Cytotoxic T cells

?Evasion of Cytotoxic T cell and Natural Killer cell recognition by Human Cytomegalovirus and Adenovirus. Abstract Natural Killer cells and Cytotoxic T lymphocytes are key cells in the first line of immune response against viruses. Cells display peptide fragments via the MHC class I molecules, the recognition of these peptides act like signals to the innate immune system. Viruses such as Adenovirus and Cytomegalovirus have evolved a variety of mechanisms to mask cellular invasion and cause persistent infections.

Many aspects and steps of antigen presentation are targeted by viral products to inhibit their recognition. Viruses can up regulate the expression of ligands that activate lymphocytes and also down regulate the expression of MHC class I. NK and CTL activation are distinct ensuring an alternative line of defence when either is inhibited. Introduction The immune system responds to viral infection via effector cells, such as Natural Killer cells (NK cells) and Cytotoxic T lymphocytes (CTL)1, which recognise infected cells.

NK is a lymphocyte that expresses a range of cell surface receptors, this feature differs this cell from most other lymphocytes 1, making it the best adapted to be the first line of defence against infections. The NK cell surface expresses both activating and inhibitory receptors, which recognise different antigens presented on infected cells. Nucleated cells express peptide fragments of the proteins produced within the cell, on their surface via the MHC class I. Healthy cells present house keeping peptide fragments whereas infected cells present foreign peptides.

These peptide fragments in MHC class I complex are recognised by the T cells containing CD8 surface glycoprotein (so called CTR) activating the CTL. The NK cell activity is mainly regulated by receptors that are complementary to major histocompatibility complex (MHC) class I antigens (peptide fragments), the level of specific activated receptors (activating and inhibitory) will decide whether to target the cell. 2 The MHC class I consists of 2 polypeptides: one membrane-integrated alpha (? ) chain that folds to form 3 alpha (? ) domains, and integral a non-covalently bound beta-2 microglobulin (?2m) [3].

Two ? domains form the groove on the upper surface of the MHC class I molecule where the peptide fragments bind, to give rise to an antigen presenting MHC class I. The polypeptides fold and assemble into the class I MHC in the Endoplasmic Reticulum (ER) lumen where foreign peptide fragments also assemble. In order for these fragments to get from the cytosol into the ER lumen, the transporter associates with antigen presenting (TAP) is required. TAP is a heterodimer composed of TAP 1 and TAP 2 subunits, forming a pore.

TAP also influences the final stages of class I MHC assembly, for this task to be successfully completed, tapasin is essential. Tapasin ease the links of class I MHC and TAP. Once the class I MHC is correctly loaded it migrates to the exporter site where the molecules are selected and packed into cargo vesicles to be transported to the Golgi apparatus. (Figure 1) Many viruses have evolved various mechanisms to avoid and supress immune responses, in this essay we will talk more specifically about the mechanisms used by Adenovirus (And) and human cytomegalovirus (HCMV).

Explain the underlying principles how cytotoxic T cells (CTL) and NK cells recognize their target cells. Natural Killer cells and Cytotoxic T lymphocytes are white blood cells produced in the bone narrow and are part of the innate immune system. Both cell types induce infected cells to undergo apoptosis before the virus has had the chance to replicate inside the cell. 1 Natural killer cells, as well as cytotoxic T cells, recognise infected cells by their MHC class I proteins 4. All nucleated cells in the human body contain this cell surface molecule, which presents peptide fragments of proteins synthesised inside the cell.

In infected cells, the peptide fragments for antigen presentation are a result of the processing of viral proteins done by the protease proteasome. The CTL recognises the viral peptide fragments presented by MHC class I protein, whereas NK cells monitor the level of MHC class I on the cell surface. Cells presenting a low level of MHC class I proteins are targeted to be killed by the natural killer cells regardless of the peptide fragments it presents. Many viruses have developed mechanisms to suppress the expression of MHC class I on cells that it invades.

By down regulating MHC class I, it inhibits the presentation of viral antigens and these cells are therefore not detected by CTL. The down regulation of MHC class I in infected cells makes them more vulnerable for NK attacks. Natural Killer (NK) cells express a range of receptors that cannot be rearranged. These receptors are activator (such as NKG2D) and inhibitory receptors (such as NKG2A), and each of these recognise a specific ligand on the surface of infected cells. The balance between the activation of activating receptors and inhibitory receptors regulates the activation of (NK) cells.

