Memory And Death In The Immune System-Immunology-Lecture Handout, Exercises of Immunology

Download Memory And Death In The Immune System-Immunology-Lecture Handout and more Exercises Immunology in PDF only on Docsity! 10/31/05; 9 AM Shiv Pillai Memory and death in the immune system Apoptosis refers to a form of death in which a cell initiates a suicide program and in which characteristic morphological alterations are observed in dying and dead cells. These changes include chromatin condensation, nucleolar disruption, cytoplasmic contraction, dilation of the endoplasmic reticulum, and membrane blebbing. This form of death is typically accompanied by the cleavage of DNA into a “ladder” of oligonucleosome length fragments. Apoptotic cells are systematically dismantled into bite-size packages that are recognized and disposed of by tissue macrophages without the concomitant activation of the innate immune system. This form of death is not associated with inflammation, and is also referred to as “programmed cell death” or “physiological cell death”. Apoptosis may be initiated in a variety of circumstances including, but not limited to, the induction of DNA damage, the activation of a stress response, the withdrawal of growth factors, or the triggering of specific signaling receptors. This type of cell death is of critical importance in the development of virtually every multicellular organism. Apoptotic death is repeatedly initiated during lymphoid ontogeny, during the process of negative selection, whenever lymphocytes fail to be positively selected, and during the contraction phase of immune responses. Cytolytic CD8+T cells, Natural Killer cells, and activated CD4+ T cells express specific molecules, including Perforin, Granzyme B, and Fas ligand, that are designed to assist in the execution of target cells by apoptosis. There are two major pathways that lead to apoptosis, both of which culminate in a common death pathway. The mitochondrial or intrinsic pathway involves the induction, by a variety of stimuli, of specialized proteins that induce mitochondrial leakiness and thus activate the common death pathway. In the extrinsic “death receptor” pathway, the triggering of cell surface receptors of the TNFR family contributes to the enzymatic activation of the common death pathway. docsity.com The induction of death is generally linked to the activation of a set of proteolytic enzymes that are called caspases (cysteine proteases that cleave proteins immediately after aspartic acid residues). The first such enzyme to be identified was the C. elegans cell death gene, Ced-3. Caspases exist as latent pro-enzymes that are activated in a cascade-like fashion following cleavage of a pro-piece that ends in an aspartyl residue. All caspases contain a QACRG motif in which the cysteine residue is a part of the active site. “Initiator” caspases of the mitochondrial pathway and of the death receptor pathway cleave and activate “executioner” caspases of the common death pathway. Executioner caspases in turn cleave specific substrates and thus contribute to the dismantling and packaging of the dying cell. The caspase that has been best characterized in the context of the common death pathway is caspase 3. A number of substrates of caspase-3 have been identified. A particularly interesting substrate is the Caspase activated DNase (CAD) also known as DNA Fragmentation Factor or DFF. CAD is a latent nuclease that can cleave chromosomal DNA into nucleosome size fragments but is held in check by an inhibitor called ICAD (for Inhibitor of CAD). The nuclease activity of CAD is released by caspase-3 mediated cleavage of ICAD. Activated CAD is responsible in part for the fragmentation of DNA that is characteristic of apoptosis. The mitochondrial pathway and proteins of the Bcl-2 family The predominant mechanism of apoptosis in all metazoan cells involves the regulation of mitochondrial integrity by members of the Bcl-2 family. Bcl-2 was initially identified during the molecular characterization of a chromosomal translocation in patients with follicular lymphoma that brings the immunoglobulin heavy chain locus on chromosome 14 in apposition with the Bcl-2 gene on chromosome 18. Bcl-2 represents the vertebrate homolog of Ced-9, a C. elegans gene that inhibits apoptosis in nematodes. Like Ced-9, Bcl-2 is an inhibitor of the mitochondrial pathway of apoptosis. Bcl-2 family proteins can be broadly divided into three categories. Multi-domain anti-apoptotic proteins such as Bcl-2 and Bcl XL, typically contain four BH (Bcl-2 homology) domains and reside in the outer mitochondrial membrane where they contribute to mitochondrial stability. Multi-domain pro-apoptotic proteins such as Bax and Bak, resemble Bcl-2 in having multiple domains but these proteins, when activated, can disrupt the integrity of the The common death pathway depends on the activation of caspases docsity.com Ending T cell responses and Activation Induced Cell Death By using artificially generated reagents such as labeled MHC class I-peptide tetramers, it has been possible to follow the fate of antigen specific CD8+T cells in infected mice and in human patients. It is now recognized that during an active immune response a very large number of the T cells in the host represent the clonal outgrowth of a few antigen specific CD4+ or CD8+ T cells. Examination of these clones has revealed that a large proportion of activated T cells are in the process of undergoing apoptosis. While large numbers of T cells do get activated by antigen most of these cells probably die because the amount of antigen is limiting and these cells no longer receive survival signals. Another major way in which activated T cells may be rendered quiescent is by the induction of inhibitory signaling via CTLA-4. Some cells which are restimulated by antigen also receive signals to die. AICD is probably as important for the elimination of activated CD8+ cells as it is for the elimination of CD4+ cells. However it has been examined mainly in the context of CD4+ cells. Evidence from lpr and gld mice, from human lymphoproliferative syndromes involving Fas mutations, and from