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Clonal deletion of thymocytes can occur in the cortex with no involvement of the medulla.

McCaughtry TM, Baldwin TA, Wilken MS, Hogquist KA - J. Exp. Med. (2008)

Bottom Line: However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation.We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation.Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c(+) cortical dendritic cells, and elimination of such cells impaired deletion.

View Article: PubMed Central - PubMed

Affiliation: Center for Immunology, Laboratory Medicine, and Pathology, University of Minnesota, Minneapolis, MN 55454, USA.

ABSTRACT
The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY(cd4) model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c(+) cortical dendritic cells, and elimination of such cells impaired deletion.

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The phenotype of thymocytes undergoing positive and negative selection in the HYcd4 model. (A) CD4 by CD8 profiles of T3.70+ thymocytes from female and male mice. (B) The expression of CD69 and PD-1 on thymocytes of the indicated genotype. The percentage of PD-1+ cells in the HYcd4 male is indicated. (C) The activation of Caspase 3 in male mice compared with female mice. The fold change in male over female is indicated. Data represent the mean from 16 individuals from 12 different experiments ± SD. P < 0.0001.
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fig1: The phenotype of thymocytes undergoing positive and negative selection in the HYcd4 model. (A) CD4 by CD8 profiles of T3.70+ thymocytes from female and male mice. (B) The expression of CD69 and PD-1 on thymocytes of the indicated genotype. The percentage of PD-1+ cells in the HYcd4 male is indicated. (C) The activation of Caspase 3 in male mice compared with female mice. The fold change in male over female is indicated. Data represent the mean from 16 individuals from 12 different experiments ± SD. P < 0.0001.

Mentions: Developing thymocytes are not competent to undergo clonal deletion until they express a TCRαβ heterodimer, an event which occurs primarily at the DP stage of development. Conventional TCR transgenics do not recapitulate the appropriate timing of TCRα expression. Therefore, we made use of the HYcd4 model, where expression of the TCRα chain is delayed until the DN to DP transition. This allowed us to examine the consequence of high-affinity TCR ligation on DP thymocytes. Similar to the conventional HY TCR transgenic, HYcd4 thymocytes are T3.70+ and are positively selected in female mice but have high affinity for a male antigen and undergo clonal deletion in male mice. Importantly, deletion in male mice is strikingly different from the conventional HY model because deletion occurs at the DP stage (41). In HYcd4 male mice, DP thymocytes failed to form a CD8 SP population and instead dramatically dulled their CD4 and CD8 expression (Fig. 1 A) and up-regulated CD69 and PD-1 to levels higher than in the female (Fig. 1 B). To monitor the activation of the apoptosis pathway, as a measure of clonal deletion, we used an antibody specific for the active form of Caspase 3 (42). Despite the fact that macrophages engulf apoptotic cells in the thymus (43), active Caspase 3+ cells were detected in HYcd4 male mice at a 4.8-fold greater frequency than in female mice in the steady state (Fig. 1 C). Activation of Caspase 3 appears to be a relevant measure of clonal deletion because in male HYcd4 mice on a Bimo background, we saw that Caspase 3 was not activated to a level higher than that of HYcd4 females (Fig. S1, available at http://www.jem.org/cgi/content/full/jem.20080866/DC1).


Clonal deletion of thymocytes can occur in the cortex with no involvement of the medulla.

McCaughtry TM, Baldwin TA, Wilken MS, Hogquist KA - J. Exp. Med. (2008)

The phenotype of thymocytes undergoing positive and negative selection in the HYcd4 model. (A) CD4 by CD8 profiles of T3.70+ thymocytes from female and male mice. (B) The expression of CD69 and PD-1 on thymocytes of the indicated genotype. The percentage of PD-1+ cells in the HYcd4 male is indicated. (C) The activation of Caspase 3 in male mice compared with female mice. The fold change in male over female is indicated. Data represent the mean from 16 individuals from 12 different experiments ± SD. P < 0.0001.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2571932&req=5

fig1: The phenotype of thymocytes undergoing positive and negative selection in the HYcd4 model. (A) CD4 by CD8 profiles of T3.70+ thymocytes from female and male mice. (B) The expression of CD69 and PD-1 on thymocytes of the indicated genotype. The percentage of PD-1+ cells in the HYcd4 male is indicated. (C) The activation of Caspase 3 in male mice compared with female mice. The fold change in male over female is indicated. Data represent the mean from 16 individuals from 12 different experiments ± SD. P < 0.0001.
Mentions: Developing thymocytes are not competent to undergo clonal deletion until they express a TCRαβ heterodimer, an event which occurs primarily at the DP stage of development. Conventional TCR transgenics do not recapitulate the appropriate timing of TCRα expression. Therefore, we made use of the HYcd4 model, where expression of the TCRα chain is delayed until the DN to DP transition. This allowed us to examine the consequence of high-affinity TCR ligation on DP thymocytes. Similar to the conventional HY TCR transgenic, HYcd4 thymocytes are T3.70+ and are positively selected in female mice but have high affinity for a male antigen and undergo clonal deletion in male mice. Importantly, deletion in male mice is strikingly different from the conventional HY model because deletion occurs at the DP stage (41). In HYcd4 male mice, DP thymocytes failed to form a CD8 SP population and instead dramatically dulled their CD4 and CD8 expression (Fig. 1 A) and up-regulated CD69 and PD-1 to levels higher than in the female (Fig. 1 B). To monitor the activation of the apoptosis pathway, as a measure of clonal deletion, we used an antibody specific for the active form of Caspase 3 (42). Despite the fact that macrophages engulf apoptotic cells in the thymus (43), active Caspase 3+ cells were detected in HYcd4 male mice at a 4.8-fold greater frequency than in female mice in the steady state (Fig. 1 C). Activation of Caspase 3 appears to be a relevant measure of clonal deletion because in male HYcd4 mice on a Bimo background, we saw that Caspase 3 was not activated to a level higher than that of HYcd4 females (Fig. S1, available at http://www.jem.org/cgi/content/full/jem.20080866/DC1).

Bottom Line: However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation.We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation.Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c(+) cortical dendritic cells, and elimination of such cells impaired deletion.

View Article: PubMed Central - PubMed

Affiliation: Center for Immunology, Laboratory Medicine, and Pathology, University of Minnesota, Minneapolis, MN 55454, USA.

ABSTRACT
The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY(cd4) model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c(+) cortical dendritic cells, and elimination of such cells impaired deletion.

Show MeSH
Related in: MedlinePlus