Biphasic regulation of Vα24 NK T cell development by IL-7. (a) Expression of IL-7 receptor α in the thymus. Analysis of NK T cell progenitor numbers and phenotype after 7 days FTOC with 0–250 ng/ml IL-7. The number of Vα24 NK T cell progenitors recovered in tissue is shown in b, and the medium surrounding the tissue (c) was quantified by cell counting and flow cytometric analysis. (d) Percentage of Vα24 NK T cell progenitors in the medium. Percentages of NK T cell precursors expressing a CD161+ (e) and a CD4/8+/+ phenotype (f) after a 7-day FTOC with graded doses of IL-7 are given. Figures represent data from six independent experiments. *, P < 0.05; **, P < 0.005, compared with culture without IL-7, as determined by paired t test.