Figure 5. Phenotypes resulting from Cre expression in thymocytes correlate with an increased propensity towards cell death.

To test whether Cre protein is toxic to thymocytes in a dose-dependent fashion, analysis of DNA double strand breaks (TUNEL) was performed on wildtype thymocytes or thymocytes from mice carrying one (hemizygous) or two (homozygous) Lck[Cre] transgenes. Freshly isolated thymocytes were indistinguishable from the three types of mice, suggesting that clearance of apoptotic cells from the thymus was not affected. However, when thymocytes were cultured overnight in medium, the presence of Cre resulted in an increase in cell death that was proportional to the amount of Cre protein expressed. These results further strengthen the conclusion that Cre protein is toxic to thymocytes in a dose-dependent manner. Data represent mean ± s.d. for 3–4 independent experiments for each mouse type.