Fractalkine: one chemokine, many functions.

purification quality of CLP is important, especially for analyses of T-cell development. The original phenotypic CLP has recently been refined by addition of positivity for Flk25 and now a smaller population of cells including CD27 are shown to contain the T-cell potential. A specific criticism of the CLP as the thymus-seeding cell has been the lack of evidence that CLP can be detected in thymus. The study further challenged the MPP-seeding hypothesis by identifying the presence of a low level of circulating Flk2 CD27 CLP, thus formally demonstrating that CLPs can be detected in transit, presumably to the thymus. Since CLPs have not yet been detected in thymus, likely due to rapid adoption of DN1 phenotype, this is an important step forward by identifying a missing link. Overall, the study provides important information about the inner potential of the originally defined CLP and offers additional markers to help all labs standardize their CLP isolations. Let’s hope that these advances will help to resolve the thymic progenitor debate. Conflict-of-interest disclosure: The author declares no competing financial interests. ■