A DEVELOPMENTAL PATHWAY INVOLVING 4 PHENOTYPICALLY AND FUNCTIONALLY DISTINCT SUBSETS OF CD3-CD4-CD8- TRIPLE-NEGATIVE ADULT-MOUSE THYMOCYTESDEFINED BY CD44 AND CD25 EXPRESSION

We have subdivided mouse CD4-CD8-CD3- triple-negative (TN) thymocytes into four subsets based upon expression of CD44 and CD25, including CD 44+CD25-, CD44+CD25+, CD44-CD25+ and CD44-CD25-. Characterization of t hese cells revealed several features distinct to each subset, in parti cular the expression of high levels of c-kit (the receptor for stem ce ll factor) by CD44+CD25-TN and CD44+CD25+TN but not by CD44-CD25+TN an d CD44-CD25-TN. The CD44+CD25+TN subset also included the IL-7 and ste m cell factor-responsive cells, whereas only minimal responsiveness wa s observed by the CD44- populations. These subsets also showed differe ntial cytokine production potential (CD44+CD25- > CD44+CD25+ > CD44-CD 25+ > CD44-CD25-) after stimulation with calcium ionophore, PMA and IL -1. The repopulation potential of these subsets in 2-deoxyguanosine-tr eated fetal thymic lobes supports the following maturation sequence: C D44+CD25- --> CD44+CD25+ --> CD44-CD25+ --> CD44-CD25-. Furthermore, t he sequence of progression from CD44+CD25+ to CD44-CD25+ cells was con firmed by their TCR beta-chain gene configuration. The former populati on exhibits germ-line TCR beta-chain configuration, whereas the latter subset shows are arranged-pattern.