Figure 3. Intracellular Cre protein levels correlate with reduced thymic cellularity in thymus-initiated Cre-transgenic strains.

Thymocytes from control (C57Bl/6) mice, or from mice expressing various Cre-transgenes, were isolated, fixed, and stained with an antibody recognizing the Cre protein, followed by flow cytometric analysis. Levels of intracellular Cre correlated directly with the appearance of phenotypic effects, with lowest levels in the Cd4[Cre] and Cd2[Cre] strains, and highest levels in both Lck[Cre] strains. Homozygous mice for the highest expressing lck[Cre] strain (Lck[Cre]^Wil, dashed line) predictably expressed even higher levels of Cre than hemizygous mice, and had the most profound phenotype (Figs. 2 and 4). These results suggest that above a certain threshold, Cre protein is toxic to thymocytes in a dose-dependent manner. These experiments were repeated at least four times with essentially identical results.