DIFFERENTIATION TO T-HELPER CELLS IN THE THYMUS - GRADUAL ACQUISITIONOF T-HELPER CELL-FUNCTION BY CD3(+)CD4(+) CELLS

We investigated at which point during thymocyte differentiation functi ons were acquired that are characteristic for mature Th cells. Differe ntiation from CD3(+)CD69(-), CD4(+)CD8(+) double-positive (DP) cells t o terminally differentiated CD3(+), CD4(+)CD8(-) single-positive (SP) cells was broken down into six discrete stages that were purified by f our-color sorting: CD69(-)CD3(+)DP (stage 0), CD69(+)CD27(-)DP (stage 1), CD69(+)CD27(-)CD4(+)SP (stage 2), CD27(+)CD1(+)CD4(+)SP (stage 3), CD1(-)CD45RO(+) CD4(+)SP (stage 4), and CD1(-)CD45RO(-)CD4(+)SP cells (stage 5). Phenotypically, these stages seem to describe consecutive steps in differentiation from immature stage 0 to the terminally matur ed stage 5. Functionally, the capacity to proliferate on IL-2 after st imulation was absent in CD69(-) stage 0 cells, but was acquired gradua lly during stages 1 to 4. Clonal expandability and the capacity to res pond to stimulation with the production of cytokines were acquired lat er and rather abruptly by CD1(-) stage 4 and 5 cells. Activation marke rs such as CD69 expression and in vivo IL-2 gene transcription came up simultaneously at the DP stage and peaked at stage 3 to 4. These data suggest that functional maturation of Th cells occurs over an extende d period in differentiation, stages 1 to 4, and coincides with a gradu al increase in activation markers. After completion of functional diff erentiation, at stage 5, in vivo IL-2 mRNA transcription and CD69 expr ession are down-regulated, and the cells become functionally resting n aive T cells expressing CD45RA(+).