T Lymphocytes
KEY TERMS
CTL = cytotoxic T lymphocyte
describe the molecules that are part of the TCR complex
= TCR along with its signaling molecule, CD3 CD3 = 5 different polypeptides = "invariant chain" (...remember β2-microglobulin and Ii that associates with newly made MHC class II proteins are also called invariant chains and should not be confused with each other) CD3 has ITAM motifs and these can be phosphorylated/dephosphorylated to determine whether or not the signaling cascade will begin. Both the TCR and the CD3 molecules are members of the Ig gene superfamily.
GENES SUMMARY
Determination of whether to become an αβ or a γδ T cell is determined by the expression of the TCR genes. The α and δ loci are closely associated - the δ locus being embedded within the α locus and The β and γ chains are on the same chromosome, they are not linked together. The make-up of the variable domains of the TCRs is also similar to the variable domains of an antibody in that they are made up of the combination of V(D)J gene segments joined to a constant region gene with the α and γ chains having V and J genes and the β and δ having the V, D, and J genes. A given T cell will express only one type of TCR and through the process of gene rearrangement, if the α chain is rearranged; the δ locus is cut out and thus will prevent the T cell from expressing the γδ TCR. Based on the number of V(D)J genes, there are estimated to be ~ 3M different possible TCRs that could be generated. As far as the process is concerned, gene rearrangement of the TCR genes is similar to Ig gene rearrangement in that a single V region is associated with a single D region, and a single J region. These three genes join together to code for the variable domain of the TCR.
detail positive and negative selection in T cells
T cell education = thymic selection - begins when the T cell is "tested" for its ability to interact with self MHC. Self-MHC molecules are expressed by thymic epithelial cells and those that can interact with self-MHC will stop α chain rearrangement (allelic exclusion) Because those T cells that can react with self-MHC continue in development = positive selection. These T cells can now be tested for their reactivity with self-peptides. T cells that interact with self-peptides will die via apoptosis (negative selection).
detail the significance of superantigens
An example of what happens without the two signals to T cells is seen with superantigens (sAg). Superantigens are molecules that cause a population of T cells to proliferate (those T cells expressing a particular Vβ family that can bind to the superantigen) and secrete cytokines. Because a relatively large population of T cells can secrete cytokines in this scenario, significant pathology can result (cytokines are very potent molecules and systemic production can be lethal). Superantigens play a role in human disease and microbes associated with producing superantigens are some species of Staphylococcus and Streptococcus as well as some other pathogens. The importance of sAg was first recognized when patients came down with a disease now called 'toxic-shock syndrome'. Besides causing a subpopulation of T cells to proliferate and produce cytokines, these T cells will become apoptotic. For years, activation of the T cell was when the superantigen brought together a particular TCR family with the MHC. Newer evidence shows that the CD28 molecule may also interact with the superantigen and therefore, also plays a role in the inappropriate activation of the T cell.
describe the difference between CD4+ and CD8+ cells with respect to Ag recognition and the MHC
As with B cells, T cells also have accessory molecules to help in their activation. These are CD4 and CD8 - both interact with the MHC molecules (class II and I) and these also have some signaling role. - without the CD4/CD8 molecules, it's estimated that one would go from needing only 100 TCR/Ag/MHC interactions all the way up to about 10,000 TCR/Ag/MHC interactions for activation of the T cell.
ALLELIC EXCLUSION
As with B cells, T cells generally adhere to ALLELIC EXCLUSION allowing a given T cell to express only one TCR, however, the α locus is not as tightly controlled and therefore, it's possible that two α chains may be expressed with a given β chain. Generally, however, only one α chain will make it to the surface of the cell and be functional (so we can still consider that only one TCR with one antigen specificity is on the surface of the T cell).