IL-2 and IL-2Rα chain knockout mice has all come together to suggest the following scenario. When both Signal One and Signal Two combine to generate maximal T cell activation, transcriptional induction of both the IL-2 gene and of the IL-2R is achieved. This form of complete activation leads to signal transduction via the IL-2R and the maximal induction of both CD40L and FasL. During the course of these events the T cell makes cytokines, and may activate specific B cells and professional APCs via CD40L- CD40 interactions as well as by triggering specific cytokine receptors. The majority of properly activated T cells commit suicide. They do so because the FasL induced on individual T cells triggers Fas receptors expressed by the same cell and thus induces death. Signaling via the IL-2 R is critical for the induction of AICD, probably because signals delivered via this cytokine receptor are required for the maximal induction of FasL. In addition IL-2 signaling may contribute to an inhibition of the cellular levels of FLIP. FLIP is a protein that is structurally similar to Caspase-8 but which lacks proteolytic activity. It can prevent the recruitment of Caspase-8 to FADD and thus protect a cell from Fas induced apoptosis. The reduction of FLIP levels by IL-2 may docsity.com be key in allowing the suicide process to go through successfully. In CD8+T cells AICD might require signals to be delivered both by IL-2 as well as by TNF-α. Given that “proper” activation of a T cell leads to death, it is pertinent to ask how memory T cells are ever generated. Making the choice between activation and memory: memory T cells The existence of “true” memory lymphocytes has often been questioned and while their existence remains controversial, there is a growing acceptance of the view that such cells do exist. The persistence of antigen, either preserved by a low-grade viral infection or as part of long- lived immune complexes sequestered by follicular dendritic cells, could potentially contribute to the identification of recently restimulated cells as “memory” cells. The best evidence for the existence of “true” memory cells has come from studies on the transfer of CD8+ cells in the apparent absence of antigen which can result in the transfer of immune responsiveness for extended time periods. Although many of the central issues regarding memory remain controversial, there is some evidence to suggest that memory CD8+ cells can be maintained by being “tickled” via their TCRs even by non-specific, MHC class I-peptide complexes. Antigen, even if it is only required in a cross-reactive non-specific form, may well be required for the maintenance of what may well be “true” memory T cells. These cells may be best described as long-lived antigen specific T cells that emerge after activation by specific antigen but which do not require specific antigen for their extended in vivo survival. The following general features characterize memory T lymphocytes (whether or not they pass the litmus test of antigen-independent survival): 1. Memory cells apparently survive for a long time in vivo in the apparent absence of specific antigen. They may survive as long as the lifespan of some small vertebrates. 2. Memory lymphocytes generally present with an activated phenotype, but can be distinguished from effector cells on the basis of size and function. They are smaller than effector cells. Effector CD4+ cells may secrete large amounts of cytokines whereas memory CD4+ cells may need to be triggered in order to do so. Effector CD8+ cells may be able to kill ex vivo targets directly while memory cells need to be triggered in order to be able to kill. docsity.com 3. Memory cells do not require antigen for survival 4. Memory T cells can be activated by signals which may be below the threshold for the activation of naive T cells. This may in part be due to the high levels of adhesion molecules expressed on memory T cells. 5. Most memory cells probably live for a long time because they have been programmed by antigen exposure to express high levels of survival factors which include but may not be restricted to members of the Bcl-2 family. Memory T cells may be categorized, into "central" memory cells which express CCR7 and traffic like naïve T cells to lymph nodes, and into "effector" memory cells which do not express CCR7 and return to tissue sites. The vast majority of effector T cells that are generated during an immune response are eliminated by AICD. A number of models have been proposed to explain how memory cells are generated. The most likely scenario is as follows: Following T cell activation a very large number of effector T cells are generated. As antigen is cleared, most of the vigorously activated T cells either die because they are no longer being stimulated or undergo AICD. A small number of these cells, perhaps those that were not activated as well because they arrived late on the scene, or those which were activated by distinct APCs in a slightly less vehement manner, fail to induce the suicide pathway and may go on to become memory cells. For CD8+ cells it has been established that effector cells go on to become memory cells. It is also possible that certain effectors are induced to “rest” and become memory cells, or that distinct activated cells give rise to effector cells and memory cells The exact molecular pathways that are responsible for memory cell generation remain to be identified. It is very likely that anti-apoptotic Bcl-2 family members such as Bcl- 2 and Bcl-XL are induced by antigen receptor and costimulatory signals. Costimulation through CD28 may be required for the induction of Bcl-XL. The induction of cytosolic kinases such as Akt might contribute to the phosphorylation and inactivation of pro-apoptotic Bcl-2 family members such as Bad. It is possible that much of the memory phenotype may be explained by two critical sets of biochemical changes: 1. Memory cells can be much more easily triggered through the TCR This might be because memory cells express higher levels of adhesion molecules than naive cells or because of alterations in intracellular signaling pathways. docsity.com