9536d_ch102212478/28/023:58 PM Page228mac76ma76:385e 228 PART 11 Generation of B-Cell and T-Cell Responses while others can deliver a complete signal. In this model, pos- pression was artificially raised to twice its normal level, the itive selection takes place when the TCRs of developing thy- concentration of mature CD8 cells in the thymus was one mocytes encounter MHC-peptide complexes that deliver thirteenth of the concentration in control mice bearing nor- weak or partial signals to their receptors, and negative selec- mal levels of CD8 on their surface. Since the interaction of T tion results when the signal is complete. At this point it is not cells with class I MHC molecules is strengthened by partici- ossible to decide between the avidity model and the differen- pation of CD8, perhaps the increased expression of CD tial-signaling model; both have experimental support. It may would increase the avidity of thymocytes for class I mole be that in some cases, one of these mechanisms operates to the cules, possibly making their negative selection more likely. complete exclusion of the other. It is also possible that no sin- Another important open question in thymic selection is gle mechanism accounts for all the outcomes in the cellular how double-positive thymocytes are directed to become ei- teractions that take place in the thymus and more than one ther CD4 8 or CD4 8 T cells. Selection of CD4 8 thy- mechanism may play a significant role. Further work is re- mocytes gives rise to class I MHC-restricted CD8 T cells quired to complete our understanding of this matter. and class II-restricted CD4+T cells. Two models have been The differential expression of the coreceptor CD8 also can proposed to explain the transformation of a double-positive ffect thymic selection. In an experiment in which CD8 ex- precursor into one of two different single-positive lineag (a) Experimental procedure-fetal thymic organ culture(FTOC) Place in Porous membrane Growth medium (b) Development of CDS* CD4- MHC I-restricted cell Thymocyte Degree of Thymus Thymic CD8+ T-cell FIGURE 10-8 Role of peptides in selection. donor peptide added development Thymuses harvested before their thymocyteNormal populations have undergone positive and negative selection allow study of the develop- ment and selection of single positive Norma CD4"CD8 and CD4" CD8T)T cells(a) Peptide Outline of the experimental procedure for in vitro fetal thymic organ culture(FTOC).(b) The development and selection ofTCR-transgenic CD8"CD4 class I-restricted T cells depends TAP.1 deficient on TCR peptide-MHC I interactions. TAP. None Minimal knockout mice are unable to form peptide- MHC complexes unless peptide is added The mice used in this study were transgenic for the a and B chains of a TCR that recog- Weak signal nizes the added peptide bound to MHC molecules of the TAP. knockout/TCR trans Approaches genic mice. Varying the amount of added pep- normal tide revealed that low concentrations of peptide, producing low avidity of binding, re- sulted in positive selection and nearly normal Strong signal levels of CD4"CD8" cells. High concentra- tions of peptide, producing high avidity of binding to the TCR, caused negative selection, Minimal and few CD4-CD8+ t cells (Adapted from Ashton Rickardt et al.(1994) e25:651while others can deliver a complete signal. In this model, pos￾itive selection takes place when the TCRs of developing thy￾mocytes encounter MHC-peptide complexes that deliver weak or partial signals to their receptors, and negative selec￾tion results when the signal is complete. At this point it is not possible to decide between the avidity model and the differen￾tial-signaling model; both have experimental support. It may be that in some cases, one of these mechanisms operates to the complete exclusion of the other. It is also possible that no sin￾gle mechanism accounts for all the outcomes in the cellular interactions that take place in the thymus and more than one mechanism may play a significant role. Further work is re￾quired to complete our understanding of this matter. The differential expression of the coreceptor CD8 also can affect thymic selection. In an experiment in which CD8 ex￾pression was artificially raised to twice its normal level, the concentration of mature CD8 cells in the thymus was one￾thirteenth of the concentration in control mice bearing nor￾mal levels of CD8 on their surface. Since the interaction of T cells with class I MHC molecules is strengthened by partici￾pation of CD8, perhaps the increased expression of CD8 would increase the avidity of thymocytes for class I mole￾cules, possibly making their negative selection more likely. Another important open question in thymic selection is how double-positive thymocytes are directed to become ei￾ther CD48 or CD48 T cells. Selection of CD48 thy￾mocytes gives rise to class I MHC–restricted CD8 T cells and class II–restricted CD4 T cells. Two models have been proposed to explain the transformation of a double-positive precursor into one of two different single-positive lineages 228 PART II Generation of B-Cell and T-Cell Responses FIGURE 10-8 Role of peptides in selection. Thymuses harvested before their thymocyte populations have undergone positive and negative selection allow study of the develop￾ment and selection of single positive (CD4CD8 and CD4CD8) T cells. (a) Outline of the experimental procedure for in vitro fetal thymic organ culture (FTOC). (b) The development and selection of CD8CD4 class I–restricted T cells depends on TCR peptide-MHC I interactions. TAP-1 knockout mice are unable to form peptide￾MHC complexes unless peptide is added. The mice used in this study were transgenic for the  and  chains of a TCR that recog￾nizes the added peptide bound to MHC I molecules of the TAP-1 knockout/TCR trans￾genic mice. Varying the amount of added pep￾tide revealed that low concentrations of peptide, producing low avidity of binding, re￾sulted in positive selection and nearly normal levels of CD4CD8 cells. High concentra￾tions of peptide, producing high avidity of binding to the TCR, caused negative selection, and few CD4CD8 T cells appeared. [Adapted from Ashton Rickardt et al. (1994) Cell 25:651.] (a) Experimental procedure—fetal thymic organ culture (FTOC) (b) Development of CD8+ CD4− MHC I–restricted cells Thymus donor Amount of peptide added Thymocyte Thymic stromal cell Degree of CD8+ T-cell development None Peptide Normal None Minimal Optimal Approaches normal High Minimal Remove thymus Place in FTOC Porous membrane Growth medium Normal TCR-transgenic TAP-1 deficient Weak signal No signal Weak signal Strong signal 8536d_ch10_221-247 8/28/02 3:58 PM Page 228 mac76 mac76:385_reb: