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Updated: Oct 2, 2026
Examination of Thymic Positive and Negative Selection by Flow Cytometry
Published on: October 8, 2012
Cyclosporin A inhibits positive selection and delays negative selection in alpha beta TCR transgenic mice
K B Urdahl1, D M Pardoll, M K Jenkins
1Department of Microbiology, University of Minnesota, Minneapolis 55455.
Abstract:
Cyclosporin A (CsA) is an immunosuppressive drug that inhibits TCR-mediated signal transduction. This drug has two major effects on developing alpha beta thymocytes in normal mice: it blocks the development of most mature CD4+8- and CD4-8+ thymocytes and inhibits the deletion of some but not all self-specific thymocytes. The latter effect may explain how CsA treatment can paradoxically induce autoimmunity in certain situations. Here we investigated the effects of CsA on thymocyte development in transgenic mice that express on most of their T cells an alpha beta TCR specific for male H-Y Ag bound to H-2 Db class I molecules, a model system in which positive and negative selection have been clearly defined. Positive selection occurs in female mice, resulting in the development of mature CD4-CD8+ T cells, whereas negative selection occurs in male mice, resulting in the deletion of self-reactive CD4+8+ thymocytes. CsA blocked positive selection, as evidenced by the finding that most of the cells present in the thymuses of CsA-treated female mice were functionally immature precursors that expressed heat-stable Ag. In male mice, CsA delayed but did not prevent the deletion of most CD4+8+ thymocytes. A few transgenic thymocytes, however, were not deleted and achieved the TCR+, CD4-8+ phenotype of positively selected cells. Therefore, for a small subset of thymocytes, CsA may convert a normally negatively selecting TCR signal to a positively selecting one.
Insights
Cyclosporin A (CsA) impacts T cell development by blocking positive selection and delaying negative selection in thymocytes. This immunosuppressive drug may paradoxically promote autoimmunity by altering T cell selection processes.
Area of Science:
- Immunology
- T cell biology
- Drug effects on immune system
Background:
- Cyclosporin A (CsA) is an immunosuppressant inhibiting T cell receptor (TCR)-mediated signal transduction.
- CsA affects developing T cells (thymocytes) by blocking mature cell development and inhibiting self-reactive cell deletion.
- Altered T cell selection by CsA may contribute to drug-induced autoimmunity.
Purpose of the Study:
- Investigate CsA's effects on thymocyte development using a transgenic mouse model.
- Define the impact of CsA on positive and negative T cell selection processes.
- Clarify the mechanisms by which CsA might induce autoimmunity.
Main Methods:
- Utilized transgenic mice expressing a specific alpha beta TCR recognizing H-Y antigen bound to H-2 Db.
- Administered CsA to female (positive selection) and male (negative selection) transgenic mice.
- Analyzed thymocyte populations for developmental stage and TCR expression (CD4, CD8, heat-stable antigen).
Main Results:
- CsA blocked positive selection in female mice, leading to immature thymocyte precursors.
- CsA delayed but did not completely prevent negative selection in male mice.
- A subset of thymocytes in male mice escaped deletion and exhibited characteristics of positive selection.
Conclusions:
- CsA significantly impairs positive selection of thymocytes.
- CsA's effect on negative selection is a delay rather than a complete block.
- CsA may convert a negative selection signal into a positive selection signal for a small thymocyte subset, potentially explaining its paradoxical autoimmune effects.