Having less MHC class I on cell surface may mean that less inhibitory receptors are activated, leading to the activation of NK cells. Describe at least three mechanisms whereby Ads and HCMV counteract NK cell recognition. The extensive genome of certain viruses such as Adenovirus and Cytomegalovirus enable them to spare a proportion of their genome so that certain genes can specifically code for proteins that are immune-evasive. Such proteins allow viruses to evade important aspects of the immune system, leading to chronic infections.

5 Human Cytomegalovirus (HCMV) blocks peptide translocation in the Endoplasmic Reticulum membrane that transports viral and cellular peptide fragments of the proteasome, from the cytosol into ER’s lumen. The lack of these peptides inhibit the assembly of class I MHC proteins. This effect causes the class I MHC proteins to be transported to the cytosol where they are degraded by proteasomes. 1 HCMV gene US6 codes for a glycoprotein that inhibits protein translocation by binding to TAP. US6 stops conformational rearrangements of TAP that are required for peptide binding; it also inhibits the binding of ATP to TAP.

ATP is required for translocation, lack of starves TAP from its energy source. HCMV also codes for US2 and US. These proteins reverse the translocation of the MHC class I heavy chains from the ER to the cytosol, promoting the degradation of MHC class I heavy chain by the proteasome, stopping the molecule from reaching the cells surface. 5 Adenovirus encodes for proteins that block Class I MHC gene transcription. The E3/19K (E19) gene product is an integral membrane protein that interacts with MHC class I in the ER preventing antigen presentation.

Two different mechanisms are used by E19 to inhibit MHC class I antigen presentation. The dilysine motif in the E19 acts as an ER retrieval motif by binding to TAP and therefore inhibiting its association with MHC class I and it inhibits glycosylation of MHC class I molecules It is said that E19 mimics tapasin, not allowing tapasin to bind leads to the incomplete folding of MHC class I. 8 Explain the structure of the NKG2D receptor and its ligands. Nutural Killer Group II D (NKG2D) is a homodimreric activating receptor expressed by NK cells and some T cells.

NKG2D is a type II transmembrane protein and its ligands are also proteins. The ligands are expressed under cell stress. Many ligands of NKG2D have been identified: MHC-I chain-related A (MICA) and B (MICB) proteins, human leukocyte antigen (HLA-E), UL16 binding proteins ULBP1, 2 and 3, and E (ULBP4) and G (2, 3, 16). 5 These ligands are expressed in response to viral infection and also cell transformation (such as in cancer). MICA and MICB have a very similar structure to MHC class I and similarly also have 3 alpha units. 7 Conclusion The innate immune system has various ways of recognising cellular invasion.

The main cells involved in the first line of defence are NK cells and CTL. The molecule responsible for presenting foreign peptide fragments (antigens) are the MHC class I molecules. Persistent viruses like Adenovirus and Cytomegalovirus have evolved mechanisms to regulate MHC class I expression and also the expression of receptors such NKG2D and its ligands. References 1- Viral modulation of NK cell immunity. Nature, Micro. 3: 59-69 2- Activation of cytotoxic T lymphocytes (`killer cells’), Matthew Mescher, Centre for Immunology. 3- Broke – Biology of microorganism, Madigan, Martiko, Dunlap and Clark.

571 4- Molecular Biology of the Cell, Alberts, Johnson, Lewis, Raff, Roberts and Walter – Fifth edition. 1535 5- Immunology 2003 110, 163-169. The MHC class I antigen presentation pathways: strategies for viral immune evasion – Eric W. Hewitt, school of Biochemistry and Molecular Biology, University of Leeds. 6- Viral modulation of NK cell immunity. Nature, Micro. 3: 59-69 7- MHC class I antigen presentation: learning from viral evasion strategies. Natature. 9:503-13 8- Adenovirus E3/19K promotes evasion of NK cell recognition by intracellular sequestration of the NKG2D ligands MHC class I chain-related proteins A and B.