Presentation on theme: "Lecture 10 Thymocyte selection II"— Presentation transcript:
1Lecture 10 Thymocyte selection II Thymocyte positive selectionA word about signalingDemonstration in radiation bone marrow chimerasImplications for transplantationThe importance of eliminating mature T cells in bone marrow transplantationThymocyte negative selectionThe limitations of thymic (central) toleranceThe role of ectopic antigen expression & AIREReminder about the existence of gd T cells
2Positive and Negative Selection The thymus:Selects the usefulIgnores the uselessRemoves the harmfulPositive SelectionDeath by neglectNegative selection
3Figure 5-12 part 1 of 2CD4CD8abTCRCD8 T cellCD4 T cell
4Effects of defined mutations on T cell selection MHC class I deficient No mature CD8 cellsMHC class II deficient No mature CD4 T cellsTAP deficient No mature CD8 T cellsCD8 deficient No killer cellsCD4 deficient No helper cellsCD3 deficient No T cellsRAG deficient No T cells (or B cells)pTa deficient Few ab T cellsNo thymus (DiGeorge) No T mature cellsNo thymocytes (ADA-) No T mature cells
5The signals from cell surface receptors that lead to positive and negative selection are believe to start with protein phosphorylations followed by poorly understood signal integration that finally leads to changes in transcription of genes determining cell fate and differentiation.
8Bone marrow transplantation and radiation chimerism This technique relies on several differences between white blood cells and other cell types.White blood cells, especially lymphocytes, are very sensitive to radiation-induced or chemotherapy-induced cell death.Most host cells, including the thymic epithelium are radiation resistant.White blood cells can be reconstituted by repopulation from bone marrow stem cells.Treated recipients are depleted of leukocytes then reconstituted (rescued) with bone marrow (usually depleted of recirculating T cells) or hematopoietic stem cells that give rise to all blood cells.
9lymphocytes in lymph nodes, spleen Cells of the immune systemBone marrow/fetal liverThymusNaïve, restinglymphocytes in lymph nodes, spleenAdaptiveInnate
12Figure 5-13 What happens in a radiation chimera? Positive selection to host cortical epithelial cellsFigure 5-13Radioresistanthost typeBone marrow derivedDonor typeNegative selection to donor and host MHC
13Host type controls MHC restriction of T cell repertoire AXBAAXBB
14See next slide for explanation Development of Major HistocompatibilityComplex Restriction Patterns in Bone Marrow Plus Thymic ChimerasSee next slide for explanation
15Explanation of the chimera experiment This is the radiation chimera experiment that you will need to understand.The nomenclature is bone marrow donor -> irradiated recipient. Thus an F1 -> parent is an irradiated mouse of strain A reconstituted with bone marrow from strain (AxB)F1. Please understand this.In an F1 -> parent chimera, the F1 T cells mature in the epithelial environment of the parent that expresses only one of the two SETS of alleles. This skews the population to express TCRs that recognize the parental alleles at some moderate affinity.Although I didn’t go into the second line of this experiment, it is accurate, and actually makes a separate point. This would be the parent -> F1. If bone marrow from the A strain is used to reconstitute an irradiated (AxB)F1, then the T cells will be selected to recognize both strain A and strain B. The APCs will express only MHC from strain A (bone marrow origin), and thus the virus will be presented only on strain A. Thus, despite the fact that the T cell population has the potential to respond to either A or B, the antigen is presented only on A, and thus in the virus-response, the only the T cells restricted to virus + A will be expanded.The last two lines show that the MHC allele expressed by the thymus epithelium restricts the population to the recognition of antigen peptides presented by MHC allele of the thymus--despite the fact that, in these mice, the antigen peptides are presented by both a and b alleles.
19What happens to thymic positive selection if there is an MHC incompatibility between donor and host?
20Figure 5-13 What happens in a radiation chimera? Positive selection to host cortical epithelial cellsFigure 5-13Radioresistanthost typeBone marrow derivedDonor typeNegative selection to donor and host MHC
27Molding of thymocyte repertoire by self MHC + peptides Double positivethymocyteabTCRCD4CD8Too strong signaldeath by negative selectionNo signaldeath by neglectAppropriate, weak signal throughMHC+self peptide promotessurvivalloss of irrelevant coreceptor (CD8 or CD4)maturation to helper or killer phenotype
28Self-tolerance mechanisms in the thymus (Central tolerance)Cells with high affinity to self (+MHC) are eliminatedProblem: The thymus is a relatively small, sequestered organ. How can T cell tolerance be induced to self-proteins produced by other tissues?Three redundant solutionsAIRE induced, "ectopic" expression of peripheral antigens in the thymusThe generation of regulatory (suppressor) cellsMature T cells are subject to additional tolerance mechanisms (Peripheral tolerance)
29AIREMutations of this gene are responsible for the clinical disorder autoimmune polyendocrinopathy syndrome type IPatients develop multiple autoimmune syndromes to endocrine organs, such as parathyroid and adrenal glands.The protein is a transcription factor that is believed to turn on many tissue specific genes IN THYMIC MEDULLARY EPITHELIAL CELLS and plays a role in negative selection.
36Summary of thymocyte development The major pathway of thymocyte development involves ab T cells. The thymus is also necessary for gd T cell development.ab T cell development starts with b gene recombination, expression, and clonal expansion.Cells then recombine TCRa and are screened for positive selection on cortical epithelial cells. Thus "self MHC" is defined by the cortical epithelial cell.Positive selection involves signals initiated with tyrosine phosphorylations that promote directed differentiation.Lymphocytes can be repopulated by bone marrow transplantation.Negative selection occurs in the thymus, where T cells reactive to many self-antigens are expressed on (or by) medullary epithelial cells and other antigen presenting cells.Negative selection can also occur at later developmental stages.