describe their biological importance of the αβ T cell subsets
T cells are divided into different groups/subsets depending on the focus of the discussion. When discussing TCRs, T cells are divided into αβ and γδ T cells. The αβ is the "normal" type of T cell and the γδ T cells are produced earliest in development of T cells and also they seem to play a role in helping to fight microbes associated with the skin, respiratory, or gastrointestinal systems Another subset of T cells is the NK1-T cells. While functionally are similar to NK cells (e.g. cytotoxic), these cells do have a rearranged TCR however, it does not interact with classical MHC molecules and instead, interacts with CD1 molecules and are thought to recognize lipid antigens. T cells can be divided based on the co-receptors (e.g. CD4 or CD8 cells) What determines which type of CD4+ T cell will be produced? Most of the evidence points to the interaction of the APC, CD4+ T cell and the cytokines that can affect transcription factor expression. The other major group of αβ T cells are the CD8+ T cells. Most often these are the cytotoxic T lymphocytes (CTLs or Tc). There is experimental evidence to support that some CD8+ T cells can be suppressive, however, most of the recent data would give the role of regulatory T cell to the CD4+ T cell.
describe the two signals needed for T cell activation
T cells need two signals for activation. The first signal is the recognition of the antigenic peptide presented in the association of self-MHC. The second signal is often interaction of CD28 with CD80/CD86. B7 can also interact with CTLA-4 (CD152) but when this occurs, a negative signal is sent to the T cell. Without the two signals, T cells will likely become anergic (= dormant, tolerized) or may even die via apoptosis.
describe the steps of T cell development/maturation
T cells originate in the bone marrow and move to the thymus for further development ("education") As T cells enter the thymus, they express CD2 and are DOUBLE NEGATIVE = CD4-CD8- The T cells will begin the rearrangement process and will become either αβ or a γδ T cells. For the cells destined to become αβ T cells, during the gene rearrangement process but before alpha chain rearrangement, the T cells will become DOUBLE POSITIVES (CD4+CD8+). These cells can proliferate at this stage and will result in the same beta chain associating with a number of different α chains. At this point, the T cell can begin the process of "selection" to insure that self-reactive cells are destroyed and that the T cell can interact with self-MHC = "T cell education". ***Prior to leaving the thymus, the T cell will become SINGLE POSITIVE expressing either CD4 or CD8.
describe the significance of αβ and γδ T cells
TCR = heterodimer made up of either an αβ heterodimer (the "major" type of TCR) or γδ heterodimer Structurally, both of these molecules resemble the Fab region of an Ig molecule. In addition, as with an antibody, the TCR has a variable and constant region and within the variable domain are 3 hypervariable regions (complementarity determining regions; CDRs, CDR1, CDR2, and CDR3). Another similarity between T cells and B cells = the antigen binding domain is associated with another molecule that functions as the signaling molecule. In the case of T cells, the signaling function occurs through the CD3 complex (remember for B cells it is the Igα/Igβ molecules) Functionally, there are differences in T cells depending on the type of TCR chain expressed by the T cell. The αβ T cells are the "traditional" T cells and they are divided into the CD4+ and CD8+ T cells. The CD4+ T cells (T helper cells) recognize their antigen in the context of MHC class II molecules whereas the CD8+ T cells (cytotoxic T cells), recognize their antigen in the context of the MHC class I molecules. γδ T cells, on the other hand, are not as important for the "typical" immune response and are thought to play a role in recognizing phospholipids antigens. These T cells are not restricted by the MHC and in fact, most of these T cells are CD4-CD8-.
MHC restriction - REVIEW
TCRs are restricted to self-MHC T cells only recognize their antigen when presented in MHC molecules T cells recognize their antigen in the context of self-MHC (called MHC restriction). - T cells are "educated" in the thymus and only those T cells that properly interact with self-MHC are allowed to leave the thymus, therefore, in the periphery a T cell will only be activated when it recognizes its antigenic peptide presented with self-MHC.
