Until it binds a peptide, a class I MHC protein remains in the ER, tethered to an ABC transporter by a chaperone protein (Figure 24-58). The chemical nature of the viral antigens recognized by cytotoxic T cells was not discovered for another 10 years. Subsequent evidence indicated that the T cells were recognizing degraded fragments of the internal viral proteins that were bound to class I MHC proteins on the infected cell surface. We first consider how a virus-infected antigen-presenting cell or target cell processes viral proteins for presentation to a cytotoxic T cell. We then discuss how ingested foreign proteins are processed for presentation to a helper T cell. When accessory receptors also have a direct role in activating the T cell by generating their own intracellular signals, they are called co-receptors.
Instead, it has a full-size USB 3 port and a Mini DisplayPort with a proprietary connector that offers power, video and data and is compatible with the $199 Surface Dock in case you need more ports. Trioxlight is concerned about the CABC altering the backlighting and changing how an image looks. It wouldn’t necessarily matter if you were just watching a movie, but considering they want to do photo and video editing, it’s something to think about. Luckily, Razer has a firmware update tool to disable the feature. Razer knows how to make a pretty display, as evidenced by its 13.3-inch 3200 x 1800 touch panel. In our testing, the display reproduced 121 percent of the sRGB gamut and averaged 328 nits of brightness.
The Surface Laptop’s 13.5-inch, 2256 x 1504 touch display is both more colorful and brighter, with 135 percent and 361 nits, which definitely gives it an advantage. The deletion of self-reactive T cells in the thymus cannot eliminate all potentially self-reactive T cells, as some self molecules are not present in the thymus. As we discuss later, antigen recognition without costimulatory signals can delete or inactivate a T or B cell. The most convincing evidence for negative selection derives once again from experiments with transgenic mice. Very few, however, are found in male mice, where the cells die in the thymus before they have a chance to mature. Like positive selection, negative selection requires the interaction of a T cell receptor and a CD4 or CD8 co-receptor with an appropriate MHC protein.
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The most important and best understood of the co-receptors on T cells are the CD4 and CD8 proteins, both of which are single-pass transmembrane proteins with extracellular Ig-like domains. Like T cell receptors, they recognize MHC proteins, but, unlike T cell receptors, they bind to nonvariable parts of the protein, far away from the peptide-binding groove. CD4 is expressed on helper T cells and binds to class II MHC proteins, whereas CD8 is expressed on cytotoxic T cells and binds to class I MHC proteins (Figure 24-55). Antibodies to CD4 and CD8 are widely used as tools to distinguish between the two main classes of T cells, in both humans and experimental animals.
The most direct way to study the selection process is to follow the fate of a set of developing T cells of known specificity. This can be done by using transgenic mice that express a specific pair of rearranged α and β T cell receptor genes derived from a T cell clone of known antigen and MHC specificity. If the mouse does not express the appropriate MHC protein, the transgenic T cells die in the thymus. Thus, the survival and maturation of a T cell depend on a match between its receptor and the MHC proteins expressed in the thymus. Rather than being derived from foreign protein synthesized in the cytosol of a cell, the foreign peptides presented to helper T cells are derived from endosomes.
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Class I MHC proteins are expressed on virtually all nucleated cells. This is presumably because effector cytotoxic T cells must be able to focus on and kill any cell in the body that happens to become infected with an intracellular microbe such as a virus.
How do peptides generated in the cytosol make contact with the peptide-binding groove of class I MHC proteins in the lumen of the endoplasmic reticulum (Figure 24-57)? The answer was discovered through observations on mutant cells in which class I MHC proteins are not expressed at the cell surface but are instead degraded within the cell. The mutant genes in these cells proved to encode subunits of a protein belonging to the family of ABC transporters, which we discuss in Chapter 11. This transporter protein is located in the ER membrane and uses the energy of ATP hydrolysis to pump peptides from the cytosol into the ER lumen. The genes encoding its two subunits are in the MHC chromosomal region, and, if either gene is inactivated by mutation, cells are unable to supply peptides to class I MHC proteins. The class I MHC proteins in such mutant cells are degraded in the cell because peptide binding is normally required for the proper folding of these proteins.
- Selective sync is typically used if Sync is installed on multiple computers, and you want to control which folders get synchronized to each computer.
- When a folder is deselected via selective sync it will show a grey circle with a minus sign overlay icon, signifying that it is no longer syncing and safe to delete from your computer .
- By default all folders are checked, which means that all folders will sync to the computer you are configuring selective sync on.
- If you re-check a folder in the future, Sync will re-download it from the cloud.
Some come from extracellular microbes or their products that the antigen-presenting cell has endocytosed and degraded in the acidic environment of its endosomes. Others come from microbes growing within the endocytic compartment of the antigen-presenting cell. These peptides do not have to be pumped across a membrane because they do not originate in the cytosol; they are generated in a compartment that is topologically equivalent to the extracellular space. They never enter the lumen of the ER, where the class II MHC proteins are synthesized and assembled, but instead bind to preassembled class II heterodimers in a special endosomal compartment. Once the peptide has bound, the class II MHC protein alters its conformation, trapping the peptide in the binding groove for presentation at the cell surface to helper T cells.