DEVELOPMENTALLY-REGULATED EXPRESSION OF THE PD-1 PROTEIN ON THE SURFACE OF DOUBLE-NEGATIVE (CD4(-)CD8(-)) THYMOCYTES

PD-1, a member of the Ig superfamily, was previously isolated from an apoptosis-induced T cell hybridoma 2B4.11 by subtractive hybridization , Expression of the PD-1 mRNA is restricted to thymus in adult mice, U sing an anti-PD-1 mAb (J43), we examined expression of the PD-1 protei n during differentiation of thymocytes in normal adult, fetal and RAG- 2(-/-) mice with or without anti-CD3 mAb stimulation, While PD-1 was e xpressed only on 3-5% of total normal thymocytes, similar to 34% of th e CD4(-)CD8(-) double-negative (DN) fraction are PD-1(+) cells with tw o distinct expression levels (low and high), PD-1(high) thymocytes bel onged to TCR gamma delta lineage cells, In the UN compartment of the T CR alpha beta lineage, PD-1 expression started at the low level from t he CD44(+)CD25(+) stage and the majority of thymocytes expressed PD-1 at the CD44(-)CD25(-) stage in which thymocytes express TCR beta chain s, The anti-CD3 epsilon antibody administration augmented the PD-1 exp ression as well as the differentiation of the CD44(-)CD25(+) DN cells into the CD44(-)CD25(-) UN stage, not only in normal mice but also in RAG-2-deficient mice, The fraction of the PD-1(low) cells in the CD4()CD8(+) double-positive (DP) compartment was very small (<5%) but incr eased by stimulation with the anti-CD3 antibody, although the total nu mber of Up cells was drastically reduced, The results show that PD-1 e xpression is specifically induced at the stages preceding clonal selec tion.