list the αβ T cell subsets
The CD4 cells = T helper cells - there are other subsets including CD4+CD25+ T regs (regulatory T cells). When focusing on the typical CD4+ T cells, we have: 1. Th0 which are precursor T helper cells expressing both IL-2, IFNγ and IL-4; 2. Th1 cells which produce IL-2, IFNγ, and TNF-β (tumor necrosis factor) and these cells are helpful in regulating cell-mediated immunity by causing cytotoxicity and inflammation, and; 3. Th2 cells which produce IL-4, IL-5, IL-6, IL-10 and IL-13. These cytokines are most helpful in the humoral immune response. In the past, there was a subset of CD4+ T cells called "delayed type hypersensitivity" T cells. More recently, analysis of these cells indicates that these are most like Th1 cells and the use of the term DTH cells is less frequent. The CD4+CD25+ T regs: more recently described subset and are thought to play a role in regulating the immune response. These cells seem to be able to suppress autoimmune disease. The CD25 molecule is part of the IL-2 receptor and CD25 is expressed by T cells early in development (in the thymus), by activated T cells after recognizing their cognate antigen and also by the Tregs. An important fact about the Tregs is that CD25 seems to be constitutively expressed by these cells and the level of expression is much higher than the other cells that express CD25. Th17 cell: seems to play a role in autoimmunity and also, in defense against certain extracellular infections caused by bacteria and fungi. The Th17 cell produces IL-17 and IL-22. Recently, there is evidence that Tregs and Th17 cells are related in that a Treg could be induced to become a Th17 by the presence of TGF-β (transforming growth factor) and IL-6.
describe the process of gene rearrangement for the TCR genes
The process of TCR gene rearrangement occurs within the thymus (especially for the αβ TCRs; evidence exists that the γδ TCRs don't have to go through the thymus) Control of the rearrangement process is determined, in part by the recombination signal sequences (RSS) (e.g. the 12-23 bp rule). As with B cells, a complex of enzymes including the RAG1 and RAG2 enzymes are involved in the cutting and sealing back together of the rearranged genes. The process of TCR gene rearrangement is sequential in that the β, γ, and δ chain rearrange simultaneously. - If the γδ genes are successful first, then the T cell will become a γδ T cell. - If the β chain is successful, a β chain polypeptide will be produced and associates with pre-Tα, a "surrogate" α chain. These two will go to the surface of the T cell and will trigger rearrangement of the α chain locus. Once this starts, the T cell will loose its δ locus and CANNOT become a γδ T cell
discuss the importance of T cell accessory molecules (e.g. CD4 and CD8 proteins)
These accessory molecules are important for: - their signaling function - to help "hold" together the T cell with its APC/target cell The overall affinity of TCRs are relatively low, however, with the accessory and adhesion molecules, T cells are able to interact with their antigen long enough to become activated. Other important molecules in T cell activation are the interaction of CD28 with its ligand, B71/B72 (CD80/CD86). The CD28 also has a signaling role and often serves as the second signal for the T cell. The adhesion molecules LFA-1/ICAM-1 and CD2/LFA-3 help hold together the T cell/APC.
detail the potential diversity of TCRs and how it is similar/different from Ab diversity
Through calculations of potential diversity, it turns out that TCRs have MORE DIVERSITY than do Igs Partly due to the fact that TCRs have HIGHER numbers of J region genes. In addition, it is more typical to see multiple "D" region genes expressed in TCRs whereas the process is rare in Igs. TCRs also have junctional flexibility as well as both P and N nucleotide addition A distinction for the T cells is that Tdt is expressed during rearrangement of all the TCR genes whereas it only occurs during Ig heavy chain gene rearrangement in B cells. While some estimates put the potential of different Igs at 1015, most of the recent evidence seems to indicate that the number is significantly lower, possibly down to about 107. In the case of TCRs, the estimated number of different TCR possibilities is at ~1013. A difference for T cells, unlike the B cells, is that TCRs DO NOT HAVE somatic hypermutation. In part, it is more difficult to alter the TCR sequence because not only does the TCR have to interact with its antigen, it also has to contact with self-MHC. CDR1 and CDR2 interact with the MHC molecule and CDR3 interacts with the antigenic peptide. In addition, the coding region for CDR3 is within the J region and having a higher number of J region genes helps insure a large number of possibilities for interacting with the largest number of different antigenic peptides.
