8536ach1022-2478/28/023:58 PM Page225mac76mac76:3854 T-Cell Matur During positive selection, the RAG-1, RAG-2, and TdT T-cell precursor proteins required for gene rearrangement and modification continue to be expressed. Thus each of the immature thymo- tes in a clone expressing a given p chain have an opportu Rearrangement of TCR genes nity to rearrange different TCR a-chain genes, and the resulting TCRs are then selected for self-MHC recognition Only those cells whose ap TCR heterodimer recognizes T-cell receptor self-MHC molecule are selected for survival. Consequentl the presence of more than one combination of ap TCR thymocyte chains among members of the clone is important because it increases the possibility that some members will"pass"the Positive selection of Death by apoptosis test for positive selection. Any cell that manages to rearrange cells whose receptor/ of cells that do not interact an a chain that allows the resulting ap TCR to recognize self- binds mhc molecules with MHC molecules MHC will be spared; all members of the clone that fail to do so will die by apoptosis within 3 to 4 days Class i and /or class Il MHC molecules Negative Selection Ensures Self-Tolerance The population of MHC-restricted thymocytes that survive positive selection comprises some cells with low-affinity re- ceptors for self-antigen presented by self-MHC molecules and other cells with high-affinity receptors. The latter mocytes undergo negative selection by an interaction with thymic stromal cells. During negative selection, dendritic ls and macrophages bearing class I and class II MHC mol selection a ecules interact with thymocytes bearing high-affinity recep- high-affinity recepto rs for self-antigen plus self-MHC molecules or for for self- MHC or self-MHC self-antigen self-MHC molecules alone(see Figure 10-5). However, the precise details of the process are not yet known. Cells that ex 原画喜 perience negative selection are observed to undergo death by apoptosis. Tolerance to self-antigens encountered in the thy nus is thereby achieved by eliminating T cells that are reac- TH cell Tccell Mature CD +or Experiments Revealed the Essential Elements CDS* Tlymphocytes of Positive and Negative Selection Direct evidence that binding of thymocytes to class I or class II MHC molecules is required for positive selection in the Dendritic cell thymus came from experimental studies with knockout mice In the Incapable of producing functional class I or class II MHC FIGURE 10-5 Positive and negative selection of thymocytes in the molecules (Table 10-1). Class l-deficient mice were found to thymus. Thymic selection involves thymic stromal cells (epithelial have a normal distribution of double-negative, double-posi- cells, dendritic cells, and macrophages), and results in mature T cells tive, and CD4 thymocytes, but failed to produce CD8* thy hat are both self-MHC restricted and self-tolerant mocytes. Class II-deficient mice had double-negative, double-positive, and CD8 thymocytes but lacked CD4 thymocytes. Not surprisingly, the lymph nodes of these class II-deficient mice lacked CD4tT cells. Thus, the absence of MHC molecules on the cortical cells at sites of cell-cell con- class I or II MHC molecules prevents positive selection of tact. Some researchers have suggested that these interactions CD8 or CD4 T cells, respectively allow the immature thymocytes to receive a protective signal Further experiments with transgenic mice provided addi- that prevents them from undergoing cell death; cells whose tional evidence that interaction with MHC molecules plays a receptors are not able to bind MHC molecules would not in- role in positive selection. In these experiments, rearranged teract with the thymic epithelial cells and consequently aB-TCR genes derived from a CD8 T-cell clone specific for would not receive the protective signal, leading to their death influenza antigen plus H-2 class I MHC molecules were in- by jected into fertilized eggs from two different mouse strains,MHC molecules on the cortical cells at sites of cell-cell con￾tact. Some researchers have suggested that these interactions allow the immature thymocytes to receive a protective signal that prevents them from undergoing cell death; cells whose receptors are not able to bind MHC molecules would not in￾teract with the thymic epithelial cells and consequently would not receive the protective signal, leading to their death by apoptosis. During positive selection, the RAG-1, RAG-2, and TdT proteins required for gene rearrangement and modification continue to be expressed. Thus each of the immature thymo￾cytes in a clone expressing a given  chain have an opportu￾nity to rearrange different TCR -chain genes, and the resulting TCRs are then selected for self-MHC recognition. Only those cells whose  TCR heterodimer recognizes a self-MHC molecule are selected for survival. Consequently, the presence of more than one combination of  TCR chains among members of the clone is important because it increases the possibility that some members will “pass” the test for positive selection. Any cell that manages to rearrange an  chain that allows the resulting  TCR to recognize self￾MHC will be spared; all members of the clone that fail to do so will die by apoptosis within 3 to 4 days. Negative Selection Ensures Self-Tolerance The population of MHC-restricted thymocytes that survive positive selection comprises some cells with low-affinity re￾ceptors for self-antigen presented by self-MHC molecules and other cells with high-affinity receptors. The latter thy￾mocytes undergo negative selection by an interaction with thymic stromal cells. During negative selection, dendritic cells and macrophages bearing class I and class II MHC mol￾ecules interact with thymocytes bearing high-affinity recep￾tors for self-antigen plus self-MHC molecules or for self-MHC molecules alone (see Figure 10-5). However, the precise details of the process are not yet known. Cells that ex￾perience negative selection are observed to undergo death by apoptosis. Tolerance to self-antigens encountered in the thy￾mus is thereby achieved by eliminating T cells that are reac￾tive to these antigens. Experiments Revealed the Essential Elements of Positive and Negative Selection Direct evidence that binding of thymocytes to class I or class II MHC molecules is required for positive selection in the thymus came from experimental studies with knockout mice incapable of producing functional class I or class II MHC molecules (Table 10-1). Class I–deficient mice were found to have a normal distribution of double-negative, double-posi￾tive, and CD4 thymocytes, but failed to produce CD8 thy￾mocytes. Class II–deficient mice had double-negative, double-positive, and CD8 thymocytes but lacked CD4 thymocytes. Not surprisingly, the lymph nodes of these class II–deficient mice lacked CD4 T cells. Thus, the absence of class I or II MHC molecules prevents positive selection of CD8 or CD4 T cells, respectively. Further experiments with transgenic mice provided addi￾tional evidence that interaction with MHC molecules plays a role in positive selection. In these experiments, rearranged -TCR genes derived from a CD8 T-cell clone specific for influenza antigen plus H-2k class I MHC molecules were in￾jected into fertilized eggs from two different mouse strains, T-Cell Maturation, Activation, and Differentiation CHAPTER 10 225 FIGURE 10-5 Positive and negative selection of thymocytes in the thymus. Thymic selection involves thymic stromal cells (epithelial cells, dendritic cells, and macrophages), and results in mature T cells that are both self-MHC restricted and self-tolerant. T-cell receptor Immature thymocyte Positive selection of cells whose receptor binds MHC molecules Death by apoptosis of cells that do not interact with MHC molecules CD8 CD3 CD4 T-cell precursor Class I and/or class II MHC molecules Epithelial cell Rearrangement of TCR genes Negative selection and death of cells with high-affinity receptors for self-MHC or self-MHC + self-antigen CD4+ CD8+ TH cell TC cell Mature CD4+ or CD8+ T lymphocytes Macrophage Dendritic cell 8536d_ch10_221-247 8/28/02 3:58 PM Page 225 mac76 mac76:385_reb: