Tracking proliferative history in lymphocyte development with cre-mediated sister chromatid recombination

Tracking and isolating live cells based on their proliferative history in live animals remains a technical challenge in animal studies. We have designed a genetic marking system for tracking the proliferative frequency and history of lymphocytes during their development and homeostatic maintenance. This system is based on activation of a fluorescent marker after Cre-dependent recombination between sister chromatids at a specially designed tandem loxP site, named Tlox. We have demonstrated the utility of the Tlox system in tracking proliferative windows of B and T lymphocyte development. We have further applied the Tlox system in the analysis of the proliferative behavior and homeostatic maintenance of Vγ1.1 positive γδ T cells. Our data show that Vγ1.1 T cells generated in neonatal but not adult life are able to expand in the thymus. The expanded Vγ1.1 T cells are preferentially maintained in the liver but not in lymphoid organs. It has been shown that numbers of Vγ1.1 T cells were dramatically increased in the lymphoid organs of Id3 deficient mice. By combining BrdU and Tlox assays we show that this phenotype is primarily due to enhanced neonatal expansion and subsequent retention of Vγ1.1 T cells. Thus, the Tlox system provides a new genetic tool to track clonal expansion within a defined cell population or tissue type in live animals.