8536dch10221-2478/29/0210:232 AM Page224mac114Mac114:24 d shift:1268tm:8536 224 PART II Generation of B-Cell and T-Cell Responses antigen plus a nonself-MHC molecule Nonetheless, the most EXPERIMENT distinctive property of mature T cells is that they recognize only foreign antigen combined with self-MHC molecules As noted, thymocytes undergo two selection processes in a Positive selection for thymocytes bearing receptors (A X B)FI(H-2 capable of binding self-MHC molecules, which results in MHC restriction. Cells that fail positive selection are Lethal x-irradiation eliminated within the thymus by apoptosis Strain-B thymus graft(H-2b) Negative selection that eliminates thymocytes bearing (A x B)F, hematopoictic sten high-affinity receptors for self-MHC molecules alone or cells(H-2a/b) self-antigen presented by self-MHC, which results in ← Infect with lcm virus self-tolerance Both processes are necessary to generate mature T cells that are self-MHC restricted and self-tolerant. As noted already me 98% or more of all thymocytes die by apoptosis within the thymus. The bulk of this high death rate appears to reflect LCM-infected LCM-infected a weeding out of thymocytes that fail positive selection be- rain-A cells strain-B cells cause their receptors do not specifically recognize foreign No killing antigen plus self-MHC molecules Early evidence for the role of the thymus in selection of the T-cell repertoire came from chimeric mouse experi CONTROL ments by r. M. Zinkernagel and his colleagues(Figure 10-4). These researchers implanted thymectomized and ir Infect with LCM virus radiated(A X B)FI mice with a B-type thymus and then reconstituted the animals immune system with an intra venous infusion of F, bone-marrow cells. To be certain that (A X BF the thymus graft did not contain any mature T cells, it was irradiated before being transplanted. In such an experi mental system, T-cell progenitors from the (A X B)FI pleen cells bone-marrow transplant mature within a thymus that ex- sses only B-haplotype MHC molecules on its stromal cells. Would these(A X B)Fi T cells now be MHC- LCM-infected restricted for the haplotype of the thymus? To answer this strainA cells question, the chimeric mice were infected with LCM virus Killing and the immature T cells were then tested for their ability to kill LCM-infected target cells from the strain A or strain B FIGURE 10-4 Experimental demonstration that the thymus selects mice. As shown in Figure 10-4, when Tc cells from the or maturation only those T cells whose T-cell receptors recognize chimeric mice were tested on LCM virus infected target antigen presented on target cells with the haplotype of the thymus cells from strain A or strain B mice, they could only lyse Thymectomized and lethally irradiated (A X B)F, mice were grafted LCM-infected target cells from strain B mice. These mice with a strain-B thymus and reconstituted with (A X B)Fi bone- have the same MHC haplotype, B, as the implanted thymus. marrow cells. After infection with the LCM virus, the CTL cells were Thus, the MHC haplotype of the thymus in which T cells assayed for their ability to kill C-labeled strain-A or strain-B target develop determines their MHC restriction. cells infected with the LCM virus. Only strain-B target cells were Thymic stromal cells, including epithelial cells, macro- lysed, suggesting that the H-2b grafted thymus had selected for phages, and dendritic cells, play essential roles in positive and maturation only those T cells that could recognize antigen combined negative selection. These cells express class I MHC molecules with H-2bMHC molecules and can display high levels of class II mHC also. The interac ion of immature thymocytes that express the TCR-CD3 Positive Selection Ensures MHC Restriction complex with populations of thymic stromal cells results in positive and negative selection by mechanisms that are under Positive selection takes place in the cortical region of the thy intense investigation. First, we'll examine the details of each mus and involves the interaction of immature thymocytes selection process and then study some experiments that pro- with cortical epithelial cells(Figure 10-5). There is evidence vide insights into the operation of these processes. that the T-cell receptors on thymocytes tend to cluster withantigen plus a nonself-MHC molecule. Nonetheless, the most