CHARACTERIZATION OF DISTINCT STAGES DURING THE DIFFERENTIATION OF HUMAN CD69(+)CD3(+) THYMOCYTES AND IDENTIFICATION OF THYMIC EMIGRANTS

During thymocyte development, CD69 expression is induced at an early s tage of positive selection. To examine the differentiation of human CD 69(+)CD3(+) thymocytes, discrete phenotypes were defined by the relati ve expression of CD69, CD27, CD1, and CD45RA/RO:CD3(+)CD69(-) thymocyt es were CD27(-)CD1(+)CD45RA(-)/RO(+) (phenotype I), whereas the CD3(+) CD69(+) population could be subdivided into CD27(-)CD1(+)CD45RA(-)/RO( +) (phenotype II), CD27(+)CD1(+)CD45RA(-)/RO(+) (phenotype III), CD27( +)CD1(-)CD45RA(-/dull)/RO(+) (phenotype. IV), and CD27(+)CD1(-)CD45RA( +)/RO(dull) (phenotype V) thymocytes. Phenotype I thymocytes were CD4( +)CD8 alpha beta(+) double positive (DP). Phenotype II thymocytes cont ained DP and CD4(+)CD8 alpha(dull)CD8 beta(-) cells, whereas phenotype III thymocytes were DP and mostly CD4(dull)CD8 alpha beta(+), indicat ing that CD27 on DP cells may be a marker for CD8-committed cells. Res ults obtained with SCID-hu mice, transplanted with human fetal thymus and liver, showed that immature human CD69(-)CD3(+) thymocytes were co rticosteroid-sensitive, whereas essentially all CD69(+)CD3(+) cells we re resistant. During differentiation of one cohort of corticosteroid-r esistant CD69(+)CD3 + thymocytes, phenotype II and III thymocytes, dis appeared within a week, whereas the percentage of phenotype IV and esp ecially V thymocytes increased, suggesting that the latter represent t he end stages in differentiation. Recent thymic emigrants in SCID-hu m ice were identified as CD69(-)CD27(+)CD1(-)CD45RA(-/dull)RO(+) or CD45 RA(+)/RO(dull) cells. Because phenotype IV and phenotype V thymocytes rapidly lose CD69 expression in cell culture, these thymocytes are pro bably the cells that-are exported from the thymus. In conclusion, afte r the acquisition of CD69, thymocyte differentiation appears to contin ue in an ordered pattern, and cells that eventually leave the thymus a re CD69(-)CD1(-) CD45RA(+) or CD45RA(-/dull).