We have used organ culture of fetal thymic lobes from T cell receptor (TCR) transgenic β2M(-/-) mice to study the role of peptides in positive selection. The TCR used was from a CD8+ T cell specific for ovalbumin 257-264 in the context of Kb. Several peptides with the ability to induce positive selection were identified. These peptide-selected thymocytes have the same phenotype as mature CD8+ T cells and can respond to antigen. Those peptides with the ability to induce positive selection were all variants of the antigenic peptide and were identified as TCR antagonist peptides for this receptor. One peptide tested, E1, induced positive selection on the β2M(-/-) background but negative selection on the β2M(+/-) background. These results show that the process of positive selection is exquisitely peptide specific and sensitive to extremely low ligand density and support the notion that low efficacy ligands mediate positive selection.
Bibliographical noteFunding Information:
Address correspondence to M. J. B. The authors wish to thank Kim McConnell for help with the mouse breeding, Stan Korsmeyer and David Hockenberry for providing the antibody to mouse Bcl-2, Sasha Rudensky for use of the Synergy peptide synthesizer, and Jon Kaye for the pES4 vector. We are grateful to Suguru lmaeda for providing information prior to publication, and to Stacey Dillon, Michael Starn-bath, and Sasha Rudensky for critically reading the manuscript. This research was supported by the Howard Hughes Medical Institute and by the National Institutes of Health (Al-28902). F. R. C. is supported byfundsfrom the Australian ResearchCouncil, the Australian National Health and Medical Research Council, and a Cancer Research Institute investigator award. S. C. J. is a special fellow of the Leukemia Society of America. W. R. H. was funded by a fellowship from the Australian Research Council.