distinctive property of mature T cells is that they recognize only foreign antigen combined with self-MHC molecules. As noted, thymocytes undergo two selection processes in the thymus: ■ Positive selection for thymocytes bearing receptors capable of binding self-MHC molecules, which results in MHC restriction. Cells that fail positive selection are eliminated within the thymus by apoptosis. ■ Negative selection that eliminates thymocytes bearing high-affinity receptors for self-MHC molecules alone or self-antigen presented by self-MHC, which results in self-tolerance. Both processes are necessary to generate mature T cells that are self-MHC restricted and self-tolerant. As noted already, some 98% or more of all thymocytes die by apoptosis within the thymus. The bulk of this high death rate appears to reflect a weeding out of thymocytes that fail positive selection be￾cause their receptors do not specifically recognize foreign antigen plus self-MHC molecules. Early evidence for the role of the thymus in selection of the T-cell repertoire came from chimeric mouse experi￾ments by R. M. Zinkernagel and his colleagues (Figure 10-4). These researchers implanted thymectomized and ir￾radiated (A B) F1 mice with a B-type thymus and then reconstituted the animal’s immune system with an intra￾venous infusion of F1 bone-marrow cells. To be certain that the thymus graft did not contain any mature T cells, it was irradiated before being transplanted. In such an experi￾mental system, T-cell progenitors from the (A B) F1 bone-marrow transplant mature within a thymus that ex￾presses only B-haplotype MHC molecules on its stromal cells. Would these (A B) F1 T cells now be MHC￾restricted for the haplotype of the thymus? To answer this question, the chimeric mice were infected with LCM virus and the immature T cells were then tested for their ability to kill LCM-infected target cells from the strain A or strain B mice. As shown in Figure 10-4, when TC cells from the chimeric mice were tested on LCM virus infected target cells from strain A or strain B mice, they could only lyse LCM-infected target cells from strain B mice. These mice have the same MHC haplotype, B, as the implanted thymus. Thus, the MHC haplotype of the thymus in which T cells develop determines their MHC restriction. Thymic stromal cells, including epithelial cells, macro￾phages, and dendritic cells, play essential roles in positive and negative selection. These cells express class I MHC molecules and can display high levels of class II MHC also. The interac￾tion of immature thymocytes that express the TCR-CD3 complex with populations of thymic stromal cells results in positive and negative selection by mechanisms that are under intense investigation. First, we’ll examine the details of each selection process and then study some experiments that pro￾vide insights into the operation of these processes. Positive Selection Ensures MHC Restriction Positive selection takes place in the cortical region of the thy￾mus and involves the interaction of immature thymocytes with cortical epithelial cells (Figure 10-5). There is evidence that the T-cell receptors on thymocytes tend to cluster with 224 PART II Generation of B-Cell and T-Cell Responses FIGURE 10-4 Experimental demonstration that the thymus selects for maturation only those T cells whose T-cell receptors recognize antigen presented on target cells with the haplotype of the thymus. Thymectomized and lethally irradiated (A B) F1 mice were grafted with a strain-B thymus and reconstituted with (A B) F1 bone￾marrow cells. After infection with the LCM virus, the CTL cells were assayed for their ability to kill 51Cr-labeled strain-A or strain-B target cells infected with the LCM virus. Only strain-B target cells were lysed, suggesting that the H-2b grafted thymus had selected for maturation only those T cells that could recognize antigen combined with H-2b MHC molecules. Lethal x-irradiation Thymectomy EXPERIMENT (A × B)F1 (H–2a/b) Strain-B thymus graft (H–2b) (A × B)F1 hematopoietic stem cells (H–2a/b) Infect with LCM virus Spleen cells CONTROL Infect with LCM virus (A × B)F1 Spleen cells Killing Killing LCM-infected strain-B cells LCM-infected strain-A cells No killing Killing LCM-infected strain-B cells LCM-infected strain-A cells 1 2 8536d_ch10_221-247 8/29/02 10:23 AM Page 224 mac114 Mac 114:2nd shift:1268_tm:8536d: