antigens can fuse with the endosomes carrying MHC-II so all the bits and pieces can get together.
The acidification of endosomes again activates proteases like cathepsin-S, which cleave off the ends of the invariant chain, leaving a shorter fragment called CLIP (class II-associated invariant chain peptide). CLIP also prevents peptide binding, so we need a new helper! That helper comes in the form of HLA-DM, which as I mentioned in my last post, is found near the genes encoding MHC-II and is thought of as "class II-like." HLA-DM is found in endosomes. It removes CLIP, allowing peptides to bind to the MHC molecule. And success! We now have a peptide bound to MHC-II! This combo can then move to the cell membrane and show off its cargo to T-cells.
For antigen presentation molecules to do their job well, their off rate has to be slow. By off rate, I mean the rate at which peptides dissociate. This is pretty important, because once the peptide dissociates, the MHC molecule is rapidly lost.
And that's it for these two lectures!
MHC-II, in contrast to MHC-I, binds peptides from antigens derived exogenously, or from outside the cell. These antigens are initially taken into the cell by phagocytosis or by endocytosis, which are pretty similar processes aside from the size of the particles which they take in (phagocytosis is the taking in of larger particles, sometimes even whole cells, whereas endocytosis involves smaller particles). Phagosomes and endosomes can fuse with lysosomes, forming phagolysosomes or endolysosomes, respectively. Antigens can be degraded into peptides in these vesicles.
Firstly, let's have a slightly closer look at uptake of antigens. Uptake of antigens is often mediated by receptors on the cell membrane, such as scavenger receptors like SR-A1. Antigens remain intact in early endosomes, as the pH of early endosomes is not low enough to activate proteases. As the endosomes mature, however, the pH decreases from 7 to around 3, allowing proteases such as cathepsin-S to become active and chew up the antigen.
Now let's backtrack a bit and look at MHC-II. The two chains of MHC-II are synthesised in the endoplasmic reticulum, where they become bound to the invariant chain (Ii). This chain prevents endogenous peptides from binding to MHC-II, forcing them to bind to MHC-I instead. MHC-II bound to Ii can then bud off into an endosome, which becomes progressively more acidified, just like the endosomes taking in antigens. In fact, the endosomes taking in antigens can fuse with the endosomes carrying MHC-II so all the bits and pieces can get together.
The acidification of endosomes again activates proteases like cathepsin-S, which cleave off the ends of the invariant chain, leaving a shorter fragment called CLIP (class II-associated invariant chain peptide). CLIP also prevents peptide binding, so we need a new helper! That helper comes in the form of HLA-DM, which as I mentioned in my last post, is found near the genes encoding MHC-II and is thought of as "class II-like." HLA-DM is found in endosomes. It removes CLIP, allowing peptides to bind to the MHC molecule. And success! We now have a peptide bound to MHC-II! This combo can then move to the cell membrane and show off its cargo to T-cells.
For antigen presentation molecules to do their job well, their off rate has to be slow. By off rate, I mean the rate at which peptides dissociate. This is pretty important, because once the peptide dissociates, the MHC molecule is rapidly lost.
And that's it for these two lectures!
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