The pTa gene encodes the pre-T cell receptor alpha chain, an essential component of the pre-TCR complex. Using pTa expression as a specific marker for early T lineage cells, we developed a lineage-tracing strategy to visualize T lineage-committed precursor cells and their progeny. A cDNA encoding an improved version of the Cre recombinase (iCre) was targeted into the pre-T cell receptor alpha-chain (pTa) gene locus. In parallel, we introduced a Cre-inducible expression cassette encoding for a very bright variant of red fluorescent protein (tdRFP) into the ubiquitously expressed mouse locus ROSA26. We generated pTa-iCre deleter and ROSA26-tdRFP indicator mouse strains, respectively. In cells bearing both "knock-in" alleles, pTa-driven iCre recombinase expression leads to permanent activation of the ROSA26-tdRFP reporter locus. As a result, pTa-expressing cells and their progeny exhibited persistent red fluorescence. Red fluorescence was detected in cells considered to belong developmentally to the T-lineage (gd, ab, natural killer-T cells, but not in other hematopoietic cells, demonstrating the exquisite specificity of pTa expression. Exclusive reporter activation in T but not B-cells indicated that onset of pTa-iCre expression occurs after the T/B lineage decision. Over 95% of gd-T cells were tdRFP-positive, demonstrating for the first time that almost all gd-T cells develop - like TCRab lymphocytes - from a pTa-expressing precursor. The extent of T-cell labeling in athymic pTa-iCre x ROSA26-RFP nude mice indicate that pTa is also expressed during extrathymic T-cell development. In the thymus, onset of reporter activation was found at the TN2 to TN3 stage but not in the most immature population (ETP). Finally, red fluorescence was detected in a minute subset of Lin- bone marrow cells. pTa-iCre deleter mice should be useful for both efficient T-lineage-specific genome manipulations and identification of pTa-expressing T precursor cells in extra-thymic tissues
