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. 1996 Dec 1;184(6):2091–2100. doi: 10.1084/jem.184.6.2091

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Subpopulations of CD4⁺CD8⁺ thymocytes, their commitment status, and their relationship to the Asymmetric Commitment Model (12). The results of the present study provide a phenotype that can be assigned to each of the commitment stages proposed in the asymmetric commitment model (12). Preselection CD4⁺CD8⁺ thymocytes are too immature to undergo lineage commitment and express low levels of CD5, CD69, TCR and bcl-2. Signals transduced by the TCR/CD3 complex (as result of engagement of those surface molecules that signal through the TCR/CD3 complex, e.g. TCR, Thy-1, etc.) increase CD5 and CD69 expression and promote the differentiation of preselection CD4⁺CD8⁺ thymocytes to the point that they are able to undergo lineage commitment (commitment checkpoint). At the commitment checkpoint TCR-signaled CD4⁺CD8⁺ thymocytes whose TCR and CD8 molecules are coengaged become CD8-committed, whereas TCR-signaled CD4⁺CD8⁺ thymocytes whose TCR and CD8 molecules are not coengaged become CD4-committed. Upregulation of TCR and bcl-2 occurs coincident with lineage commitment so that lineage committed thymocytes express TCR and bcl-2 at high levels.