Asymmetric thymocyte death underlies the CD4:CD8 T-cell ratio in the adaptive immune system

Significance Thymocytes express a diverse repertoire of T-cell antigen receptors. Stringent selection processes eliminate autoreactive cells and guide useful thymocytes to develop into CD4 or CD8 lineages. Development always generates more CD4 than CD8 T cells, but it is not understood why. Our study used mathematics to investigate the basis of this asymmetric lineage development. Although similar numbers of CD4 and CD8 precursors start selection, our analysis revealed unexpectedly high death rates in developing thymocytes. In particular, CD8 precursors were more susceptible to death than CD4 lineage cells, and this was a major contributor to the high CD4:CD8 ratio of development. It has long been recognized that the T-cell compartment has more CD4 helper than CD8 cytotoxic T cells, and this is most evident looking at T-cell development in the thymus. However, it remains unknown how thymocyte development so favors CD4 lineage development. To identify the basis of this asymmetry, we analyzed development of synchronized cohorts of thymocytes in vivo and estimated rates of thymocyte death and differentiation throughout development, inferring lineage-specific efficiencies of selection. Our analysis suggested that roughly equal numbers of cells of each lineage enter selection and found that, overall, a remarkable ∼75% of cells that start selection fail to complete the process. Importantly it revealed that class I-restricted thymocytes are specifically susceptible to apoptosis at the earliest stage of selection. The importance of differential apoptosis was confirmed by placing thymocytes under apoptotic stress, resulting in preferential death of class I-restricted thymocytes. Thus, asymmetric death during selection is the key determinant of the CD4:CD8 ratio in which T cells are generated by thymopoiesis.

[1]  M. Egerton,et al.  Kinetics of mature T-cell development in the thymus. , 1990, Proceedings of the National Academy of Sciences of the United States of America.

[2]  Suzanne Cory,et al.  bcl-2 transgene inhibits T cell death and perturbs thymic self-censorship , 1991, Cell.

[3]  Susumu Tonegawa,et al.  RAG-1-deficient mice have no mature B and T lymphocytes , 1992, Cell.

[4]  Kristin A. Hogquist,et al.  Murine thymic selection quantified using a unique method to capture deleted T cells , 2013, Proceedings of the National Academy of Sciences.

[5]  David Klatzmann,et al.  Comprehensive Assessment and Mathematical Modeling of T Cell Population Dynamics and Homeostasis1 , 2008, The Journal of Immunology.

[6]  S. Sakaguchi,et al.  The Long-Term Survival Potential of Mature T Lymphocytes Is Programmed During Development in the Thymus , 2011, Science Signaling.

[7]  Mark Coles,et al.  Transgenic mice with hematopoietic and lymphoid specific expression of Cre , 2003, European journal of immunology.

[8]  Kristin A. Hogquist,et al.  T cell receptor antagonist peptides induce positive selection , 1994, Cell.

[9]  S. Korsmeyer,et al.  bcl-2 inhibits multiple forms of apoptosis but not negative selection in thymocytes , 1991, Cell.

[10]  M. Kubo,et al.  Signaling by intrathymic cytokines, not T cell antigen receptors, specifies CD8 lineage choice and promotes the differentiation of cytotoxic-lineage T cells , 2010, Nature Immunology.

[11]  K. Garcia,et al.  Reconciling views on T cell receptor germline bias for MHC. , 2012, Trends in immunology.

[12]  P. Marrack,et al.  Normal development of mice deficient in beta 2M, MHC class I proteins, and CD8+ T cells. 1990. , 2010, Journal of immunology.

[13]  C. Benoist,et al.  Targeted complementation of MHC class II deficiency by intrathymic delivery of recombinant adenoviruses. , 1997, Immunity.

[14]  J. Seidman,et al.  Construction of normalized RNA-seq libraries for next-generation sequencing using the crab duplex-specific nuclease. , 2011, Current protocols in molecular biology.

[15]  F. Miedema,et al.  Maintenance of peripheral naive T cells is sustained by thymus output in mice but not humans. , 2012, Immunity.

[16]  D. Green,et al.  The BCL-2 family reunion. , 2010, Molecular cell.

[17]  J. Harrison,et al.  Grasshoppers in research and education: methods for maintenance and production , 2007, Lab animal.

[18]  K. Rajewsky,et al.  How αβ T cells deal with induced TCRα ablation , 2001, Proceedings of the National Academy of Sciences of the United States of America.

[19]  L. Peltonen,et al.  Aire regulates negative selection of organ-specific T cells , 2003, Nature Immunology.

[20]  W. Huber,et al.  which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. MAnorm: a robust model for quantitative comparison of ChIP-Seq data sets , 2011 .

