Ontogeny of a novel CD4+CD8-CD3- thymocyte subpopulation: a comparison with CD4- CD8+ CD3- thymocytes.

We have studied the ontogeny of a novel thymocyte subset, CD4+CD8-CD3-. Three-colour flow cytometric analysis demonstrated that these cells constituted approximately 1% of the total thymocyte content in adult CBA mice, and were not present in lymph nodes. They were mainly blastic, cortisone-sensitive, and localized in the outer thymic cortex. During foetal life they were first observed at day 15 and reached a maximum (6%) at day 17, beyond which they decreased to the adult level. This kinetic profile was similar to that of the CD4-CD8+CD3- subpopulation, except that the CD4+CD8-CD3- cells appeared slightly earlier and their percentage was lower. Both these populations appeared after the CD4-CD8-CD3- cells but before the CD4+CD8+CD3- cells. Similar observations were made during thymic reconstitution following dexamethasone treatment. In this case, both CD4+CD8-CD3- and CD4-CD8+CD3- thymocytes disappeared 48 h after the treatment. While their absolute number increased up to 14 days post-treatment, their percentage was maximal at day 7 post-treatment and returned to normal values by day 10 post-treatment. These results argue strongly that not only the CD4-CD8+CD3- population but also the CD4+CD8-CD3- population can be considered an intermediate precursor in CBA thymuses.