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Old 05-12-2014
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Default T-Cell Maturation in the Thymus


I'm having a bit of a problem understanding a step in the process of T-Cell maturation. Here's my understanding of what is going on...

(Short Version)
1) In the thymus the thymocyte enters the cortex then starts expressing CD3+,CD4+, CD8+, TCR and thus becomes a double-positive thymocyte.
2) The TCR on the thymocyte binds to the MHC complex on the cortical thymic epithelial cells. These cells either express MHC I or MHC II.
* If they bind MHC I, the cell retains CD8+ and becomes a cytotoxic T cell.
* If the TCR binds MHC II the cell retains CD4+ and becomes a Helper T cell.
3) In the thymus t-cells go through positive selection in the cortex then negative selection in the medulla. They then make their way to the lymph nodes.

Here's where i'm having some trouble...

What stimulates the cortical thymic epithelial cells express MHC I or MHC II? I'm guessing some of them express one and others the other at the same time. But is this expression regulated by anything? Or is just always the same ratio of CD4+ to CD8+ cells that come out of the thymus? Or is it just random because you don't really know how many of the specific cells (CD4+ or CD8+) will get negatively selected in the medulla anyways.
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Old 05-12-2014
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As far as I can tell it depends on which MHC molecule they bind when undergoing selection, remember that early thymocytes (immature) initially express both molecules on their surface, so when the selection happens:
-If they bind MHC I they are directed to express CD8 and
-If they bind MHC II they are directed to express CD4.

So there is not really an "stimulus", it depends on the cell affinity for either one during the process
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