Cell-mediated Cytotoxicity

Chapter 10 Cell-mediated Cytotoxicity




Summary




Cell-mediated cytoxicity is an essential defence against intracellular pathogens, including viruses, some bacteria and some parasites.


CTLs and NK cells are the lymphoid effectors of cytotoxicity. Most CTLs are CD8+ and respond to non-self antigens presented on MHC class I molecules. Some virally infected and cancerous cells try to evade the CTL response by downregulating MHC class I. NK cells recognize these MHC class I negative targets.


NK cells recognize cells that fail to express MHC class I. NK cells express a variety of inhibitory receptors that recognize MHC class I molecules. When these receptors are not engaged, the NK cell is activated. Killer Immunoglobulin-like Receptors (KIRs) recognize classical MHC class I molecules. CD94 interacts with HLA-E. LILRB1 recognizes a wide range of class I molecules.


Cancerous and virally infected cells express ligands for the activating receptor NKG2D. Stressed cells, including cancerous and virally infected cells, upregulate ULBP1–3, MICA and MICB, which are ligands for NKG2D. This results in NK cell activation.


NK cells can also mediate ADCC.


The balance of inhibitory and activating signals determines NK cell activation.


Cytotoxicity is effected by direct cellular interactions, granule exocytosis and cytokine production. Fas ligand and TNF can induce apoptosis in the target cell. Granules containing perforin and granzymes are also released. Perforin forms a pore in the cell membrane, allowing granzymes access to the cytosol. Granzymes trigger the cell’s intrinsic apoptosis pathways.


Macrophages, neutrophils and eosinophils are non-lymphoid cytotoxic effectors. Macrophages and neutrophils usually destroy pathogens by phagocytosis, but can also sometimes release the contents of their granules into the extracellular environment. Eosinophils release cytotoxic granules in response to antibody-coated cells.



Cytotoxic lymphocytes


Cytotoxicity describes the ways in which leukocytes can recognize and destroy other cells. It is an essential defense against intracellular pathogens, including:



Tumor cells and even normal host cells may also become the targets of cytotoxic cells. Cytotoxicity is important in the destruction of allogeneic tissue grafts.


Several types of cells have cytotoxic potential, including:



The two cytotoxic lymphoid effector cells recognize their targets in different ways, but use similar mechanisms to kill them. The myeloid cells use different recognition and killing mechanisms from the lymphoid cells, and indeed these also differ between different types of myeloid cell.





CTLs recognize antigen presented on MHC class I molecules


The most important role of CTLs is the elimination of virally infected cells. CTLs recognize specific antigens (e.g. viral peptides on infected cells) presented by MHC class I molecules, which are expressed by nearly all nucleated cells. Cellular molecules that have been partly degraded by the proteasome are transported to the endoplasmic reticulum, where they become associated with MHC class I molecules and are transported to the cell surface. Normal cells therefore present a sample of all the antigens they produce to CD8+ T cells.



Additional interactions may be required to stabilize the bond between a CTL and its target. Like CD4+ T cells, CD8+ CTLs form an immunological synapse with their target. Signaling molecules including the TCR and CD3 are found in the central zone of the supra-molecular activation cluster (cSMAC), while adhesion molecules segregate in the peripheral zone (pSMAC). In contrast to CD4+ T cells, the cSMAC of CTLs and NK cells is divided into signaling and secretory domains (Fig. 10.2). After signaling has occurred, the microtubule organizing center polarizes towards the synapse, directing cytotoxic granules to the secretory domain of the cSMAC. Early CTL signaling occurs within ten seconds of cell–cell contact and granule release follows some two minutes later.






NK cell development


Like T and B cells, NK cells belong to the lymphoid lineage. However, the pathways and locations of NK cell development are still less well-defined than those of T and B cells. There may also be some differences between humans and mice.


In mice, the bone marrow is essential for the production of NK cells, and a complete pathway of NK development has been described at this location. Human bone marrow does contain CD34+ hematopoietic progenitor cells that have the potential to differentiate into NK cells, but NK-committed progenitor cells have not been identified here. On the other hand, a complete scheme of NK cell development has been described in human, but not mouse, secondary lymphoid tissue. Circulating CD34+ cells are thought to be recruited from the blood to the lymph nodes, where they progress through an NK-committed immature stage to give rise to CD94+CD56hi mature NK cells, capable of effector functions (Fig. 10.w1). CD56hi cells can further develop into CD56low cells, which express KIRs and are more cytotoxic than CD56hi cells. A thymic pathway of NK cell development has also been described in both humans and mice. However, this pathway is clearly not required for NK cell production as athymic individuals have normal NK cell numbers and function.



The factors required for NK cell development have been identified using in vitro cultures of human hematopoietic stem cells and by examining knockout mice. Initially, NK cell development in culture was thought to be absolutely dependent on contact with a stromal cell feeder layer, but it was then discovered that at later stages of development, exogenously added cytokines can substitute for the presence of stroma and that IL-2 or IL-15 alone can mediate the differentiation of NK-committed cells to CD56hi mature NK cells. Although NK cells will develop in the presence of either IL-2 or IL-15, only IL-15 is made by the stroma. The phenotype of knockout mice and humans with genetic deficiencies also suggests that it is IL-15, and not IL-2, which is the critical cytokine for NK cell development. Flt3L and stem cell factor (SCF) synergistically promote NK cell development, but are not absolutely required.


For the acquisition of KIRs, which occurs at the final stage of development, as CD56hi NK cells become CD56low, contact with stromal cells is once again required. Defective Ly49 expression by NK cells in the Tyro/Axl/Mer triple knockout mouse suggests that this may be partially because there is a requirement for an interaction between Protein S or Gas6 on stroma and one of their receptors (Tyro, Axl or Mer) on developing NK cells. Stromal MHC class I molecules are also likely to be important for acquisition of KIR. Several of the genes that are required for immune recognition by NK cells are grouped together on the leukocyte receptor complex on the long arm of chromosome 19 (Fig. 10.w2).



Most of what is known about the transcriptional control of NK cell development comes from knockout mice, although where attempts have been made to transfect human hematopoietic stem cells with genes of interest, this has confirmed that similar transcription factors are likely to be important in human NK cell development. The master transcriptional regulator of NK cell development has recently been identified as E4bp4, which is also known as Nfil3. E4bp4 knockout mice do not have any NK cells, but are normal in all other aspects of their immune system. Downstream of E4bp4 are other factors involved in commitment to the NK lineage, most notably Id2, and factors required for their migration from the bone marrow to the periphery. T-bet, IRF2 and GATA3 are all thought to be required at this stage, as knockouts of these genes accumulate NK cells in the bone marrow but have low numbers in the periphery. Finally, there are transcription factors that are required for the acquisition of NK cell function. For example, CEBPγ knockout mice have normal numbers of phenotypically mature NK cells, but the cells are unable to kill MHC class I negative target cells, or produce IFNγ.



NK cell receptors



Jun 18, 2016 | Posted by in IMMUNOLOGY | Comments Off on Cell-mediated Cytotoxicity

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