[21]  K. Rajewsky,et al.  How alpha beta T cells deal with induced TCR alpha ablation. , 2001, Proceedings of the National Academy of Sciences of the United States of America.

[22]  K. Hogquist,et al.  Thymic emigration revisited , 2007, The Journal of experimental medicine.

[23]  K. Nakayama,et al.  Essential role for ZAP-70 in both positive and negative selection of thymocytes , 1995, Nature.

[24]  E. D. L. Cueva,et al.  Refinement of intrathymic injection in mice , 2007, Lab Animal.

[25]  W. Paul,et al.  Distinct functions for the transcription factors GATA-3 and ThPOK during intrathymic differentiation of CD4+ T cells , 2008, Nature Immunology.

[26]  J. Alberola-Ila,et al.  GATA-3 expression is controlled by TCR signals and regulates CD4/CD8 differentiation. , 2003, Immunity.

[27]  A. Singer,et al.  MHC restriction is imposed on a diverse T cell receptor repertoire by CD4 and CD8 co-receptors during thymic selection. , 2012, Trends in immunology.

[28]  Bruce W. Kennedy,et al.  Field studies and the IACUC: protocol review, oversight, and occupational health and safety considerations , 2007, Lab Animal.

[29]  D. Kioussis,et al.  The Action of Bax and Bcl-2 on T Cell Selection , 1998, The Journal of experimental medicine.

[30]  B. Williams,et al.  Mapping and quantifying mammalian transcriptomes by RNA-Seq , 2008, Nature Methods.

[31]  E. Reinherz,et al.  Separation of functional subsets of human T cells by a monoclonal antibody. , 1979, Proceedings of the National Academy of Sciences of the United States of America.

[32]  T. Nakayama,et al.  CD69 cell surface expression identifies developing thymocytes which audition for T cell antigen receptor-mediated positive selection. , 1993, International immunology.

[33]  S. Habu,et al.  Dual functions of Runx proteins for reactivating CD8 and silencing CD4 at the commitment process into CD8 thymocytes. , 2005, Immunity.

[34]  R. Zamoyska,et al.  TCR Signals Mediated by Src Family Kinases Are Essential for the Survival of Naive T Cells1 , 2002, The Journal of Immunology.

[35]  D. Kioussis,et al.  Positive and Negative Selection in Transgenic Mice Expressing a T-Cell Receptor Specific for Influenza Nucleoprotein and Endogenous Superantigen , 1993, Developmental immunology.

[36]  G. Linette,et al.  Bcl-XL and Bcl-2 repress a common pathway of cell death , 1995, The Journal of experimental medicine.

[37]  A. Strasser,et al.  BH3-only Bcl-2 family member Bim is required for apoptosis of autoreactive thymocytes , 2002, Nature.

[38]  A. Strasser,et al.  Proapoptotic Bcl-2 relative Bim required for certain apoptotic responses, leukocyte homeostasis, and to preclude autoimmunity. , 1999, Science.

[39]  A. Strasser,et al.  The BH3-only proteins Bim and Puma cooperate to impose deletional tolerance of organ-specific antigens. , 2012, Immunity.

[40]  P. Marrack,et al.  Normal development of mice deficient in beta 2M, MHC class I proteins, and CD8+ T cells. , 1990, Science.

[41]  G. Gil-Gómez,et al.  Bax alpha perturbs T cell development and affects cell cycle entry of T cells. , 1996, The EMBO journal.

[42]  R. E. Tillman,et al.  The role of the Runx transcription factors in thymocyte differentiation and in homeostasis of naive T cells , 2007, The Journal of experimental medicine.

[43]  A. Strasser,et al.  Loss of Bim Increases T Cell Production and Function in Interleukin 7 Receptor–deficient Mice , 2004, The Journal of experimental medicine.

[44]  Yoram Groner,et al.  Runx3 and Runx1 are required for CD8 T cell development during thymopoiesis , 2003, Proceedings of the National Academy of Sciences of the United States of America.

[45]  Q. Hu,et al.  Bim-Mediated Apoptosis Is Not Necessary for Thymic Negative Selection to Ubiquitous Self-Antigens1 , 2009, The Journal of Immunology.

[46]  C. Benoist,et al.  Evidence for a single-niche model of positive selection. , 1994, Proceedings of the National Academy of Sciences of the United States of America.

[47]  Shimon Sakaguchi,et al.  Regulation of Zap70 Expression During Thymocyte Development Enables Temporal Separation of CD4 and CD8 Repertoire Selection at Different Signaling Thresholds , 2010, Science Signaling.

[48]  Masanori Nakamura,et al.  Immunosuppressant FTY720 inhibits thymocyte emigration , 2000, European journal of immunology.

[49]  C. Benoist,et al.  Mice lacking all conventional MHC class II genes. , 1999, Proceedings of the National Academy of Sciences of the United States of America.