Ontogeny of thymic NK1.1+ cells.

Thymic NK1.1+ cells are a recently described lymphocyte subset whose biologic function is not well defined. There is some controversy as to whether thymic NK1.1+ cells mature in a thymic or an extrathymic pathway. In this study, we examined the ontogeny of murine thymic NK1.1+ cells utilizing direct examination of freshly obtained fetal thymi as well as fetal thymi established in organ cultures (FTOC). We found a reproducible peak (5-40%) of NK1.1+ cells, demonstrable in day 15 to 16 freshly obtained fetal thymi, which was markedly decreased by day 17 of gestation; this peak preceded the appearance of the CD4+ CD8+ thymocytes by 12 to 24 h. Reverse-transcriptase PCR analysis of NK1.1 demonstrated its presence as early as day 9 of gestation, thus placing it as one of the earliest lymphocytic genes to be transcribed. Utilizing FTOC, we found that: 1) day 12 fetal thymi contained a progenitor that can differentiate into an NK1.1+ CD4+ CD8+ lymphocyte; 2) NK1.1+ cells dwindle to <5% in FTOC established from day 14 thymi; 3) NK1.1+ cells dominate in FTOC supplemented with IL-2; and 4) most of the NK1.1+ cells seen in FTOC did not express CD3epsilon on their surface, except for the FTOC supplemented with IL-12. These findings suggest that NK1.1+ cells may play an important role in thymic maturation. Moreover, these findings suggest that fetal thymi contain several novel lymphocyte subsets that can be induced to overgrow the normal thymocytes upon exposure to certain cytokines.